Aston University

Department of Electronic Engineering and Applied Physics

HSPICE User Guide: Transmission Lines Supplement

INTRODUCTION

This document should be read in conjunction with the 'HSPICE User Guide'. It covers additional information required to model and simulate transmission lines with HSPICE.

A transmission line is a device intended to deliver an output signal at a distance from the point of signal input. Transmission lines include power cables, telephone lines, and waveguides. Less obviously perhaps, printed circuit boards, multi-chip modules and even integrated circuit packages have to be considered as transmission lines when operating frequencies are high.

The extra effects which are introduced by transmission line models are: time delay, phase shift, power loss, distortion and reduction of bandwidth.

HSPICE provides facilities for modelling lossless (ideal) and lossy transmission lines.

It also provides facilities to model numerous different physical layouts of the conductors forming the transmission lines. In this document only coaxial cables and twin-lead cables are described. Full details are in the Meta-Software HSPICE manual. See Mr. Wilton if you wish to consult that manual.

This document is also available in PostScript format on the Aston University WWW server. (URL http://www.eeap.aston.ac.uk/eeap/documents/user-guides.html).

HSPICE TRANSMISSION LINE ELEMENTS

There are two transmission line elements. The T element is used for ideal transmission lines and the U element for lossy transmission lines. In addition the U model may be used to combine ideal and lossy elements. However, it is often convenient (and more consistent) to use the U model with the relevant parameters even for ideal lines.

The ideal line is modelled as a voltage source and a resistor. The lossy line is modelled as a multiple lumped filter section.

Fig 1. Ideal and lossy transmission line models

LOSSLESS TRANSMISSION LINE (T ELEMENT)

Syntax

Txxx in refin out refout <mname> z0=val td=val L=val

in Signal node (in side)

refin Ground reference for input.

out Signal node (out side)

refout Ground reference for output.

mname U model reference name

z0 Characteristic impedance

td Transmission delay (secs/m)

L Physical length of the transmission line in metres. The default value is 1m.

LOSSY TRANSMISSION LINE (U ELEMENT)

Syntax

For 1 wire and ground plane

Uxxx in refin out refout mname L=val

For 2 wires without ground plane (unshielded twin)

Uxxx in1 in2 out1 out2 mname L=val

For 2 wires and ground plane

Uxxx in1 in2 refin out1 out2 refout mname L=val

in, in1, in2 Signal node(s )(in side)

refin Ground reference for input.

out,out1,out2 Signal node(s) (out side)

refout Ground reference for output.

mname U model reference name

L Physical length of the transmission line in metres. The default value is 1m.

TRANSMISSION LINE MODEL (U MODEL)

The U model is intended for specifying the extra parameters for lossy lines but it is convenient to use it also to specify the parameters required for lossless lines.

The syntax is:

.MODEL mname U LEVEL=3 PLEV=x ELEV=x <DLEV=x>

+ <Pname=val> ...

mname Model name.

PLEV Physical level. Used to distinguish between planar conductors (PLEV=1), Coaxial conductors (PLEV=2),Twin and twisted pair conductors (PLEV=3)

ELEV Electrical level. Used to distinguish modelling via geometry such as thickness, width, dielectric (ELEV=1), pre-calculated equivalent resistance, capacitance and inductance (ELEV=2), measured impedance and delay (ELEV=3).

DLEV Device level. A further level of refinement for different conductor/dielectric configurations.

In this document only geometric and measured parameter modelling are considered.

GEOMETRIC MODELLING (ELEV=1)

Coaxial cable

Geometric coaxial cable modelling is selected with U model parameters PLEV=2, ELEV=1. the DLEV parameter is not required.

.MODEL mname U LEVEL=3 PLEV=2 ELEV=1 <Pname=val> ...

The physical geometry parameters for coaxial cable are shown in Fig. 2 and described in Table 1.

