How to simulate a Zoom in FFT display?

GNU Radio
1

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

2

Bill,

Am 12.08.2012 19:28, schrieb William Pretty Security:

Hello all;

I have a 40KHz carrier that I want to be able to check for
modulation.

[…]

What I am trying to implement is the equivalent of the “Span” and
“Center Frequency” functions on a spectrum analyzer.

maybe the GUI included in the specest toolbox does the trick or you
can find some programming hints there:

https://www.cgran.org/wiki/SpecEst

Jens

3

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.

No?

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 ??
Thanks.

Bill

4

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 :slight_smile:

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…

Thanks

5

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