Problem with 'logpwrfft' Block


I’ve got a problem with the logpwrfft block. I’m using a RTL-SDR with a
Raspberry Pi as streaming server.

On my computer I’m receiving the samples with GNU Radio over network. I
then want to calculate the spectrum of the signal. For this I tried the
QT-FFT Sink and it worked very good. But I need to save the picture of
the spectrum every second.

So I tried out the logpwrfft block. I’m plotting the data with
Matplotlib and I’m saving the plot to a file. The problem is that the
spectrum looks totally different from what the FFT Sink shows.
Experiments showed that the spectrum with the logpwrfft block is just
wrong. Has anybody got an idea why this is like that? Or is there a
possibility to save the output of the FFT Sink in a thread?

My Flowgraph with the logpwrfft block looks like this:

RTL-SDR Source —> Logpwrfft —> Message Sink (—> Pad Sink, just
that no error occurs)

Thank you in advance!