Usrp_spectrum_sense.py

GNU Radio
1

Hi,

I am new to DSP, so I am finding it a little difficult to understand
exactly whats going on in the usrp_spectrum_sense.py code. When I
print out m.data[0], m.data[1] along with m.center_freq, this is what
I get:

./usrp_spectrum_sense.py 2412M 2444M

Using RX d’board A: Flex 2400 Rx
gain = 45.0
2413500000.0 10906340.0 4703766.5
2416500000.0 5325887.0 4675695.0
2419500000.0 1110628864.0 726685696.0
…

I understand that output of the block gr.fft_vcc would be the FFT of
the signal; And then we do a c2mag to get the power spectrum …

My question is: If my center frequency is say 2413500000.0 (first line
of the output), how do I measure the power on that frequency? Is it
proportional to data[0]? (The comments say that data[0] is DC … is
this DC relative to the center frequency?)

What frequency does the bin data[1], data[2] etc represent? Is this
dependent on the ADC sampling frequency?

Thank you for the help.

Regards,
Shravan

2

On Tue, Nov 21, 2006 at 06:06:58PM -0600, Shravan Rayanchu wrote:

gain = 45.0
proportional to data[0]? (The comments say that data[0] is DC … is
this DC relative to the center frequency?)

What frequency does the bin data[1], data[2] etc represent? Is this
dependent on the ADC sampling frequency?

Thank you for the help.

Regards,
Shravan

I suggest that you take a look at the docs on www.fftw.org. Our fft
block uses FFTW to do the work, and the output is exactly as they
specify.

For a good intro to DSP, see “Understanding Digital Signal Processing”
by Richard Lyons. ISBN 0131089897

Eric

3

Hi Eric,

Thanks for getting back to me. I’ll look into the fftw docs and the
links you gave to understand the details.

–Shravan