Don’t rely on the automatic settings of your SPICE simulator

Article By : John Dunn

Trusting in the automatic can be a mistake sometimes. Here's a simple case in point.

A lot of things happen automatically all around us. Even our own breathing and heart rate are automatic. Still, trusting in the automatic can be a mistake sometimes and the following is a simple case in point.

That SPICE simulator you’ve been using is one pretty spiffy little tool, isn’t it? All kinds of analytic decisions take place without requiring all that much attention from you, the user, but please don’t be too complacent about that.

The automatic settings that your SPICE simulator provides might not be appropriate for your specific analysis requirement. Be alert for anomalous simulation results that might be correctable by overriding the automatic settings based on your own good judgment.

Consider this simulation of an R-C oscillator circuit (Figure 1):

Figure 1 An oscillator simulation.

The Figure 1 oscillator’s performance looks pretty ragged what with the unstable rise and fall times of each cycle and the frequency dithering. Please note though that the simulator has made an automatic choice of the 10 µsec time steps.

This wasn’t my choice. It just happened automatically. However, if I intervene in the simulation settings, I can markedly affect the simulation results (Figure 2):

Figure 2 A modified oscillator simulation.

The Figure 2 oscillator’s performance is obtained with time steps that I have personally changed from what had been chosen automatically as 10 µsec down to 0.01 µsec, a very substantial change indeed.

Now look at how much better the oscillator’s performance has become. The circuit is unchanged. Only one simulation setting is different, and the simulation result is a much better reflection of the circuit’s real-world performance.

The upshot is to not be afraid to vary one’s simulation settings from whatever the automatic setting(s) might be.

In matters such as these, please think “Simulator Smart, Engineer Smarter.”

This article was originally published on EDN.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).


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