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SciLab power spectral density of quantization noise

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NikosTS

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Hello there!

I have been trying to plot the PSD of quantization noise produced from a MASH ( cascaded SD modulators) using the pspect() function of Scilab.
Although i get the correct slope and generally a correct looking spectrum, the dB for each corresponding frequency is way off.

By that i mean , for example, that while in the ideal spectrum the maximum magnitude is -40 dB at (Fsampling/2) , in my plot that value is around +20 dB for that frequency. That difference of around 60dB is almost constant for every frequency.

The question is : Am I calculating the PSD wrong? And if that is the case , how is that offset generated and what should i change?

Regards,
Nikos
 

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