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ADCs and Digital Wireless Base Stations

While the world is moving more and more into an all-digital
domain, the fact remains that many natural phenomenons are analog
in nature. Probably the foremost example of a basic analog signal
is that of human speech. Converting analog speech into digital
bits, and then compressing the resultant data stream so that it can
be stored, processed, and/or transmitted has been the subject of
numerous research projects and has led to the development of
multiple compression/decompression schemes, but scant attention is
usually paid to the analog-to-digital converter itself. However, in
the case of digital wireless telephony, the A/D converter device
itself (ADC) is a crucial component in being able to make the next
systems level technology leap. In this technical overview, we look
at some of the characteristics needed in an ADC intended for the
next generation wireless system.

Material for this article was provided by both Analog Devices
and Lucent Technologies. Both companies have looked at the
particular demands of the wireless base station market and both
have introduced A/D silicon optimized for that application.

Traditional base station designs use a separate receiver for
each channel, each receiver tuned to a fixed channel that may range
from 30 kHz to 3 MHz wide, depending on the wireless standard. The
output of each tuner is fed to an ADC and then to a digital signal
processor (DSP) for further processing. Because a base station
often has 10 or more channels per cell, a wireless operator has to
manage a cabinet of heat generating, power consuming, and expensive
receiver electronics.

A newer approach to base station design uses a single, very
high-performance wide-band radio receiver to capture and digitize
the entire cellular band (30 MHz wide versus 30 kHz for a single
channel in some standards) as a single block of data. The high
bandwidth of the radio signal coming into the base station requires
high sampling speed from the ADC, but the cost of a single radio
stage is shared across all wireless channels. Because the whole
block of signals is digitized at once, this approach is often known
as block radio.

With block radio, all the processing is done in the digital
domain. Channel selection to baseband is performed using
digitally-controlled oscillators and digital multipliers. Once the
channel selection is made, digital filters decimate the 20-70 MHz
incoming signal to a slower rate that can be processed by a DSP
chip.

By reducing the analog content of a base station's electronics,
block radio improves on cost, reliability, and power dissipation.
Digital components are less susceptible to drift, eliminate the
need for trimming, and perhaps more important, are able to be
upgraded with new software so that the base station can be
reprogrammed to work with new standards. However, digitizing a
wideband wireless waveform and extracting the signal from the
background noise places some severe design constraints on the ADC
device.

In block radio the Intermediate Frequency (IF) filter must be
wide enough to allow the entire band to be converted. Consequently,
the ADC must have high sampling speed. Unfortunately, the wide-band
input is also likely to contain signals transmitted by other
wireless users from other wireless providers, and may even contain
a mix of AMPS, Global System for Mobile Communications (GSM), Time
Division Multiple Access (TDMA), and Code Division Multiple Access
(CDMA) transmissions. The base station receiving these foreign
signals does not have the ability to reduce their transmission
power, as it does with the signals under its system control. These
foreign signals may cause significant in-band distortion products
(called blockers) that can block the desired signal. Therefore, the
ADC needs exceptional distortion performance, including low
non-linearity and wide dynamic range.

Lucent Technologies
With this block architecture in mind, Lucent Technologies
Microelectronics Group in 1998 introduced the CSP1152A, an ADC
designed to address the performance requirements of block radio.
The CSP1152A helps carry wireless base stations to the next level
of performance by enabling many analog components to be replaced
with digital electronics. The CSP1152A is a CMOS ADC that has the
high speed needed to digitize high-bandwidth radio signals, the
high dynamic range to help pull low-power signals from high-power
noise environments, and on-chip dither to reduce distortion and
extraneous signals up to 100 dB below maximum signal levels,
thereby improving signal-to-noise performance without additional
circuit cost. The CSP1152A chip is a member of Lucent's growing
wireless-communications integrated circuit family and can be used
for GSM, CDMA, and TDMA wireless standards.

Analog Devices
Analog Devices (ADI) was actually first to offer a product in this
space. In mid-1994 the company introduced the AD9026, a TTL chip,
and the AD9027, a functionally equivalent chip using ECL
technology. At the time, the ADI products, with 12-bit accuracy and
exceptional linearity at up to 31 Msps sampling rates and a
spurious free dynamic range of 73 dB, were real breakthroughs for
block radio applications. Both devices use a three-pass sub-ranging
architecture and digital error correction to achieve 12-bit
accuracy with relatively low power consumption.

Unique Application and Performance Issues in
IF Sampled ADC Designs

SFDR
Perhaps the best measure of an ADC's in-band distortion performance
is its spurious free dynamic range (SFDR). SFDR is the difference
between the desired signal level and the highest spurious signal in
the band of interest. The desired signal may be single- or
multi-tone that can excite intermodulation (IM) products. The
signals in the multi-tone test are carefully chosen so that the IM
products fall inside the frequency band of interest. The band of
interest may be the whole Nyquist band (0 – fs/2 where fs is the
sampling frequency) or a subsection of the band. For the case of
block radio, the subsection approach is often useful because, after
digital channel selection and filtering, only a small part of the
converted band is of interest. Modern SFDR tests are performed with
discrete Fourier transform (DFT) techniques and are often performed
at different power levels and presented as two-dimensional plots.
Bumps in the SFDR curve as the signal level is lowered are an
indication of converter linearity errors.

