The Designer's Guide Community Forum - Phase Noise of Δ-Σ modulators

Mayank

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Re: Phase Noise of Δ-Σ modulators
Reply #15 - Jun 17^th, 2010, 4:14am

Quote:

we have to set analysis time step lesser than time resolution of jitter.

What do you mean by resolution of Jitter ??

Quote:

This is not SDM for Fractional-N Synthesizer.

I can't find proper example of typical 3-cascaded-MASH-SDM for Fractional-N Synthesizer.
Posted by: Mayank

yeah, it's a band-Pass SDM, probably for some ADC application.

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pancho_hideboo

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Re: Phase Noise of Δ-Σ modulators
Reply #16 - Jun 17^th, 2010, 4:17am

Mayank wrote on Jun 17^th, 2010, 4:14am:

Quote:

we have to set analysis time step lesser than time resolution of jitter.

What do you mean by resolution of Jitter ??

If you use jitter generator model for fixed time step simulator, answer is self-evident.
If you use jitter generator for continous time simulator without sepecifying lowest time resolution, we can't know time resolution.

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« Last Edit: Jun 17^th, 2010, 5:31am by pancho_hideboo »

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sheldon

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Re: Phase Noise of Δ-Σ modulators
Reply #17 - Jun 17^th, 2010, 5:21am

Mayank,

You might find this paper interesting, it deals with some of the issues
with writing PLL models for discrete time simulators.

http://www.cppsim.com/Publications/JNL/perrott_dtc02.pdf

You might also want to look at cppsim while it is not MATLAB, you find
it interesting. It is fast, has libraries to support PLL simulation, high
level tools for PLL design, and some useful tutorial examples.

Best Regards,

Sheldon

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Mayank

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Re: Phase Noise of Δ-Σ modulators
Reply #18 - Jun 17^th, 2010, 5:50am

@ Pancho :
Quote:

If you use jitter generator model for fixed time step simulator, answer is self-evident.
If you use jitter generator for continous time simulator without sepecifying lowest time resolution, we can't know time resolution.

Now, i got what you meant by jitter resolution. Thanks.
Will get myself acquainted with the Schrier toolbox.

@ Sheldon :
Thanks for the link. I have worked on cppsim & sue models. The paper explains decently the PLL modelling. But cppsim is mainly for phase domain modelling of PLL. I was more interested in SDM Design. Thanks anyways.

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Ken Kundert

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Re: Phase Noise of Δ-Σ modulators
Reply #19 - Jun 17^th, 2010, 4:59pm

Pancho,
Your comments about the simulator are only part of the issue. The Fourier transform itself samples the waveforms on a fixed interval. Assuming that the signal is a square wave, any jitter that occurs between the sample points is unobservable by the Fourier transform. Hence, Sheldon's original comment is correct.

-Ken

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pancho_hideboo

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Re: Phase Noise of Δ-Σ modulators
Reply #20 - Jun 17^th, 2010, 8:14pm

Ken Kundert wrote on Jun 17^th, 2010, 4:59pm:

The Fourier transform itself samples the waveforms on a fixed interval.

Right if we use conventional FFT.

Ken Kundert wrote on Jun 17^th, 2010, 4:59pm:

Assuming that the signal is a square wave, any jitter that occurs between the sample points is unobservable by the Fourier transform.
Hence, Sheldon's original comment is correct.

You are also wrong.
That is no more than an advertisement of Fourier Integral Calculation in Cadence Spectre.

If we use fixed time step solver in signal flow model simulator and set analysis time step lesser than time resolution of jitter,
we can never miss jitter in FFT.

http://www.designers-guide.org/Forum/YaBB.pl?num=1276575200/14#14

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Mayank

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Re: Phase Noise of Δ-Σ modulators
Reply #21 - Jun 17^th, 2010, 8:59pm

Quote:

If we use fixed time step solver in signal flow model simulator and set analysis time step lesser than time resolution of jitter,
we can never miss jitter in FFT.

Suppose, if the accuracy of discrete time simulator is 1ps,
& we upsample the waveform, be it square or sine, at less than 1ps,
then i dont think we are missing anything.
Since for taking FFT, we are sampling at rate higher than the accuracy of the simulator. I think that's what Pancho meant by Quote:

we have to set analysis time step lesser than time resolution of jitter.

