Harmonic Index of PSS data is not found?

Hi Sharjeel,

One more thing to add (if it hasn't been stated already), if you are simulating a high Q circuit, it's typically better to use hb instead of pss. Ideally both will give the same answer, but hb is more suited for highQ oscillators.  Shooting Newton is better for strongly nonlinear circuits (rapidly changing waveforms in time), but it can struggle with high Q oscillators.   If you have a high Q oscillator followed by strongly nonlinear circuitry, try using hb, but you'll need to use many harmonics.  

• Set the number of harmonics to the period divided by the fastest rise/fall time in the circuit. The risetime here is calculated by taking the highest slew rate in the transition, and calculating the 0 to 100% risetime​ based on that maximum slew rate.  
• Set oversample to 2.    

Do you have access to Cadence Online Support?  If so, check out these articles: 

• 20486872    Getting The Most Out Of Spectre® APS RF 18.1 - Maximizing Performance  
• 20444400  How to simulate a high Q (LC or crystal) oscillator followed by strongly nonlinear circuitry (e.g. hard switching buffers )

best regards,

Tawna

Hi Tawna,

Sorry for circling back to this after three months. Although, I did manage to make PSS+PNOISE work by adjusting various parameters using shooting method, my simulations have always been plagued with inconsistency of results. Coming back to this page, I thought maybe I should try using HB noise once again as per your suggestions and articles,  and see if there is more consistency. 

However I am not sure if choosing ~150000 (comparator output edge rise time in my circuit ~ 200 ps; Tclk = 1/32768 ) harmonics for an HB analysis is reasonable. Could you please advise?

Thanks,

Sharjeel

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First and foremost, I do not intend not imply that I can provide the insight that either Tawna or Andrew can provide!

Nevertheless, in the hope of providing some information to you in light of it being a US Holiday, the number of harmonics specified on the pss options GUI, sets the maximum time step of the integration within the pss transient simulation. Hence, if you do not separately specify a value for parameter “maxstep”, spectre will use the number of harmonics to compute a value for parameter “maxstep”. This is detailed in the article at URL:

support.cadence.com/.../ArticleAttachmentPortal

I’ve included some text from that article below for your convenience. In your case, if the number of harmonics is specified as 150,000 relative to a frequency of 32.868 kHz, the value of maxstep computed by spectre will be 8.138 ps or 1/25th of a period of 150e3*32868 Hz = 4.9152 GHz. This is not an unreasonable value for maxstep given your high Q resonator, but you may find it to be more efficient simulation timewise to reduce the value of reltol from its value with errpreset=“conservative” if you wish.

Again, Tawna or Andrew may provide more insight than I!

Shawn

From URL:

support.cadence.com/.../ArticleAttachmentPortal

Maxacfreq forces 5 timesteps in the period of the frequency set by maxacfreq (in pss options form).

Maximum harmonics sets maxstep to force 20 timepoints in the period of the highest harmonic of the pss (on the choosing analyses form).

Let’s take an example. Say the PSS frequency is 1GHz and you want pnoise to go up to 100GHz. (Think easy math.)

One way is to set maxacfreq to 100GHz. This forces 5 timepoints in the period of 100GHz.

The other way is to set 25 harmonics in pss. This forces 20 timepoints in the period of 25GHz.

Note that 20 timepoints in the period of 25GHz is exactly the same as 5 timepoints in the period of 100GHz.

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

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