Transmission Line Effect

Definition: As signal frequency or signal trace increases to a certain threshold, the electrical signal on conductor starts to exhibit transmission line effect: voltage and current are treated as traveling waves where wave speed, signal integrity, trace impedance is depended on the traveling medium.

Background:

Transmission line is a conductor that is used to carry an electrical signal. At high frequency, signal travels as a traverse electromagnetic wave (TEM) inside the dielectric materials guided by the transmission line structure such as impedance controlled Microstrip or Strip line on the PCB.

Wave guide is a special type of transmission line usually constructed with hollow metal tube, usually used for very high frequency signal.

In the context of PCB routing, stripe line trace acts as a parallel plate wave guide.

Application:

In the context of PCB micro-strip or strip-line signal transmission interface, we determine whether or not a signal exhibits transmission line effect by examine of interfacelength (L) with respect to electrical wavelength ( λ ) of the highest spectral content of the signal as shown below:

• L > 0.1 λ
• if the trace length greater than a tenth of electrical wavelength of highest spectral frequency of the signal, we say that the trace exhibits transmission line effect.

Q&A

What are impacts of transmission line effect on PCB high speed design?

• Characteristic impedance of the transmission must be taken into account
  • Note: Characteristic impedance limits instantaneous current flow.
• Poor signal integrity due to signal reflection due to a improperly impedance controlled trace or poor termination for the signal driver.

How do you determine highest spectral content of the signal?

• It's mainly determined by signal rise time Tr [ns]. A commonly used formula to determine F [gHz]:
  • F=0.35/Tr

Reference and Further Reading:

"Bandwidth of a signal from its rise time", https://www.edn.com/rule-of-thumb-1-bandwidth-of-a-signal-from-its-rise-time/