Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 254

interference it can outperform an infinite-bit linear ADC by more than 10 dB at high
jamming-to-desired signal ( J / S ) power ratios (60 dB or higher) (see footnote 2). It
performs well for modulated CW, such as swept CW and narrowband jammers.
The principle of operation of the nonlinear ADC in the presence of CW is as fol-
lows. In the design in [3], the application is for very short period PRN codes that are
BPSK modulated and received at the much higher SNRs typical of spread spectrum
communications systems. Thus, the upper and lower magnitude comparator volt-
ages are changed dynamically by complementary digital counters at the ADC out-
put, while the AGC attack and recovery rate is slow. Referring to the Figure 6.1
adaptation of this concept for receiver applications, the upper and lower magnitude
comparator voltages, V B , are held constant, the AGC attack and recovery rate is fast,
and the feedback to the AGC controls the RMS amplitude for proper clipping of the
ADC. As part of the AGC feedback control, the weighting factor, N , adjusts the sta-
tistical averaging time to properly follow fluctuations in the noise level of the signal
plus noise that exceeds the plus and minus magnitude bit comparators. A compara-
tor passes the output of the weighting factor function that exceeds a certain percent-
age ( T %). The result is an AGC control error that is digitally integrated, then
converted to an analog AGC control voltage (see footnote 2). Figure 6.3 depicts the
GPS signal buried in (dominated by) thermal noise being added to (riding on top of)
a CW signal, along with typical complementary threshold settings of T 5%. The

T % threshold adjusts the signal statistics such that a similar effect is taking place at
the plus magnitude and minus magnitude comparators as would occur at the sign bit
comparator with only thermal noise present. In this manner, the nonuniform ADC
provides correlation in the presence of constant envelope interference. The values of
T % and N can be constants or variable, depending on the level of sophistication

=

7

6

Upper threshold (T% = 5)

5

4

Signal + thermal noise +
CW interference

3

2

1

0

− 1

− 2

− 3

− 4

− 5

Lower threshold (T% = 5)

− 6

− 7

0 90 180 270 360 450 540

CW phase (degrees)

Figure 6.3 Setting nonuniform ADC threshold to exploit constant envelope interference statistics.