In message , gareth
writes
"Brian Howie" wrote in message
...
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

Thank-you Brian, but what you have URLed is already at baseband, being VLF.

Well not quite. Base-band in this case is the audio pass -band which is
down-converted from the RF ( if you can call it that) at up to 20KHz by
the software local oscillator.

You can do it mechanically as well.

Http://www.wireless.org.uk/mechrx.htm

Brian
--
Brian Howie