TA039 by robbiecokkin

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									  DefenseElectronics

   Digital Polar Design Facilitates
   Multimode Transmitters
   A novel digital design technique enables a new class of multimode,
   multiband radios for reconfigurable military transmission systems.
   By Steven Hurwitz



  T
            actical military radio requirements
            have seen significant changes over       Table 1. Disadvantages of I/Q Architecture
            the last several years. In order to
                                                                    Function                                   Disadvantage
            meet the communication needs of
  Future Combat Systems, programs like the           I/Q modulators                             Lower efficiency/noise
  Joint Tactical Radio System demand that ra-
  dios be software reconfigurable. The RF hard-      SAW filters                                Lower efficiency/higher cost/size
  ware associated with software-defined ra-
  dios must be capable of transmitting over a        Linear PAs                                 Lower efficiency
  variety of waveforms with varying channel          No “true” multimode capability             Performance capability/higher cost/size
  bandwidths over frequency bands ranging
  from 2 MHZ to 2.8 GHZ and beyond. In
  addition, the handheld and small form factor
  versions of these radios require reduced parts
  count to optimize size, weight and cost. They
  must also provide greater power efficiency to
  increase battery life.
     These requirements pose great challenges
  for radio designers. Traditional analog in-
  phase/quadrature (I/Q) radio architectures
  are an excellent solution for radios designed
  for band-specific operation. However, they
  are not well suited to multimode multiband
  operation due to their band-specific nature.
  This article introduces design of a digital
  polar transmitter architecture to overcome
  the limitations of classical analog transmitter
  architectures in multiband and multimode
  radios.                                           Fig. 1. Digital transmitter architecture.

  I/Q Transmitter Limitations
      I/Q-based transmitter design can be
  problematic when multiband operation is
  required. Baseband mixing produces un-
  wanted spurious products requiring use of
  surface acoustic wave (SAW) bandpass
  filters to reject these adverse signals and
  minimize wideband noise. The use of bandpass
  filters forces designers to diplex the signal
  path or build individual transmitter chains
  for each band of interest. This design means
  additional circuitry is required in order to
  achieve multiband operation. Additional
  circuitry increases the size, weight and, in
  most cases, power consumption of the trans-
  mitter chain, which is counter to some design
  objectives.
      To achieve multimode operation, analog
  transmitters architectures must be able to
  accommodate constant envelope signals
  (phase and frequency modulations), as well
  as non-constant envelope signals (amplitude       Fig. 2a. Phase modulation applied to DPA with digital envelope deactivated.




Power Electronics Technology   March 2004                                 52                                                 www.powerelectronics.com
                                                                                               (class A/B) amplifiers. Both of these imple-
                                                                                               mentations sacrifice power efficiencies and
                                                                                               result in decreased battery life for the user
                                                                                               when the transmitter is operating in constant
                                                                                               envelope mode. Table 1 summarizes the dis-
                                                                                               advantages of current design techniques.

                                                                                               Digital Polar Transmitter
                                                                                                  True multimode multiband operation can
                                                                                               be achieved in a single transmit chain through
                                                                                               digital polar design. Fig. 1 depicts a high-level
                                                                                               architecture abstraction of a digital polar trans-
                                                                                               mitter. The digital I and Q datastreams gener-
                                                                                               ated by the baseband IC (modem) are input to a
                                                                                               CORDIC processor. The CORDIC processor
                                                                                               transforms the Cartesian coordinates (sine and
                                                                                               cosine) into polar coordinates (amplitude and
                                                                                               phase). The amplitude (A) and phase infor-
Fig. 2b. Output from DPA when digital envelope is activated (p/4-DQPSK modulation).
                                                                                               mation are separated and sent down separate
                                                                                               paths until they are recombined in the digital
                                                                                               power amplifier.
                                                                                                  The phase information extracted from the
                                                                                               original signal (either constant envelope or
                                                                                               non-constant envelope) is transformed into
                                                                                               a constant envelope signal. This is achieved
                                                                                               by phase modulating a phase lock loop de-
                                                                                               signed to output the desired transmit fre-
                                                                                               quencies. The resulting signal may now be
                                                                                               amplified by compressed amplifiers without
                                                                                               concern of distorting amplitude information.
                                                                                                  The extracted amplitude information is
                                                                                               quantized into control bits. These bits are
                                                                                               used to modulate a digital power amplifier
                                                                                               (DPA). Each bit is a digital representation
                                                                                               of the amplitude envelope. The control
                                                                                               bits are used to switch amplifier elements of
                                                                                               the DPA into on or off states. The examples
                                                                                               use a 7-bit control word offering 128 unique
                                                                                               amplitude-modulation states. Fewer quanti-
                                                                                               zation states can be implemented if decreased
                                                                                               amplitude-modulation resolution is accept-
Fig. 3. Current consumption of linear power amplifier.
                                                                                               able. More quantization states also can be
                                                                                               implemented for greater resolution.

