Lossy Transmission Line

In this topic:

Netlist Entry

Oxxxx p1 n1 p2 n2 modelname [IC=v1,i1,v2,i2]
p1 Positive input port 1
n1 Negative input port 1
p2 Positive input port 2
n2 Negative input port 2
modelname Name of model defined in a .MODEL statement. Must begin with a letter but can contain any character except whitespace and period '.'.
v1,i1,v2,i2 Initial conditions for voltage at port 1, current at port 1, voltage at port 2 and current at port 2 respectively. These only have an effect if the UIC parameter is specified on the .TRAN statement.

Model Syntax

.model modelname LTRA ( parameters )

Model Parameters

Name Description Units Default
R Resistance/unit length ???MATH???\Omega???MATH???/unit length 0.0
L Inductance/unit length Henrys/unit length 0.0
G Conductance/unit length Siemens(mhos)/unit length 0.0
C Capacitance/unit length Farads/unit length 0.0
LEN Length Required
REL Relative rate of change of derivative for breakpoint 1.0  
ABS Absolute rate of change of derivative for breakpoint 1.0  
The parameters REL and ABS control the way the line is simulated rather than its electrical characteristics. More accurate results (at the expense of simulation time) can be obtained by using lower values.


The uniform RLC/RC/LC/RG transmission line model (LTRA) models a uniform constant-parameter distributed transmission line. The LC case may also be modelled using the lossless transmission line model. The operation of the lossy transmission line model is based on the convolution of the transmission line's impulse responses with its inputs.

The following types of lines have been implemented:
RLC Transmission line with series loss only
RC Uniform RC line
LC Lossless line
RG Distributed series resistance and parallel conductance

All other combinations will lead to an error.

REL and ABS are model parameters that control the setting of breakpoints. A breakpoint is a point in time when an analysis is unconditionally performed. The more there are the more accurate the result but the longer it will take to arrive. Reducing REL and/or ABS will yield greater precision.


The above could represent a 10 metre length of RG58 cable. The parameters would be described in a .model statement e.g.

.model RG58_10m LTRA(R=0.1 C=100p L=250n LEN=10)

Subcircuit-based RLGC Model

A subcircuit-based lossy transmission line model is also provided in the model library and accessible in the schematic editor from the menu system. The sub-circuit model uses the Laplace Transfer Function Convolution Implementation device as its base. The sub-circuit model is faster, more accurate and also supports both shunt and series losses. The subcircuit model may be entered using

Xxxx p1 n1 p2 n2 LOSSY_LINE params:
+ C=cap-per-unit-length
+ L=ind-per-unit-length
+ G=shunt-conductance-per-unit-length
+ R=series-r-per-unit-length LENGTH=length