Re: [Discuss-gnuradio] usrp_spectrum_sense.py

[Eric Blossom]

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

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.

http://www.fftw.org/fftw3_doc/The-1d-Discrete-Fourier-Transform-_0028DFT_0029.html#The-1d-Discrete-Fourier-Transform-_0028DFT_0029

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

Eric