Harmonic Index of PSS data is not found?

Well, he was talking about an HB analysis, where ~150000 harmonics are clearly not reasonable...

Hi Shawn,

Many thanks for this. This is valuable information. However, 150000 harmonics does look like a lot to me, although I will play around with this and see if scaling it down by an order or 2 makes a difference. Fyi, with 150000 harmonics spectre gives up after a certain time complaining about insufficient memory which I believe is telling upon resource capacity of the farm  I am using rather than Spectre itself. 

Frank, are you specifically pointing out 150k harmonics as unreasonable for HB analysis or in general both (PSS/HB)? If it's just HB, then why not the other?

Thanks,

Sharjeel

Hi Sharjeel,

There's no absolute upper limit, although practically I'd say that this is unlikely to work unless you have a trivially small circuit. Even then, it's questionable as to whether it would be useful. Given that the solution has to be represented for every node in the circuit with a fourier series with 150,000 complex numbers, you're going to run out of memory pretty quickly. I was able to simulate a square wave source with a resistor and capacitor (so just two nodes) with this number of harmonics, and even that took 0.5Gbytes of memory. It's rare to see any more than 100-200 harmonics being used with harmonic balance, as its sweet spot is weakly nonlinear circuits, and this is far away from being weakly nonlinear.

Shooting is far more likely to be relevant here; hb may be fine, but if you really need to capture the very high bandwidth at the output of the comparator with respect to how fast it switches, I definitely think shooting makes more sense.

Andrew.

I have also made the experience that the HB analysis often has more convergence problems as the circuit becomes more nonlinear (like an amplifier going into compression).

 THere could be more at play here as getting PSS/HB to behave for large autonomous circuits seems very iterative in terms of getting the correct settings, but indeed i couldn't get HB to converge even with just 150 harmonics for the same circuit for which shooting seems to.

And while we are at it, I have some doubts about the validity of using Jc from Direct Plot form post pnoise to my application which has a specification of peak to peak period jitter = 10 ns (in a 10000 cycle window).

As per the formula 1.18 in  "Jitter Measurements using Spectre RF AN" also stated here https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1O0V000007MnBUUA0&pageName=ArticleContent  ,  Jc (from the definitions in Cadence docs, Jc matches exactly the definition of period jitter in other literature) for my design comes out to be sqrt(2) or 3 dB lesser than what Jc from Direct plot spits out. I established this by exporting the phase noise data from my results in cadence spectre and using the said formula in MATLAB/ Excel on exported data.

It seems that Jc in direct plot does not convolve the phase noise with the high pass function [sin(pi*f*k.Tc)]^^2 to calculate Jc making it effectively a measure of absolute jitter rather than period jitter ( Eqn. 3 and 8  here: D. C. Lee, "Analysis of jitter in phase-locked loops," in IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing, vol. 49, no. 11, pp. 704-711, Nov. 2002, doi: 10.1109/TCSII.2002.807265.)

Would appreciate if some more light could be shed on this?

Thanks,

Sharjeel

Hi Sharjeel,

I would suggest contact customer support about this. Other may have the bandwidth to investigation this in depth, but I don't myself (a bit busy at the moment with the day job and so I can only answer questions that are quick to answer and don't involve too much experimentation and investigation).

Andrew.

I want to second what Andrew Beckett has stated.  This really needs to be handled by an Application Engineer in Customer Support.  We answer these questions in our "off hours" after we've been at our "day jobs".   

The appnote that you are referring to "Jitter Measurements using Spectre RF AN"  is about 15 years old (or older).  It is always best to use the Spectre and APS RF User Guide for guidance.  It's the most up-to-date information available.   We've changed our pnoise setups (and how we calculate some of the pnoise parameters) since those two appnotes/papers were written. 

IF the analyses are set up properly, the Shooting Newton engine should give the same results as the Harmonic Balance engine.  You need to use the most appropriate tool for the job, however.  Very fast rise times, periodic switching circuits do better with Shooting pss/pnoise.   Mildly nonlinear circuits are best handled with HB/hbnoise.    All things that I believe Andrew has mentioned.

best regards,

Tawna

Thanks Andrew/ Tawna.

There is indeed a gamut of info available in Cadence Spectre docs and i do have access to all of these from Jan, 2020 (spectreRFTheory, sectreRFinExplorer, spectreRFexplorerWorkshop, etc) but to my knowledge (correct me if I am wrong) unfortunately none of these deal with the underlying formula for different jitter definitions except the AN which I mentioned, which is why I have also referenced Lee's paper to corroborate the theory in the said AN.

However, i will get in touch with an applications engineer and see what they say.

Regards,

Sharjeel