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Design >> Mixed-Signal Design >> FFT simulation of SNR for Sigma delta Modulator
https://designers-guide.org/forum/YaBB.pl?num=1150350571

Message started by hk2004 on Jun 14^th, 2006, 10:49pm

Title: FFT simulation of SNR for Sigma delta Modulator
Post by hk2004 on Jun 14^th, 2006, 10:49pm

Dear,

I am facing a stranger problem as following.

In Cadence , I run transient analysis for model based SDM circuit and strobe the output.
My setting is like this: quantizer clock frequency: 64M or period 15.625ns
skipstart: 319.5ns = 312.5+7ns. the 312.5 is 20 cycle for skipping the inital data. The quantizer is rising edge triggered. So 7ns is used to settle.
The problem is like this: if I store 8192 data , the FFT results shows the SNR is 48dB. But if I store 4096 data, the FFT results shows the SNR is 79dB that is expected. FFT operation is performed by matlab, and hann window is used.

Does anyone help on this? Thanks a million.

Title: Re: FFT simulation of SNR for Sigma delta Modulato
Post by sheldon on Jun 15^th, 2006, 9:52pm

HK,

It is a little difficult to analyze the problem from the available data. One
experiment that may be useful would be to run the 4096 sample FFT using
different data sets. In particular,
FFT A: 319.5n to 64.3195u
FFT B: 64.3195n to 128.3195n
If the FFT results are not consistent, then 20 cycles may not be long enough
to settle the start-up transients.

One other point, you say that you allowed 7ns for settling the transient before
performing the FFT. Shouldn't you allow half a period + 7ns, (15.625n/2) +7ns,
or 14.8125ns?

Best Regards,

Sheldon

Title: Re: FFT simulation of SNR for Sigma delta Modulato
Post by ee05s030 on Jul 26^th, 2006, 10:46pm

how do u get a decreased SNR when u take a 8192pt FFT? do u see the noise shaping? is the noise floor increased? if noise shaping is maintained u might be doing some scaling factor mistakes. check that.

Title: Re: FFT simulation of SNR for Sigma delta Modulato
Post by sheldon on Aug 2^nd, 2006, 2:37am

ee05s030,

If there are long internal time constants inside the circuit, then the effect
of settling the time constants will degrade the SNR. A settling time issue
looks like a 1/f noise plot, the FFT noise floor decreases with increasing
frequency. The request was to perform to FFTs. If settling time was an
issue then the SNR from the two FFTs would be different. The SNR of
the first FFT would be lower than the SNR of the second FFT. Settling
issues can occur for many reasons, for example, the common-mode
feedback loop might have a long settling time.

Best Regards,

Sheldon

Title: Re: FFT simulation of SNR for Sigma delta Modulato
Post by Jerry Kwon on Sep 8^th, 2006, 11:38pm

Hello HK2004!

I read your problem so I'd like to suggest something.
First of all, sometimes there are faults with transient simulation results because of step problem.
Thus, I think, more point sizes make FFT's SNR reduce.
If you can get text output, how about you perform following as:
Using Matlab-> command box -> (typing) psd(output,length(output),frequency) and then you can see the exact specrum you want.

Good luck!

Best,
Jerry Kwon

In Cadence , I run transient analysis for model based SDM circuit and strobe the output.
My setting is like this: quantizer clock frequency: 64M or period 15.625ns
skipstart: 319.5ns = 312.5+7ns. the 312.5 is 20 cycle for skipping the inital data. The quantizer is rising edge triggered. So 7ns is used to settle.
The problem is like this: if I store 8192 data , the FFT results shows the SNR is 48dB. But if I store 4096 data, the FFT results shows the SNR is 79dB that is expected. FFT operation is performed by matlab, and hann window is used.

Does anyone help on this? Thanks a million.

Title: Re: FFT simulation of SNR for Sigma delta Modulato
Post by filipe on Mar 9^th, 2007, 1:42pm

a good way to calculate the snr, and plotting your psd is using a script from a toolbox about delta sigma modulators, with matlab.
bye

Title: Re: FFT simulation of SNR for Sigma delta Modulato
Post by sheldon on Mar 9^th, 2007, 4:56pm

Jerry,

Sorry did not see your post. Your results are certainly contrary to expectation.
A couple of questions about the FFT issue.

1) What is the stop time for the simulation for the 4096 point FFT and for the 8192 point FFT?
--> The stop time for the 8192 point FFT is 2x longer than for the 4096 point FFT
64.3125us [4096] or 128.3125us [8192]
2) What input frequencies do you use for each simulation? Adjusting the input frequency for
for each simulation may help.
7/4096 --> 13/8192
3) Also, I have found that using the zvcvs, ideal S/H, can give better results than using the
strobe function.

Just my experience maybe but adjusting the input frequenices based on the number
of points seems to improve the results.

Again really sorry for the slow response time.

Best Regards,

Art Schaldenbrand