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Agilent-Testing & Troubleshooting RF Communication Receiver Design

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					                                                      Testing and Troubleshooting
                                                      Digital RF Communications
                                                      Receiver Designs
                                                      Application Note 1314




                                                                   I


                                                                   Q




                      Wireless Test Solutions




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      Table of Contents


      Page                                                           Page

       1     Introduction                                            15     3. Troubleshooting Receiver Designs
       2     1. Digital Radio Communications Systems                 15     3.1 Troubleshooting Steps
       3     1.1 Digital Radio Transmitter                           15     3.2 Signal Impairments and Ways to Detect Them
       3     1.2 Digital Radio Receiver                              16        3.2.1 I/Q Impairments
       3         1.2.1 I/Q Demodulator Receiver                      17        3.2.2 Interfering Tone or Spur
       4         1.2.2 Sampled IF Receiver                           17        3.2.3 Incorrect Symbol Rate
       4         1.2.3 Automatic Gain Control (AGC)                  18        3.2.4 Baseband Filtering Problems
       5     1.3 Filtering in Digital RF Communications Systems      19        3.2.5 IF Filter Tilt or Ripple
                                                                     19     3.3 Table of Impairments Versus Parameters Affected
       6     2. Receiver Performance Verification
                Measurements                                         20     4. Summary
       6     2.1 General Approach to Making Measurements
       7     2.2 Measuring Bit Error Rate (BER)                      20     5. Appendix: From Bit Error Rate (BER) to
                                                                               Error Vector Magnitude (EVM)
       8     2.3 In-Channel Testing
       8         2.3.1 Measuring Sensitivity at a Specified BER      22     6. Symbols and Acronyms
       9         2.3.2 Verifying Co-Channel Rejection
       9     2.4 Out-of-Channel Testing
                                                                     23     7. References
       9         2.4.1 Verifying Spurious Immunity
      10         2.4.2 Verifying Intermodulation Immunity
      11         2.4.3 Measuring Adjacent and Alternate Channel
                 Selectivity
      14     2.5 Fading Tests
      14     2.6 Best Practices in Conducting Receiver Performance
                 Tests




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      Introduction


      This application note presents the          The digital radio receiver must
      fundamental measurement principles          extract highly variable RF signals
      involved in testing and troubleshooting     in the presence of interference and
      digital RF communications receivers—        transform these signals into close
      particularly those used in digital RF       facsimiles of the original baseband
      cellular systems. Measurement               information. Several tests verify
      setups are explained for the various        receiver performance in the presence
      receiver tests, and troubleshooting         of interfering signals. These
      tips are given.                             performance verification tests are
                                                  categorized as either in-channel or
      The demand for ubiquitous wireless          out-of-channel measurements.
      communications is challenging the
      physical limitations of current wire-       This application note includes:
      less communications systems.
      Wireless systems must operate in a          • A block diagram of a digital radio
      very limited area of the radio spectrum       communications system.
      and not interfere with other systems.       • Common receiver designs.
      The maturing wireless markets are
      becoming much more competitive,             • In-channel tests, including sensitivity
      and product cycle times are now               and co-channel immunity.
      measured in months instead of years.        • Out-of-channel tests, including
      Consequently, network equipment               spurious and intermodulation
      manufacturers must produce wireless           immunity and adjacent and alternate
      systems that can be quickly deployed          channel selectivity.
      and provide bandwidth-efficient
      communications.                             • Best practices in the receiver
                                                    performance tests.
      Digital modulation has several
      advantages over analog modulation.          • Troubleshooting techniques for
      These include bandwidth efficiency,           receiver designs.
      superior immunity to noise, low             • An appendix that relates Bit Error
      power consumption, privacy, and               Rate (BER) to Error Vector
      compatibility with digital data services.     Magnitude (EVM).
      These advantages, coupled with
      advances in digital signal processing       The setups required to perform
      and in analog-to-digital conversion,        the receiver performance tests are
      have spawned the current migration          included in this application note
      to digital RF communications formats.       along with descriptions of potential
                                                  errors in the measurement process.
      Digital RF communications systems           Troubleshooting techniques applicable
      use complex techniques to transmit          to the design of digital radio receivers
      and receive digitally modulated signals     are also provided.
      through the radio channel. These
      complexities challenge designers
      in the isolation of system problems.
      Signal impairments can be traced
      back to a component, device, or
      subsystem of the digital RF communi-
      cations system. Successful receiver
      design depends on the ability to
      locate sources of error.




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      1. Digital Radio Communications Systems


      The digital radio signal experiences              Consequently, the measurement                                Certain parts of the digital radio may
      many transformations in its migration             challenges are similar for both parts                        be implemented in a Digital Signal
      from a baseband signal at the trans-              of the digital radio system. However,                        Processor (DSP), an Application-
      mitter to its replication at the receiver.        unique problems exist at various                             Specific Integrated Circuit (ASIC),
      A rudimentary block diagram of a                  locations in the system. For example,                        or a Digital Down Converter (DDC).
      digital radio communications system               the receiver must detect weak signals                        The DSP, ASIC, or DDC has different
      (Figure 1) reveals the transformation             in the presence of noise and is there-                       levels of involvement in the various
      process the signal undergoes from                 fore tested with very low level signals.                     digital radio designs. Sometimes it is
      origination to reception.                         The transmitter must not interfere                           difficult to distinguish those problems
                                                        with other radio systems and is                              originating in the digital portion of
      The system-level diagram in Figure 1              consequently tested for the amount                           the radio from those originating in
      displays the symmetry of the digital              of interference it produces in the                           the analog portion. This application
      radio. To a certain degree, the receiv-           nearby frequency channels.                                   note describes how to isolate and
      er can be considered a reverse imple-                                                                          clarify sources of error in digital
      mentation of the transmitter.                                                                                  radio receiver tests and designs.


      Figure 1. Block Diagram of a Digital Radio System


                                                                        Transmitter

                                                                        Baseband          I/Q
                                                                          Filter        Modulator       IF Filter      Upconverter   Amplifier
                                                                    I               I
              Input               Channel Coding/       Symbol
          (Data or Voice)          Interleaving/        Encoder
                                    Processing
                                                                    Q              Q

                                                                                                                                 Power Control
                                                                                                    IF LO                        RF LO


                                                            Channel


                                                                         Receiver
               Preselecting                                                                                     Baseband
                  Filter                Downconverter   IF Filter   Downconverter                                 Filter
                                                                                                            I              I
                                                                                                                                 Bit          Output
                                                                                        Demodulator                            Decoder       (Data or
                                                                                                                                              Voice)
                                                                                                        Q                  Q
                              Low-Noise
                            Amplifier with
                              Automatic
                             Gain Control           RF LO                          IF LO




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      1.1 Digital Radio Transmitter               Of the many different ways to imple-                 After downconversion to the IF, the
                                                  ment a digital radio receiver, most                  signal is separated into two distinct
      The digital radio transmitter (Figure 1)
                                                  designs fall into two basic categories:              paths. To convert to baseband,
      accepts a baseband waveform and
                                                  I/Q demodulation and sampled IF.                     each path is mixed with an LO whose
      translates that signal into a waveform
                                                                                                       frequency equals the IF frequency.
      that it can effectively transmit
                                                  1.2.1 I/Q Demodulator Receiver                       The upper-path signal (I) is simply
      through the channel. Before the
                                                                                                       mixed with the LO and then filtered.
      transformation from baseband to a
                                                  I/Q demodulation implemented with                    In the lower path, a 90° phase shift is
      Radio Frequency (RF) channel, the
                                                  analog hardware is a commonly used                   introduced in the mixing signal. This
      waveform is digitized to utilize the
                                                  digital radio receiver design. The                   lower-path signal (Q) is converted to
      advantages of digital modulation.
                                                  function of the analog I/Q demodulator               baseband by mixing with the phase-
      Coding is applied to the signal to
                                                  (Figure 3) is to recover the baseband I              shifted LO signal, and then filtered.
      more efficiently use the available
                                                  and Q symbols.                                       This process produces the in-phase
      bandwidth and to minimize the
                                                                                                       (I) and out-of-phase (Q) baseband
      effects of noise and interference that
                                                                                                       components of the data stream.
      will be introduced by the channel.
                                                                                                       For a detailed explanation of I/Q
      The coded signal is filtered, modulated,
                                                                                                       modulation, consult (Ref. 2, pg. 23).
      and changed back to an analog wave-
      form that is converted to the desired
      frequency of transmission. Finally,         Figure 2. Receiver Block Diagram
      the RF signal is filtered and amplified
      before it is transmitted from the
      antenna. A more detailed description                Preselecting
      of digital transmitters can be found in                Filter                Downconverter     IF Filter
      the companion Agilent Technologies                                                                                             Output
      application note, Testing and                                                                              Demodulator
                                                                                                                                    (Data or
                                                                                                                 and Decoder
      Troubleshooting Digital RF                                                                                                     Voice)
      Communications Transmitter                                         Low-Noise
      Designs (Ref. 1, pg. 23).                                        Amplifier with
                                                                         Automatic
                                                                        Gain Control
      1.2 Digital Radio Receiver                                                               LO
      The digital radio receiver (Figure 2)
      can be implemented several ways,
      but certain components exist in all
      receivers. The receiver must extract        Figure 3. I/Q Demodulator
      the RF signal in the presence of
      potential interference. Consequently,                                                                                   Baseband
      a preselecting filter is the first compo-                                                                  Mixer          Filter
      nent of the receiver, and it attenuates
      out-of-band signals received by the                                                                                                      ADC       I
      antenna. A Low-Noise Amplifier
      (LNA) boosts the desired signal level
      while minimally adding to the noise              Preselecting
      of the radio signal. A mixer down-                  Filter               Downconverter   IF Filter                 LO
      converts the RF signal to a lower
      Intermediate Frequency (IF) by

