Question on PSS+PNoise simulation for a Track and Hold circuit

You only get all the noise in the frequency range up to half the "Beat Frequency" if you use a sampled pnoise analysis. Take a look at the stop frequencies of unsmpldNoise and smpldNoise in Listing 1 of the document.

Hi Frank,

Thanks for your reply.

I have a few questions based on your reply.

   a. it seems that noise folding only happens with sampled pnoise sim. I would like to know the reason and more detail on the difference between the "sampled" and "unsampled" pnoise sim. Is there any material about this? Could you point me to that?

   b. In the version of tool I am using, there are only "timeaverage" and "sampled (jitter)" for Noise type. I think I should use "sampled (jitter)" to get the sampled Noise. But there is no option for "noisetimepoints" and "numberofpoints", which was used in the Listing1 in the document. How can I do an equivalent setup?

Thanks and regards,

Yutao

Hi Yutao,

I don't have bandwidth this week to look into your questions or issues in depth, but a quick answer to your points above:

a) If the circuit itself folds noise, then that is handled by timeaverage mode. All that sampled (jitter) does is introduce an ideal sampler which samples at given time steps within the period at the PSS fundamental rate. This is similar to having a stage of circuitry which follows the block you're analysing - so you can think of the analysis being more representative of how the output of the block is used. The time average is more like looking at the output with a spectrum analyser - you'll see the effect of the circuit itself folding noise, but the measurement equipment is not doing any sampling.

b) Within the sampled (jitter) mode, there are three choices - edge crossing, edge delay, and sampled phase. The edge crossing uses the transition of a signal to determine the time which the sampling at another (or the same) signal occurs at; the edge delay measures the delay between a transition of one signal edge and another (could be different signals), and sampled phase allows you to specify the number of sample points, and a list of specific time points to sample at.

BTW, from a very quick glance at your original picture, you almost certainly should be using the fullspectrum choice on the noise form.

Andrew.

Dear Yutao,

Please allow me to try to add to both Andrew's and Frank's very valuable comments...maybe my comments will not but will let you be the judge!

YutaoLiu said:

a. it seems that noise folding only happens with sampled pnoise sim. I would like to know the reason and more detail on the difference between the "sampled" and "unsampled" pnoise sim. Is there any material about this? Could you point me to that?

Andrew's response is exactly correct. If you need a bit more detailed explanation, Cadence On-line support has a nice Application note that graphically shows why the use of a sampled noise analysis is more appropriate than a linear noise analysis for a switched-capacitor circuit:

https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1Od0000000nTk0EAE&pageName=ArticleContent

There is no "unsampled" pnoise simulation. In all cases, the impact of noise folding is included in a pss analysis. However, for non-autonomous circuits (i.e. driven) the magnitude of the resulting phase noise will be different as the time over which the noise is measured varies between a "timeaverage' (more conventional) and sampled jitter modes. In the former, the noise is averaged over the entire period and in the latter the noise represents the noise at the sample points/times only. As a result, the noise measured in a sampled (jitter) analysis will typically appear larger than that for a more conventional pnoise "timeaveraged" analysis. This is probably more clearly explained than in the few words I am providing at URL:

https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1O0V000009EStcUAG&pageName=ArticleContent&oMenu=People%20who%20viewed%20this%20also%20viewed

YutaoLiu said:

b. In the version of tool I am using, there are only "timeaverage" and "sampled (jitter)" for Noise type. I think I should use "sampled (jitter)" to get the sampled Noise. But there is no option for "noisetimepoints" and "numberofpoints", which was used in the Listing1 in the document. How can I do an equivalent setup?

Are you studying the most recent version of this Application Note? I am seeing a much newer version (January 2021, page 343 "Characterizing a Switched-Capacitor Filter") and believe you should refer to it; This version of the manual is at Cadence URL:

