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

From: Jorge Gallo
Subject: [Discuss-gnuradio] log POWER FFT Output - Samples flipped in frequency
Date: Mon, 19 Jan 2015 13:24:56 +0100


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,


Attachment: FFT_estimated_power.png
Description: PNG image

Attachment: GRC.png
Description: PNG image

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