Performance of an IF sampling ADC in receiver applications by YAdocs

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									Performance of an IF
sampling ADC in receiver
applications
David Buchanan
Staff Applications Engineer
Analog Devices, Inc.



Introduction                                                         list of possible frequencies by considering real world limita-
The concept of direct intermediate frequency (IF) sampling is        tions, such as the bandwidth of the analog source, or the band-
not a new one. In fact many modern receiver designs employ           width of the ADC’s analog input circuit2. Another helpful con-
this architecture. Performance of commercially available ana-        cept is to divide the unfiltered analog input spectrum into
log to digital converters (ADC) , however, often eliminates this     Nyquist zones. As defined in Figure 3, a signal at frequency
architecture due to dynamic performance limitations. This pa-        fanalog in an odd Nyquist zone N will alias to fanalog’= fanalog - (N-
per explores the concept of direct IF sampling and ADC perfor-       1)fs/2 . Even Nyquist zone frequencies will alias to fanalog’ =
mance characteristics which designers should consider in this        (N)fs/2 - fanalog.
application, and highlights the performance of a new 12 bit,
125Msps ADC design optimized for this application.

Basics of Direct IF Sampling
The Nyquist sampling theorem, as traditionally interpreted,
requires that the sampling rate (fs) of an ADC be at least twice
the highest frequency component in the waveform being
sampled to recover or accurately represent the original
waveform.1 This is often referred to as baseband sampling. To
meet this requirement, the analog signal must be low pass filtered
before being sampled. In Figure 1, f2 is filtered to meet the
Nyquist criteria. If you examined the frequency content of the       Figure 2. Aliasing in the time domain.
ADC output data with a fast Fourier transform (FFT), the desired
signal (f1) would be represented within the accuracy of the ADC.




                                                                     Figure 3. Dividing the analog input spectrum into Myquist zones.
Figure 1. Traditional Interpretation of Nyquist Sampling Theorem

    Without the low pass filter to guarantee the Nyquist sam-            Figure 4 illustrates how frequencies are aliased in the fre-
pling theorem is met, aliasing may occur. Aliasing is a term         quency domain, with f2 and f3 being aliased to f2' and f3'.
used to describe frequency content in the ADC output spec-               The traditional interpretation of the Nyquist sampling theo-
trum caused by under-sampling signals (fanalog>fs/2). If you re-     rem is not completely accurate, in that it limits the absolute
construct the samples of an under sampled frequency, it will         frequency of the waveform being sampled to be less than 1/2
always alias to a baseband frequency as shown in Figure 2.           the sampling frequency. In fact it is the bandwidth of the wave-
Alias is an appropriate term because the actual analog input         form that must be limited, and not the actual frequency.3* In IF
frequency which has been aliased to fanalog’ could theoretically     under-sampling applications, a band-pass filter limits the
be generated from any frequency Nfs (fanalog’, where N is an inte-
ger. In practice, fanalog can always be narrowed to a more finite    * Also known as the Shannon sampling theorem.


                                                                         International IC – China • Conference Proceedings 63
output spectrum to a particular Nyquist zone, and in effect takes
advantage of the aliasing affect.




                                                                        Figure 6. Traditional Super Heterodyne Digital Receiver.




Figure 4. Aliasing occurs when the conditions of the Nyquist Sampling
Theorem are not met.
                                                                        Figure 7. IF Sampling Digital Receiver.




                                                                        Figure 8. Multi Carrier IF Sampling Receiver
Figure 5. Bandpass or IF Under-Sampling

