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

Engelke, Bill bill.engelke at ua.edu
Tue Apr 14 15:50:04 EDT 2020

Tom – thanks for the analysis on that.  (I must admit, I will have to study this a bit to really grasp it)…
But now, let me pose a couple of questions for Scotty & Nathaniel –

  *   Scotty – could we enable the DE to be able to do what Tom describes below?
  *   Nathaniel – regarding the precision & sample rate – for science results, is this the right trade-off between precision and how often we get a sample?  Or is there another, better way to do it?

-73- Bill, AB4EJ

From: Tom McDermott <tom.n5eg at gmail.com>
Sent: Tuesday, April 14, 2020 1:27 PM
To: TAPR TangerineSDR Modular Software Defined Radio <tangerinesdr at lists.tapr.org>
Cc: Engelke, Bill <bill.engelke at ua.edu>
Subject: Re: [TangerineSDR] Question on bandwidth versus sample rate: can they be independently set?

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<mailto: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
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