Continuing on this theme...

Having abandonned my idea of a direct conversion Rx due to uncontrollable DC offsets, and not yet finished a locked clock for my SDR-IQ, I wondered about the next stage.

The idea is to overcome errors and drift in sound cards, and ideally make any coherent receiver non-dependent on the S/Card.  So I was contemplating this solution as being worthy of further tests.  Once the receiver (SDR-IQ here)  is locked,  its output can be delivered as an audio tone whose exact frequency is known.    This adjusted to end up at exactly 1kHz and is applied to one input channel of a soundcard, 

Now, I can generate an exact 1kHz tone by dividing down from the same reference signal and send this to the other soundcard channel.   So now, we have independant reference and test inputs to the Soundcard and it doesn't matter how unstable  its own clock is; the phase of the input signal can always be compared with that of the  reference input.  In fact, my gut feeling is that the less stable the soundcard clock is, after a bit of averaging a better result will ensue. 

Thoughts ?

The idea of making the soundcard's clock irrelevant appeals.   

And generating a nice reference 1kHz tone is dead easy.

Andy  G4JNT