                                                                        ADC Performance Considerations
IF vs. Baseband Sampling                                                While ADC technology has improved significantly over time,
Many modern receivers take advantage of digital signal pro-             only a few can provide the performance needed for IF sampling
cessing, but most rely on traditional super heterodyne architec-        applications. Even fewer guarantee performance specifications
tures to translate the signal of interest to a base band IF before      for under-sampling applications. This puts the burden on de-
the signal is sampled (base band sampling). Direct IF receivers         signers to understand the specifications and characteristics of
take advantage of under-sampling to eliminate one or more of            the ADC. This section will highlight some of the more impor-
the tuned analog IF stages. The ADC acts like a mixer, trans-           tant dynamic performance characteristics designers should con-
lating the signal to base band for digital processing. Depend-          sider when selecting an ADC for IF sampling. It also offers
ing on system performance, each IF stage eliminated has the             examples of performance data for the AD9433, a new 12 bit,
potential to reduce system cost by $10 to $100.                         125 Msps ADC from Analog Devices, Inc., which is targeted at
     The eliminated IF stages are replaced by digital Receive           this application.
Signal Processors (RSPs). These specialized devices take ad-                 As discussed above, the minimum sample rate of an ADC
vantage of low cost VLSI solutions for filtering, frequency trans-      must be twice the signal of interest. However, a higher sample
lation, error correction, and demodulation. In addition to sys-         rate is often instrumental in reducing the required selectivity ,
tem cost reduction, these RSPs eliminate the many of the sensi-         and therefore the cost and complexity, of the analog anti-aliasing
tivities of analog solutions, such as device matching, phase noise,     filter. By frequency planning so that the signal of interest is not
environmental sensitivity, and performance variation over time.         near the boundaries of the Nyquist zones, the selectivity of the
     If the ADC can maintain the required performance level over        band pass filter is relaxed. Since IF components, such as sur-
a wide bandwidth, it may be possible to implement a multi car-          face acoustic wave (SAW) filters, are only available at limited
rier receiver. In this architecture, a single ADC samples mul-          number of frequencies, it is not practical to design receivers
tiple signal channels, which are then separated and                     with arbitrary IFs. Alternatively, increasing the sample rate is
demaodulated in parallel in the digital domain. This architec-          often an easy way to move the IF away from the Nyquist bound-
ture compounds the system cost and performance advantage of             ary. Relaxing the selectivity of the analog filter may also allow
IF sampling by eliminating multiple RF / IF sections.                   additional noise and adjacent interfering signals into the ADC