Fig. 2. Coaxial Cable geometry

Name      Unit      Default            Description                            
RA        m         required           Outer radius of inner conductor        
RB        m         required           Inner radius of outer conductor        
                                       (shield)                               
RD        m         RA+RB              Outer radius of outer conductor        
                                       (shield)                               
HGP       m         1.5.RD             Radius to circuit ground point         
RHO       [[Omega]  17E-9              Resistivity of conductor material      
          ]m                           (default=Copper)                       
RHOB      [[Omega]  RHO                Resistivity of shield material         
          ]m                                                                  
SIG       [[Omega]  0.0                Conductivity of dielectric             
          ]-1m-1                                                              
KD                  4.0                Relative dielectric constant
Table 1. Geometric Coaxial cable parameters

Twinlead cable

Geometric twinlead cable modelling is selected with U model parameters PLEV=3, ELEV=1. Differences caused by the presence of a shield are indicated by a third parameter DLEV. The different geometric parameters are shown in Fig 3a (sea of dielectric), Fig. 3b (insulating spacer) and Fig. 3c (shielded). The parameters are described in Table 2.

Name      Unit      Default            Description                            
DLEV                0                  Device Level.                          
RA1       m         required           Outer radius of conductor 1            
RA2       m         RA1                Outer radius of conductor 2            
D12       m         required           Distance between conductor centres     
RHO       [[Omega]  17E-9              Resistivity of conductor material      
          ]m                           (default=Copper)                       
KD                  4.0                Relative dielectric constant           
SIG       [[Omega]  0.0                Conductivity of dielectric             
          ]-1m-1                                                              
HGP       m         1.5.D12            Radius to circuit ground point         
RHOB      [[Omega]  RHO                Resistivity of shield (if present)     
          ]m                                                                  
OD1       m         required           Maximum outer dimension of shield      
                                       (if present)                           
OD2       m         OD1                Minimum outer dimension of shield      
                                       (if present)
Table. 2. TwinLead cable parameters

Fig. 3a. TwinLead cable (sea of dielectric) geometry

Fig. 3b. TwinLead cable (insulating spacer) geometry

Fig. 3c. TwinLead cable (with shield) geometry

PRE-COMPUTED ELEMENT MODELLING (ELEV=2)

Not covered in this document.

MEASURED ELEMENT MODELLING (ELEV=3)

When measured parameters are given, HSPICE calculates the resistance, capacitance and inductance parameters using TEM transmission line theory. If some parameters are redundant HSPICE guesses which are most likely to be accurate and discards the others.

This modelling method is most useful for standard cable types where the parameters can be extracted from tables.

Name      Unit      Default            Description                            
ZK        [[Omega]  calculated         Characteristic impedance               
          ]                                                                   
VREL                calculated         Relative velocity of propagation       
DELAY     sm-1      calculated         Delay/length                           
CAPL      Fm-1      1.0                Capacitance/length                     
AT1       m-1       1.0                Attenuation factor/length

Table 3. Measured parameters

The measured parameters for some standard cables are given below.

Coaxial cable type RG58 (Thin Ethernet)

.model rg58c u level=3 plev=2 elev=3 zk=50 capl=100.7p

+ vrel=0.66 fr1=100meg at1=0.173db

Twisted pair (Shielded)

.model tw/sh u level=3 plev=3 elev=3 zk=300 capl=25.5p

+ vrel=.698 fr1=57meg at1=0.0566db

Twisted Pair (Unshielded)

.model tw/un u level=3 plev=3 elev=3 zk=300 capl=17.3p

+ vrel=.733 fr1=100meg at1=0.0458db

OTHER TRANSMISSION LINE PARAMETERS

There are a few U model parameters which may be used irrespective of the type of transmission line. These are used to control the simulation algorithms.

WLUMP=val Number of lumps per wavelength for error control. The default value is 20.

MAXL=val Maximum number of lumps per element. The default value is 20.

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Andy Wilton. a.p.wilton@aston.ac.uk