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I tried to generate a Gaussian noise signal having a standard deviation of 1 by setting the amplitude value in the noise source configuration to 1.

When I tried to display it using the QT GUI Frequency Sink, I initially expected that the resulting PSD would fluctuate around 0 dB across all frequency points. However, I have found that the PSD displayed at QT GUI Frequency Sink fluctuates around -40 dB (instead of 0 dB) over all frequency points.

From the signal processing theory's point of view, this result is clearly incorrect.

Is there a bug in my app that I don't know?

I am using GNU Radio application v3.8.2.0-57-gd71cd177 (Python 3.9.0) on windows 10.

Marcus Müller
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switchkane
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1 Answers1

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Two things: there's a bug in your expectation:

the sum of all powers needs to be around 1, i.e. 0dB, because that's the power and thus the variance, so the individual bins need to be lower - in fact, the lower the longer your FFT is.

But there's also undocumented scaling in the frequency sink, so we can call that a GNU Radio bug, if we want. GNU Radio is aware of the problem. It's not easy to address because it's bad to have unexpected behaviour, but it's also bad to break existing applications.

Marcus Müller
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