Transmission Line

A transmission line is an interconnect used to guide electromagnetic wave or electrical energy from one point to another with the intention of least possible loss and distortion. Transmission lines are extensively used by RF (Radio Frequency) engineers in the designs of RF circuits and RF PCBs. For example, to transfer the RF energy from transmitter to antenna, RF engineers use transmission line in place of normal electric wire or normal PCB trace. A normal PCB trace or electric wire creates too high a loss and distortion to the electric energy. If a transmission line is used instead, it will transfer the energy from the transmitter to the antenna with almost zero loss of energy. In high speed digital transmission the use of transmission lines lead to less distortion and lower loss.

The characteristics of the transmission line depend upon the physical dimensions. A transmission line can be formed using PCB traces, coaxial cables, wave guides, twisted pair, shielded pair etc. As PCB designers we will see how transmission lines can be formed using PCB traces. The same electric wire on the PCB traces which we call ordinary point to point wiring can become transmission line if we follow certain basic rules e.g., define the ratio of the PCB trace width and its separation from ground plane

9.1 When is Transmission Line essential

If the frequency of the signals traveling on an electric wire is low enough, we can assume that the electric voltage on the wire at a given time is same at all points on the wire. Transmission lines are not essential in such cases. If however, the frequency component of the signal traveling is very high and at the same time the length of the interconnect PCB trace is large, this may create a problem if the PCB trace is ordinary interconnect instead of transmission line. We should calculate the wavelength corresponding to the frequency component of the signal using the formula

λ = c/f

where
c = speed of propagation
f = maximum frequency component of the signal

If the length of the interconnect PCB is greater than 1/10 th of the wavelength, we must consider using the transmission line instead of ordinary interconnect.
Example 9.1 – Transmitter from IC A has frequency component of 1GHz. The signal from IC A to another IC B on a PCB, travel at speed of 1.5 x 108 meter /second. Find the minimum distance above which the signal must be routed as transmission line.

Solution – The wavelength of 1 GHz frequency component is given by

λ = c/f
= 1.5 x 108 (m/second) / 1 x 109
= 0.15 meter

Therefore, the minimum length over which the signal must be routed as transmission line is given by ?/10 = 0.015 meter or 1.5 cm

Any PCB trace length greater than 1.5 cm or about 0.6 inches must be routed as transmission line.

The above example does not mean that the PCB traces less than 1.5 cm should not be routed as transmission line. The signal may still cause EMI radiation if not routed using transmission line even for smaller lengths. It should be noted that by the term frequency component we mean the frequency component corresponding to the rise time of the signal.


Previous - Question & Answer - Chapter 8                                Transmission Line Properties