AC Table Lookup (including S-Parameters)

In this topic:

Netlist Entry

Uxxx node_pairs modelname
node_pairs Pairs of nodes for each port. So for example, a two port device has four nodes.
modelname Model name

Model Format

.MODEL modelname actable LOAD=filename [
NUMPORTS=number_of_ports ] [ DCMETHOD=extrapolate|extend ]
[ ]INTERPMETHOD=yparams|sparams]
[DCPARAMS=[dcgainvalues] ]
filename Name of file containing frequency table. File uses Touchstone format and may contain s-parameters or y-parameters. Other parameter types are not currently supported.
number_of_ports Number of ports. Default value is 2.
DCMETHOD Has values of 'extrapolate' (default) or 'extend'. This determines how the DC gains are calculated if they are not explicitly defined using the DCPARAMS parameter or with an explicit F=0 term in the definition file. With 'extrapolate', the dc values are calculated by extrapolating back to zero; with 'extend', the DC gain is the same as the lowest frequency gain.
INTERPMETHOD Has values 'yparam' (default) or 'sparam' . Only effective for s-parameter files. SIMetrix implements s-parameter tables by converting the parameters to y-parameters. Y-parameters can be directly modelled by the simulator core. For a frequency sweep, SIMetrix must interpolate the data in the table to work out the device behaviour for frequencies that lie between table points. This parameter determines whether the original s-parameter values are interpolated or the converted y-parameter values are interpolated. Interpolating the y-parameters is usually faster but sometimes interpolating the s-paraneter data yields smoother results.
dcgainvalues Vector providing the DC gain values for the network. This is expected to be an n x n matrix where n is the number of ports.

AC Table Notes

The AC Table device implements a circuit device that is defined by a frequency lookup table. This device operates only in the small signal analysis modes, AC, Noise and TF. In transient and DC analyses it behaves like a simple linear DC gain block with no frequency dependence.

The lookup table for this device must be defined by a file and uses the industry standard 'Touchstone' format. The full details of this format are supplied as a separate document and maybe found on the SIMetrix web site. Please visit Further Documentation for details. The document is freely distributable under the terms described therein and may also be found at various Internet sites.

The SIMetrix implementation of the touchstone format includes the following:
  1. Y-parameters to any number of ports
  2. S-parameters to any number of ports

Although there is no hard limit to the number of ports, there is likely to be a practical upper limit for s-parameters as these require conversion to y-parameters. A definition with a very large number of ports will likely be slow to run and maybe subject to substantial errors.

Z, H and G parameters are not supported. Also, noise parameters are not supported.

Touchstone files traditionally use the extension snp (s-parameters) or ynp (y-parameters) where n indicates the number of ports. Be aware that SIMetrix uses the NUMPORTS parameter in the .model statement to determine the number of ports and will ignore the value of n in the filename extension.