Role of Number of harmonics in loadpull

Hi, In Virtuoso ADE-L while setting up HB analysis for loadpull, there is an option called "Number of Harmonics".After running simulation, the optimum impedance value that I get when Number of Harmonics=1 is different from the one I get when Number of Harmonics=3 or 5. I am not sure whether is the right behavior and what is the impact of other harmonics on optimum impedance for fundamental.

What simulator do you use ?
Can you understand HB Analysis?

I use 15 as number of harmonics at least.

HB Analysis is based on Fourier series expansion of finite sum and fixed time step sampling in time domain.

So you have to set number of harmonics and over sampling factor appropriately.

The difference matters with strongly nonlinear power stages, e.g. class-C. Consider that the exact circuit behavior analysis would require time domain simulation. Harmonic balance with reasonable number of harmonics gives a good approximation for periodical signals.

Spectre. I think I know basics about harmonic balance, may not be expert. I usually keep number of harmonics and over sampling factor to their default values auto and 1 respectively.

There is information here https://community.cadence.com/cadenc...balance-part-1. But I can't really relate it to loadpull and optimum impedances.

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.

The number of harmonics is the primary parameter controlling accuracy for Harmonic Balance
Setting Mxham correctly is important. Too little and you can get incorrect results and convergence problems. Too many increses simulation time and memory used.
Start with 5-8 harms on large tones and 3 harms for linear tones
For strongly nonlinear tones (I.E. square wave LO, VCO with divider), use more harms. Can improve convergence. 10-20 harms is recommended.
For circuits with frequency divider, we suggest harms= 5 x divide_ratio for that tone.
Periodically increase # of harms for key tones to see if answer changes. If not, it means the # of harmonics is sufficient.

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.

For most cases, use Oversample = 1 (default)
For strongly nonlinear tones, set Oversample = 2 and you can reduce the # of harms required
Does a better job recovering the time domain waveform for IFFT.

Before load pull, select appropriate number of harmonics and over sampling factor for large inductive load.
This require many trials.

And I don?t recommend ?auto? in any analysis of Cadence Spectre.