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Re: [Discuss-gnuradio] What's the meaning of the m.data in usrp_spectrum


From: Ling Huang
Subject: Re: [Discuss-gnuradio] What's the meaning of the m.data in usrp_spectrum_sense.py?
Date: Tue, 23 Dec 2008 17:01:45 -0800 (PST)

Eric 
Thanks´╝üNow I get it.
Ling

Eric Blossom wrote:
> 
> On Tue, Dec 23, 2008 at 05:52:03AM -0800, Ling Huang 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
> 
> 
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