https://cadence.okta.com/app/cadencedesignsystemsinc_techpubs_1/exk8bkujilm7hqHIZ0x7/sso/saml?SAMLRequest=hZJLb9swEIT%2FisC7TElO%2FCBsA3IUoQLSInDaHnIxGHpTsxYf4ZKp%2Fe9LynCbXtwTgeEOd74BF8hVb1kd%2FF5v4C0A%2Buyoeo1suFiS4DQzHCUyzRUg84I91Z8fWDUqmHXGG2F68sFy3cERwXlpNMm6Zkm2k%2FVNW0zWs9ntvJi367qt2sl9Wc%2Ba6f28mjQzkn0Hh3F%2BSaI9mhADdBo91z5KRVXmxTgvb7%2BWczau2PjmmWRNZJCa%2B8G1994io1TwHWgBI3PwfCSMotzai7gDlD80ntCDQqnF1oPY2%2FCC25LC8TB7OYSfslfT%2Fdun7rk4TimioYmVZHdGI6Qo16DFeYiJ4Fw8c6lsL4X0JGuNEzB0vySvvEdIhI%2BxJPkOf5T60llaFhS4J3DvUsC3zcNfPgzWGufL0QU0MSaOPIFQBX1vNLUG%2FQbQpkBktUgMbKjUra4%2BFFNLf%2BqaBf1oWZz%2FzpcI3DWPJjKdEpHi%2F%2BkjKXKXvw6jzDuuUcYNkTSG%2FHXngPtI710AQlfnlf%2F%2B0NVv&RelayState=https%3A%2F%2Fsupport1.cadence.com%2Ftech-pubs%2FDocs%2FspectreRFTheory%2FspectreRFTheory20.1%2FspectreRFTheory.pdf

Do these add any useful intuitive information to Andrew and Frank's comments for you? I hope so!

Shawn

Hi Shawn,

Thanks for your reply.

I need to spend some time to digest Andrew's and your comment.

By the way, the last link in your reply is not valid. It there any other way I can find this updated application note?

Thanks and regards,

Yutao 

Dear YutaoLion,

YutaoLiu said:

I need to spend some time to digest Andrew's and your comment.

Absolutely! I understand...

YutaoLiu said:

By the way, the last link in your reply is not valid. It there any other way I can find this updated application note?

I just checked and it appears to work for me.

Do you have a Cadence On-line support account? If not, that may be why you are unable to access it. I would highly recommend you apply for a Cadence On-line support account. There is a wealth of information available. You may need a site key to create an account which you can get from the individual(s) who maintain or install your Cadence tools.

Shawn

Hi Shawn,

I think I have the on-line support account because. Could you tell me the name of the application notes, so that I can search it through the website or I can ask people in my company to download it.

Thanks and regards,

Yutao

Dear YutaoLin,

YutaoLiu said:

I think I have the on-line support account because. Could you tell me the name of the application notes, so that I can search it through the website or I can ask people in my company to download it.

I am sorry you are experiencing issues with my link!!

The document is not an Application note, but rather a section of the RF Analysis Theory document. I have attached its title and publication date as Figure 1. Perhaps if you just search for its title you might locate the document? ( (Section entitled "Characterizing a Switched-Capacitor Filter" starts on page 343.)  

Shawn

Figure 1

Hi Frank, Andrew and Shawn,

Thanks for your explanation and suggestion.

I kind of understand why Frank suggest to use "sampled (jitter)" noise type if setting stop frequency only up to half the "Beat Frequency".

Following is my understanding,

a) My circuit is a track and hold circuit. In the hold phase, circuit samples the noise at the end of track phase, and then hold it for the rest of the period. It can be separated into two parts.

    First, it samples the noise in time domain, so noise is folded back into 0~ half of beat frequency in frequency domain.

    Second, the sampled noise goes through a zero-order hold convolving in time domain (multiplying a sinc function in frequency domain). The resulting noise spectrum spread to +/- infinity due to the effect of sinc function. I thinks this is what we should see in the sprectrum analyzer as mentioned by Andrew. So that is the reason why stop frequency need to set 10x of  "beat frequency" to get a closer result as possible.

b) As explained by Andrew, with "sampled (jitter)" noise type, an ideal sampler (working at beat frequency) folds back all the noise into 0~half of beat frequency range again. So with this setup I can see the integrated noise power from 0~half of "beat frequency" matches to kT/C.

Is my understanding correct?

Thanks and regards,

Yutao

Hi Andrew,

If I select "sampled (jitter)" mode, and I would like to set the sampling point, which has 15.625ps delay from the falling edge of the 25% duty cycle 8GHz clock, for example.

I am thinking which one is better for my case, "edge delay" or "sampled phase"?

For "edge delay", there are also setting "Edge Number", "Threshold Value" and "Edge Direction"  for Measurement node, which I don't want to define. Is it fine to leave them empty?

For "sampled phase", I can set only one sampled point for each period, but how should I define the delay between clock's falling edge and the sampling point? May I define the delay time in "Add Specific Point"?

Thanks and regards,

Yutao