Automatic Fine Tuning for FM demod

I’ve just updated simple_fm_rcv to implement a simple Automatic Fine
Tuning function.

This function measures DC offset on the output of the demodulator block,
and uses that to drive a parameter of the freq-xlating
filter block that’s on the input. It seems to work OK in my simple
tests this evening. This is a piece that typically wouldn’t be required
if using UHD hardware, but the RTL-SDR radios have, ahem,
less-than-ideal master oscillators, so I wanted to see if AFT would be

So, in effect, I’ve implemented a feedback loop within the flow-graph.
But it’s via a polled value, so it doesn’t violate the rules.

Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium