I've designed a 5GHz LCVCO but the fundamental frequency found by pss and transient is 40MHz apart. Does this happen to you? Which one is right? I heard pss is more accurate but later I want to run transient for a PLL using this VCO. If transient is not accurate (I think 40MHz error is a pretty big), should I trust the transient results of my PLL, such as the transient of Vcontrol? Appreciate any suggestion on how I should approach the problem.  

I am using SpectreRF which I guess adopts shooting method. Control voltage and all other conditions are the same for my transient and pss analysis. I set moderate for the transient analysis accuracy (Changing it to conservative can narrow the frequency difference from 40MHz to 15MHz).  

I changed reltol from the default 1e-3 to 1e-6 and now pss and transient VCO freqs have no difference but at the cost of X6 longer simulation time and the annoyance of changing reltol whenever I worry about the transient accuracy. Is this about what I should do?  

I also read a VCO simulation tutorial from MIT opencourse website (http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-77...). With SpectreRF default, their transient and pss VCO freq difference is 20MHz @ 4GHz.

Even more puzzling to me is that when I put the VCO into a PLL and first made VCO electrically unconnected with the rest of the PLL, the transient gave me different oscillation frequency, e.g. 10MHz difference. I could get the right pss oscillation frequency only after I changed reltol from the default to 1e-6.

Any comments would be appreciated.

Thank you very much for your guys' replies.

There is no verilog-A model involved in my simulation.

I used the function, freq, to measure the oscillation frequency, which basically calculated every rising edge interval with 0 V as threshold for my differential output signal. I run a long transient and took the calculated frequency value after the transient was stable. The transient frequency is 40 MHz lower than my pss result of 5GHz.

If I set reltol to be 1e-3, how much difference is normal between pss and transient estimate of VCO frequency? 5MHz (i.e. 0.1% for my 5GHz VCO)?  

-Alex

I think the linear interpolation is the answer to explain the difference between pss and transient results. Then what is the solution in case that I need good transient simulation results?

Specifically speaking, I designed a simple transistor-level PLL using the VCO mentioned here and wanted to see a reasonably accurate Vcont transient. The clock frequency at steady state was right at 5GHz. However, with the default reltol of 1e-3, the steady-state Vcont was 1V (my vdd is 1.3V), far away from the right value of 0.5V which I could obtain after adjusting reltol to 1e-6. Do people often adjust simulation accuracy setup in order to  get right transients for high-frequency PLL? If not, I might need to re-examine my circuit. Thanks.


-Alex

Thank you very much for your previous reply.

Did you mean that verilog-a module, such as periods.va which I obtained from verilog-ams.com, would improve the frequency measurement accuracy? I've tried it but the transient result was still 40MHz @5GHz away from that of pss with the default reltol of 1e-3. I checked the verilog manual which says the last_crossing() function used in periods.va doesn't control the timestep and still uses linear interpolation to estimate crossing points. Any comments would be appreciated.

-Alex

I see but my freq calculation is still about 35MHz@5GHz away from my pss and transient results with reltol adjusted to 1e-6. I can see the smaller time step around the crossing point. I even reduced the tolerance of the cross() function but still there is this above difference.

-Alex