Bert Simonovich's Design Notes

Innovative Signal Integrity & Backplane Solutions

Characteristic Impedance and Propagation Delay of a Transmission Line

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A transmission line is any two conductors with some length separated by a dielectric material. One conductor is the signal path and the other is its return path. As the leading edge of a signal propagates down a transmission line, the electric field strength between two oppositely charged conductors creates a voltage between them. Likewise, the current passing through them produces a corresponding magnetic field. A uniform transmission line terminated in its characteristic impedance will have a constant ratio of voltage to current at a given frequency at every point on the line.

To ensure good signal integrity, it is important to maintain a constant impedance at every point along the way. Any change in the characteristic impedance results in reflections which manifests itself into noise on the signal. In any printed circuit board design, it is almost impossible to maintain a constant impedance of the transmission path from transmitter to receiver. Things like vias, non-homogeneous dielectric, thickness variation and other component paracitics all contribute to impedance mismatch.   In high-speed designs, uncontrolled impedance can significantly reduce voltage and timing margins to the point where the circuit may be marginal or worst inoperable. The best you can do is to try to minimize each impedance discontinuity when they occur.

Lossy Transmission Line Circuit Model:

imageThe circuit model for a lossy transmission line assumes an infinite series of two-port components as illustrated. The series resistor  represents the distributed resistance with the units as ohms (Ω) per unit length. The series inductor represents the distributed loop inductance with the units as henries (H) per unit length. Separating the two conductors is the dielectric material represented by conductance G in siemens (S) per unit length. Finally, the shunt capacitor represents the distributed capacitance between the two conductors with units of farads (F) per unit length.

A 2D field solver is the best tool to extract these parameters from a given transmission line geometry. It assumes, however, that the same geometry is maintained through its entire length. Many spice like simulators need these RLGC parameters for their lossy transmission line models.

Given the RLGC parameters, the characteristic impedance can be calculated by the following equation:

image

Where:

Zo is the intrinsic characteristic impedance of the transmission line.

Ro is the intrinsic series resistance per unit length of the transmission line.

Lo is the intrinsic loop inductance per unit length of the transmission line.

Go is the intrinsic conductance per unit length of the transmission line.

Co is the intrinsic capacitance per unit length of the transmission line.

Lossless Transmission Line:

For the lossless transmission line model, Ro and Go are assumed to be zero. As a result, the equation reduces to simply:

image

Propagation Delay:

Propagation delay, as it relates to transmission lines, is the length of time it takes for the signal to propagate through the conductor from on point to another. Given the inductance and capacitance per unit length, the propagation delay of the signal can be determined by the following equation:

image

Where:

tpd is the propagation delay in seconds/unit length.

Lo is the intrinsic loop inductance per unit length of the transmission line.

Co is the intrinsic capacitance per unit length of the transmission line.

Relative permittivity is also known as relative dielectric constant . The number is a measure of an insulator material’s ability to transmit an electric field compared to a vacuum, which is 1. For simplicity, it is usually referred to it as just the dielectric constant, Dk.

Electromagnetic signals propagate at the speed of light through free space. When these signals are surrounded by insulating material other than air or a vacuum, the propagation delay increases proportionally. You can determine the propagation delay with a known Dk by the following equation:

image

Where:

Dk is the dielectric constant of the material.

c is the speed of light in free space = 2.998E8 m/s or 1.180E10 in/s.

Written by Bert Simonovich

January 2, 2011 at 5:36 pm

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