On Tue, Dec 23, 2008 at 05:52:03AM -0800, Ling H. wrote:

Hi,all

I’ve change the usrp_spectrum_sense.py to print out the m.data. And I got

the numbers like 9376200 2976546 34170452… But I just don’t understand

what these numbers stand for . Are they stand for the power or the energy?

And what’s these numbers’ dimension?

Sorry for my bad English.

Ling

They are the single precision floating point magnitude squared of the

FFT output. That is, for each FFT bin[i], mag_squared[i] = re[i]*re[i]

- im[i]*im[i].

If you want power, take the square root. We don’t do it by default

since most users are going to be computing a log somewhere downstream,

and the square root can be trivially folded into that calculation.

To see what the FFT bins contain, see the FFTW documentation (fftw.org).

The actual mapping from the levels at the daughterboard antenna port

to sample values depends on a lot of factors including any

daughterboard gain and decimation specific gain in the digital down

converter. You’ll need to calibrate the system if you need something

that maps to dBm.

Eric