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Nexxim Simulator >
Nexxim Harmonic Balance >
   Harmonic Balance Technical Notes >
       Controlling Nexxim Harmonic Balance Analysis >
           Convergence Aids

Convergence Aids

The alpha, beta, and cmin options aid in convergence by adding conductances and capacitances to the circuit. The values can be increased to aid the convergence of circuits with rapidly changing circuit values.

• hb.alpha=val sets a conductance across all branches. The default value is 1e-12 mho.

• hb.beta=val sets a conductance between every node and ground. The default value is 0.0 mho.

• hb.cmin=val sets a capacitance between every node and ground. The default value is 0.0 Farad.

Note

If floating nodes are present in the circuit, a non-zero beta conductance value is automatically added to those nodes. If a value of beta has been specified in an .options statement in the netlist, that value is used. Otherwise, if alpha is greater than zero, the added beta conductances have the value alpha/10. If alpha is not greater than zero, the added beta conductances have the value 1e-13 mho.

Nexxim issues a warning message if the circuit contains floating nodes or nodes with single connections. Only nodes with a single branch connection that is both conservative and constitutive are flagged. For example, the controlling nodes in a VCVS have a constitutive/behavioral/relational connection to the controlled nodes, but not a physical/structural/conservative connection, and therefore are not flagged.

The continuation option controls the strategy to be used when convergence is not achieved with normal methods.

• continuation=source_stepping_last_tone

Source stepping solves the last tone in the TONES argument to the .HB statement by reducing the input voltages and currents and adding resistances between all nodes and ground, solving this low-voltage case, then gradually increasing the inputs until a solution is found that uses the original input values. Source stepping is the default continuation strategy.

• continuation=pseudo_transient

In the pseudotransient continuation strategy, harmonic balance is run with an additive transient term. Pseudotransient continuation is very useful for problems that are not strongly nonlinear but that have high memory requirements because of the size of the problem or of singularities in the circuit. If source-stepping does not provide a solution or if memory bounds are reached, continuation automatically switches to pseudotransient. This option has two subordinate options:

• continuation.pseudo_transient.step_reltol_scale=val

• continuation.pseudo_transient.pt_reltol_scale=val

The harmonic balance pseudotransient continuation solver relies on the HB solver as the inner solver. This option specifies the relative tolerance of the HB solver with respect to the pseudotransient tolerance. Because HB is called repeatedly, this scale factor tightens the tolerances. The default for pt_reltol_scale is 0.8. If pseudotransient continuation does not converge in a reasonable number of iterations or shows convergence problems toward the end of the simulation, lowering pt_reltol_scale to 0.5 might help. (Most users will not need to adjust this option.)

• continuation.pseudo_transient.max_h=val

HB pseudotransient continuation has a time-stepping scheme with an internal variable, h, that can be thought of as a resistance value. The variable h is manipulated internally, but it cannot exceed a maximum value. This option sets the maximum value for h. Larger values of h cause HB pseudotransient continuation to more closely resemble standard HB. Smaller values of h lead to quicker and more accurate solutions, typically with lower memory overhead but with increased simulation time. The default is 5e3.

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