Help about digital SDM output quantization noise by fft

[hhq414]

Hi all,
When I use simulink to simulate a MASH111 SDM for pll, the output are digital for example ...-2,-1,0,1,2,3..., and I want to convert this value to output quantization noise. The code I used to do convertion is as follow:
Fref=1e6
NFFT=8096
SIMOUTave=mean(simout)
y=(simout(1:NFFT)-SIMOUTave).*hanning(NFFT);
Y=fft(y,NFFT)/NFFT;
pyy=Y.*conj(Y);
f = Fref/2*linspace(0,1,NFFT/2+1);
semilogx(f,10*log10(pyy(1:1:NFFT/2+1)));
axis([100 1e6 -250 -90]);
xlabel('frequency');
ylabel('quantization noise')
grid on
And the result is also attached. The blue line represent what I get using the above code, and the red line is by direct calculation(attached).But there is a great differece in power. Is anyone know what is wrong with the above code?

 

[JoannesPaulus]

The power of your simout signal needs to be scaled to +/- 1 (now you have [-2,3]).
In general, for an sdm, the absolute output power (in simulation) is not important (it is the power of a bitstream). The difference of powers is what really matters!

 

[hhq414]

The power of your simout signal needs to be scaled to +/- 1 (now you have [-2,3]).
In general, for an sdm, the absolute output power (in simulation) is not important (it is the power of a bitstream). The difference of powers is what really matters!

Hi, JoannesPaulus. If you want to know how much phase noise your SDM contributes to the whole PLL, you should know exactly how much quantization it has. So if anyone know, please help!