# Usrp_spectrum_sense.py

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

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

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