                                                                                                                 φ
      mixing the RF signal with a Local
                                                                      Low-Noise
      Oscillator (LO) signal. The IF filter                            Amplifier
                                                                                                                         90-Degree Phase Shifter
      attenuates unwanted frequency
      components generated by the mixer                                                                                       Baseband
                                                                                          LO                                    Filter
      and signals from adjacent frequency
      channels. After the IF filter, the                                                                                                       ADC       Q
      variations in receiver design
      manifest themselves.                                                                                       Mixer




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      Although the I/Q demodulator receiver             1.2.2 Sampled IF Receiver                  1.2.3 Automatic Gain Control (AGC)
      is a popular design, it has potential                                                        AGC is used in digital radio receivers
      problems. Unequal gain in the I and               To decrease analog hardware
                                                                                                   to handle the wide range of signal
      Q paths and/or a relative phase shift             complexity, the digitally modulated
                                                                                                   levels encountered at the receiving
      other than 90° (quadrature error)                 signal can be sampled earlier in the
                                                                                                   antenna. AGC compresses the signal
      will cause image suppression problems             signal path, which increases the
                                                                                                   range by reducing the gain of the IF,
      in the baseband mixers. I/Q demodu-               digital or software complexity of
                                                                                                   and sometimes the RF, stages as the
      lators inherently produce a spurious              the receiver design. The sampled IF
                                                                                                   signal level increases. A strong RF
      response at DC (that is, in the center            receiver converts the analog signal to
                                                                                                   signal can overdrive the mixer and
      of the passband) regardless of the                a digital data stream earlier than the
                                                                                                   cause excessive signal distortion.
      input frequency. As a result, I/Q                 I/Q demodulator does (Figure 4).
                                                                                                   The receiver must also process weak
      demodulators are most commonly                                                               RF signals in the presence of noise.
                                                        In this receiver, the IF signal is
      used in single-channel base station                                                          Therefore, the RF portion of the
                                                        digitized. The sampled data stream
      receivers that have a separate receiver                                                      receiver may incorporate AGC to
                                                        from the ADC is digitally demodulated
      for each frequency channel, rather                                                           process the full range of signal levels
                                                        into its I and Q components, and the
      than in multi-channel base station                                                           presented to it. Used in the IF stage,
                                                        original signal is reconstructed.
      receivers that use a single, wide-                                                           AGC can prevent overload and
      bandwidth receiver for the entire                 This type of receiver is becoming          maintain a reasonably constant
      band of frequencies.                              more popular because of advances           signal input to the demodulator stage.
                                                        in ADCs and DSPs. The sampled IF           For all applications, the AGC circuitry
      The I and Q data streams are sampled
                                                        receiver design requires less analog       must maintain allowable levels of
      by Analog-to-Digital Converters (ADCs).
                                                        hardware than the I/Q demodulator          signal distortion over a broad range
      This allows filtering and signal
                                                        type and does not split the analog         of power levels. Also, the AGC should
      corrections to be performed with
                                                        signal into two paths. The I/Q             respond quickly to signal level changes
      digital signal processing. Baseband
                                                        demodulation is actually performed         as it processes signals over its entire
      filtering by a DSP, ASIC, or DDC
                                                        in a DSP, ASIC or DDC. Digital I/Q         dynamic range.
      removes many of the problems
                                                        demodulation avoids phase and
      associated with analog filter imple-
                                                        amplitude imbalance between I and
      mentations (for example, phase and
                                                        Q signals. The trade off is more digital
      group delay problems) and provides
                                                        signal processing and power-hungry
      filter characteristics closer to ideal
                                                        ADCs fast enough to capture all the
      than those of analog filters. Baseband
                                                        information in the analog signal
      filtering, whether it is analog or digital,
                                                        (two factors that reduce battery life
      is better behaved than IF filtering.
                                                        in mobile phones). As with the I/Q
                                                        demodulator, the sampled IF receiver
                                                        requires a downconverter that does
                                                        not degrade the incoming signal.


      Figure 4. Sampled IF Receiver

            Preselecting
               Filter               Downconverter   IF Filter

                                                                ADC

                       Low-Noise
                        Amplifier


                                               LO




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      1.3 Filtering in Digital RF                Gaussian filters, such as those used
      Communications Systems                     in GSM systems, do not provide the
                                                 theoretical zero ISI like the Nyquist
      Distortion-free transmission of the        filters do. The Gaussian filter has a
      digital I and Q signals theoretically      Gaussian shape in the time and
      requires infinite bandwidth. An             frequency domains, and it does
      infinite-bandwidth RF communications       not go to zero at the symbol spacing.
      system would interfere with other          This causes some ISI, but each symbol
      systems and would not provide              interacts significantly with only the
      efficient use of radio spectrum.           preceding and succeeding symbols.
      Filtering narrows the bandwidth of         The bandwidth-time product (BT) of
      RF systems, but it also slows down         the Gaussian filter corresponds to the
      signal transitions.                        alpha of the Nyquist filter, and typical
                                                 BT values range from 0.3 to 0.5.
      Baseband filtering rounds off the          Unlike Nyquist filters, Gaussian filters
      rapid transitions in the transmitted       are not split into matched pairs in the
      data, but this can cause Inter-Symbol      transmitter and receiver. They are
      Interference (ISI). A Nyquist filter,      only used in the transmitter. GSM
      which is a type of raised-cosine filter,   receivers typically use Butterworth
      minimizes ISI by forcing the filter’s      filters that have a sharper roll-off than
      impulse response to zero at the symbol     the Gaussian filters. Consequently,
      points (except at the center of the        sensitivity is improved because less
      filter). Thus, the time response of the    out-of-channel noise and interference
      Nyquist filter (Figure 5) goes through     is allowed into the receiver’s pass-
      zero with a period that exactly            band.
      corresponds to the symbol spacing.
      Adjacent symbols do not interfere          A more thorough examination of
      with each other at the symbol times        filtering is provided in (Ref. 2, pg. 23).
      because the response equals zero at all
      symbol times except the desired one.

      The sharpness of a raised-cosine           Figure 5. Impulse Response of a Nyquist Filter
      filter is described by its alpha (α)
      and quantifies the occupied band-
                                                       1
      width of the signal. An ideal (“brick
      wall”) filter would have an alpha of
      zero. Typical alpha values range from           0.5
      0.35 to 0.5. Filter alphas also affect     hi
      transmitted power. A low alpha                   0
      value results in low occupied band-                     Symbol Period

      width but requires high peak transmit
      power. Consequently, the filter alpha             -10       -5           0              5   10
                                                                               ti
      must be carefully chosen to achieve a
      balance between spectral occupancy
      and required transmit power. In
      some systems, a root-raised-cosine
      filter is implemented at both ends of
      the digital radio, and the resulting
      overall filter response is a raised
      cosine.




