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Re: spectrum analyzer problem

From: Fabian Schwartau
Subject: Re: spectrum analyzer problem
Date: Wed, 30 Jun 2021 11:57:05 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

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,

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 and ubuntu 16.04.
> Would be grateful if you could help me in this matter.
> regards,
> Ali

Attachment: flowgraph.png
Description: PNG image

Attachment: result.png
Description: PNG image

Attachment: test.grc
Description: application/gnuradio-grc

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