[Discuss-gnuradio] QT GUI Frequency Sink Noise Floor

Hi,


I'm an Electrical Engineering Technology teacher and I like to use GNU Radio in my classrom for demos and labs.


Today, while trying to explain the relation between the noise floor on a frequency sprectrum and the RMS value of the same signal (noise), I realised that the QT GUI Frequency Sink noise floor is off.


When I connect a Noise source (amplitude of 1) to a QT GUI Frequency Sink, the noise flloor is -33 dB (window = none, fft size = 1024). I was expecting a noise floor of -30 dB as 1024 bin means that the 0dB total power is spread in 1024 bins (which gives -30dB).


For fun, I have switched the flowgraph to WX and used the FFT sink. The noise floor on the WX FFT sink was -30 dB as expected.


Anyone knows where the 3dB different comes from?


Thanks,

Frédéric

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