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      2. Receiver Performance Verification Measurements


      This section contains test setups and      2.1 General Approach to                    2.2 Measuring Bit Error Rate
      procedures for performance tests on
      digital radio receivers. Each receiver
                                                 Making Measurements                        (BER)
      must meet strict performance criteria      The most comprehensive receiver            BER is the fundamental measurement
      defined by the various standards of        test is to evaluate the reconstructed      used when testing receiver performance
      the telecommunications industry’s          baseband signal that has been              parameters such as sensitivity and
      governing bodies (the ITU, ETSI,           processed by the receiver. In this         selectivity. It is the percentage of
      TIA, and others). Design teams             test, one piece of test equipment          erroneous bits received compared
      must develop performance criteria          stimulates the antenna port of the         to the total number of bits received
      for their receiver, or for a portion of    receiver and is considered to be an        during an observation period.
      that receiver, and conduct unique          ideal transmitter. Another instrument      Virtually all BER test instruments use
      performance tests to verify correct        monitors the demodulated digital bit       a Pseudo-Random Binary Sequence
      implementation and modeling of             stream. If desired, impairments can be     (PRBS) as the test signal. PRBS signals
      components in the receiver. More-          introduced by inserting interference       are usually labeled PNx, where x is
      over, these performance tests verify       in the channel between the source          the number of bits being permutated
      receiver compliance prior to the           and the receiver, or by altering           in the sequence (for example,
      design’s submission for type               parameters in the source, to determine     PN9 = 29–1 or 511 bits). Since an
      approval.                                  the receiver’s ability to operate prop-    entire PNx sequence can be recon-
                                                 erly under less-than-ideal conditions.     structed from any sequence of “x”
      Performance verification tests are                                                    bits, using a PRBS signal eliminates
      divided into in-channel and out-of-        The following tests assume the receiver    the need to synchronize the received
      channel measurements. In-channel           is complete. If the digital portion of     and transmitted bits. Alternatively,
      measurements test the receiver’s           the receiver is unavailable for testing    the entire PRBS is reconstructed in
      operation within the frequencies           (for example, if it’s still under devel-   the BER tester (BERT) receiver from
      occupied by the desired signal. Out-       opment), then the analog RF designer       the first correct “x” bits received. The
      of-channel measurements verify that        needs to establish performance goals       received signal is then compared to the
      the receiver is not being adversely        for the analog portion of the receiver.    reconstructed correct bit sequence.
      affected by (or affecting) other signals   Typical performance goals are the          For a thorough explanation of BER
      outside its specific frequency channel.    estimated optimum noise figure for         testing, see (Ref. 3, pg. 23).
      Although the performance tests pre-        the receiver to pass the performance
      sented in this application note focus      verification tests and the estimated       Two popular methods exist for BER
      on digital RF cellular applications,       optimum Signal-to-Noise Ratio (SNR)        testing of mobile phones: baseband
      many of the concepts and tests apply       for proper ADC operation (at the           BER and loopback BER. The feature
      to other forms of digital RF               digital conversion point).                 set of the unit-under-test (UUT)
      communications.                                                                       dictates which test method to use.
                                                                                            For the baseband BER test, the
                                                                                            demodulated PRBS signal at the
                                                                                            receiver remains at baseband and is
                                                                                            compared to the reconstructed PRBS
                                                                                            by the BERT (Figure 6). Typically,
                                                                                            CDMA mobile phones and sub-
                                                                                            assemblies use the baseband BER
                                                                                            measurement method.

                                                                                            Conversely, for the loopback BER test
                                                                                            the received signal is retransmitted,
                                                                                            or looped back to a receiver (Figure 7).
                                                                                            In the loopback test, the UUT
                                                                                            demodulates the incoming RF signal,
                                                                                            decodes it, then re-encodes the data
                                                                                            stream (with possible errors), and
                                                                                            retransmits the signal, often to the
                                                                                            original transceiver. To attain the
                                                                                            BER, this received signal is compared




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      to the expected PRBS that is                 Figure 6. Baseband BER Test Configuration
      reconstructed by the BERT (Ref. 3,
      pg. 23). GSM handsets are tested
      using the loopback method.
                                                                                                            RF Signal
      The Agilent E4438C ESG signal gener-
      ator can be configured to provide the
                                                                    Agilent E4438C ESG
      RF signal that carries the PRBS and
                                                                     Signal Generator
      perform the BER measurement.

      Data is managed in telecommunications
      systems by a hierarchical system of bit                            RF Signal
      grouping. Speech frames are nearly                                  Source
      the lowest-level building blocks in
      this hierarchical system. Not all bits
      in a speech frame are equally impor-
      tant. Some bits are so important that                              Encoder
                                                     Baseband                                                   Baseband Signal
      the entire frame is erased if any of                                                                      00110110110001
                                                     Modulator
      them are bad. This leads to a new                                  Pattern
      parameter for expressing receiver                                 Generator
      performance—Frame Erasure Rate
      (FER). It is the percentage of erased
      frames compared to the total number             Bit Error
                                                     Rate Tester        Comparator
      of frames sent during an observation          (Option UN7)
      period. Frame erasure also leads to a
      modification of our BER measurement.
      When frames are erased, only the
      BER of the remaining frames is
      measured. This parameter is called
      residual BER (RBER).

      Figure 7. Loopback BER Test Configuration (this test set-up applies only to GSM/EDGE)



                                                RF Signal              RF Signal


                      Agilent E4438C ESG                                                 Agilent E4440A PSA
                       Signal Generator                                                  Spectrum Analyzer



                                                                                              RF Signal
                                                                                              Receiver


                                     PRBS

                           Encoder
       Baseband
                                                                                                          IF Signal
       Modulator
                           Pattern
                          Generator




                         Demodulator

        Option
                           Decoder
         300

                         Comparator
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      2.3 In-Channel Testing                                                   with a very accurate signal set to a     may be Continuous Wave (CW),
                                                                               relatively low power level and see if    narrowband, or of the same type as
      The most significant in-channel test
                                                                               the receiver output is acceptable.       the desired signal. The ability of the
      measures the sensitivity of the receiver.
                                                                               Alternatively, the signal level is       receiver to remain sensitive to the
      Sensitivity specifies the minimum
                                                                               adjusted for a specified SNR or other    desired signal while subjected to the
      signal level for a specified percentage
                                                                               performance metric. For analog FM        interfering signal is a measure of its
      of errors in the demodulated informa-
                                                                               receivers, the performance metric is     co-channel immunity.
      tion. As the separation between
                                                                               known as SINAD (12 dB is typical).
      transmitter and receiver increases, or
                                                                               SINAD is the ratio of signal-plus-       2.3.1 Measuring Sensitivity at a
      as fading occurs in the radio channel,
                                                                               noise-plus-distortion to the noise-
      the signal will drop into the noise                                                                               Specified BER
                                                                               plus-distortion at the same output.
      floor from the perspective of the
                                                                               Similarly, for digital receivers the     Sensitivity is one of the key specifica-
      receiver. Information will be lost
                                                                               specified performance metric is the      tions for a digital radio receiver and
      when the signal approaches the noise
                                                                               BER or FER (Figure 8).                   is specified at a particular BER (or
      floor. The ability of the receiver to
      capture the information in a signal                                                                               FER). Sensitivity is the minimum
                                                                               Co-channel immunity testing is similar
      as it drops to very low levels is a                                                                               received signal level that produces
                                                                               to sensitivity testing. The level of
      function of the receiver’s sensitivity.                                                                           a specified BER when the signal is
                                                                               signal distortion is monitored with an
      The go/no-go method for sensitivity                                                                               modulated with a bit sequence of
                                                                               interfering signal present in the same
      testing is to stimulate the receiver                                                                              data.
                                                                               RF channel. The interfering signal
                                                                                                                        Because sensitivity is often
      Figure 8. Understanding SINAD                                                                                     expressed in voltage units, such as
                                                                                                                        µV, the following equation will be
      The top curve in Figure 8 is the desired audio output of the receiver. As the RF input to the receiver is         used to convert to dBm:
      reduced, this curve falls off. The bottom curve is the residual hum and noise of the receiver. As the RF
      input is reduced, the AGC of the receiver adds gain, which increases the residual hum and noise.                  dBm = 10 * log (Vrms2/Zo) + 30
      SINAD is the difference between these two curves. The level of RF input required to maintain a SINAD
      of 12 dB is generally defined as the sensitivity of an FM receiver.                                               where Vrms = receiver sensitivity
                                                                                                                                     in volts rms
                                                                                                                                Zo = receiver impedance
                                                                                                                                     (typically 50Ω).

                                                                                       Desired Audio Signal             For example, if a receiver has a sensi-
                                                                                                                        tivity expressed as 1 µV, the sensitivity
       Audio Output of Receiver (dB)




                                                                                                                        can be converted to –107 dBm for a
                                                                                                                        system with a 50Ω impedance.

                                                                                                                        To perform the sensitivity test,
                                                  12 dB SINAD                                                           connect a signal source to the anten-
                                                                                                                        na port of the receiver with a cable of
                                                                                                                        known loss. Then connect the output
                                                                                       Residual Hum & Noise             of the receiver to the BERT (Figure 9).

