volume55-number1 - Flipbook - Page 5
SFDR Considerations in
Multi-Octave Wideband
Digital Receivers
Benjamin Annino, Applications Director
Introduction
Next-Generation ADC Performance
Electronic warfare (EW) receivers must intercept and identify unknown enemy
signals among a congested wideband spectrum of multiple interfering signals
without the benefit of dynamic range and sensitivity improvement techniques
employed in communications and radar receivers. The incident RF band limiting
employed in communications receivers is an unwanted trade for the EW receiver
that seeks to process ever wider instantaneous bandwidth in less time. In the
radar realm, receiver dynamic range benefits from matched filtering, whereby
the received radar return is correlated with a copy of the transmitted signal.
Alas, the EW receiver has no prior knowledge of the signal to be intercepted and
thus nothing with which to correlate! It’s like searching a crowd of people for a
stranger you’ve never seen before … and worse yet, he is hiding, or maybe isn’t
even there!
Many of today’s EW receivers feature sub-octave instantaneous bandwidth (IBW)
that is limited by the older generation data converter. These will be replaced
tomorrow with multi-octave wideband digital receivers spanning several GHz of
IBW. For example, in the coming years a growing number of sensing platforms
will employ ADI converter chips featuring ADCs and DACs with the ability to
process greater than 4 GHz IBW while maintaining SFDR greater than 70 dB.2,3,4
Now for the good news: over the coming years, high sample rate analog-to-digital
converter (ADC) and digital-to-analog converter (DAC) technology will usher in a
wideband digital receiver architectural evolution. Most importantly, converters
from Analog Devices will maintain the excellent linearity, noise performance,
and dynamic range of legacy lower rate digital converters. The workhorse
super-heterodyne tuner will give ground to direct sample and direct conversion
architectures.1 Adaptive spectral tuning will continue to shift from the RF to the
digital signal processing realm.
This sea change in wideband RF sensing will enable size, weight, power, and cost
(SWaP-C) benefits: higher receive and transmit channel counts at lower cost per
channel, in the same or smaller sized form factors as today.
Anticipating the coming era of digital EW receivers with multi-octave bandwidth,
this article discusses new challenges and considerations when designing for bestin-class dynamic range. In this article, dynamic range refers to instantaneous spur
free dynamic range, the key figure of merit for receivers tasked with detecting
small signals among a crowded spectrum of larger blockers.
Analog Dialogue Volume 55, Number 1
A popular low SWaP, wideband digital receiver ADC use case might be:
X
An ADC sample rate of ~15 GSPS
X
A direct sample of the first Nyquist zone (that is, dc to 6 GHz)
X
A direct sample of the second Nyquist zone (that is, 8 GHz to 14 GHz)
X
RF block convert middle (6 GHz to 8 GHz) and higher (>14 GHz) bands
EW receivers need to cover higher and higher swaths of spectrum from 18 GHz
to 50 GHz and beyond. The ADC’s high second Nyquist zone eases the frequency
plan, allowing simple RF front-end block converters with relaxed, smaller SWaP
RF filters. The following discussion considers an RF front end cascaded with a
high sample rate ADC similar to the previous example.
Dynamic Range in Wideband Digital Receivers
Receiver designers optimizing dynamic range must balance sensitivity (NF) with
linearity (IP2, IP3) as these RF device attributes usually move against each other.
Dynamic range is bound by sensitivity at lower RF levels and linearity at higher
RF levels. As a rule of thumb, the maximum allowed receiver operating level is
set so that the multisignal intermodulation distortion (IMD) spurious levels are
equal to the noise power, as shown in Figure 1. Modern systems use adaptive
instantaneous bandwidth channelization and processing bandwidths (Bv), which
moves the noise floor up and down 10Log(Bv). The nuanced topic of processing
bandwidth is critical and receives its own discussion later.
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