Many things happen automatically all around us. Even our own breathing and heartbeat are automatic. However, trusting the automatic can be a mistake sometimes, and the following is a simple example.

That SPICE simulator you used is a pretty nifty little tool, isn’t it? All kinds of analytical decisions are made without requiring that much attention from you, the user, but please don’t be too complacent about it.

The automatic settings that your SPICE simulator provides may not be suitable for your specific analysis requirements. Be alert for anomalous simulation results, which may be corrected by overriding the automatic settings based on your own good judgment.

Consider this simulation of an RC oscillator circuit (Figure 1):

Figure 1 Oscillator simulation.

The performance of the oscillator in Figure 1 appears quite erratic with erratic rise and fall times of each cycle and frequency jitter. Please note, however, that the simulator has made automatic choosing time steps of 10 µsec.

That wasn’t it mine choice. It just happened automatically. However, if I tamper with the simulation settings, I can significantly affect the simulation results (Figure 2):

Figure 2 Modified oscillator simulation.

The performance of the oscillator in Figure 2 is obtained with time steps that I personally changed from what was automatically selected as 10 µsec down to 0.01 µsec, a very substantial change indeed.

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

The result is that you don’t have to be afraid to change your simulation settings from whatever the automatic settings are.

Harry Belafonte once sang “Man Smart, Woman Smarter.” Please see below:

For questions like these, please think “The simulator is smart, the engineer is smarter”.

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

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