wrote:
>
> Alberto:
>
> I never did ask anyone about the offset clock frequency to the
quad mixers you suggest for use with the Winrad software? Is it 12
kHz such as the Soft Rock 40 suggest or 96 KHz in this case? It sort
of came up in a discussion at soft rock group. I am ready to try a
vfo design on my SR 40 v6 and want to know what I am getting into.
>
> ka9rza
Well, there are two different ways of using an SDR program. One is
like Flex does, choosing a fixed last IF, 11.025 Hz in their case
(neglecting for a moment the anti-spur trick), and commanding the HW
LO accordingly. Then the final conversion to zero IF is done in the
software, together with demodulation.
Another way is to have a wide last IF, as wide as the sampling
frequency permits, e.g. from -48 to +48 kHz with 96 kHz sampling, and
then tune inside this chunk the wanted signal, using a point-and-click
technique in the software. Then the HW LO will need to jump in
discrete steps of 96 kHz (or a bit less, to have some overlap). You
see on the screen the entire chunk, the individual CW or SSB QSOs, and
you choose which one you want to listen to. My personal preferences go
to this way, and SDRadio and Winrad have been designed with this kind
of utilization in mind.
However nobody prevents you from using the first method with SDRadio,
choosing whatever last IF frequency pleases you most. Just stay a bit
away from the zero spot, to avoid the well known problems. 11025 Hz is
a good choice as any other. And if you are intending to build a VFO,
not a synthesizer, you are almost obliged to use the first method, as
it would be a bit difficult to tune a VFO in discrete, all equal, steps.
73 Alberto I2PHD
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