Appendix D. Accuracy of Pi Model

The transfer function H for a transmission line may be expressed in terms of the Taylor-series approximation for the exponential function.

Equation D.1



g is the propagation coefficient for the transmission line (complex nepers/m), and


l is the length of the line (m).

The inverse of this function is similarly defined for negative exponents:

Equation D.2



Taking only the first four terms of each expression, you can derive approximate formulas for the forms of H used in the calculation of system gain (Appendix C).

Equation D.3



Equation D.4



Now substitute approximations [D.3] and [D.4] into the expression for the gain of a fully configured transmission system [C.17].

Equation D.5



The next calculations compare [D.5] with the expression for the gain of a pi-model circuit in Section 3.4.2, Figure 3.4. The two-port transmission matrix for the pi-model circuit, fully configured with source and load impedances Z S and Z L respectively, is a composite of five two-port mini-models. The first (leftmost) matrix represents the series-connected source impedance. The next three represent the components in the pi-model. The last matrix represents the shunt-connected load impedance.

Equation D.6



The expression for the circuit gain follows :

Equation D.7



Next use the definitions of graphics/744equ01.gif and graphics/744equ02.gif to derive the following substitutions:

Equation D.8



Equation D.9



Plugging these new substitutions back into [D.7] leads to the following expression for the gain of the pi-model circuit.

Equation D.10



Comparing [D.10] with [D.5] reveals a perfect match of all zero-, first-, and second-order terms, with the lowest -order difference showing up in the third-order part of the expression. Assuming a small value of the propagation coefficient l g < 1/4, and thereby an overall gain somewhere near unity, you might reasonably conclude that the modeling error should not exceed the difference between [D.5] and [D.10]

Equation D.11



Further assuming the ratios Z S /(2 Z C ) and Z C / Z L to each be less than 3.8, the resulting pi-model error (measured as a function of frequency) should remain less than 1%.

As the coefficient l g grows, so grows the error in proportion to (at least) the third power of l g . At a value of l g < 1/2, you should expect a modeling accuracy no better than 1 part in 10.


Transmission Line Parameters

Performance Regions

Frequency-Domain Modeling

Pcb (printed-circuit board) Traces

Differential Signaling

Generic Building-Cabling Standards

100-Ohm Balanced Twisted-Pair Cabling

150-Ohm STP-A Cabling

Coaxial Cabling

Fiber-Optic Cabling

Clock Distribution

Time-Domain Simulation Tools and Methods

Points to Remember

Appendix A. Building a Signal Integrity Department

Appendix B. Calculation of Loss Slope

Appendix C. Two-Port Analysis

Appendix D. Accuracy of Pi Model

Appendix E. erf( )


High-Speed Signal Propagation[c] Advanced Black Magic
High-Speed Signal Propagation[c] Advanced Black Magic
ISBN: 013084408X
Year: 2005
Pages: 163 © 2008-2020.
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