[Discuss-gnuradio] QT GUI Frequency Sink Noise Floor

[Daigle , Frédéric]

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