                                                                                                                        If the approximate sensitivity is
                                                                                                                        unknown, the signal level should be
                                                                                                                        set to a nominal level (such as –90
                                                           RF Input to Receiver (µV)                                    dBm) and decreased until the
                                                                                                                        specified BER occurs. The sensitivity
                                                                                                                        is the power level of the signal minus
      Figure 9. Sensitivity Measurement Setup                                                                           the loss in the cable. For example, if
                                                                                                                        the signal generator is transmitting a
                                        Signal Generator                                                                –106 dBm signal when the specified
                                                                                  Modulated RF Signal                   BER is reached and the cable loss is
                                                                                                                        4 dB, then the sensitivity is –110 dBm
                                                                                       DUT                     BERT
                                                                                                                        for the receiver.
                                                                                                        Data
                                       Agilent E4438C ESG


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      2.3.2 Verifying Co-Channel Rejection       2.4 Out-of-Channel testing                        Intermodulation immunity tests for
                                                                                                   distortion products that are generated
      Most receivers are required to maintain    Out-of-channel, or blocking, tests
                                                                                                   when more than one tone is present
      a specified BER in the presence of an      verify correct receiver operation in
                                                                                                   at the input of the receiver. In this
      interfering signal within the channel.     the presence of out-of-channel signals
                                                                                                   test, two interfering signals are
      Frequently, this co-channel interfering    and monitor the receiver’s susceptibility
                                                                                                   combined with the desired signal
      signal will be a CW signal. Figure 10      to internally generated spurious
                                                                                                   at the input of the receiver. The
      illustrates the test setup for the co-     responses. Three prominent out-of-
                                                                                                   frequencies of the interfering signals
      channel rejection measurement. This        channel tests verify receiver perfor-
                                                                                                   are set such that one of the third-order
      setup includes a power combiner that       mance: spurious immunity, intermod-
                                                                                                   intermodulation products lies within
      has some power loss associated with        ulation immunity, and adjacent/
                                                                                                   the passband of the receiver. The
      it. Maximum insertion loss of most         alternate channel selectivity. For
                                                                                                   power of these interferers is raised
      2-way resistive combiners is near          certain digital formats, the single-
                                                                                                   until the sensitivity of the receiver
      6 dB when combining two noncoher-          tone blocking test verifies receiver
                                                                                                   is compromised.
      ent signals such as in this test. For      performance with a large signal in
      all measurements using a power             a nearby frequency channel. A large               Adjacent channel selectivity measures
      combiner, the combiner loss should         single tone slightly offset from the              the ability of the receiver to process
      be characterized and offset by an          carrier frequency could desensitize               the desired signal with a strong signal
      increase in signal power from the          a receiver to the desired signal. The             in the adjacent channel. Alternate
      signal generators.                         single-tone blocking test is straight-            channel selectivity is a similar test in
                                                 forward and will not be covered in                which the interfering signal is two RF
      The frequency of the desired signal, a     this application note.                            channels away from the passband of
      digitally modulated test signal, is set                                                      the receiver.
      to the center of the passband of the       Spurious immunity is the ability
      receiver. The power of this signal is      of the receiver to prevent single,
      typically set to a level relative to the   unwanted signals from causing an                  2.4.1 Verifying Spurious Immunity
      measured sensitivity of the receiver       undesired response at the output of
                                                                                                   Spurious responses, also called spurs,
      (for example, 3 dB above). The             the receiver. Spurious immunity is
                                                                                                   manifest themselves in radio receivers
      frequency of the interfering signal        similar to co-channel immunity, but
                                                                                                   in two ways: they are generated
      is set within the passband of the          the interfering signals occur across a
                                                                                                   internally by the receiver, or they
      receiver. The power level of the           broad range of frequencies instead of
                                                                                                   result from the interaction of the
      interfering signal is set to a nominal     in-channel.
                                                                                                   receiver with external signals.
      level at which the BER of the receiver
      must not exceed the specified level.
      The required BER level is usually the      Figure 10. Co-Channel Rejection Measurement Setup
      same level specified for the receiver
      sensitivity measurement. The differ-                                           Modulated RF Signal (Desired)
      ence in power levels between the two
      signals is the interference ratio.                     Signal Generator                            Combiner

      For example, suppose a 931.4375                                                                        Σ             DUT
      MHz pager has a sensitivity of –105
      dBm with a BER of 3%. The desired
      signal is set to 931.4375 with a power          Agilent E4438C ESG with BERT
      level of –102 dBm. At this power                                                                                    Demodulated,
      level the BER is less than 3%. The                    Signal Generator                                              Decoded Data
      channel width for the pager is
      25 kHz. The interfering signal is set
                                                                                                                In-band CW or
      to 931.4380 MHz. The power level                                                                        Modulated RF Signal
      of the interfering signal is first set                                                                     (Interfering)
      to –105 dBm and gradually increased                  Agilent E4438C ESG
      until the BER is again 3%. If a level
      of –97 dBm is required to return the
      BER to 3%, then the co-channel
      rejection is 5 dB.




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      Both types of spurs should be identified.            internally generated spurs must be               output amplitude of the interfering
      Replacing the antenna of the receiver                identified (as described above) and              signal is set at a specific level at which
      with a load will ensure that the                     should be below the specified level.             the BER of the receiver under test
      receiver is not picking up any stray                 To perform the spurious immunity                 must be less than a specified level
      signals. Connect the final analog                    measurement, one signal generator                (usually the BER specified in the
      output of the receiver to a spectrum                 supplies a modulated test signal in              sensitivity test). The amplitude
      analyzer. Any spur viewed on the                     the desired RF channel at a level                difference between the test signal and
      spectrum analyzer is internally                      above the sensitivity of the receiver            the interfering signal is the spurious
      generated by the receiver and may                    (usually 3 dB above). The second                 immunity (SI) of the receiver:
      be a harmonic of the power supply,                   signal generator supplies an interfering
      a harmonic of the system clock, or a                 signal. This interfering signal is               SI = Pint – Ptest (dB)
      spur from an LO.                                     adjusted to several frequencies to
                                                                                                            Spurs from the signal generator used
                                                           verify the receiver’s immunity to
      Spurious response immunity is a                                                                       to provide the interfering signal can
                                                           spurs (Figure 11).
      measure of the receiver’s ability to                                                                  cause a good receiver to appear bad.
      prevent single, unwanted signals                     The interfering signal may be                    Any spurs created by the interfering
      from causing an undesired response                   modulated or unmodulated, depending              signal generator should be less than
      at the output of the receiver. Prior                 upon the frequency range and the                 the receiver’s spurious immunity.
      to making this measurement, the                      communications standard. The
                                                                                                            2.4.2 Verifying Intermodulation
      Figure 11. Spurious Immunity Measurement Setup                                                        Immunity
                                                                                                            Intermodulation products may be
                                             Modulated RF Signal (Desired)
                                                                                                            generated within the receiver when
                    Signal Generator                                                                        more than one signal is present at the
                                                                    Combiner
                                                                                                            input of the receiver. Intermodulation
                                                                       Σ               DUT                  products are caused by receiver non-
                                                                                                            linearities. Two-tone intermodulation
                                                                                                            is a common method of testing a
                                                                                                            receiver. The test signal is the same
              Agilent E4438C ESG with BERT                                                                  signal used in other measurements
                                                                                           Demodulated,
                    Signal Generator                                                       Decoded Data     (for example, spurious immunity).
                                                                                                            The frequencies of the interfering
                                                                                                            signals are set such that one of the
                                                                        Out-of-band CW or                   third-order intermodulation products
                                                                        Modulated RF Signal
                                                                                                            (frx1 = 2f1 – f2 and frx2 = 2f2 – f1) falls
                                                                           (Interfering)
                                                                                                            within the passband of the receiver
                   Agilent E4438C ESG                                                                       (Figure 12).

