Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 248

interference levels that their digital baseband tracking loops can tolerate. For mili-
tary GPS receivers, it is essential that the front-end dynamic range accommodate the
maximum expected levels of RF interference (i.e., the specified levels). The digital
tracking portion of the military receiver must be designed to match the front-end
performance. This means that none of the front-end gain stages go into gain com-
pression for the specified nonburst-type jamming. For burst-type (pulse) jammers,
such as on-board radars, the front-end design must prevent any gain stage from
going into saturation, and the receiver must adjust its gain to instantly blank (com-
press) when excessive burst energy is present, then instantly recover (expand back to
linear operation) when the burst energy disappears.
The RF susceptibility characteristics of GNSS receivers to adjacent-band RF inter-
ference must be considered when evaluating the overall effectiveness of the system for
any potential application. Only front-end filtering or antenna cancellation techniques
can prevent the GNSS receiver from being overdriven by these adjacent out-of-band
signals. One advantage of multistage down conversion is the opportunity to not only
isolate the gain stages, but also add increasingly higher-Q filters as the IF is lowered.
Surface acoustic wave (SAW) filters are highly effective with high-Q characteristics
even at L-band. But SAWs also have large insertion loss, so they must be used well
past the front-end preamplifier that sets the noise figure of the receiver. Low insertion
loss passive bandpass filters can and should be used prior to the preamp. Cavity filters
are a very suitable passive filter choice but tend to be physically large. Because of this,
they are often physically included inside the passive GNSS antenna. The preferred
conventional filter for GNSS at lower IF stages is one that is maximally flat in-band
and has the sharpest stop-band roll-off characteristics, such as a Chebyshev bandpass
filter. The front-end filtering not only prevents unwanted out-of-band interference but
also reduces aliasing (discussed later in the A/D conversion section). The front-end fil-
tering problem is particularly difficult on platforms where high-powered transmitter
antennas are near the GNSS receiver antenna. Highly specialized signal cancellation
antenna design techniques that actually use samples of the interfering signals or deep
null steering front-end filters are often the only possible solutions to this problem.
6.2.2.2 AGC
The gain objective of the front-end design is that the RMS amplitude of the thermal
noise plus jamming noise must remain essentially constant at the input of the
analog-to-digital converter (ADC). The optimum RMS levels are discussed in Sec-
tion 6.2.2.3. This must be accomplished without any analog gain stages being driven
into compression. High-powered burst jammers will drive these stages into com-
pression, but all of the gain stages must be prevented from saturating. Otherwise,
they will be slow to recover to their linear gain when the burst energy disappears.
The effect of continuous gain compression (or saturation) is the suppression of
the GNSS signals in a manner that is highly disproportional to the level of the noise
that caused the effect (i.e., a very nonlinear detrimental effect). This nonlinear effect
renders the linear circuit theory presented elsewhere in this topic meaningless. It
does not matter what elegant algorithms have been implemented in the digital part
of the receiver to accomplish acquisition and tracking if the analog GNSS signals
have been totally corrupted by the nonlinear behavior of the front end.