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/