                                                                                                            The power levels of the interfering
      Figure 12. Intermodulation Products                                                                   signals are set equal to each other at
                                                                                                            a specified level, and the BER of the
                        f2                                                             f1      f2           desired signal is checked. As with
                                                                                                            other receiver tests, the required BER
              f1                                                                                            level is usually the BER at which the
                             Antenna                                                                        sensitivity is measured.

         f1        f2                                                          frx1                  frx2
                                        Preselecting   Low-Noise
                                           Filter       Amplifier                             IF Filter




                                        frx1=2f1– f2
                                        frx2=2f2– f1                                  LO



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      Whenever two signals are input to         The second signal generator inputs            For example, the sensitivity of
      a combiner, the nonlinearities of         either the adjacent channel signal,           an NADC base station receiver is
      the signal generators may create          offset by one channel spacing, or the         specified at –110 dBm with a BER
      intermodulation products (Figure 13).     alternate channel signal, offset by two       of 10-3, or 0.1%. The adjacent channel
      There are several techniques for          channel spacings. The out-of-channel          specification requires that the BER be
      reducing signal generator                 signal is set to a specified level at         no worse than 10-3 with the in-channel
      intermodulation products:                 which the BER of the test signal is           signal set to –107 dBm, a 3 dB
                                                below a certain rate (usually the same        increase, and the adjacent channel
      1) maintain a frequency separation        level specified in the sensitivity test).     signal set to –94 dBm, or 13 dB above
      between the interfering signals that                                                    the in-channel signal level. This
      is greater than the bandwidth of the
      Automatic Level Control (ALC) of the
      sources; 2) add attenuators to the        Figure 13. Intermodulation Immunity Measurement Setup
      outputs of the signal generators; 3)
      use hybrid combiners; 4) use isolators;                                   Modulated RF Signal (Desired)
      and 5) turn off the ALC of the sources.                                                         Combiner
                                                            Signal Generator
      All these techniques may be applied                                                                 Σ                DUT
      simultaneously to reduce intermodu-
      lation products. Maintaining a large
      frequency separation is usually the
      most effective means to combat this            Agilent E4438C ESG with BERT                                          Demodulated,
      problem. For example, if the ALC                                                                                     Decoded Data
                                                           Signal Generator
      bandwidth is 1 kHz, the signal
      separation should be at least 10 kHz.
                                                                                                                    Out-of-band CW or
      If this cannot be done, adding attenu-                                                                       Modulated RF Signals
      ation at the signal generator outputs                                                                            (Interfering)
      theoretically reduces the intermodu-
      lation products 3 dB for every 1 dB                 Agilent E4438C ESG
      of attenuation.
                                                           Signal Generator

      2.4.3 Measuring Adjacent and
      Alternate Channel Selectivity
      Adjacent and alternate channel
                                                          Agilent E4438C ESG
      selectivity measure the receiver’s
      ability to process a desired signal
      while rejecting a strong signal in an
      adjacent channel (one channel away)       Figure 14. Adjacent and Alternate Channel Selectivity Test Setup
      or alternate channel (usually two
      channels away). The selectivity                                           Modulated RF Signal (Desired)
      tests are very important for commu-                   Signal Generator                         Combiner
      nications receivers in which channel
      spacings are narrow and adjacent                                                                   Σ               DUT
      and alternate channel power is hard
      to control (for example, Specialized
      Mobile Radio, or SMR). An adjacent             Agilent E4438C ESG with BERT
      and alternate channel selectivity test                                                                             Demodulated,
                                                           Signal Generator                                              Decoded Data
      setup is shown in Figure 14. One signal
      generator inputs a test signal at the
      desired channel frequency at a level
      relative to the sensitivity of the                                                                     Modulated RF Signal
      receiver (usually 3 dB above).                                                                            in Adjacent or
                                                                                                              Alternate Channel
                                                          Agilent E4438C ESG                                     (Interfering)




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      implies that the adjacent channel signal       where                                    For a receiver with a noise-equivalent
      cannot increase the receiver noise floor        Φn = signal generator SSB phase         bandwidth of 14 kHz, a Pac at the
      by more than 3 dB. For alternate                       noise (dBc/Hz) at the channel    adjacent channel of 70 dB, a margin
      channel selectivity, the alternate                     spacing offset                   of 10 dB, and a channel spacing of
      channel signal is set to –65 dBm,               Pac = adjacent or alternate channel     25 kHz, the required SSB phase noise
      or 42 dB above the in-channel signal                   selectivity specification (dB)   is –122 dBc/Hz at a 25 kHz offset.
      level. Figure 17 displays the specified         Be = receiver noise-equivalent          This is typical for an analog FM
      NADC adjacent and alternate channel                    bandwidth (Hz)                   receiver. Unlike the FM receiver in
      selectivity spectrum.                           Pmar = test margin (dB)                 this example, most digital communi-
                                                                                              cations receivers have adjacent
      In addition to level accuracy, the             Since Pac and Be are fixed by the        channel selectivity values less than
      spectral characteristics of the test           specifications or design, the test       15 dB. For a GSM receiver with a
      and interfering signals are important.         margin determines the power that         noise-equivalent bandwidth of 200
      For many receivers, the single sideband        the signal generator phase noise will    kHz, a Pac at the adjacent channel of
      (SSB) phase noise of the signal                be allowed to contribute to the IF       9 dB, a margin of 10 dB, and a channel
      generator used to produce the                  passband of the receiver. A large test   spacing of 200 kHz, the required SSB
      interfering signal is a critical spectral      margin increases the confidence that     phase noise is –72 dBc/Hz at a 200 kHz
      characteristic. If the phase noise             the receiver operates properly in the    offset. The required SSB phase noise
      energy inside the passband of the IF           presence of SNR degradation due to       is driven primarily by Pac.
      filter is excessive, the receiver may          fading in the channel or due to
      appear to fail the test (Figure 15).           imperfections in receiver components.    Table 1 lists the values of adjacent
                                                     For a system using a new technology      and alternate channel selectivity for
      The required signal generator SSB              or new operating frequencies, a large    various communications systems as
      phase noise may be calculated from:            test margin should be used to            well as the required signal generator
                                                     compensate for uncertainties.            SSB phase noise. A 10 dB test margin
      Φn = Pac – 10 * log(1/Be) + Pmar
                                                                                              was used. Clearly, for the digital RF
                                                                                              communications formats, the signal
      Figure 15. Phase Noise in Adjacent Channel Selectivity                                  generator SSB phase noise is not as
                                                                                              important as for analog FM systems.

                             Channel Spacing                                                  For selectivity tests the spectral
                                                                                              shape of the signal is the special
                                                                                              characteristic that is of primary
                                                                                              importance. The digital modulation
                                                                                              formats used by GSM, CDMA, NADC,
                                                                                              and PDC characteristically leak a
       Level (dBm)




                                                                                              small amount of power into the
                                                                                              adjacent channels. Figures 16–18 plot
                                                                      IF Rejection Curve      amplitude versus frequency for the
                                                                                              selectivity values specified in Table 1.
                                                                                              The impact of the spectral shape on
                                                                                              the adjacent and alternate channels
                                                                                              of the receiver is evident. To properly
                                                                                              test your digital radio receiver, the
                                                                                              Adjacent Channel Power (ACP) of
                                                                      SSB Phase Noise
                                                                                              your signal generator must be below
                                                                                              the required system specification
                                               Frequency
                                                                                              plus the desired test margin.




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        Table 1. Maximum Tolerable SSB Phase Noise

        System                            Channel         Approximate                 Adjacent            Maximum SSB                Alternate               Maximum SSB
        Type                              Spacing        Receiver Noise               Channel              Phase Noise                Channel                 Phase Noise
                                                           Bandwidth                 Selectivity             @ Offset                Selectivity                @ Offset
        Analog FM                          25 kHz              14 kHz                     70 dB    –122 dBc/Hz @ 25 kHz
        GSM                               200 kHz             200 kHz                      9 dB    –72 dBc/Hz @ 200 kHz                  41 dB           –104 dBc/Hz @ 400 kHz
        NADC                               30 kHz              35 kHz                     13 dB     –68 dBc/Hz @ 30 kHz                  42 dB             –97 dBc/Hz @ 60 kHz
        PDC                                25 kHz              33 kHz                      1 dB     –56 dBc/Hz @ 25 kHz                  42 dB             –97 dBc/Hz @ 50 kHz




      Figure 16. GSM Adjacent and Alternate Channel Selectivity Spectrum                            Figure 17. NADC Adjacent and Alternate Channel Selectivity Spectrum

                        –44                                                                                            –65




                                                                           41 dB                                                                         42 dB
                                                                                                     Amplitude (dBm)
      Amplitude (dBm)




                        –76                                                                                            –94
                                   9 dB
                                                                                                                                         13 dB
                        –85
                                                                                                                –107




                                              fc          +200             +400                                               fc          +30             +60
                                             Offset from Nominal Center Frequency (kHz)                                      Offset from Nominal Center Frequency (kHz)



      Figure 18. PDC Adjacent and Alternate Channel Selectivity Spectrum




                             –58




                                                                             42 dB
           Amplitude (dBm)




                                   1 dB

                         –99
                        –100




                                               fc          +25               +50
                                             Offset from Nominal Center Frequency (kHz)




