Changes

The standard SPICE provides two types of general-purpose transmission line models: lossless (TRA) and lossy (LTRA). These models are primarily intended for transient analysis. The lossless transmission line model is characterized by either its delay in seconds or by its normalized length at a given frequency. On the other hand, the lossy transmission line model is characterized by distributed RLCG [[parameters]]: resistance per unit length (R), inductance per unit length (L), capacitance per unit length (C), and conductance per unit length (G). Both [[B2.Spice A/D]] and [[RF.SpiceA/D]] offer the native SPICE transmission line models TRA and LTRA as passive devices.

However, [[RF.SpiceA/D]] also offers a number of other transmission line models specifically intended for use in RF circuit analysis. These include the generic T-Line, the generic coupled T-lines, and a variety of physical transmission line models.

The Generic Open Stub and Generic Short Stub are two one-port devices based on the Generic T-Line device. They represent terminated generic transmission line segments. In the open stub case, the termination load is Z_L → ∞, and in the short stub case, the termination load is Z_L = 0. The [[parameters]] of both open and short stub devices are the same as those of the generic T-line device.

In real practical RF circuits, you often need to transition from one transmission line to another or connect two or more [[Transmission Lines|transmission lines]] together. These transitions can be modeled as [[Multiport Networks|multiport networks]]. [[RF.SpiceA/D]] currently offers five generic T-Line discontinuity devices as follows:

The schematic symbols of these devices are shown in the opposite figure. Similar to other RF devices or [[Multiport Networks|multiport networks]], the negative pins of the ports must always be grounded. Unlike the T-line device described earlier which have models based on or derived from the standard SPICE LTRA, the generic T-line discontinuities have S-parameter-based models.

When you first place these discontinuity parts on your circuit, they have default S-parameter values. The default values have been chosen to be very general and may not necessarily represent the physics of your specific circuit. You must enter your own S-parameter values over the desired frequency range and replace the default values. These data can easily be generated in and imported from an electromagnetic simulation suite like [[EM.Cube]]. Unlike physical [[Transmission Lines|transmission lines]] like microstrip or coaxial line (to be discussed later) that have particular geometries and physical structures, the Generic T-Line device and Generic T-Line Discontinuities are very general by definition and do not have physical, material or dimensional [[parameters]]. You can model and simulate very complicated transmission line structures as well as open end, bend, step, tee or cross junctions based on those structures using [[EM.Cube]] and then import their S-parameter data into the corresponding discontinuity parts.