[SOLVED] [Moved]: Actual SNR value using FFT in Matlab/Cadence

[jdp721]

Hello friends.

We know that SNR can be found using FFT on transient analysis data from Cadence (using calculator or Spectrum in plot), as well as Matlab (after importing data into it).

I am doing this for a delta-sigma modulator, and have taken all precautions like f_in = (D/N)*f_s : D = prime integer and N = no of points of FFT...

Now comes the question (rather, confusion):

In some references (ref 1, ref 2 etc.), it is stated that:

"FFT acts as a narrowband spectrum analyzer with a bandwidth of fs/N that sweeps over the spectrum. This has the effect of pushing the noise down by an amount equal to the process gain..."
"...For correct frequency domain measurements it is also necessary to apply corrections for FFT processing gain...theoretical noise floor for N-point FFT is 10log(N/2) dB below the quantization noise due to processing gain of the FFT"

So my question, do we really need to subtract 10log(N/2) dB from the SNR computed using FFT? But, none of the Matlab codes found from internet does so while finding SNR! :-?
Is there anything that I am missing in this regard, which nullifies the need of subtracting 10log(N/2) ?

Any help/idea is highly anticipated...

 

[erikl]

Re: Actual SNR value using FFT in Matlab/Cadence

So my question, do we really need to subtract 10log(N/2) dB from the SNR computed using FFT? But, none of the Matlab codes found from internet does so while finding SNR! :-?
Is there anything that I am missing in this regard, which nullifies the need of subtracting 10log(N/2) ?

Don't mix-up SNR (and SINAD, SFDR, THD) calculation with the value of the NOISE FLOOR gap. Just for the latter one this 10log(N/2) dB value can be subtracted (or added, actually, because it means gap resp. distance).

For the other values this is not necessary (resp. allowed, unfortunately ;-) ) See Fig. 5 of the AD Tutorial.

Don't Get Lost in the Noise Floor

 

[jdp721]

Re: Actual SNR value using FFT in Matlab/Cadence

thanks Erikl

if you don't mind me asking....what is the full form of your "resp." ?

 

[erikl]

Re: Actual SNR value using FFT in Matlab/Cadence

what is the full form of your "resp." ?

respective(ly)