AFC (automatic frequency control)

AFC tracks a weak or drifting signal and steers the first mixer so the signal stays centered at 0 Hz. It is not tied to one mode: the same loop runs under CW, SSB, AM and FM, which is why it is described here on its own rather than as part of any single demodulator. Tracking a signal down to a narrow filter is what makes weak-signal work and coherent detection possible.

The loop#

The high-resolution spectrum is the input: when the second FFT is enabled AFC uses fft2 transforms, otherwise it falls back to the first-FFT spectrum. Averaging more transforms finds weaker signals at the cost of delay. collect_initial_spectrum() acquires the signal — averaged transforms, the peak located by parabolic interpolation, and an initial polarization estimate on two-channel input — and the loop then follows the peak from transform to transform. The measured frequency error, bounded by AFC_LOCK_RANGE and rate-limited by AFC_MAX_DRIFT, becomes the fine-tuning phase rotation applied inside the first mixer (do_mix1_afc() / fft2_mix1_afc()), so the tuned signal re-enters the next transform closer to zero.

The signal-to-noise used to validate the track comes from make_afc_signoi() (noise estimated from filters outside the signal); make_ag_point() turns each measurement into a point on the AFC graph, the display that shows the tracked frequency over time.

Settings#

Three general parameters control it (globdef.h:307):

AFC is active only when AFC_ENABLE != 0 and AFC_LOCK_RANGE != 0; setting AFC_ENABLE to 0 forces AFC_LOCK_RANGE to 0. With AFC_ENABLE == 2 the automatic spur search also runs, so stable carriers are removed from the spectrum before the peak is tracked.

Related#

AFC works on the second FFT spectrum and drives the first mixer; its AFC_ENABLE == 2 mode ties it to spur removal. A second, tighter AFC stage inside the narrowband chain is what enables coherent CW on very weak signals.