On 2/6/2016 4:49 PM, Michael Black wrote:
On Sat, 6 Feb 2016, Brian Howie wrote:

In message ple.org,
Michael Black writes
On Sat, 6 Feb 2016, gareth wrote:

"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.

I thought that was the norm, not much doing A/D at signal frequency.
Initially, it was too fast for the hardware to handle, but there are
probably some good reasons still to downconvert.

Michael

Correct , but Gareth asked about the software equivalent of a DDS
frequency synthesiser or VFO.

The "directly from antenna" in his post threw me. If there's a
heterodyne conversion, which is what he was asking I see now, then there
has to some sort of local oscillator. The way I read it was that he was
asking how to tune something that directly converted to digital. Sorry.

I'm a bit confused by the many comments about the original post, but
there is such a thing as a direct down conversion receiver. As long as
the input is sampled fast enough for the signal frequency or at least
fast enough for the bandwidth when using sub-sampling and there are
adequate filters on the input, this can work. The trouble is the
filtering. I believe that is why IF frequencies have been used, to
filter the signal more easily.

--

Rick