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This is the first of a series of Blogs to talk about how to fill out the forms for Harmonic Balance. I will include our suggested settings and some helpful hints. I will add to these over time as new things come up both from us and our customer base.
Harmonic Balance works in the frequency domain and is a very efficient way to describe systems operating in steady-state. HB is VERY efficient for systems that have sinusoidal tones. As the tones become more nonlinear, or more "square" in nature, HB requires more harmonics of each tone to describe the waveform. This will begin to slow down the simulation as more unknowns are added to the matrix. Luckily, in most RF systems, the tones are rounded off by parasitics at high frequencies, filtering, rise-times, etc. so harmonic balance remains a very efficient method to describe RF systems.
If you go to the ADE -> Choose -> Analysis form and select the QPSS button or if you have MMSIM71 or later, you will also see a new HB button. Choosing QPSS with the Harmonic Balance engine or choosing just the HB button will do the same thing. We are trying to simplify the interface for Harmonic Balance users who wish to only use HB by simplifying the choices and options.In the middle of the form, you should see something like...
Only Mxham, Ovsap and Tstab are editable. The rest of the parameters are read in from the sources.
Number of Harmonics (MxHam): This is the number of harmonics of currents at each node that the simulator will use find a "balanced" solution. The trick is to use enough harmonics to allow the simulator accurately create the waveforms. But, the more harmonics you have, the larger the number of unknowns and the more memory and simulation time required.
Oversample (Ovsap): Oversample helps the simulator more accurately represent a nonlinear waveform by adding more samples or you can think of more time points to the waveform.
Transient Assist (Tstab): Tstab runs an initial transient so that a better first guess can be computed for the steady-state response. For most circuits run with HB, the intial DC solution is good enough to get convergence. Only when there are strong initial start up conditions such as for a divider or VCO is a tstab necessary.