Automatic frequency control (AFC)
AFC is used for tuning the STALO to compensate for magnetron frequency drift.
RVP10 analyzes the burst pulse samples, and produces a running estimate of the power-weighted center frequency of the transmitted waveform. This frequency estimate is the basis of the RVP10 AFC feedback loop, whose purpose is to maintain a fixed intermediate frequency from the radar receiver.
The STALO is typically tuned 30 or 60 MHz away from the magnetron frequency. The maximum tuning range of the AFC feedback is approximately 5 MHz on each side of the center frequency. This is limited by the analog filters that are installed just before the signal and burst IF inputs on the IFDR.
The instantaneous frequency estimate is
computed using four autocorrelation lags from each set of N bn
samples. This estimate is valid over the entire Nyquist interval (for example,
18 ... 36 MHz), but becomes noisy within 10% of each end. Since the span of the burst pulse
samples is only approximately one microsecond, several hundred estimates must be averaged
together to get an estimate that is accurate to several kHz. The AFC feedback loop typically
has a time constant of several seconds or more.
Most burst pulse analysis routines, including the AFC feedback loop, are inhibited from running immediately after making a pulsewidth change. The center-of-mass calculations are held off according to the value of Settling time (to 1%) of burst frequency estimator, and the AFC loop is held off by the wait time before applying AFC. This prevents the introduction of transients into the burst analysis algorithms each time the pulse width changes.