In message , gareth
writes
Presumably those SDR rigs which do not work on the IF but
directly from antennae must have, separately from the DSP processor,
some semblance of a DDS generator (but without the final DAC) to
act as the equivalent of the VFO, for I cannot perceive that a fractional-Hz
tuning rate could be achieved with machine code running in the DSP
processor?




I'm not an expert ,but I think what you're asking is " how is the local
oscillator generated" in a direct conversion SDR and "what determines
its resolution"

There is an example here, :-

http://www.radioelementi.it/public/saqrx.pdf

The "c" source code is here,which I can just about understand ( My
software background is FORTRAN and Matlab) :-

https://sites.google.com/site/sm6lkm/saqrx/

Softies shouldn't have a problem with it although I was able to mess
about with it and recompile it successfully

In this case the spectrum is dc to 22050Hz in 512 steps. It's not the LO
precision ( it's floating point in this one) that limits it but the
size of the FFT , the sample rate and thus the record length, that sets
the minimum FFT bin width . This one tunes in lumps of about 43Hz

HTH Brian


--
Brian Howie