Then i agree with what Pancho said.

But, obviously this method requires unnecessary upsampling at such high frequency in the Post-Processing Phase generating large no. of timepoints.

If Ken says, Spectre employs a better algo taking only transition timepoints into consideration, then it's worth it.

Speaking alongwith the on-going but digressed discussion,

Ken, if you say, taking a FFT directly over VCO/FDN to determine phase noise is incorrect, then while doing transient noise simulations on PLLs, when we take DFT of the output clock to obtain phase noise PSD, IS THAT INCORRECT TOO ??

--
Mayank.

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Ken Kundert

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Re: Phase Noise of Δ-Σ modulators
Reply #22 - Jun 18^th, 2010, 9:07am

If you are using a fixed-time step simulator, you have already greatly limited your ability to represent jitter unless you convert the jitter to a real quantity that is passed on each transition. This is the idea behind Michael Perrott's CppSim and the matlab scrip I used in my PLL jitter paper.

Furthermore, if you are using a fixed-time step simulator there is no need to up-sample as no new information is gained. It is best to synchronize the sample rate of the Fourier Transform with the time step of the simulator.

-Ken

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Ken Kundert

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Re: Phase Noise of Δ-Σ modulators
Reply #23 - Jun 18^th, 2010, 4:09pm

Mayank,
I never mentioned Spectre or its algorithms. Nor did I say that using a FFT to determine phase noise was incorrect. Rather I am just reiterating the point that Sheldon made, that FFTs sample the waveform and so have limited ability to resolve jitter. I believe Sheldon made an important point, and I believe that point was being lost.

-Ken

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sheldon Community Fellow Offline Posts: 485	Re: Phase Noise of Δ-Σ modulators Reply #24 - Jun 18^th, 2010, 5:43pm Mayank, Please see the following reference, the Fourier Integral is available in MATLAB. http://www.mathworks.se/access/helpdesk/help/toolbox/symbolic/fourier.html Try it and see how if works your application. Best Regards, Sheldon
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Mayank

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Re: Phase Noise of Δ-Σ modulators
Reply #25 - Jun 24^th, 2010, 10:23pm

Quote:

If you are using a fixed-time step simulator, you have already greatly limited your ability to represent jitter unless you convert the jitter to a real quantity that is passed on each transition. This is the idea behind Michael Perrott's CppSim and the matlab scrip I used in my PLL jitter paper.

Got your point.

Quote:

Furthermore, if you are using a fixed-time step simulator there is no need to up-sample as no new information is gained. It is best to synchronize the sample rate of the Fourier Transform with the time step of the simulator.

Right, but i do it just to increase the span of FFT for better viewing.

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pancho_hideboo Senior Fellow Offline Posts: 1356 Real Homeless	Re: Phase Noise of Δ-Σ modulators Reply #26 - Jun 26^th, 2010, 10:27pm Consider actual instruments for evaluation of jitter characteristics. They are based on fixed time step sampling even if edge trigger is used with not only Real-Time-Sampling but also Equivalent-Time-Sampling technique. You know that they are very practical level, if you have experiences of actual measurements.
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hyy95 Junior Member Offline Posts: 10	Re: Phase Noise of Δ-Σ modulators Reply #27 - Jul 1^st, 2010, 1:16am In the realworld measurements, lots of averaging is done to bring down the noise floor. In simulation, if the FFT points is not large enough or if you can't do tons of averaging, the quantize noise itself will kill the result.
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pancho_hideboo

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Re: Phase Noise of Δ-Σ modulators
Reply #28 - Jul 1^st, 2010, 4:21am

hyy95 wrote on Jul 1^st, 2010, 1:16am:

In the realworld measurements, lots of averaging is done to bring down the noise floor.
In simulation, if the FFT points is not large enough or if you can't do tons of averaging, the quantize noise itself will kill the result.

You are misunderstanding.

The main reason why many period sets are required is to capture many jitters and to increase frequency resolution for low frequency, since it is no more than monte carlo trials.

Of course, you can decrease floor level of noise, if you utilize frame averaging of spectrums for short time.

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