Fig 1 (or its equivalent, Listing 1) is a simple and efficient Spice model for a transmission line with constant resistive losses. The model is an extension of the Spice transmission-line model (designated Txxx).
Fig 1 (or its equivalent, Listing 1) is a simple and efficient Spice model for a transmission line with constant resistive losses. The model is an extension of the Spice transmission-line model (designated Txxx).
Some commercial versions of Spice offer models for lossy transmission lines. However, these models are lumped-element models. These models require many cells to give accurate results for relatively long delays. Having many cells causes long calculation times.
The model in Fig 1 executes quickly, is compatible with any Spice-type commercial program, and is a very interesting alternative to the lumped-element models.
A trace's length, ln, and parameters Ll, Cl and Rl—inductance, capacitance, and resistance per unit length, respectively—define the model. You must enter the line's parameters in the subcircuit's CALL statements this way:
Xyyy in ref out RLINE Ll = val Cl = val Rl = val ln = val
The model's basis is the first order of the frequency-dependent characteristic-impedance and exponential-propagation function of a lossy transmission line. This model is valid for high frequencies and corrects for the line's dc response. The model is very accurate for an interconnection that acts as a low-loss transmission line only if
tS<8×ln×(Ll3Cl)˝
and
Rl×ln<2(Ll/Cl)˝,
where tS is the switching time of the fastest signal. Be careful if you use the model for other cases. (DI #1566)