                                                                  To avoid distortion of
                                                                non-constant envelopes,
                                                               analog transmitters must
                                                             employ linear (class A) or use
                                                               predistortion techniques to
                                                              “linearize” slightly saturated
                                                                  (class A/B) amplifiers.
                                                            modulated) simultaneously. To         The digitized amplitude envelope and
                                                            avoid distortion of non-constant   the phase-modulated RF carrier are synchro-
                                                            envelopes, analog transmitters     nized and recombined within the DPA
                                                            must employ linear (class A) or    to produce linear and efficient RF transmis-
Fig. 4a. AM/AM with and without digital correction for      use predistortion techniques to    sion. The result is a precise and repeatable
digital sawtooth envelope.                                  “linearize” slightly saturated     phase and amplitude-modulated signal.


www.powerelectronics.com                                               53                                   Power Electronics Technology   March 2004
                                                                                                      Fig. 5. Out-of-band noise performance without
                                                                                                      (blue) and with (yellow) baseband filtering.
                                                                                                      of AM/AM and AM/PM signal distortions
                                                                                                      when driven into saturation. These distor-
                                                                                                      tions can degrade the quality of transmitted
                                                                                                      signal. The digital transmitter architecture
                                                                                                      has the advantage of providing a method for
   Fig. 4b. AM/PM without correction for applied triangle waveform.                                   correcting nonlinearities at the digital base-
                                                                                                      band. To correct for these nonlinear effects,
                                                                                                      the DPA is characterized in each of its digital
                                                                                                      states. This information is then used to select
                                                                                                      the correct digital state to achieve the desired
                                                                                                      modulation characteristic.
                                                                                                         This digital approach offers several advan-
                                                                                                      tages over analog implementations of polar
                                                                                                      modulation-based transmitters. Due to the
                                                                                                      intrinsically digital nature of the digital power
                                                                                                      amplifier, the AM/AM and AM/PM charac-
                                                                                                      teristics are exclusively defined by the behav-
                                                                                                      ior of this GaAs HBT-based integrated cir-
                                                                                                      cuit. The other analog components on the
                                                                                                      transmitter transfer characteristics, including
                                                                                                      linear regulators, have no influence on the
                                                                                                      output signal quality. This characteristic has
                                                                                                      significant implications for the repeatability
                                                                                                      and simplicity of look-up tables, which are
                                                                                                      used to implement digital correction. The
                                                                                                      overall effect demonstrates the fundamental
                                                                                                      advantage of digital over analog predistortion
                                                                                                      systems that typically require complex and
                                                                                                      expensive adaptive feedback circuits to
                                                                                                      achieve equal levels of performance.
                                                                                                         Fig. 4 shows the measured AM/AM
                                                                                                      and AM/PM characteristics of the DPA for
   Fig. 4c. AM/PM with correction for digital triangle envelope.
                                                                                                      an applied ramp signal. Ideally, as the digital
   Fig. 2 illustrates how the phase and digital       the operating output powers. The DPA can        state (the amplitude of the envelope) increases,
   amplitude are combined within the DPA              provide improved efficiency for constant en-    the RF output voltage should increase lin-
   to produce a linear output signal. These           velope and non-constant envelope waveforms      early and there should be no phase deviation.
   results were taken under p/4-DQPSK modu-           simultaneously. Fig. 3 shows the current con-   This linear behavior generally holds true at
   lation. Fig. 2a depicts the constant envelope      sumption of a linear power amplifier in a       low power levels, but more distortion exists
   phase signal that is used to drive the DPA.        typical CDMA transmitter, compared to the       at high power levels as the DPA moves to-
   When the digital envelope is activated, the        current consumption of a digital power am-      ward compression. At high power levels, digi-
   RF signal is amplitude modulated to produce        plifier. Clearly, high dynamic range (>50 dB)   tal optimization is easily applied to com-
   an amplified RF output signal containing phase     can be provided through the control of the      pensate for any amplitude and phase
   and amplitude information (Fig. 2b).               base bias voltages of the DPA while retaining   nonlinearities. Fig. 4a shows the performance
      Since the transmitted signal quality does       superior efficiency.                            before and after digital optimization is ap-
   not depend on the linearity of the amplifying                                                      plied. The improvements in the digital ampli-
   devices, the transmitter maintains efficiency      Digital Transmitter Linearity                   fier characteristics are readily apparent.
   by consuming less current across the range of         All amplifiers are subject to some level        The DPA behaves as a digital-to-analog



