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.
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.
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.
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]:
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/