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Are there any rules and techniques for setting the accuracy and step length when performing transient simulation of unstable systems (that is, the transfer function has RHP) in Spectre?

zuiying

I built a two-stage op amp in the spectre and deliberately did not add a compensation capacitor. Then I connected this op amp into a unity gain negative feedback form, as shown in the figure below:

I performed a pz analysis on it, and the result showed that the transfer function from Vin to Vout in the above figure has a pair of conjugate right half-plane poles.

So, if I add a step excitation to Vin at this time, Vout should output a sine wave that gradually diverges and finally stabilizes. Then I set the Vpulse on the Vin side as follows:

Next, I set the transient simulation as follows (accuracy is set to conservative):

So I got the following tran simulation results:

Vout did not diverge first as I thought, but gradually converged.

I feel something is wrong. After that, I changed the accuracy of tran simulation to moderate, and the simulation waveform this time is as follows:

This time Vout diverged first.I think this is the transient simulation result that is consistent with the theory, right?

The accuracy of conservative is higher than that of moderate. But why is the result given by conservative inconsistent with the theory?

Later, I set the simulation step size to 1ns (it was not set manually before, I think spectre should decide the simulation step size by itself).

When the simulation step size becomes 1ns, the simulation results of conservative accuracy and moderate accuracy are both that Vout first diverges and then tends to stabilize the sine wave (I think this is the phenomenon that RHP should produce).

Why is the simulation result inconsistent with theory only when the accuracy is conservative and the step size is not set? (I found that if the simulation step size is not set manually, the conservative simulation result step size is larger, and the moderate simulation step size is smaller.) How should the accuracy be set during transient simulation and when should the simulation step size be set manually? Are there any rules or techniques?

Thank you very much.

  • Andrew Beckett

    This is probably due to the fact that conservative uses method=gear2only  whereas moderate uses method=traponly. The gear2 method is usually the best bang for the buck in terms of accuracy (you get better accuracy more cheaply than tightening reltol further in most cases) but it does introduce a small amount of numerical damping and that's probably what is causing the oscillation to be damped. 

    Simulation oscillation in time-domain circuit simulators is often difficult because it can be difficult to get the oscillation to start (because a DC might find a metastable state and there's nothing to cause the simulator to move away from that). In this case you're giving a kick start to get the oscillation to start, but the numerical damping is having an effect.

    So I'd suggest setting method=traponly (keep conservative if you need greater accuracy) to prevent the numerical damping.

    Regards,

    Andrew

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