Re: Fwd: spectrum analyzer problem

[Fabian Schwartau]

OK, yes, I think this is a bug.
In
https://github.com/gnuradio/gnuradio/blob/master/gr-qtgui/lib/freq_sink_c_impl.cc#L553
The data is converted using the fft() function and averaged after that.
But the fft() function already converts the transformed data into the
logarithmic scale in
https://github.com/gnuradio/gnuradio/blob/master/gr-qtgui/lib/freq_sink_c_impl.cc#L359
Averaging after the conversion to dB will yield wrong values, as
log(a+b) != log(a) + log(b).

Fabian

Am 30.06.21 um 15:29 schrieb Ali G. Dezfuli:
> 
> 
> ---------- Forwarded message ---------
> From: *Ali G. Dezfuli* <ali69550@gmail.com <mailto:ali69550@gmail.com>>
> Date: Wed, Jun 30, 2021 at 4:56 PM
> Subject: Re: spectrum analyzer problem
> To: Fabian Schwartau <fabian@opencode.eu <mailto:fabian@opencode.eu>>
> 
> 
> thanks a lot, Fabian,
> 
> I asked about the mapper itself and not pulse shaping (RRC). After some
> tests, I finally get to the problem of the spectrum analyzer (QT GUI
> Freq. Sink).
> I attached the grc photo to make my problem clearer.
> About the version, I really really like to be up-to-date but that's the
> matter of the OOT delay to move to the latest versions of GR and I
> really need them as well.😀
> 
> On Wed, Jun 30, 2021 at 2:29 PM Fabian Schwartau <fabian@opencode.eu
> <mailto:fabian@opencode.eu>> wrote:
> 
>     Hi Ali,
>     you should provide an example flow graph.
>     And I think I heard several times on this list, that you should not use
>     version 3.7 any more, as it is fairly old and not maintained any more.
>     Nevertheless, I hacked together a small flowgraph, where I cannot
>     reproduce your problem. I generate a QPSK constellation, the output RMS
>     value is 1 (which makes the peak values slightly larger than 1!) and the
>     1000 point FFT with a rectangular window shows -30 dB. When you generate
>     a constellation with 2 samples per symbol and make the symbol values to
>     have a value of 1, you will probably get 3 dB less than expected, as the
>     intermediate samples have less power than 1.
>     Attached is the flowgraph and the results.
> 
>     Best regards,
>     Fabian
> 
>     Am 30.06.21 um 11:19 schrieb Ali G. Dezfuli:
>     > Hi all,
>     >
>     > I wonder if this is a bug or what:
>     > I just make a QPSK constellation on the unit circle with quite random
>     > data and
>     > look at the spectrum by "QT GUI Frequency Sink" block in GRC.
>     > With these parameters:
>     >     FFT size = 1000
>     >     Window type = None OR Rectangular
>     > I should see a flat line as the spectrum with -30 dB (= -30 dBW) but
>     > surprisingly it
>     > shows -33 dB !!!
>     > I use GNU Radio Companion 3.7.13.4 and ubuntu 16.04.
>     > Would be grateful if you could help me in this matter.
>     >
>     > regards,
>     > Ali
>