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Simulators >> Circuit Simulators >> problem with simulation step in tran analysis
https://designers-guide.org/forum/YaBB.pl?num=1161978354

Message started by zychang on Oct 27^th, 2006, 12:45pm

Title: problem with simulation step in tran analysis
Post by zychang on Oct 27^th, 2006, 12:45pm

Hi all,

I just got a very annoying problem in tran analysis. Basically, I was doing simulation on my amplifier. First i tried to run transient analysis for 1us and the result seemed reasonable. Then I reduced the simulation time to 200ns just to take a close look at the waveform, and the result shows that the amplifier starts to oscillate. After playing around with the spectre for a while, I noticed that the only difference between these two simulation setup is the step size, 1ns for the 1us ananlysis and 200ps for the latter. So I arbitarily set the step size to 200ps and run it for 1us. This time the amplifier begins to oscillate as well. Now, I am wondering why the difference in step size could cause such a begin difference in tran response and how should I set the step size in order to do things right. Anyone having this experience before please give me a hint. thanks in advance.

Mike

Title: Re: problem with simulation step in tran analysis
Post by Ken Kundert on Oct 27^th, 2006, 5:49pm

Time steps that are too large can damp out a parasitic oscillation (or perhaps it is better to say they can result in a parasitic oscillation failing to start). If you shrunk the step and the circuit starts to oscillate, then your circuit is unstable I'm afraid.

Good thing you ran that shorter simulation. It is always a good idea to run at least one simulation with tighter tolerances to reduce the chance that something like this is missed.

-Ken