64 International IC – China • Conference Proceedings
output spectrum. These can be removed by additional filtering        that will determine how it distortion performance. Devices with
in the digital domain.                                               large INL errors, or many perturbations in the transfer function
    There are obvious disadvantages to higher sample rates as        will have poor harmonic and spurious distortion performance.
well. Like many other components, the highest speed ADCs             For instance, a “S” shaped linearity curve will cause the ADC
available are leading edge technology, and therefore their cost      to have pronounced third harmonic distortion.
is higher. Designers will also find that resolution and dynamic          In the case of the AD9433, two key features minimize its
performance will be degraded as compared to lower speed al-          linearity errors. The first is an on board circuit that trims out
ternatives. Power dissipation of the converter may also increase     DNL errors to –0.25LSBs. During device test, DNL is mea-
with sample rate. The cost, performance, and power dissipa-          sured and then adjusted by programming registers in the trim
tion of other devices in the system (RSPs, drive amplifiers, and     circuit. Once optimized, the trim setting is fixed with poly-sili-
ADC clock sources) are also impacted by the ADC sample rate.         con fuses.
    An ADC’s performance will also degrade as you increase               The second linearity feature optimizes INL errors during
the sample rate. This may be due to a variety of factors, but T/     device operation, and is referred to an SFDR (spurious free
H settling time is most often the main reason for this degrada-      dynamic range) optimization circuit. This circuit shuffles some
tion. It may be advantageous to use a sample rate below the          of the internal devices that determine the linearity of the ADC
rated maximum to insure that system dynamic performance is           between each clock cycle, which randomizes the location of
not impacted by this effect. Another concern is the added noise      the worst DNL errors in the transfer function. This effectively
of higher speed digital circuits, which can often degrade the        spreads the worst DNL errors over the entire range of the con-
performance of nearby analog circuits and the ADC itself. In         verter transfer function. To illustrate the effectiveness of this
fact, an ADCs own outputs may often degrade its performance.         circuit, an AD9433 was intentionally mis-trimmed to exagger-
    The bandwidth of an ADC can be defined in several ways.          ate DNL errors, and then INL was measured with and without
One is simply to specify the -3dB bandwidth of the ADC input         the SFDR optimization circuit. The results were that the SFDR
stage or track and hold circuit. Another is to consider the fre-     circuit could reduce the INL error from LSBs to (0.25LSBs.
quency at which the spectral power of the ADCs digital output        Unless otherwise noted, the SFDR improvement circuit is ac-
signal is reduced by 3dB. Perhaps the most stringent definition      tive for all AD9433 data presented in this paper.
would be the analog frequency for which the ADCs signal to
noise ratio (SNR, see below) performance was reduced by 3dB
(as compared to it’s base band performance). However it is
defined, direct IF sampling receiver applications require the
ADC to provide enough bandwidth to allow sampling of com-
mon IF frequencies, 70-300MHz. This frequency range is driven
by availability of other IF components, such as mixers, ampli-
fiers, and SAW filters. Additional bandwidth may be neces-
sary in the analog front end for the ADC to meet other dynamic
specifications, such as harmonic distortion, but it is important
to consider that this extra bandwidth will also allow additional
wide band noise to be aliased into the spectrum of the ADC
output signal. Designers are cautioned not to assume that an
ADC’s dynamic specifications will be constant over its rated
bandwidth.
    The bandwidth of the AD9433 is an extremely wide
                                                                     Figure 9. AD9433 INL without SFDR Optimization Circuit.
750MHz, allowing it to extend other important performance
characteristics to IFs as high as 400MHz. One of the more
unique features of this ADC is a user configurable input band-
width optimization. Control pins on the device allow the user
to optimize the performance of the ADC for 3 different input
bandwidths: Base band (<100MHz, IF Sampling I (100-
250MHz), and IF Sampling II (>250MHz). The IF Sampling II
mode also reduces the differential analog input voltage range
from 2Vp-p to 1Vp-p, reducing the drive requirements of the
signal source.
    The transfer function of the ADC is described in terms of its
linearity. Differential non-linearity (DNL) is the deviation of
any output code from an ideal least significant bit (LSB) step.
Manufacturers typically specify the worst short and long code
in terms of an ideal LSB. Although it is not specified, the rms
value of all the DNL errors in the ADC will determine the
amount of quantization noise in the ADCs output spectrum.            Figure 10. AD9433 INL With SFDR Optimization Circuit.
Integral non-linearity is typically specified as the deviation, in
LSBs, of the ADCs transfer function from a best straight line
(offset and gain errors are ignored). While the INL specifica-          Frequency domain testing and characterization of high speed
tion is a good measure of overall dynamic performance, it does       ADCs has become standard over the past decade. Most of these
not tell the whole story. It is the shape of the linearity curve     specifications are measured by performing a FFT on the output

                                                                         International IC – China • Conference Proceedings 65
data. It is important that designers consider test conditions,       the output spectrum that are not traceable to the fundamental or
such as sample rate, analog input frequency, and analog input        its harmonics. They may be due to other frequency sources in
amplitude, when interpreting and comparing these specifica-          the system which unintendendly couple into the ADCs clock,
tions.                                                               analog input, power supplies, or reference. Even worse are in-
    The most common benchmark is SNR. It is usually speci-           ter-modulation products between two system frequencies, which
fied in dB for a sine wave input, and at rated sample rate. It is    may be nearly impossible to identify. In many ADC specifica-
defined as the ratio of the rms signal amplitude to the rms value    tions, higher order harmonics are often considered spurious fre-
of the sum of all other spectral components in the ADC output        quency content.
spectrum. To give designers a more accurate understanding of             Another key specification related to receiver sensitivity is
ADC performance, manufacturers typically specify SNR ex-             two tone inter modulation distortion. In this test, the analog
cluding the harmonics of the fundamental frequency. SNR with         input contains two sine wave inputs, f1 and f2. The inter-modu-
harmonics, or SINAD (signal to noise and distortion), as well        lation distortion products fall at 2f1 - f2, and 2f2 - f1 as illustrated
as the relative levels of the individual harmonics (Harmonic         in Figure 12.
Distortion) and other spurious frequency content (Spurious Free
Dynamic Range) are often specified as well.
    The theoretical SNR of an ADC for a full-scale sine wave
input is
    SNRTheoretical = 6.02N + 1.76dB,
    where N is the number of bits.4 Based on this, manufactur-
ers often specify an Effective Number of Bits (ENOB), calcu-
lated from the measured SNR or SINAD based on the follow-
ing equation:




                                                                     Figure 12. Diagram Two Tone Inter-modulation Distortion.


