rfckt.txline

General transmission line

Description

Use the txline class to represent transmission lines that are characterized by line loss, line length, stub type, and termination.

Creation

Syntax

h = rfckt.txline

h = rfckt.txline(Name,Value)

Description

h = rfckt.txlinereturns a transmission line object whose properties are set to their default values.

h = rfckt.txline(Name,Value) sets properties using one or more name-value pairs. For example, rfckt.txline('Z0',75) creates a transmission line object with characteristic impedance of 75 ohms. You can specify multiple name-value pairs. Enclose each property name in a quote. Properties not specified retain their default values.

Properties

AnalyzedResult — Computed S-parameters, noise figure, OIP3, and group delay values

rfdata.data object

Computed S-parameters, noise figure, OIP3, and group delay values, specified as rfdata.data object. This is a read-only property. For more information refer, Algorithms.

Data Types: function_handle

Freq — Frequency data

M-element vector

Frequency data for the RLCG values, specified as a M-element vector in Hz. The values must be positive and correspond to the order of loss and phase velocity values. By default, this property is empty.

Data Types: double

IntpType — Interpolation method used in rfckt.rlcgline

'Linear' (default) | 'Spline' | 'Cubic'

Interpolation method used in rfckt.rlcgline, specified as one of the following values:

Method	Description
Linear	Linear interpolation
Spline	Cubic spline interpolation
Cubic	Piecewise cubic Hermite interpolation

Data Types: char

LineLength — Physical length of transmission line

scalar

Physical length of transmission line, specified as a scalar in meters. The default value is 0.01.

Data Types: double

Loss — Reduction in strength of signal

0 (default) | nonnegative M-element vector

Reduction in strength of signal as it travels through the transmission line, specified as a nonnegative M-element vector in decibels per meter.

Data Types: double

Name — Object name

'Transmission Line' (default) | 1-by-N character array

Object name, specified as a 1-by-N character array. This is a read-only property.

Data Types: char

nport — Number of ports

positive integer

Number of ports, specified as a positive integer. This is a read-only property. The default value is 2.

Data Types: double

PV — Phase velocity

M-element vector

Phase velocity or propagation velocity of a uniform plane wave on the transmission line specified as a M-element vector in meters/sec. The phase velocity values correspond to the frequency values. The default value is 299792458.

Data Types: double

StubMode — Type of stub

'NotaStub' (default) | 'Series' | 'Shunt'

Type of stub, specified as one of the following values: 'NotaStub', 'Series', 'Shunt'.

Data Types: double

Termination — Stub transmission line termination

'NotApplicable' (default) | 'Open' | 'Short'

Stub transmission line termination, specified as one of the following values: 'NotApplicable', 'Open', 'Short'.

Data Types: double

Z0 — Characteristic impedance

vector in ohms

Characteristic impedance, specified as a vector in ohms. The default value is 50 ohms.

Data Types: double

Object Functions

analyze	Analyze RFCKT object in frequency domain
calculate	Calculate specified parameters for rfckt objects or rfdata objects
plotyy	Plot parameters of RF circuit or RF data on X-Y plane with two Y-axes
circle	Draw circles on Smith Chart
loglog	Plot specified circuit object parameters using log-log scale
plot	Plot circuit object parameters on X-Y plane
listparam	List valid parameters for specified circuit object
getz0	Calculate characteristic impedance of RFCKT transmission line object
semilogx	Plot RF circuit object parameters using log scale for x-axis
semilogy	Plot RF circuit object parameters using log scale for y-axis
polar	Plot specified object parameters on polar coordinates
smith	Plot circuit object parameters on Smith chart
write	Write RF data from circuit or data object to file

Examples

Frequency Domain Analysis of a Transmission Line

Transmission Line Properties

trl = rfckt.txline('Z0',75)

trl = 
   rfckt.txline with properties:

        LineLength: 0.0100
          StubMode: 'NotAStub'
       Termination: 'NotApplicable'
              Freq: 1.0000e+09
                Z0: 75
                PV: 299792458
              Loss: 0
          IntpType: 'Linear'
             nPort: 2
    AnalyzedResult: []
              Name: 'Transmission Line'

Plot

f = [1e9:1.0e7:3e9];     % Simulation frequencies
analyze(trl,f);          % Do frequency domain analysis
figure
plot(trl,'s21','angle'); % Plot angle of S21

Algorithms

The analyze method treats the transmission line, which can be lossy or lossless, as a 2-port linear network. It computes the AnalyzedResult property of a stub or as a stubless line using the data stored in the rfckt.txline object properties as follows:

• If you model the transmission line as a stub less line, the analyze method first calculates the ABCD-parameters at each frequency contained in the modeling frequencies vector. It then uses the abcd2s function to convert the ABCD-parameters to S-parameters.

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

  $$\begin{array}{l}A=\frac{e^{kd}+e^{-kd}}{2}\\ B=\frac{Z_0*\left(e^{kd}-e^{-kd}\right)}{2}\\ C=\frac{e^{kd}-e^{-kd}}{2*Z_0}\\ D=\frac{e^{kd}+e^{-kd}}{2}\end{array}$$

  Z₀ is the specified characteristic impedance. k is a vector whose elements correspond to the elements of the input vector freq. The analyze method calculates k from the specified properties as k = α_a + iβ, where α_a is the attenuation coefficient and β is the wave number. The attenuation coefficient α_a is related to the specified loss, α, by

  $${\alpha }_a=-\ln \left({10}^{\alpha /20}\right)$$

  The wave number β is related to the specified phase velocity, V_p, by

  $$\beta =\frac{2\pi f}{V_p},$$

  where f is the frequency range specified in the analyze input argument freq. The phase velocity V_p is derived from the rfckt.txline object properties. It is also known as the wave propagation velocity.

• If you model the transmission line as a shunt or series stub, the analyze method first calculates the ABCD-parameters at the specified frequencies. It then uses the abcd2s function to convert the ABCD-parameters to S-parameters.

  When you set the StubMode property to 'Shunt', the 2-port network consists of a stub transmission line that you can terminate with either a short circuit or an open circuit as shown in the following figure.

  

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

  $$\begin{array}{c}A=1\\ B=0\\ C=1/Z_{in}\\ D=1\end{array}$$

  When you set the StubMode property to 'Series', the 2-port network consists of a series transmission line that you can terminate with either a short circuit or an open circuit as shown in the following figure.

  

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

  $$\begin{array}{c}A=1\\ B=Z_{in}\\ C=0\\ D=1\end{array}$$

References

[1] Ludwig, R. and P. Bretchko, RF Circuit Design: Theory and Applications, Prentice-Hall, 2000.

Version History

Introduced in R2009a