Peter Restall wrote:
> I've since got a better (experimental) setup - better front-end, AGC,
> and fewer chips - and I'm going to pipe everything into a PIC, using
> its PWM as a BFO and comparators as threshold detectors (and ADC
> inputs to monitor the level and control the AGC), but that's a story
> for another thread :)

With a dsPIC you should be able to go another step.  After suitable
amplification and at least some frequency filtering, run the antenna signal
into the dsPIC A/D.  The rest is all software.  You can multiply the signal
by both a sine and cosine at 60KHz, low pass filter each of these
independently, then square them and add them to get the squared carrier
amplitude.  Sampling at 4x the carrier frequency still leaves you over 120
instructions per sample.

The low pass filter after the sine and cosine multiplies will make this
software tuner highly selective.  You only need to pass enough to account
for the crystal error and modulation bandwidth.  60KHz derived from a 50ppm
crystal only has a error of +-3Hz, so you can easily set your LPF rolloff
frequency to 10Hz.  You could even detect and long term null out the beat
frequency.


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