Seeking for Help in PSS Analysis

 Dear Xujing,

I noticed a few things you might consider. I do not know your specific circuit parameters (for example, if frf1 = 5 GHz, is frf2 5 GHz +/- 40 MHz?, what are inductor values?), so my apologies if these do not make sense to you.

1. Your beat frequency appears to be 40 MHz. However, your PSS simulation time span is 25 ns - which is only one period of the beat frequency. Therefore, the pss analysis which looks for a steady-state solution, includes the initial settling time of your circuit. With at least 1 coupling capacitor C2 of 10 pf, it appears there will be a period of time at the start of the simulation where the operating points of devices will not be at their steady-state operating points. To estimate the final steady-state solution most accurately, I think it worth including a period of time at the beginning of the simulation (before the pss simulation starts) to allow for these transient conditions to settle to close to their steady-state values. The parameter tstab shown on page 3 of your document contains no value and hence is set to 0. I might suggest running a transient analysis and examing how long it takes the nodes in your circuit to settle to a steady-state value and use this value for tstab. The fact that you are observing many overvoltage values in your output log further suggests your circuit has not settled to a steady-state solution before starting the pss solution effort.

2.  The input frequencies appear to be at 5 GHz and (possibly) 5 GHz +/- 40 MHz. Hence the period difference of the two waveforms is 1/( 5 GHz) - (1/(4.96 GHz)) = 1.61 ps. Your simulation, therefore, needs to accurately estimate waveform periods whose difference is 1.61 ps or less. However, I noticed you have not set the maximum integration timestep of your simulation from its default value and you are using an "errpreset" of "moderate" (page 3). With a "moderate" value for errpreset, and the number of harmonics you ahve set, the value of maxstep is shown as 1 ns (page 5). I would suggest reducing the value of maxstep from 1 ns to  something on the order of  the difference in the periods of the two applied frequencies or less. If you decide to set tstab to a non-zero value, you will also want to make sure the value of maxstep is also set to the same value in your intiial transient simulation (prior to starting the pss analysis).

3. Finally, you may decide to also set errpreset to "conservative" in lieu of "moderate" to further improve simulator accuracy. With the time constants you appear to have in your circuit, this appears to be a set of stiff differential equations that will stress the solver.

I am sure others will have good suggestions, but I hope this helps a little.

Shawn

 Dear Xujing,

I noticed a few things you might consider. I do not know your specific circuit parameters (for example, if frf1 = 5 GHz, is frf2 5 GHz +/- 40 MHz?, what are inductor values?), so my apologies if these do not make sense to you.

1. Your beat frequency appears to be 40 MHz. However, your PSS simulation time span is 25 ns - which is only one period of the beat frequency. Therefore, the pss analysis which looks for a steady-state solution, includes the initial settling time of your circuit. With at least 1 coupling capacitor C2 of 10 pf, it appears there will be a period of time at the start of the simulation where the operating points of devices will not be at their steady-state operating points. To estimate the final steady-state solution most accurately, I think it worth including a period of time at the beginning of the simulation (before the pss simulation starts) to allow for these transient conditions to settle to close to their steady-state values. The parameter tstab shown on page 3 of your document contains no value and hence is set to 0. I might suggest running a transient analysis and examing how long it takes the nodes in your circuit to settle to a steady-state value and use this value for tstab. The fact that you are observing many overvoltage values in your output log further suggests your circuit has not settled to a steady-state solution before starting the pss solution effort.

2.  The input frequencies appear to be at 5 GHz and (possibly) 5 GHz +/- 40 MHz. Hence the period difference of the two waveforms is 1/( 5 GHz) - (1/(4.96 GHz)) = 1.61 ps. Your simulation, therefore, needs to accurately estimate waveform periods whose difference is 1.61 ps or less. However, I noticed you have not set the maximum integration timestep of your simulation from its default value and you are using an "errpreset" of "moderate" (page 3). With a "moderate" value for errpreset, and the number of harmonics you ahve set, the value of maxstep is shown as 1 ns (page 5). I would suggest reducing the value of maxstep from 1 ns to  something on the order of  the difference in the periods of the two applied frequencies or less. If you decide to set tstab to a non-zero value, you will also want to make sure the value of maxstep is also set to the same value in your intiial transient simulation (prior to starting the pss analysis).

3. Finally, you may decide to also set errpreset to "conservative" in lieu of "moderate" to further improve simulator accuracy. With the time constants you appear to have in your circuit, this appears to be a set of stiff differential equations that will stress the solver.

I am sure others will have good suggestions, but I hope this helps a little.

Shawn