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Nexxim Simulator >
Nexxim Oscillator Analysis >
   Oscillator Analysis Technical Notes >
       Controlling Nexxim Oscillator Analysis >
           Initial Guess for Oscillator Frequency

Initial Guess for Oscillator Frequency

The initial guess for the resonant frequency can be controlled by the initial_guess option and its subordinate options. Settings for the initial_guess option are transient, fewer_tones_transient, freq_sweep, and dc. For single-tone oscillator analysis (with no driven frequency), transient is the default (listed as transient on the Oscillator Options dialog in the schematic editor). For the two-tone oscillator analysis that includes a driven oscillation, fewer_tones_transient is the default method (two-tone oscillator analysis is available only in netlists).

Another option, osc.result_as_initial_guess, allows Nexxim to use the result from the previous oscillator sweep step as the initial guess for the current step.

Transient Initial Guess

osc.initial_guess=transient obtains an initial guess for the resonant frequency by running a transient analysis.

• The subordinate option osc.initial_guess.transient.num_cycles specifies the number of cycles to use in the transient analysis (default is calculated).

• The subordinate option osc.initial_guess.transient.tmax specifies the maximum timestep to use in the transient analysis (default is calculated).

• The subordinate option osc.initial_guess.transient.tstop specifies the stop time to use in the transient analysis (default is calculated).

• The subordinate option osc.initial_guess.transient.tstep specifies the stp time to use in the transient analysis (default is calculated).

• The subordinate option osc.initial_guess.transient.output_start specifies an initial period of transient analysis to be ignored in the initial guess.

Fewer Tones Transient Initial Guess

initial_guess=fewer_tones_transient solves for the oscillator frequency using transient analysis, then proceeds to solve the full two-tone circuit. This option has two subordinate options.

• initial_guess.fewer_tones_transient.tstop=val specifies the stop time. There is no default.

• initial_guess.fewer_tones_transient.tmax=val sets the maximum step size. There is no default.

Frequency Sweep Initial Guess

osc.initial_guess=freq_sweep sweeps the frequency of the probe looking for a frequency where the real component of the probe current [Re(Iprobe)] crosses zero from positive to negative. If a zero-crossing is not observed in the sweep range, the initial frequency guess is the frequency at which the minimum value min{abs[Re(Iprobe)]} is obtained. The osc.initial_guess=freq_sweep option has four subordinate options

• Subordinate option osc.initial_guess.freq_sweep.start sets the starting frequency. The default is 1e9 Hz.

• Subordinate option osc.initial_guess.freq_sweep.stop sets the ending frequency. The default is 5e9 Hz.

• Subordinate option osc.initial_guess.freq_sweep.num_steps sets the number of sweep frequencies to be swept. num_steps defaults to 50.

• Subordinate option osc.initial_guess_freq_sweep.method sets the sweep type. At present, the only method is linear.

Note

The value of probe parameter A is always used as the initial probe voltage.

DC Initial Guess

osc.initial_guess=dc uses the value of the FREQ parameter on the oscillator probe as the initial guess for the resonant frequency. It also uses DC analysis to calculate initial voltage and current values.

Previous Result as Initial Guess

By default, oscillator analysis uses the initial guess strategy at the beginning of each iteration of a sweep. The option osc.result_as_initial_guess allows Nexxim to use the result from the previous oscillator sweep step as the initial guess for the current step. Use of this option can avoid repeated analysis runs.

The default for osc.result_as_initial_guess is false (0). Set this option to true (1) to enable the analysis to use the result from each previous step as the initial guess for each new step.

Note

For result_as_initial_guess to be effective, the increments in the variable being swept must be closely spaced or in a region where the frequency does not vary much with each sweep step.

If an analysis step fails while this option is in effect, Nexxim defaults to the normal guess based on transient analysis.

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