Olin Lathrop wrote:
> 
> 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.

Olin, can you (or anyone else) explain the idea behind this?  I'm 
referring to the mixing of the carrier by the sine and cosine.

I have seen plenty of hardware ICs that do this (I/Q mixing) but I do 
not know what the purpose is.

Thanks!
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