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      2.5 Fading Tests                             2.6 Best Practices in Conducting                 When measuring the adjacent channel
                                                                                                    selectivity performance of an analog
      A unique challenge for the receiver is       Receiver Performance Tests                       radio receiver, the phase noise of the
      to overcome the random effects of the        By following certain guidelines in               out-of-channel test signal is extremely
      radio channel. In a cellular environ-        conducting receiver performance                  important. Conversely, when making
      ment, a radio signal may take a number       verification tests, you can be sure              out-of-channel tests on digital radio
      of paths en route from the transmitter       that your test results are valid.                receivers, the phase noise of the test
      to the receiver. These multipath signals     Performing in-channel and out-of-                signal is much less important. The
      may add up constructively (in phase)         channel receiver tests within the                power in the modulation sidebands
      or destructively (out of phase) at the       confines of a shielded room greatly              of the test signal greatly exceeds the
      receiver as a function of the distance       reduces interference from outside                power contribution from the phase
      each signal travels. The effect of this      sources. The shielded, or screen                 noise sidebands. The portion of the
      phenomenon can be fading of the              room provides isolation from RF                  test signal that spills over into the
      received signal strength, which can          signals that could potentially interfere         adjacent channel has the greatest
      greatly stress signal reception. Fast        with the receiver. Also, impedance               impact on the out-of-channel testing
      fading distorts the shape of the base-       mismatches between the signal                    of digital radio receivers. Because
      band pulse. This distortion is linear        generator and the receiver create                of this, ACP is the most important
      and creates ISI. Adaptive equalization       reflections that degrade measurement             specification for out-of-channel test
      reduces ISI by removing linear               accuracy. The test equipment used to             signals.
      distortion induced by the channel.           conduct receiver tests should be care-
      Slow fading results in a loss of SNR.        fully chosen to reduce measurement
      Error-correction coding and receive          uncertainties and increase confidence
      diversity are used to overcome the           in proper receiver operation.
      effects of slow fading.
                                                   When making a sensitivity measure-
      Fading tests can be performed by             ment, the level accuracy of the signal
      routing the test signal through a radio-     generator is extremely important.
      channel emulator before the signal is        The measurement system will intro-
      processed by the receiver. This device       duce some amount of error, and the
      provides several paths for the signal        amplitude level accuracy of the signal
      to travel in the simulated RF channel        generator is the main source of this
      before being recombined at the               error. In addition to level accuracy,
      receiver. The receiver must be able          the signal generator must also have
      to process fading signals with an            accurate modulation. Distortion in
      acceptable BER. The fading measure-          the signal modulation will degrade
      ment setup (Figure 19) is similar to the     the sensitivity of the receiver being
      sensitivity measurement setup with           tested.
      the exception of the channel simulator.


      Figure 19. Fading Measurement Setup

                                          Modulated RF Signal
                 Signal Generator                                Electrolit, Spirent, Others
                                                                    Channel Simulator


                                                                DUT

           Agilent E4438C ESG with BERT                    Demodulated,           Faded RF Signal
                                                           Decoded Data




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      3. Troubleshooting Receiver Designs


      Digital RF communications systems             2. Co-channel Immunity. Check for           A noise figure measurement on the
      require complex digital radio                    compression occurring in the             RF front end, or any analog component
      transmitters and receivers. Complex              analog components or check for an        or subsystem of the receiver, is a
      designs challenge engineers in the               algorithm implementation problem         two-port measurement (from input
      isolation of system problems. Most               in the digital realm.                    to output). For more information
      physical impairments can be traced                                                        on noise figure measurements, see
                                                    3. Spurious Immunity. Look for any
      back to a component, device, or sub-                                                      (Ref. 6, pg. 23). The TOI measurement
                                                       interfering tone (see section 3.2.2).
      system. Successful receiver design                                                        is also a two-port measurement see
                                                       If no interfering tone is found,
      often depends upon the ability to                                                         (Ref. 7, pg. 23). ADC measurements
                                                       perform a Fast Fourier Transform
      find the source of error. This section                                                    process the digital output of the ADC
                                                       (FFT) on the data from the ADC to
      suggests some basic techniques for                                                        and are unaffected by probe placement.
                                                       convert to the frequency domain.
      troubleshooting a receiver that does
                                                       Then check for spurs generated by
      not pass a certain test. Also, a table                                                    3.2 Signal Impairments and
                                                       the ADC.
      that links measurement characteristics                                                    Ways to Detect Them
      to possible causes of error in the            4. Intermodulation Immunity. Measure
      different sections of the receiver is                                                     Certain signal impairments appear
                                                       the third-order intercept (TOI) of
      provided.                                                                                 in specific measurements. In these
                                                       the RF front end. If it meets the
                                                                                                measurements, variations from the
                                                       expected value, measure the TOI
      3.1 Troubleshooting Steps                                                                 expected results can help locate
                                                       and gain of each analog stage.
                                                                                                problems in different parts of the
      If the receiver under test fails any          5. Selectivity. Look at the shape of        receiver. The following sections
      of the performance tests, you should             the IF filter (see section 3.2.5), and   explain some common impairments
      attempt to isolate the source of the             check for excessive LO phase noise       and how to recognize them through
      error in the receiver. The following is          or sidebands.                            their effects on the different measure-
      a suggested troubleshooting procedure                                                     ments. With the exception of the IF
      to follow if your receiver does not           Specific guidelines should be followed      filter measurement, Agilent 89400 or
      meet the expected performance                 when connecting to the receiver during      89600 series vector signal analyzers
      criteria.                                     troubleshooting. When connecting to         (VSA) are used to troubleshoot
                                                    analog nodes of the receiver, the test      receiver designs in this application
      Test Failed:                                  probe alters signal characteristics to      note. The IF filter measurement is
                                                    a certain degree, which increases           performed with the Agilent 8753E
      1. Sensitivity. Measure the BER versus        uncertainty in the test results. In a       vector network analyzer (VNA).
         the input power. If the BER is high        conventional analog receiver there
         at high input powers, check for I/Q        are many accessible test points, such
         impairments (see section 3.2.1),           as the outputs of the LNA, the LO,
         excessive group delay in analog            the mixers, and the various filters.
         components, or phase noise from an         Accessibility of components in the
         LO. If the BER is high at low input        digital radio receiver depends on the
         powers, measure the noise figure           level of circuit integration. Many of
         of the analog front end (from the          the components of receiver subsystems
         antenna port to the ADC). If the           are embedded in Integrated Circuits
         noise figure is higher than expected,      (ICs). For receivers containing ICs,
         measure the noise figure and gain          tests are normally conducted at the
         (or loss) of each stage of the receiver.   subsystem levels of the receiver. To
         If no noise figure problems are            test embedded components, strategic
         detected, the gain of the front end        test points must be designed into
         may be low, there could be a detection     the IC.
         algorithm problem in the digital
         portion of the receiver, or a spur
         may be desensitizing the receiver
         (see section 3.2.2).




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      3.2.1 I/Q Impairments                       imbalances are detectable by viewing        slightly different conversion losses in
                                                  the constellation diagram of the symbol     the I and Q mixers or by different filter
      Constellation diagrams are useful           time and comparing with the ideal           losses in the I and Q signal paths of
       in displaying the characteristics of       grid of the constellation. These ideal      an I/Q demodulator. Even subtle
      signal impairments related to I and Q.      grids indicate where the symbol             imbalances are often visually detected
      Matching problems due to component          states should occur.                        by zooming in (magnifying the scale)
      differences between the I side and Q                                                    on the constellation and using the
      side of a receiver can cause gain           I/Q gain imbalance results in a             markers. Without the ideal grids it
      imbalance or quadrature errors.             distorted measured constellation            would be difficult to detect small
      These differences may be attributed         relative to the reference (Figure 20).      imbalances.
      to mixers, filters, or ADCs. Subtle         This imbalance may be caused by


      Figure 20. I/Q Gain Imbalance (excess I gain and reduced
      Q gain relative to the ideal constellation locations)              Figure 21. I/Q Quadrature Error




                                                                         Figure 23. A Sinusoidal Spur Indicated by the Circle Around
      Figure 22. I/Q Offset                                              One of the Constellation Points




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      I/Q quadrature errors result in a tipped    The radius of the circle is proportional    processor clock, an intermodulation
      or skewed constellation (Figure 21).        to the amplitude of the interfering         product, or an internally generated
      Quadrature errors are caused by a           signal, but this display format contains    spur. This interfering tone could
      phase shift other than 90° between          no information about the interfering        cause the receiver to fail many of
      the I and Q paths. Different group          frequency, which may be the key to          the performance verification tests.
      delays in the baseband I and Q filters      identifying the cause.
      also create quadrature errors. This                                                     3.2.3 Incorrect Symbol Rate
      distortion of the constellation             The presence of spurs on a modulated
      increases the possibility of errors         signal may be difficult to determine        The symbol clock of a digital radio
      in the interpretation of the received       on a constellation display or through       dictates the sampling rate of the base-
      symbols and will increase the Error         spectrum analysis. An alternative           band I and Q waveforms required to
      Vector Magnitude (EVM).                     parameter can be used to check signal       accurately interpret the symbols and
                                                  quality: EVM. A description of EVM          recover the digital data at the receiver.
      I/Q offsets are shifts in the origin of     and how it relates to the BER can be        In the transmitter, the symbol rate
      the I/Q constellation and can occur         found in the Appendix. The magnitude        dictates the creation of the baseband
      when DC offsets are introduced by           of the error vector versus time graph       I and Q waveforms to properly put the
      rounding errors in the DSP or by LO         may hint that the error observed is         valid states in the correct locations,
      feedthrough in the transmitter              sinusoidal in nature, but what is really    ensuring proper encoding of the digital
      (Figure 22).                                needed is a method to determine the         data. It is imperative that the trans-
                                                  frequency of the spur.                      mitter and receiver have the same
      3.2.2 Interfering Tone or Spur              The error vector spectrum can indicate      symbol rate to be compatible.

