Log POWER FFT Output - Samples flipped in frequency

GNU Radio
1

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

2

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

M

3

On 01/19/2015 07:24 AM, Jorge G. wrote:

in order to give an accurate power figure).
without any jump). (The same way as using a FFT sink).
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That’s normal for FFTW-based FFTs (and a lot of others).

The output goes from:

DC to BW/2
-BW/2 to DC

Where BW is your signal bandwidth.