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A Question About Delta Sigma Modulators

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naalald

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When I simulate the modulator in simulink the SNR is 62 dB but when I simulate it in hspice it is around 24 dB. The modulator seems to work well in hspice and the output waveforms are correct. I guess maybe there is some mistakes in calculating fft. Here is what I use:

.tran 1u 1.002 UIC
.fft v(no+,no-) np=63131 start=2m stop=1.002 window=hann freq=1 fmin=1 FMAX=540

The sampling frequency is around 63131. The output waveforms and FFT of both simulink and hspice are as below:

The input and output in simulink:


The input and output in hspice:


The FFT in simulink:


The FFT in hspice:


Any ideas?
Thanks.
 

maybe the fft algorithms are different. I always output data from spectre and fft them with the same fft algorithm as in simulink.
 

hi naalald,

maybe the time points is problematical, try the time period 2m+1/63131 ~ 1.002, or 2m-0.1/63131 ~ 1.002-0.1/63131. if bad yet, export the data and run fft in matlab. luck.
 

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    naalald

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Thanks all,
I got the answer! It's mainly related with the ".FFT" syntax parameters. There are two points:
1- the start point (tstop) should be selected exactly before the end of phi2 phase.
2- the stop point should be selected so: tstop=tstart+nfft*ts in which nfft is the number of fft points taken and ts is the sampling period.
Make sure nfft is a power of 2.
jiangxb was right! Thanks jiangxb!
 

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