[TangerineSDR] Question on bandwidth versus sample rate: can they be independently set?

Tom McDermott tom.n5eg at gmail.com
Tue Apr 14 14:26:33 EDT 2020

Hi Bill - generally the bandwidth and sample rate are locked together.  The
Nyquist criteria is that the
sample rate be at least twice the highest frequency component.  When using
I+Q samples, we are able
to distinguish negative from positive frequencies, so the range of usable
frequencies goes from
 -fsamp/2 to +fsamp/2.    Nyquist means that in theory no information is
discarded at lower than half the sample
rate ** - even very fractional Hertz stuff is in there and can be
recovered, but it might take a long time.

As an example:
In order to receive a 10 Hertz wide spectrum slice (+/- 5 Hz) the receiver
would need to first
lowpass filter the signal to < 5 Hz. (i.e. from -5 to +5) then decimate and
change the sample rate, then send those to you.
You could then for example implement a Complex FFT of size=1024.  This
would yield bin sizes of
10 / 1024,  or 9.8 milliHertz.   Beware however that at a 10 Hz sample
rate, that FFT process needs 102.4 seconds worth
of data per FFT.

Does this clarify?

-- Tom, N5EG

** Except that due to imperfect LPF filter roll-off, ADC quantization, etc.

On Tue, Apr 14, 2020 at 10:17 AM Engelke, Bill via TangerineSDR <
tangerinesdr at lists.tapr.org> wrote:

> This question is sort of directed to Scotty, but I am posting it here
> because I’m sure others will have useful thoughts on this matter.
> As you may recall, in TangerineSDR when we collect IQ data for multiple
> channels, we have to run them all at the same sample rate (at least, for
> Phase 1). The main reason for this is that Digital RF is optimized for
> handling data that way, and we need to roll with that rather than struggle
> against it, so that we can have something ready and tested in the time
> frame. We can add more complexity later if we want.
> I am puzzling over something, however – probably because I still have a
> lot to learn about digital signal processing.  The frequency precision of
> an FFT is related to the number of bins (i.e., when I do the FFT with 2048
> samples, I get more precision than if I do it with 1024 samples). In
> several conversations, the sample rate and the bandwidth have been almost
> interchangeable; however, is that mandatory?
> Specifically, let’s say that I want to closely watch the carrier frequency
> of WWV at 10 MHz.   We know that the most it moves due to doppler shift is
> a fraction of a Hz, so I only want to handle a few Hz of bandwidth – but I
> want to do it to a precision of 0.01 Hz.  This precision would seem to
> dictate a very high sample rate, but the bandwidth I want to process is
> only 2 to 3 Hz.
> The QUESTION then is this:  when configuring the channels for the DE to
> run, is it feasible for me to be able to independently set the sample rate
> and bandwidth?   If so, is there a major downside to doing this?
> -73- Bill AB4EJ
> --
> TangerineSDR mailing list
> TangerineSDR at lists.tapr.org
> http://lists.tapr.org/mailman/listinfo/tangerinesdr_lists.tapr.org
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