      An interfering signal can cause the         the frequency of spurious signals that      An internal clock generator deter-
      amplitude and phase of the transmitted      cannot be observed on traditional           mines the symbol rate of a system.
      signal to be different each time the        spectrum analyzers or on a constella-       This generator must be set correctly.
      signal passes through the same state.       tion display. In Figure 24, a spur offset   Symbol rate errors often occur when
      This will result in a spread at the         from the carrier by approximately 47        the wrong crystal frequency is used
      symbol locations in the constellation       kHz is detected at the output of the        (for example, if two numbers have
      diagram (Figure 23). Random smearing        IF filter. This spur was most likely        been accidentally switched in the fre-
      of the points indicates noise, but a        caused by an in-band CW signal              quency specification). If no problem
      circling of the symbols around the          undetectable by traditional spectrum        exists with the crystal, the receiver
      constellation states indicates there        analysis (Figure 25). This in-band CW       is having a synchronization problem.
      may be a spur or interfering tone.          interferer could be a harmonic of the       Either the receiver is not properly

      Figure 24. Error Vector Spectrum Reveals Spur                      Figure 25. Signal Spectrum Conceals Spur




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      recovering the carrier frequency, or        defines the shape of the filter in the     To verify baseband filter performance,
      the receiver is not achieving symbol        frequency domain. A low alpha creates      examine the vector constellation
      lock. To recover the proper carrier         a sharp filter shape in the frequency      diagram for excessive overshoot of
      frequency, the receiver must lock           domain, but also creates high over-        the signal trajectory between symbol
      onto the phase of the carrier. To           shoot in the time domain, which can        states. The magnitude of the error
      accurately extract the symbols from         be recognized on a vector diagram.         vector versus time would be a good
      the carrier, the receiver must also         It is important to verify that the         indicator of roll-off factor discrepancies.
      determine when symbol transitions           receiver has the appropriate base-         If the wrong roll-off factor is used, the
      occur. A timing recovery loop provides      band frequency response and time           magnitude of the error vector will be
      the mechanism for the receiver to           characteristics for the specified alpha.   high between symbol points and low
      achieve the necessary symbol lock.                                                     at the symbol points (Figure 26).
      When the receiver does not achieve          In cases where baseband filtering is
      proper phase lock and/or proper             shared between the transmitter and         The correct roll-off factor can be
      symbol lock, symbol rate errors occur.      the receiver, the filters must be          found by using different roll-off
      If you suspect an incorrect symbol          compatible and correctly implemented       factors in the VSA while viewing the
      rate and no problem exists with the         in each. The type of filter and the        error vector time display. When the
      crystal, verify proper operation of the     corresponding roll-off factor (alpha)      correct value is used, the magnitude
      carrier and timing recovery circuitry       are the key parameters that must be        of the error vector between symbol
      of the receiver.                            considered. For raised-cosine filters,     decision points will be approximately
                                                  an error in the selection of alpha may     equal to the magnitude of the error
                                                  result in undesirable amplitude over-      vector at the decision points (Figure 27).
      3.2.4 Baseband Filtering Problems           shoot in the signal. It may also result    Furthermore, equalization can be
      Baseband filtering must be correctly        in ISI. Incorrect filtering due to a       applied to decrease the errors caused
      implemented to provide the desired          wrong roll-off factor may affect the       by baseband filtering problems.
      baseband frequency response and to          amount of interference from an
      avoid ISI as well as overshoot of the       adjacent channel signal. This could
      baseband signal in time. The alpha          cause an otherwise good receiver to
      parameter in a raised-cosine filter         fail many of the performance
                                                  verification tests.


      Figure 26. Vector Diagram and Magnitude of the Error Vector        Figure 27. Vector Diagram and Magnitude of the Error Vector
      Versus Time for Wrong Roll-off Factor                              Versus Time for Correct Roll-off Factor




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      3.2.5 IF Filter Tilt or Ripple                      3.3 Table of Impairments                           isolate the source of the error. EVM
                                                                                                             is a powerful signal analysis tool
      The IF filter attenuates out-of-channel             Versus Parameters Affected                         that can be scrutinized to pinpoint
      interference. Errors in the design of               Table 2 shows the physical impairments             sources of interference in receiver
      this filter can affect the overall signal.          encountered with digitally demodulated             tests. Frequency response and group
      IF filter problems include filter tilt or           signals, and the parameters that these             delay measurements prove effective
      ripple in the frequency response and                impairments affect.                                in the detection of filtering problems.
      variations in group delay. Ideally,                                                                    Phase error analysis can detect
      the filter should be flat across the                The key to troubleshooting is to
                                                                                                             sources of unwanted phase noise.
      frequency band of interest, and its                 identify the impairments that could
      group delay should be constant                      be causing signal degradation. Each                Strategic use of these analysis tools
      across the same frequency band.                     of the different impairments uniquely              will enhance your ability to track
      Filter tilt or ripple in the frequency              affects the quality of a digitally                 down sources of error in your digital
      response causes linear distortion in                demodulated signal. As the table                   radio receiver designs. The ability to
      the signal. Improper matching of any                indicates, the I/Q constellation is                quickly locate design problems can
      component between the antenna and                   typically affected by physical impair-             greatly reduce product development
      the IF filter also causes tilt or ripple.           ments. Although the constellation                  and test verification times, and facilitate
      For example, mismatch between the                   diagram is a good indicator of problems,           the type approval of receiver designs.
      preselecting filter and the LNA causes              further analysis may be necessary to
      reflections that result in distortion of
      the overall frequency response of the
      receiver.                                            Table 2. Impairments Versus Parameters Affected
                                                           Physical Impairments        Parameters Affected
      Filter tilt or ripple causes distortion
                                                           I/Q Gain Imbalance          I/Q Constellation (Figure 20)
      on the demodulated baseband signal.
                                                           I/Q Quadrature Errors       I/Q Constellation (Figure 21), Average EVM, Magnitude of the Error
      This distortion is discernible in the                                            Vector versus Time, Error Vector Spectrum
      constellation diagram. Also, the
                                                           I/Q Offsets                 I/Q Constellation (Figure 22)
      magnitude of the error vector will be
                                                           Interfering Tone or Spur    I/Q Constellation (Figure 23), Average EVM, Error Vector Spectrum
      higher than expected at the symbol                                               (Figure 24)
      points as well as during symbol tran-
                                                           Incorrect Symbol Rate       I/Q Constellation, Phase Error
      sitions. Since the IF filter is the main
                                                           Baseband Filtering Problems I/Q Constellation, Average EVM, Magnitude of the Error Vector versus
      contributor to the frequency response                                            Time (Figures 26 and 27)
      of the receiver, IF filter shape distortion
                                                           IF Filter Tilt or Ripple    I/Q Constellation, Magnitude of the Error Vector versus Time,
      is observed and analyzed by performing                                           Frequency Response (Figure 28), Group Delay
      a frequency response measurement on
      the filter alone, as shown in Figure 28.


