Selecting Wire Models

A transmission line or interconnect is one of the following:

• Wire
• Trace
• Conductor
• Line

Many applications model electrical properties of interconnections between nodes by their equivalent circuits, and integrate them into the system simulation to accurately predict system performance.

An electrical model that simulates the behavior of interconnect must consider all of the following:

• Physical nature or electrical properties of the interconnect
• Bandwidth or risetime and source impedance of signals of interest
• Interconnect's actual time delay
• Complexity and accuracy of the model, and the corresponding effects on the amount of CPU time required for simulations

You can choose from the following circuit models for interconnects:

• No model at all. Use a common node to connect two elements.
• Lumped models with R, L, and C Elements, as described in Statistical Analysis and Optimization. These include a series resistor (R), a shunt capacitor (C), a series inductor and resistor (RL), and a series resistor and a shunt capacitor (RC).
• Transmission line models such as an ideal transmission line (T Element) or a lossy transmission line (U Element)

Choosing the simplest model that adequately simulates the required performance minimizes sources of confusion and error during analysis.

Generally, to simulate both low and high frequency electrical properties of interconnects, select the U Element transmission line model. For compatibility with conventional versions of SPICE, use one a discrete lumped model or the T Element.

Following are the factors that determine the best choice of a transmission line:

Source properties

trise = source risetime

Rsource = source output impedance

Interconnect properties

Z0 = characteristic impedance

TD = time delay of the interconnection

or

R = equivalent series resistance

C = equivalent shunt capacitor

L = equivalent series inductance

Wire Model Selection Chart shows you how to select a model based on source and interconnect properties.

Figure C-1: Wire Model Selection Chart

Use the U model with either the ideal T Element or the lossy U Element. You can also use the T Element alone, without the U model. Star-Hspice offers both, a flexible definition of the conventional SPICE T Element and an accurate U Element lossy simulation.

Figure C-2: U Model, T Element and U Element Relationship

The T and U Elements do not support the <M=val> multiplier function. If a U or T Element is used in a subcircuit and an instance of the subcircuit has a multiplier applied, the results are inaccurate.

A warning message similar to the following is issued in both the status file (. st0 ) and the output file (. lis ) if the smallest transmission line delay is less than TSTOP/10e6:

**warning**: the smallest T-line delay (TD) = 0.245E-14 is too small 

Please check TD, L and SCALE specification

This feature is an aid to finding errors that cause excessively long simulations.

Using Ground and Reference Planes

All transmission lines have a ground reference for the signal conductors. In this manual, the ground reference is called the reference plane so that it is not confused with SPICE ground. The reference plane is the shield or the ground plane of the transmission line element. The reference plane nodes may or may not be connected to SPICE ground.

Selecting Ideal or Lossy Transmission Line Element

The ideal and lossy transmission line models each have particular advantages, and they may be used in a complementary manner. Both model types are fully functional in AC analysis and transient analysis. Some of the comparative advantages and uses of each type of model are listed in Ideal versus Lossy Transmission Line.

The ideal line is modeled as a voltage source and a resistor. The lossy line is modeled as a multiple lumped filter section, as shown in Ideal versus Lossy Transmission Line Model.

Figure C-3: Ideal versus Lossy Transmission Line Model

Because the ideal element represents the complex impedance as a resistor, the transmission line impedance is constant, even at DC values. On the other hand, you must prefilter the lossy element if ideal piecewise linear voltage sources are used to drive the line.

Selecting U Models

The U model allows three different description formats: geometric/physical, precomputed, and electrical. It provides equally natural description of vendor parts, physically described shapes, and parametric input from field solvers. The description format is specified by the required model parameter ELEV, as follows:

• ELEV=1 - geometric/physical description such as width, height, and resistivity of conductors. It is used by board designers dealing with physical design rules.
• ELEV=2 - precomputed parameters. These are available with some commercial packaging, or as a result of running a field solver on a physical description of commercial packaging.
• ELEV=3 - electrical parameters such as delay and impedance, available with purchased cables. It only allows one conductor and ground plane for PLEV = 1.

The U model explicitly supports transmission lines with several types of geometric structures. The geometric structure type is indicated by the PLEV model parameter, as follows:

• PLEV=1 - Selects planar structures, such as microstrip and stripline, which are the usual conductor shapes on integrated circuits and printed-circuit boards.
• PLEV=2 - Selects coax, which frequently is used to connect separated instruments.
• PLEV=3 - Selects twinlead, which is used to connect instruments and to suppress common mode noise coupling.

Figure C-4: U Model Geometric Structures

Using Transmission Lines - Example

The following Star-Hspice file fragment is an example of how both T Elements and U Elements can be referred to a single U model as indicated in U Model, T Element and U Element Relationship. The file specifies a 200 millimeter printed circuit wire implemented as both a U Element and a T Element. The two implementations share a U model that is a geometric description (ELEV=1) of a planar structure (PLEV=1).

T1 in gnd t_out gnd micro1 L=200m

U1 in gnd u_out gnd micro1 L=200m

.model micro1 U LEVEL=3 PLEV=1 ELEV=1 wd=2m ht=2m th=0.25m KD=5

The next section provides details of element and model syntax