Re: [Discuss-gnuradio] log POWER FFT Output - Samples flipped in frequency

[Martin Braun]

Looks like the log power fft doesn't fftshift. I didn't even know we had
such a block :) It's a hier block anyway, you could chain fft -> complex
to mag² -> qt gui vector sink for equal results.

M

On 01/19/2015 01:24 PM, Jorge Gallo wrote:
> Hi,
> 
> I am working with a B200 and GNURadio over Debian. At first sight, UHD
> driver and GNURadio work fine with my B200.
> 
> In my first example I am sampling at 2Msamples centered at 938MHz. I
> generate a 20KHz FM signal with my generator at 937.1MHz.
> 
> When doing an FFT I see clearly my signal at the expected level/freq.
> However I want to estimate power (I am aware that I need calibration in
> order to give an accurate power figure).
> 
> I set in GRC an USRP signal source followed by an "log POWER FFT" (1024
> points) and a sink file.
> 
> Furthermore I see those samples in real time with a "QT GUI Vector sink"
> and I realized that the samples from "log POWER FFT" are flipped. I mean
> the first samples I get are from 938 to 939MHz and then from 937 from
> 938MHz.
> 
> Does it make sense? ( I expected to get them from 937 to 939MHz without
> any jump). (The same way as using a FFT sink).
> 
> The aim of my example is to monitor the presence of power at certain bands.
> 
> Thanks in advance,
> 
> Jorge
> 
> 
> 
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