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Simulators >> Circuit Simulators >> periodic steady-state?
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Message started by jane on Aug 25^th, 2005, 8:39pm

Title: periodic steady-state?
Post by jane on Aug 25^th, 2005, 8:39pm

I have recently started learning about PSS and PNoise analysis and would like to make certain I understand some terminology that is commonly used.

What exactly is meant by finding the "periodic steady-state"? Does this mean the simulator finds waveform solutions for the circuit such that all node voltages are periodic (over the same period)? If this is true, does the simulator compare the solutions for the start and end of the period and check to see that the difference between the two is within the predefined tolerances? If the difference is not within the tolerances, then the simulation adds a deltaT and re-checks all solutions again over the period. Is this a correct understanding of what's happening in the PSS analysis?

Also, what is meant by "linearizing the circuit about the periodically time-varying operating point"? Does this mean that for the set of solutions found in the PSS analysis, a small-signal analysis is performed?

~j

Title: Re: periodic steady-state?
Post by Geoffrey_Coram on Aug 26^th, 2005, 4:27am

Jane -
You've almost got it. The simulator does compare the solutions at the start and end of the period, but if the difference is too big, then it doesn't (generally) add a deltaT, it changes the initial point (voltages and currents) and re-computes the transient for a period and compares again. One could just run the transient analysis for another period (fixed-point iterations), but more sophisticated simulators use "shooting Newton" which uses the circuit matrix at each timepoint along the period to figure out which way to perturb the initial conditions.

When doing PSS for an oscillator, the period can change (deltaT), but also the initial condition can as well. For many (driven) circuits, the period is known, eg, you know what the local oscillator (LO) frequency is because you have a sine source driving your mixer.

For each timepoint in the periodic solution from t to t+T, the circuit is linearized, and then the response at each timepoint (to a small-signal stimulus or noise) is added up (weighted, I think, by the length of the timestep).

Title: Re: periodic steady-state?
Post by jane on Aug 26^th, 2005, 9:41am

Geoffrey,

Thanks so much for your response.

~j