S-Element Options Reference

Option

Default Value

Description

s_element.by_entry

0

0=Measure error tolerance relative to the absolute difference between each entry of the fit and the corresponding entry of the S-parameter data

1=Measure error tolerance relative to the size of S-parameter matrix entry

s_element.cache_state_space

1

1=Enables cacheing of the state space fitting information

0=no cacheing

s_element.causality_check

0

1=Enable causality checking of Touchstone data

0=No causality checking

s_element.causality_checker_tolerance

0.002

Maximum acceptable causality error.

s_element.column_fit

1

0=Fit each entry separately, then combine them at the end

1=Fit one column at a time

2=Do the entire matrix at once, using symmetry if present

s_element.convolution

0

Sets Nexxim to use convolution rather than state-space matrices to model the behavior of S-parameter elements during transient analysis.

0: The state space method is used.

1: Convolution is used. The impulse response is a piecewise linear waveform, with breakpoints chosen to exactly match the given frequency-domain data. This setting gives the most accurate response, but may have passivity violations just above the frequency band of interest.

2: Convolution is used. The impulse response is a train of impulses in the time domain, given by the inverse Fast Fourier Transform. This setting yields an impulse response that is accurate in-band and usually passive. However, the transient waveforms may have discontimuity effects.

3: Convolution is used. The impulse response is the linear interpolation of the inverse FFT results. This setting yields an impulse response with a low porbability of passivity violations, but there is significant filtering towards the top of the frequency range of the input data.

s_element.enforce_causality

0

1=Perform causality check on Touchstone data, perform compensation to replace non-causal data with causal reconstruction estimate

0=No checking or compensation

s_element.enforce_passivity

0

0=No passivity enforcement

1=Enforce the passivity of the state-space model during transient analysis of S-parameter element

2= Use a point-by-point method for enforcing passivity. Use this option in cases with large numbers of ports (more than about 30).

6=Use a passivity by perturbation algorithm.

7=Use Iterated Fitting of Passivity Violations (IFPV) algorithm

Setting enforce_passivity=2 automatically sets mor=3, since this passivity enforcement algorithm requires this model-order reduction strategy.

s_element.errorif

1

1 = Reject state-space model if error is above 10%.

0 = No rejection.

s_element.g_to_gnd

0

Conductance between all terminal nodes of all S-elements and ground

s_element.max_states

128

sets the maximum number of states per entry in the state space formulation

s_element.mor

0

0=No model order reduction (MOR)

1=MOR of entire matrix at once

2=MOR of rows of matrix individually, then final MOR for the combination

3=MOR of rows of matrix individually, no final MOR

4=MOR of columns of matrix individually, then final MOR for the combination

5=MOR of columns of matrix individually, no final MOR

s_element.mor_sp

0

1=Use singular perturbation MOR. Preserves DC fit.

s_element.noisemodel

external

external=use external data if present, else use internal noise model

internal=use internal noise model. External data is ignored

none=no noise calculation

Noisemodel applies to frequency domain analyses, not to time domain.

s_element.q_limit

1e-4

Sets the limit of the quality factor of poles in the rational fit. The default is 1e4. Limiting the quality factor can be useful in avoiding overfitting, which can lead to severe passivity violations. The value should be a positive number greater than 0.5 (generally much greater).

s_element.rational_fitting_iteration_limit

2

Increasing the limit improves the fit on certain complicated S-parameter cases, at the cost of extra CPU time.

s_element_reduce

0

Port reduction. 1=eliminate ports tied to ground directly. Does not affect unconnected ports or ports tied to ground through a resistor.

s_element.reltol

1e-2

Relative tolerance to apply to currents and voltages during analysis

s_element.test_sss

1

1 = Test existing .sss file for goodness of fit with given S-parameter data.

Mainly for internal use

s_element.time_domain_s_model

0

1=Create state-space system in a frequency-domain analysis like LNA where one is not usually generated. The state-space system won’t be passive unless s_element.enforce_passivity=1

s_element.twa

1

0=Use rational fitting

1=Use Ansoft alternative to rational fitting

s_element.wide_dynamic_range

0

Set wide_dynamic_range to 1 when the S-parameter data contains a wide dynamic range within single entries, for example with values around 1 at low frequencies but approaching 10^-5 at high frequencies, and the smallest values must be fitted precisely.