hspice book hspice ch22 1 Thu Jul 23 19 10 43 1998 Chapter 21 Using Transmission Lines A transmission line delivers an output signal at a distance from the point of signal input Any two conductors can make up a transmission line The signal which is transmitted from one end of the pair to the other end is the voltage between the conductors Power transmission lines telephone lines and waveguides are examples of transmission lines Other electrical elements which should be thought of as transmission lines include traces on printed circuit boards and multichip modules MCMs and within integrated circuits With current technologies that use high speed active devices on both ends of most circuit traces all of the following transmission line effects must be considered during circuit analysis Time delay Phase shift Power voltage and current loss Distortion Reduction of frequency bandwidth Coupled line crosstalk Star Hspice provides accurate modeling for all kinds of circuit connections including both lossless ideal and lossy transmission line elements This chapter covers these topics Selecting Wire Models Performing HSPICE Interconnect Simulation Understanding the Transmission Line Theory References Star Hspice Manual Release 1998 2 21 1 hspice book hspice ch22 2 Thu Jul 23 19 10 43 1998 Selecting Wire Models Using Transmission Lines Selecting Wire Models Various terms are used for electrical interconnections between nodes in a circuit Common terms are Wire Trace Conductor Line The term transmission line or interconnect generally can be used to mean any of the above terms Many applications model electrical properties of interconnections between nodes by their equivalent circuits and integrate them into the system simulation to make accurate predictions of system performance The choice of electrical model to simulate the behavior of interconnect must take into account 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 Choices for circuit models for interconnects are No model at all Use a common node to connect two elements Lumped models with R L and C elements as described in Chapter 12 Using Passive Devices 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 21 2 Star Hspice Manual Release 1998 2 hspice book hspice ch22 3 Thu Jul 23 19 10 43 1998 Using Transmission Lines Selecting Wire Models As a rule of thumb follow Einstein s advice Everything should be made as simple as possible but no simpler 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 When compatibility with conventional versions of SPICE is required use one of the discrete lumped models or the T element The best choice of a transmission line model is determined by the following factors 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 Figure 21 1 is a guide to selecting a model based on the above factors Star Hspice Manual Release 1998 2 21 3 hspice book hspice ch22 4 Thu Jul 23 19 10 43 1998 Selecting Wire Models Using Transmission Lines Initial information required trise source risetime TD time delay of the interconnect Rsource source resistance Selection Criterion R 10 Rsource R R Rsource C 10 trise C L R Rsource 10 trise L Default U Compatibility with conventional SPICE T Infinite bandwidth required for ideal sources T Consider crosstalk effects U Consider line losses U TDs are very long or very short U Default U RC trise 5 TD low frequency trise 5 TD high frequency RL Figure 21 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 Thus Star Hspice offers both a more flexible definition of the conventional SPICE T element and more accurate U element lossy simulations 21 4 Star Hspice Manual Release 1998 2 hspice book hspice ch22 5 Thu Jul 23 19 10 43 1998 Using Transmission Lines Physical Geometry Precalculated R C L Impedance Delay Z0 TD Selecting Wire Models U Model Field Solution Calculated R C L Inverse Solution Z0 TD Lossless Ideal T Element Lossy U Element Figure 21 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 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 as not to be 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 Star Hspice Manual Release 1998 2 21 5 hspice book hspice ch22 6 Thu Jul 23 19 10 43 1998 Selecting Wire Models Using Transmission Lines Selection of 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 fashion 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 Table 21 1 Table 21 1 Ideal versus Lossy Transmission Line Ideal Transmission Line Lossy Transmission Line lossless includes loss effects used with voltage sources used with buffer drivers no limit
View Full Document
Unlocking...