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Transmission Line

Model transmission line

Elements

Description

The Transmission Line block models transmission lines. Lumped-element transmission line models are lossy and can only be simulated in the time domain. Distributed transmission models use an S-parameter representation. These models are simulated using either a time or frequency domain modeling option. For an introduction to RF simulation, see the example, Circuit Envelope Basics.

Dialog Box and Parameters

Main Tab

Model type
Ground and hide negative terminals

Select this check box to internally ground and hide the negative terminals. Clear the check box to expose the negative terminals. By exposing these terminals, you can connect them to other parts of your model.

By default, this check box is selected.

ABCD Parameter Calculations for a Transmission Line

When modeling a transmission line, the block first calculates the ABCD-parameters at at a set of internal frequencies. It then uses the abcd2s function to convert the ABCD-parameters to S-parameters.

The block calculates the ABCD-parameters from the physical length of the transmission line, d, and the complex propagation constant, k, using the following set of equations:

Transmission Line with Stub

When you set the Stub mode parameter in the mask dialog box to Shunt, the two-port network comprises a stub transmission line. You can terminate this line with either a short circuit or an open circuit as shown in the following figure.

Zin is the input impedance of the shunt circuit. The ABCD-parameters for the shunt stub are calculated as

When you set the Stub mode parameter in the mask dialog box to Series, the two-port network comprises a series transmission line. You can terminate this line with either a short circuit or an open circuit as shown here.

Zin is the input impedance of the series circuit. The ABCD-parameters for the series stub are calculated as

Modeling Tab

Modeling options

Modeling domain. Select Frequency domain or Time domain (rationalfit) from the drop-down list.

When modelling using Frequency domain, Visualization tab plots only the data defined in Data Source.

When modelling using Time domain, Visualization tab plots the data defined Data Source and the rationalfit values .

For the Amplifier and S-parameters blocks, the default value is Time domain (rationalfit). For the Transmission Line block, the default value is Frequency domain.

Fitting options

This field displays when you select Time domain (rationalfit) as the Modeling options. Valid values are Share all poles, Share poles by columns, and Fit individually.

For the Amplifier block, the default value is Fit individually. For the S-parameters block, the default value is Share all poles.

Relative error desired (dB)

This field displays when you select Time domain (rationalfit) as the Modeling options. Enter the desired relative error in decibels (dB). The default value is -40.

Rational fitting results

These fields display the results of rational fitting when you select Time domain (rationalfit) as the Modeling options.

Visualization Tab

Source of frequency data

Frequency data source. When Source of frequency data is Extracted from data source, the Data source must be set to Data file. Verify that the specified Data file contains frequency data.

When Source of frequency data is User-specified, specify a vector of frequencies in the Frequency data parameter. Also, specify units from the corresponding drop-down list.

For the Amplifier and S-parameters blocks, the default value is Extracted from source data. For the Transmission Line block, the default value is User-specified.

Plot type

Specify the type of plot that you want to produce with your data. The Plot type parameter provides the following options:

• X-Y plane — Generate a Cartesian plot of your data versus frequency. To create linear, semilog, or log-log plots, set the Y-axis scale and X-axis scale accordingly.

• Polar plane — Generate a polar plot of your data. The block plots only the range of data corresponding to the specified frequencies.

• Z smith chart, Y smith chart, and ZY smith chart — Generate a Smith® chart. The block plots only the range of data corresponding to the specified frequencies.

The default value is X-Y plane.

Parameter #

Specify the S-parameters to plot. From the Parameter1 and Parameter2 drop-down lists, select the S-parameters that you want to plot. If you specify two parameters, the block plots both parameters in a single window.

The default value for Parameter1 is S11. For the Amplifier and S-parameters blocks, the default value for Parameter2 is None. For the Transmission Line block, the default value for Parameter2 is S22.

Format #

For X-Y plots, format the units of the parameters to plot from the Format1 and Format2 drop-down lists. For polar plots and Smith charts, the formats are set automatically.

The default value is Magnitude (decibels).

Y-axis scale

Scale for the Y-axis.

The default value is Linear.

X-axis scale

Scale for the X-axis.

The default value is Linear.

Tips

• In general, blocks that model delay effects rely on signal history. You can minimize numerical error for these blocks that occurs because of a lack of signal history at the start of a simulation. To do so, in the Configuration Parameters dialog box Solver pane you can specify an Initial step size. For models with delay-based Transmission Line blocks, use an initial step size that is less than the value of the Delay parameter.

References

[1] Sussman-Fort, S. E., and J. C. Hantgan. "SPICE Implementation of Lossy Transmission Line and Schottky Diode Models." IEEE Transactions on Microwave Theory and Techniques.Vol. 36, No.1, January 1988.

[2] Pozar, David M., Microwave Engineering. Hoboken, NJ: John Wiley & Sons, Inc., 2005.

[3] Gupta, K. C., Ramesh Garg, Inder Bahl, and Prakash Bhartia. Microstrip Lines and Slotlines, 2nd Edition, Norwood, MA: Artech House, Inc., 1996.

[4] Ludwig, Reinhold and Pavel Bretchko. RF Circuit Design: Theory and Applications. Englewood Cliffs: NJ: Prentice-Hall, 2000.

[5] True, Kenneth M. "Data Transmission Lines and Their Characteristics." National Semiconductor Application Note 806, April 1992.