At SFDR levels of 90 dB and above, circuit board implementation
and the quality of passive components also become important design
issues. Clock jitter is also a significant problem in radio
frequency (RF) design. Another issue is that the high-speed signals
at the output of the ADC can leak back into the input of the
converter and degrade SFDR. To address this issue, Lucent's
CSP1152A uses a low-voltage differential signal (LVDS), a
low-noise, high-speed electrical interface that moves data from the
converter chip to a DSP with improved reliability by reducing
coupling of digital to analog signals. LVDS receivers are now
commonly available as standard products or as ASIC libraries,
making interface to the CSP1152A easy.

SNR
The signal-to-noise ratio in a wideband architecture benefits
directly from a process called oversampling, which reduces the
noise floor for a constant bandwidth signal. For example, consider
a wireless signal with a 30 KHz bandwidth. With an ADC sampled at
the Nyquist rate (60 kHz), the full noise power of the converter
will be in-band. But if we raise the sampling rate to 120 kHz, then
only half the noise power is in-band and the SNR is improved by
3dB. But in our block radio design, the sampling rate with the
CSP1152A is 65 MHz. This is 1083 times the required sampling rate
and it results in an improvement of 30dB, calculated from the
equation SNR = 10log ((sampling frequency/2) * (1/ signal
bandwidth)). This added SNR is called processing gain. This means
that an ADC with a SNR of 65dB over the Nyquist band will have an
SNR of 95dB after processing gain. Processing gain applies only to
random noise; it does nothing to improve a converter's linearity
performance and thus does not improve the harmonic distortion or
SFDR.

Bandwidth
Another important specification for an ADC used in block radio is
the sample and hold (S/H) bandwidth, typically specified as the
input frequency range that causes the converter gain to be reduced
by 1 to 3 dB. In block radio the input sampler is often used to
perform a mixing function as well as a sampling function, thereby
eliminating the need for additional external components. This
approach allows for relaxation of the linearity requirements of
some of the RF and IF analog blocks. The input signal to the ADC
may have frequency components as high as 250 MHz, so the S/H
bandwidth must be much higher.

The mixing property of an ADC's sample and hold circuit is
called undersampling. In an undersampling operation, a frequency
translation occurs. The frequency range of the input signal and the
sampling frequency must be carefully chosen so the resultant signal
at the output of the sampler is in the range of DC (0 Hz) to half
the sampling rate of the converter.

The design of the ADC sampler can greatly limit its usability in
undersampling. The sampler must not only have wide bandwidth, but
also display high linearity in the frequency range of interest.
These performance requirements are difficult to achieve when active
electronics are part of the sampler, as is the case with bipolar
technology. In contrast, to form a sampler with Lucent's CSP1152A,
a simple CMOS switch and a capacitor are all that is required. This
simple, high-performance circuit can be used because the sampler
drives a CMOS operational amplifier (op amp), which places no DC
load on the sampling capacitor and imposes no DC offset on the
signal sample.

Reducing ADC Non-Linearities with Dither
Modern high-speed, high-resolution converters are made using
multi-stage, pipelined converter techniques. A problem with this
approach is that linearity errors caused by mismatches between the
stages can significantly affect SFDR and linearity. One method to
address this problem is to add dither to the input signal to
de-correlate (randomize) the linearity errors of the converter.
Correlated spurious tone energy that cannot be handled by
downstream signal processors is converted to random noise that can
be reduced by processing gain. This use of dither for linearity
improvement is accomplished by making linearity errors more uniform
across the converter's range and is different from the use of
dither to decorrelate the quantization error present in any
ADC.

Analog Devices has proposed introducing high-level, low-pass
dither signals into the input of the ADC. Because communications
signals are bandpass in nature, the low-pass dither will in theory
not affect the in-band signal. In practice, however, in-band noise
modulation effects can be seen and the SFDR of the converter is
degraded in the presence of signals near the full-scale range of
the ADC. According to some, another problem with this approach is
that external components must be used to generate the broadband
noise, filter it and then add it to the signal path.

Lucent's chip uses a dither signal generated on-chip. The dither
signal itself does not appear at the output of the converter
because digital post processing removes it. This dither method is
made possible by the availability of switched capacitor circuits
and high-density digital logic found only in a CMOS design. Dither
improves the SFDR of the converter at all signal levels, and
because the function is on-chip, the implementation cost is
low.

1 comment on “ADCs and Digital Wireless Base Stations

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    March 29, 2020

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