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Re: [Discuss-gnuradio] intermittent pulse detection

From: Martin Braun
Subject: Re: [Discuss-gnuradio] intermittent pulse detection
Date: Sun, 5 Feb 2017 13:06:08 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

On 02/04/2017 11:39 PM, Dirk Gorissen wrote:
> 1) 2.5 Msps gives me way more bandwidth than I neeed. Assuming, for
> now, I only care about a single pulse frequency I really only need
> ~1khz bandwidth. In the frequency domain I can directly decimate down
> (with a big factor) to the 1-2 khz range using the low pass filter
> block, do an fft, and look for peaks. Is that the right approach?

That's a viable approach, yes. It's not the only one, but it's a good
place to start.

> 2) Im somewhat confused about the FFT block if I just pipe the SDR
> straight into it. The FFT size is set to 1024 and the window is set to
> "window.blackmanharris(1024)". So Im assuming the FFT just applies one
> window (?) and outputs 1024 bins. However, how many samples are
> accumulated before the FFT is run? I would have assumed I can control
> that too. And if so, should I best be doing this every 50ms, 500ms,
> 2000ms, ..?

The FFT block will take N samples, calculate an FFT, and then output
that. If you have a bigger FFT, you will output FFTs less often.

> 3) I can use the rational resampling block to bring the sample rate
> down to 48khz so I can use the audio sink. From that I can still hear
> the pulse even if it is not visible in the spectrum (gui sink). Im
> assuming this is just because the plotting cannot keep up?

The QT GUIs will only update the visuals every x seconds. If you use
gr-fosphor, though, you won't lose samples in your visuals.

> 4) In the time domain I guess I can generate a synthetic pulse of the
> same length / frequency and then cross correlate. Not obvious to me
> how to generate the required pulse in gnuradio though (would a
> continuous signal work?). I also notice there are no built in
> (auto)correlation blocks? I found the "correlation estimator" but not
> clear how to use it. As for dealing with the frequency uncertainty
> problem. Does one just try correlating with different freuencies and
> pick the best one? Or what is the good thing to do here given I may
> also have to deal with quite a bit of noise.

For cross-correlation, you can use an FIR filter block. That will not
correlate in 2 dimensions, though, only in time-direction. That may be
all you need.

Autocorrelation is tricky in a streaming model; you either have a fixed
lag (i.e. you calc the autocorrelation at a single point), then you can
just do delay-conjugate-multiply. In this setup, you can still make the
delay variable.

If you want a true auto-correlation, you could do an IFFT of your
signals's PSD. But that might not give you what you want.

-- M

> Any guidance appreciated.
> Many thanks,
> Dirk
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