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Re: [Discuss-gnuradio] How to simulate a Zoom in FFT display ? PROGRESS

From: William Pretty Security
Subject: Re: [Discuss-gnuradio] How to simulate a Zoom in FFT display ? PROGRESS :-)
Date: Tue, 14 Aug 2012 15:10:25 -0400

Hello List;

I have been working on the problem and have managed to solve most of the
problems myself.
As usual, there is just one last bug to step on :)

BY adjusting the "baseband frequency" and "sampling rate" of the FFT GUI, I
managed to get a 
Frequency display that makes sense. I (mostly) fixed the image problem by
adding a translator block
between the output of the filter and the input to the FFT.

However there is still an image at 44.5KHz. I confirmed that this is an
artifact by substituting the
audio source for a 42KHz Sine signal source ...

I could use some help here...


-----Original Message-----
From: William Pretty Security [mailto:address@hidden 
Sent: Tuesday, August 14, 2012 11:01 AM
To: 'Ryan Connelly'
Cc: 'address@hidden'
Subject: RE: [Discuss-gnuradio] How to simulate a Zoom in FFT display ?

Thanks for the assistance Ryan;

I tried that myself and the attached screen shots are the result.
As you say, if I am sampling at 96KHz and have a decimation of then, then I
should end up with a 9.6KHz window with the center frequency at 40KHz.

As you will see in the screen shots, the raw FFT shows a signal at about
42.9KHz. The "Zoomed" window shows an image on the minus side of the scale.
I would expect to see a signal about 3KHz positive from the center frequency
of 40KHz.


Could it have something to do with the fact that the filter has to accept a
float input from the audio source, but has to translate that to complex to
output it to the FFT. (There is no Float In -> Float Out parameter.)

Any ideas ??


-----Original Message-----
From: Ryan Connelly [mailto:address@hidden
Sent: Tuesday, August 14, 2012 9:51 AM
To: William Pretty Security
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] How to simulate a Zoom in FFT display ?

In the Frequency Xlating FIR filter just put a decimation factor and output
that to FFT. This will be "zoomed" to the center frequency by the decimation
factor. For example if you have a 1M sample rate file, and pass it through a
decimation of 4, you will get an FFT "zoomed" to a 250 kHz bandwidth. Is
that what you want to achieve?

Also the center frequency parameter (in that FIR filter) can be a variable
linked to a GUI slider so that you can interactively move the center
frequency you are zoomed on.

On Sun, Aug 12, 2012 at 1:28 PM, William Pretty Security
<address@hidden> wrote:
> Hello all;
> I have a 40KHz carrier that I want to be able to check for modulation.
> The 'source' is the audio source in GnuRadio Companion.
> I have tried a number of things and nothing seems to work for me.
> I have the source, followed by a 40KHz Bandpass filter and a frequency 
> translating FIR filter block.
> The output of the BPF and FIR filters go to separate FFT GUI's so that 
> I can see what is going on.
> What I am trying to implement is the equivalent of the "Span" and 
> "Center Frequency" functions on a spectrum analyzer.
> The Center Frequency is easy, using the Translating FIR block, but I 
> can't figure out how to do the Span/Zoom function.
> I Google searched and found a couple of SDR's with this function, but 
> it appears to be "hard coded".
> Does anybody know how to this in GnuRadio Companion ??
> Thanks .
> Bill
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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