[Discuss-gnuradio] FFT normalization issue: noise aliasing

[Jean-Michel FRIEDT]

I want to demonstrate how aliasing rises the noise level upon decimation.
I have a trivial flow graph with a noise source, whose bandpass I  
limit to make
sure I know its spectral characteristics, and I compare the spectral power
with and without decimation. The decimation FIR is 1 so I believe no filtering
should be performed. I expect the noise level to rise 10-fold upon a  
decimation
by 10 through aliasing. That does not happen: the spectral density remains the
same, with and without decimation.
http://jmfriedt.sequanux.org/aliasing_noise_png.png for the flowchart, and
http://jmfriedt.sequanux.org/noise1.png for the spectra.
Is this the expected behavior ? I do not expect to be so, and I wonder if the
normalization coefficient is correct.
GNU/Octave does not help me much on the issue: generating a random sequence
and comparing the fft of the sequence or fft of one in every 10 sample *drops*
the level 10 fold, as summarised in jmfriedt.sequanux.org/aliasing_octave.pdf.

Of course we all want ifft(fft(signal)) to be equal to signal, but I have seen
all sorts of normalization (1/N on the FFT and none on iFFT, 1/N on iFFT and
none on FFT, 1/sqrt(N) on both FFT and iFFT with N the number of samples on
which the FFT is computed).
Shouldn't my aliasing experiment raise the noise floor rather than keep it
constant ?

Thanks, JM

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