    The last term in the numerator compensates for any reduc-           An FFT for the AD9433 illustrates additional higher order
tion in the signal amplitude during the test. It is impractical in   harmonics and IMD products in Figure 13.
production test systems (as well as in receiver applications) to
maintain a constant full scale amplitude, so the testing is per-
formed with a -0.5 to -1.0dBFS (relative to full scale) input
level.
    Because they can affect overall sensitivity, harmonic and
spurious frequency content in the ADC output spectrum are of
key importance in receiver applications. It is important to note
that these frequencies can be aliases, so they may be difficult to
identify. The FFT result in Figure 11 is for the under sampling
condition of 100Msps and 140.3MHz, so the fundamental and
all harmonics appear as aliased frequencies.




                                                                     Figure 13. AD9433 Two Tone Intermodulation. 100Msps,ƒ1=210.3MHz,
                                                                     ƒ2=211.3Mhz


                                                                     Realizing ADC performance in Systems
                                                                     While it is not possible to give a comprehensive overview of
                                                                     realizing the performance of a high performance ADC such as
                                                                     the AD9433 in the space provided, it is worth mentioning a few
                                                                     key issues. The first is signal conditioning of the analog input
                                                                     circuit. While most real world signals are single ended, most
                                                                     high performance ADCs require a differential analog input sig-
                                                                     nal to realize rated performance. It is also difficult to find cost
Figure 11. FFT of AD9433. 100Msps, 140.3MHz                          effective differential amplifiers at IF frequencies, especially ones
                                                                     that provide the level of performance of the ad9433. A trans-
    Harmonics may be difficult to recognize in the output spec-      former, as configured in can provide cost effective solution to
trum, but since the fundamental frequency is known, it possible      this problem.
to predict where they will fall using the Nyquist zone technique         Perhaps the most important consideration in realizing the
discussed earlier. Spurious frequencies are technically those in     performance of the ADC is the sampling clock. While the

66 International IC – China • Conference Proceedings
Figure 14. Using two Transformers for Single Ended to Differential Signal
Conversion


AD9433 has an integrated clock duty cycle stabilization circuit
that will the user to provide input duty cycles from 25-75%,
jitter in the clock reference will severely degrade noise perfor-
mance at IF frequencies.5 For the test results presented in this
paper, a 100MHz sine oscillator from Wenzel Associates was
used as configured in Figure 15.




Figure 15. Wenzel Sine Oscillator Sample Clock Circuit.



Summary
The direct IF sampling architecture provides the opportunity to
simplify and cost reduce receiver designs when compared with
standard super heterodyne architectures. The AD9433, a 12
bit, 125Msps ADC, provides a new level of performance in IF
sampling ADC technology. The design innovations of this new
device, including an SFDR optimization circuit, will allow de-
signers to extend the IF sampling architecture to higher IF fre-
quencies, and over wider bandwidth signals.

References
( Also known as the Shannon sampling theorem.
1 Robert W. Ramirez. The FFT Fundamentals and Concepts.
    New Jersey: Prentice-Hall, 1985
2 N. S. Tzannes. Communication and Radar Systems. New
    Jersey: Prentice-Hall, 1985
3 Walt Kester. High Speed Design Seminar . Massachusetts.
    Analog Devices, 1990.
4 Rudy Van De Plassche. Integrated Analog To Digital and
    Digital To Analog Converters. Massachusetts. Kluwer Aca-
    demic Publishers, 1994.
5 Brad Brannon. Application Note 501: Aperture Uncertainty
    and ADC System Performance. Massachusetts. Analog De-
    vices, 2000.

Author’s contact details
David C. Buchanan, Jr.
Analog Devices, Inc.
7910 Triad Center Drive
Greensboro, NC 27410 USA
Phone: (1-336) 605 4221
Fax: (1-336) 605 4187
E-mail: david.buchanan@analog.com


                                                                            International IC – China • Conference Proceedings 67

								
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