Andrew Beckett
Senior Fellow
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Life, don't talk to me about Life...
Posts: 1742
Bracknell, UK
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It's mostly a matter of practicality of the simulation. The steady-state analysis (be that using shooting newton or harmonic balance) could be done with transient, but you need to simulate long enough for a) a complete beat period of all the input tones, and b) until all the signals have settled. You need to do this with sufficient accuracy to resolve signals with high dynamic range, as seen in RF circuits.
If you're prepared to wait long enough, you could do that. However, RF simulators allow you to then do small signal analyses (e.g. ac, xf, noise etc) about this periodic operating point - which allows you to sweep frequency rapidly - and also allows you to measure noise in a reasonable time. Whilst transient noise has its uses, to measure high frequency noise you need to have very small timesteps, and to measure flicker noise, you need to simulate for a very long time - plus you need to run a long enough simulation to average the random noise. Also, the noise is typically quite small compared with the signals (at least that's what you hope!), and so you need high accuracy to take reasonable measurements. All of this tends to lead to transient noise being a very inefficient way of measuring things that RF simulators can measure directly.
Transient noise is useful for things that are hard to measure in a small-signal RF analysis - for example, when you have non-periodic circuits, or if there is some large-signal response to the noise.
Regards,
Andrew.
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