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

From: Martin Braun
Subject: Re: [Discuss-gnuradio] log POWER FFT Output - Samples flipped in frequency
Date: Mon, 19 Jan 2015 14:30:35 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0

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 magĀ² -> qt gui vector sink for equal results.


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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