In PSS form, there is one option called Number of harmonics. I am NOT able to understand what is its use as PSS is any way a Steady state DC operating point analysis ?
Of course, No of harmonics actually controls the time step for better accuracy (like to avoid Aliasing effect etc). But the time step can easily controlled by maxacfreq OPTION.
So my doubt is:- Whether the Harmonic Option in PSS form is needed for Some other reasons ?
IS there any documents which actually explains the need/use (OTHER than controlling Time Step) of No. of Harmonics option in PSS form
It depends on whether you're using shooting or harmonic balance. From your description above, it sounds like shooting.
The main reason is if you actually want to observe the harmonics produced by PSS. That is useful in itself in some cases. If you're only doing the PSS as a means to get a steady state (it's not a "DC operating point") in order to do a small signal analysis (e.g. pnoise, pac, pxf etc), then you don't have to save any harmonics. maxacfreq would be sufficient (assuming you even need to set that; maxacfreq defaults to 4x the highest harmonic requested, or 40x the PSS fundamental, whichever is the higher). So you can generally consider the maxharms to be the number of output harmonics.
For Harmonic Balance however it affects the accuracy - it's quite different.
The SpectreRF Theory document will probably describe this in more detail (I didn't check the documentation to make sure or point you in the right place due to lack of time right now).
In reply to Andrew Beckett:
Thanks a lot Andrew
I had actually gone through SpectrRFTherory document. But I couldn't find any explanation for the role of Output Harmonics option (No of harmonics) in PSS form for Shooting Method. I will dig more through the Cadence documents.
I couldn't understand the meaning of the sentence you have mentioned."So you can generally consider the maxharms to be the number of output harmonics"
Can you please tell what you actually mean by that ?
In reply to RFQuery:
When you run shooting PSS, it usually creates two output databases. One contains the time domain waveforms, and one the frequency domain waveforms. The harms parameter controls how many harmonics (of the PSS fundamental) are output in the frequency domain waveform database. It also indirectly controls maxacfreq which in turn alters the timestep in the time domain simulation. This is to ensure that if you asked for (say) 100 harmonics, you would have enough time points in the period to be able to accurately compute the 100th harmonic in the frequency domain results. Similarly, you can also choose the harmsvec parameter (which corresponds to selecting the specific harmonics you want on the UI) if you only want certain harmonics in the frequency domain output database. In this case, the highest harmonic chosen is what affects maxacfreq (which is set to 4 times the highest harmonic requested, or 40 times the fundamental, whichever is the higher, or maxacfreq if explicitly specified).
Since the small signal analyses which follow PSS use the time domain data, the number of harmonics chosen in shooting PSS only affects the small signal analyses by virtue of the effect on maxacfreq - again, it has to have enough points to allow you to analyse the small signal response at frequencies up to maxacfreq.
Generally speaking, increasing the number of harmonics with shooting PSS does not affect accuracy (that said, there can be benefit sometimes in very high dynamic range systems of increasing the number of timepoints) - but such scenarios are often better suited to Harmonic Balance.
For Harmonic Balance, however, it's very different. The number of harmonics you specify with harms is the size of the solution - the solution is computed in the frequency domain, so you will be trying to represent the frequency domain steady state response with that number of harmonics. As a result it has a direct effect on accuracy - failure to include sufficient harmonics will mean that you are trying to represent the whole steady-state solution in too few harmonics, and thus the accuracy of the harmonics you have requested will be compromised. So it's an input parameter for harmonic balance, whereas for shooting it primarily affects the number of harmonics output into the result database.
Hope that's clearer?
Thanks A lot Andrew !!
I appreciate your patience and devoting time for the reply
Just one small question out of curiousity:-
In Shooting method PSS, for plotting the 100th harmonic in frequency domain the maxacfreq is adjusted to accomodate 100th harmonic in the plot with good accuracy. As you know there exist a relationship between max harms and maxacfrequency.
So if I set the maxacfreq for the haighest harmonics and keep the No. of Output Harmonics =0 will it give the same result to the setting with No. of Output Harmonics =100 ?
Actually I have one question about harmonic balance PSS simulation. Could we say more number of harmonics will have more accurate simulation result if we say I just wanna check my circuit's output spectrum? In other words, If I don't set this value large enough in PSS, I cannot get accurate result. In my simulation, it seems there is an aliasing effect at output spectrum.
In reply to yxue:
In harmonic balance, the number of harmonics chosen does affect accuracy. There is a blog and a solution on this:
Here is the Blog: Guidelines for Maximizing Speed vs Accuracy for Harmonic Balance - Part 2.
And the corresponding solution in Cadence Online Support is
11736578 FAQ: Guidelines for Maximizing Speed vs Accuracy for Harmonic Balance http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:ViewSolution;solutionNumber=11736578
Sr. Staff Support AE, Global Customer Support
Cadence Design Systems, Inc.
In reply to Tawna:
Thanks for you reply. Yes, I have already read this explaination, but there is one sentence made me confused.
Why we don't keep increasing oversample to 10 or high value if the simulation result changed? Instead, we start to increase harmonics. What is the mechanism here? What is the relationship between oversample and harmonics please? That is what I really want to know. Thanks.
Recently, I observed one thing ( though surprisingly to the above description) regarding Shooting PSS.
I was doing PSS and PXF analysis for a simple ring oscillator.
From oscillator transient, I have set my PSS beat frequency to 2.5GHz.
No. of Harmonics=1
1st I did PXF with maxsideband 70. I got some result.
The 1st Harmonic frequency PSS shows:- 2.503412490202536GHz
Then I increasd the No. of Harmonics = 20. The PXF results were same.
And the 1st Harmonic frequency PSS shows:- 2.503412490202536GHz
But when I increased the No. of Harmonics=40 ( This I did to accurately reproduce the PSS waveform with accurate phase from PSS harmonics. If there is some better way of reproducing PSS waveform with accurate phase please mention), the PXF result I got was differnt from previous results.
Also the 1st Harmonic frequency PSS shows:- 2.503562180960924GHz ( There is a clear difference)
I wonder why it is happening so.
Could you please tell why it is happening ? ( I am using Cadence (R) Virtuoso (R) Spectre (R) Circuit Simulator Version 18.104.22.1682.isr5 64bit -- 7 Mar 2013 )