Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 249

The constant RMS amplitude feature is usually provided by one or more AGC
stages or one AGC stage and one or more switchable attenuators upstream from the
AGC. Normally, the AGC operates on the receiver's thermal noise level, and this
corresponds to the highest gain mode of the receiver. If this were always the case,
the AGC time constant would not be critical. However, a prudent AGC design
should assume that there will be occasions where in-band RF interference will
appear above this thermal noise level that can be mitigated with proper AGC and
ADC design. When RF interference occurs, the action of the AGC is to rapidly
reduce the gain in order to maintain the original RMS level at the input of the ADC.
There is probably no optimum AGC time constant for all possible types of RF inter-
ference. As a design guideline, the attack/recovery times should be much shorter
than the shortest integrate and dump times following the correlation process. The
range of acceptable AGC attack and recovery times is typically much shorter than 1
ms but no shorter than 50 s as possible

consistent with AGC amplifier stability. The AGC amplifier is usually the last gain
stage of the receiver. Figure 6.1 illustrates such an AGC amplifier along with a non-
uniform 2-bit quantization ADC and a digital jamming-to-noise ratio ( J/N ) meter
that will be described later. As shown in Figure 6.1, the analog gain control voltage
has a nonlinear effect on the gain, G A , of the AGC amplifier as follows:

µ s. The actual time should be as close to 50 µ

β V AGC

Ge
A

= α (6.1)

where:

V AGC = AGC control voltage

α = AGC gain coefficient

β = AGC control voltage coefficient

Equation (6.1) can be expressed in terms of the control voltage

G

1
β

V = ln

AGC

(6.2)

AGC

α

The two coefficients can be calibrated at the factory and stored as constants.
For higher precision that accounts for aging and other drift factors, the calibration
can be performed during built-in test immediately following power-up operation
but prior to receiver operation. This built-in test requires that the front end be
muted and a built-in tone at two precise amplitudes injected, one at the upper end
and the other at the lower end of the AGC range. The built-in calibration and mut-
ing are important only if it is desired that the J / N meter (described later) is accurate.
Many commercial GPS receivers are not designed to account for RF interfer-
ence; they are designed to provide enough gain (usually distributed over one or two
downconversion stages) to raise the RMS thermal noise amplitude to a value suit-
able for A/D conversion at the IF. Direct L-band sampling, with all the gain and
bandpass antialiasing filtering confined to one stage, is highly vulnerable to
self-jamming (i.e., self-induced oscillation due to leak-through feedback from the