quantization noise demonstration ?

[jmfriedt]

I wanted to demonstrate quantization noise (V_q^12/12/fs noise floor)
with GNU Radio and somehow my demonstration is failing. I am trying
http://jmfriedt.org/untitled.png (or http://jmfriedt.org/untitled.grc)
following the chart of the Quantizer. Somehow I had already noticed
that the Qt Freq Sync is displaying a spectral density (because keeping
1:N will not raise the noise floor by N as would be expected from aliasing), 
but here the noise is obviously quantized (see time domain) but the
spectral density in the frequency domain does not show the impact of
quantization.
Is this expected ?

Thanks, JM

-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe,
25000 Besancon, France