Re: Subject: [Discuss-gnuradio] APCO 25

[Matt Ettus]

>       I must be missing something here.  Perhaps I am. I am getting
> pretty senile these days I'm afraid...  The normal Nyquist criterion
> would require a 0-200 khz signal centered on 100 khz (what Ian was
> proposing) to be sampled at 2F where F is the highest frequency
> component of interest (in this case 200 khz or so).   This is pretty
> fundemental sampling theory I think.   Now I recognize that delta-sigma
> type converters can provide a good intrisic low pass filtering by
> oversampling which moves the ailiases way up in frequency, but still we
> are talking about a 200 khz bandwidth here centered at 100 khz.  And Mr.
> Nyquist had a point...

Not necessarily.  If you go by the description given, a 10.7 MHz IF, 200 kHz
wide gives us 10.6 to 10.8 MHz.  If you mix that with 10.8, you get 0 to 200
kHz.  As long as you tune the radio so that the signals of interest are in the
10.75 to 10.8 MHz range (i.e. on the side of the filter), you could sample them
with a 96khz soundcard pretty easily.  The rest get filtered out by the
soundcards antialiasing.

On the other hand, you'd probably be better off mixing the 10.7 MHz IF with a
quadrature 10.7 MHz LO.  The you could use both inputs on the card to get 10.65
to 10.75 MHz in.  The 10.6 to 10.65 MHz and 10.75 to 10.8 MHz segments would get
filtered out by the soundcard.

My basic point is that you can feed the soundcard basically whatever bandwidth
you have, and it will handle the antialiasing.

>       Which brings up an ignorant question - of the available high
> end sound cards with 96 khz sampling, do any of them use external
> (not part of an ASIC) delta sigma converters, and if so are they clocked
> when running at 96 khz at the full spec'd max of the chip or could one
> hack the card to further up the sample clock rate.

There are some that support external codecs, but only by SPDIF interface.  Check
out the RME Hammerfall for one example of this.  The external codecs are quite a
bit more money.  I don't know about overclocking them, though.

>       Certainly for gnu-radio one might get by with somewhat reduced
> specs but a higher sampling rate quite nicely if the PCI or USB logic
> could handle faster sample streams and the converter could be clocked
> faster too.   And there are significant numbers of signals that 200 to
> 400 k samples/sec would digitize that 96 k samples/sec does not handle.

There are 192 ks/s cards on the market, but higher than that is unlikely in the
near future.