Power Electronics Technology   March 2004                                54                                                   www.powerelectronics.com
                                                                                                    ter, it can be tasked for different frequency
     To correct for nonlinearities,                                                                 bands and allow for different modes of op-
                                                                                                    eration by simply reconfiguring frequency,
   all digital states of the DPA are                                                                clocks and digital filtering coefficients that
                                                                                                    support the desired standards.
   mapped to provide the required
                                                                                                    Summary
      RF output current. And by                                                                        Significant advantages are available to
                                                                                                    multimode, multiband radio designers who
 eliminating the AM/AM and AM/PM                                                                    choose to use digital polar transmitter design
                                                                                                    techniques. These advantages include:
     nonlinearities, other system                                                                      ● Elimination of I/Q modulators that
                                                                                                    create mixer spurs and signal imperfections
    parameters such as efficiency                                                                   due to mismatch.
                                                                                                       ● Elimination of SAW filters through the
       can be further optimized.                                                                    use of digital signal processing algorithms.
converter and is subject to the quantization       coupled with the appropriate digital filtering      ● Efficient power operation in both

noise associated with digital signal genera-       of the phase path, the digital transmitter ar-   constant envelope and non-constant enve-
tion. The typical solution (such as implement-     chitecture eliminates the need for SAW fil-      lope modes.
ing a lowpass reconstruction filter after the      ters ahead of the DPA. This design provides         ● True multimode/multiband capability.

DPA to remove quantization noise and               a significant architectural advantage and dem-      These advantages all add up to smaller
sampling replicas) will introduce excessive loss   onstrates the unique attributes of implement-    size, lower weight and less power consump-
and degrade efficiency. To overcome this           ing digital polar modulators.                    tion enabling multiple waveforms to be ported
effect, baseband signal processing is used to         Full digital control and programmability,     onto smaller form factor platforms then
provide out-of-band noise suppression. Fig. 5      coupled with the wideband-amplitude modu-        were ever before possible.
shows an example of the impact of this signal      lation, enables digital polar transmitters to               About the Author
processing on the noise performance.               generate true multimode operation. Further-
                                                                                                      Steven Hurwitz is manager of strategic
   In this example, the baseband signal pro-       more, given that this architecture can elimi-
                                                                                                      initiatives for M/A-COM Inc.’s Aerospace
cessing on the amplitude path provides more        nate “band-specific” hardware (such as filters
                                                                                                      & Defense business unit.
than 35 dB of noise suppression. When              and narrow-band amplifiers) in the transmit-




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