      Figure 28. Undesired Tilt and Ripple in the IF Filter

                                     REF
       CH1 S21 LOG       1 dB/       –7 dB       1:–10.172 dB    190.050 000 MHz

      PRm



      Cor                                    1




            Center 190.000 000 MHz                          Span 10.000 000 MHz
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      4. Summary                                5. Appendix: From Bit Error Rate (BER) to
                                                Error Vector Magnitude (EVM)

      Digital RF communications receivers       BER is the best measurement to verify        has been stripped away. EVM is the
      are challenging to design, test, and      receiver performance, but BER testing        root-mean-square (rms) value of the
      troubleshoot. Two digital radio           is not always possible in the subsystems     error vector over time at the instants
      receiver designs were discussed in this   of a digital radio receiver. Also, BER       of the symbol clock transitions. By
      application note: I/Q demodulator         can indicate a problem exists, but it        convention, EVM is usually normalized
      and sampled IF. Receivers must            may not help identify the source of          to the outermost symbol magnitude
      meet strict conformance standards.        the problem. An alternative to BER           at the symbol times and expressed
      Common in-channel and out-of-             testing is to examine the quality of a       as a percentage:
      channel tests verify that receiver        demodulated signal. The most widely
      designs meet these standards. To          used modulation quality metric in            EVM = (rms error vector / outermost
      reduce measurement errors, best           digital RF communications systems            symbol magnitude) x 100%
      practices should be followed, with an     is the EVM. EVM provides a way to
                                                                                             The error vector information of
      awareness of measurement caveats.         quantify the errors in digital demodu-
                                                                                             the trajectory between the points
      A basic troubleshooting procedure         lation and is sensitive to any signal
                                                                                             (viewable in the magnitude of the
      helps to isolate design problems.         impairment that affects the magnitude
                                                                                             error vector versus time display of
      Application of these testing and          and phase trajectory of a demodulated
                                                                                             the Agilent 89441A VSA) helps you
      troubleshooting techniques can            signal.
                                                                                             troubleshoot baseband filtering
      reduce product cycle times and
                                                As shown in Figure 29, the error vector      problems in your receiver design (see
      increase confidence in proper
                                                is the vector difference between the         section 3.2.4). Also, the spectrum of
      operation after the receiver is
                                                reference signal and the measured            the error vector can help you locate
      manufactured and put into use.
                                                signal. The error vector is a complex        sources of interference (see section
                                                quantity that contains a magnitude           3.2.2). The magnitude error and
                                                and phase component. Expressed               phase error between the two vectors
                                                another way, the error vector is the         provide a way to view unwanted
                                                residual noise and distortion remaining      phase and amplitude modulation
                                                after an ideal version of the signal         that may occur in your receiver.


                                                Figure 29. EVM and Related Quantities




                                                                Magnitude Error                             Magnitude of Error Vector


                                                Q                                            Error Vector



                                                                                                   θ                    Phase of Error Vector
                                                    Measured Signal


                                                                      Phase Error

                                                                 φ
                                                                              Ideal Signal
                                                                              (Reference)



                                                                                                                                  I




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           EVM may also be normalized to the                      easily measured figure-of-merit that
           square root of the average symbol                      can be used to monitor design changes,
           power. In this way, EVM can be                         locate design problems and, when
           related to the SNR:                                    baselined against a BER measurement,
                                                                  indicate the likelihood that a design
           SNR = –20 * log (EVM / 100%)                           will meet the required specifications.
                                                                  Hence, the connection of BER to EVM
           The importance of the above equation
                                                                  is through the SNR, the more general
           is that it relates EVM to BER through
                                                                  indicator of likely signal quality
           the SNR.
                                                                  (Figure 31).
           Many textbooks have standard
                                                                  Measurements of EVM and related
           curves that relate BER to SNR, as in
                                                                  quantities can provide powerful
           Figure 30 (Ref. 8, pg. 23). Generally,
                                                                  insight into the performance of a
           these curves assume that the noise is
                                                                  digital radio receiver. When properly
           Additive White Gaussian Noise (AWGN)
                                                                  applied, these signal quality measure-
           with a finite peak-to-average ratio, or
                                                                  ments can pinpoint sources of error
           crest factor. The assumptions made
                                                                  by identifying the exact type of
           in generating textbook plots of BER
                                                                  degradation in a signal. For more
           versus SNR will not necessarily apply
                                                                  detail on using EVM measurements
           to a particular receiver. The noise in
                                                                  to analyze and troubleshoot vector-
           a receiver under test, for example,
                                                                  modulated signals see (Ref. 4 and
           may not be AWGN but may instead
                                                                  Ref. 5, pg. 23).
           have a strong spectral component.
           In addition, the steep slope of BER
           curves makes BER estimations from
           measured SNR (or EVM) more prone
           to error. However, EVM provides an


           Figure 30. Probability of Error Versus SNR             Figure 31. SNR Versus EVM for Crest Factor of 1.4

            10–3                                                                                     Peak-to-Average Ratio of 1.4
                                                  16-PSK                     30
            10–4
                                                 16-APK
            10–5                                  or 16 QAM

                                                    8-PSK
    P(e)    10–6                                     8-APK
                                                        Class I              28
            10–7                                         QPR
                        4-PSK
                                                                  SNR (dB)




                        (QAM)
            10–8
                        BPSK
            10–9
                                                                             26
            10–10
                    6    8 10 12 14 16 18 20 22 24 26
                                    SNR (dB)


                                                                             24
                                                                                  2.5   2.7            2.9                3.1       3.3   3.5




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      6. Symbols and Acronyms


      α      Alpha (roll-off factor) of a      IC     Integrated Circuit
             Nyquist filter
                                               IF     Intermediate Frequency
      ACP    Adjacent Channel Power
                                               ISI    Inter-Symbol Interference
      ADC    Analog-to-Digital Converter
                                               ITU    International
      AGC    Automatic Gain Control                   Telecommunications Union

      ALC    Automatic Level Control           LNA    Low-Noise Amplifier

      ASIC   Application-Specific              LO     Local Oscillator
             Integrated Circuit
                                               NADC   North American Digital
      AWGN Additive White Gaussian                    Cellular
           Noise
                                               PDC    Pacific Digital Cellular
      BER    Bit Error Rate
                                               PHS    Personal Handyphone
      BERT   Bit Error Rate Tester                    System

      BT     Bandwidth-Time product            PRBS   Pseudo-Random Binary
             (roll-off factor) of a Gaussian          Sequence
             filter
                                               Q      Quadrature-phase
      CDMA   Code Division Multiple
             Access                            RBER   Residual Bit Error Rate

      CW     Continuous Wave                   RF     Radio Frequency

      DDC    Digital Down Converter            SMR    Specialized Mobile Radio

      DSP    Digital Signal Processor          SAW    Surface Acoustic Wave

      DUT    Device Under Test                 SNR    Signal-to-Noise Ratio

      ETSI   European                          TDMA   Time Division Multiple
             Telecommunications                       Access
             Standard Institute
                                               TIA    Telecommunications
      EVM    Error Vector Magnitude                   Industry Association

      FER    Frame Erasure Rate                TOI    Third-Order Intercept

      FFT    Fast Fourier Transform            UUT    Unit Under Test

      GSM    Global System for Mobile          VNA    Vector Network Analyzer
             Communications
                                               VSA    Vector Signal Analyzer
      I      In-phase




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      7. References


      1   Testing and Troubleshooting         9   Theodore S. Rappaport, Wireless
          Digital RF Communications               Communications: Principles
          Transmitter Designs,                    and Practices, Prentice Hall
          Agilent Application Note 1313,          1996: Upper Saddle River, New
          literature # 5968-3578E.                Jersey.

      2   Digital Modulation in               10 Bernard Sklar, Rayleigh Fading
          Communications Systems—                Channels in Mobile Digital
          An Introduction,                       Communication Systems Part I:
          Agilent Application Note 1298,         Characterization, IEEE
          literature # 5965-7160E.               Communications Magazine, July
                                                 1997, Vol. 35 No. 7.
      3   Measuring Bit Error Rate using
          the Agilent ESG-D Series RF         11 Robert H. Walden, Performance
          Signal Generators Option UN7,          Trends for Analog-to-Digital
          literature # 5966-4098E.               Converters, IEEE
                                                 Communications Magazine,
      4   Using Vector Modulation                February 1999, Vol. 37 No. 2.
          Analysis in the Integration,
          Troubleshooting, and Design
          of Digital RF Communications
          Systems, Agilent Product Note
          89400-8, literature # 5091-8687E.

      5   Ten Steps to a Perfect Digital
          Demodulation Measurement,
          Agilent Product Note 89400-14A,
          literature # 5966-0444E.

      6   Fundamentals of RF and
          Microwave Noise Figure
          Measurements,
          Agilent Application Note 57-1,
          literature # 5952-8255E.

      7   Measuring Third-Order
          Intermodulation, N dB
          Bandwidth, and Percent AM
          with Built-in Functions,
          Agilent Product Note 8590-8,
          literature # 5091-4052E.

      8   K. Feher, Digital
          Communications, Prentice Hall
          1981: Englewood Cliffs, New
          Jersey.




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