Transmission Line Effect

Introduction

When the frequency or trace length of an electrical signal surpasses a certain threshold, the signal on a conductor begins to exhibit transmission line effects. At this point, voltage and current are treated as traveling waves, and factors such as wave speed, signal integrity, and trace impedance become dependent on the characteristics of the transmission medium.

Background

 Transmission lines are conductors used to carry electrical signals. At higher frequencies, the signal travels as a transverse electromagnetic wave (TEM) guided by the structure of the transmission line, such as impedance-controlled microstrip or stripline on a PCB. Waveguides, on the other hand, are specialized transmission lines typically constructed as hollow metal tubes, designed for very high-frequency signals. In PCB routing, a stripline trace acts as a parallel plate waveguide.

Practical Application

In the context of PCB microstrip or stripline signal transmission interfaces, the presence of transmission line effects is determined by examining the interface length (L) relative to the electrical wavelength (λ) of the highest spectral content of the signal. If the trace length is greater than a tenth of the electrical wavelength of the highest spectral frequency of the signal (L > 0.1λ), we consider the trace to exhibit transmission line effects.

Another way to determine if a trace exhibits transmission line effects based on propagation delay, the following timing-based rule can be applied: Propagation Delay (τ) > 0.25T_rise If the propagation delay on the transmission line from transimter to receiver is greater than 0.25 times the signal's rise time (T_rise), it suggests that the trace exhibits transmission line effects. 

Q&A

• Q: What are the impacts of transmission line effects on PCB high-speed design?

  • A: The characteristic impedance of the transmission line must be taken into account. Note that the characteristic impedance limits the instantaneous current flow. Poor signal integrity may result from signal reflection due to improperly impedance-controlled traces or poor termination for the signal driver.

• Q: How do you determine the highest spectral content of the signal?

  • A: The highest spectral content is mainly determined by the signal's rise time (Tr) in nanoseconds. A commonly used formula to determine the highest frequency (F) in gigahertz is: F = 0.35/Tr.

Summary

• At a certain threshold of signal frequency or trace length, electrical signals on conductors exhibit transmission line effects.

• Transmission lines carry electrical signals and can operate as microstrip or strip-line structures on PCBs.

• Waveguides, constructed from hollow metal tubes, are used for very high-frequency signals.

• Transmission line effects are determined by examining the trace length (L) relative to the electrical wavelength (λ) of the highest spectral content of the signal.

• A timing-based rule states that if the propagation delay (τ) is greater than 0.25 times the signal's rise time (T_rise), the trace exhibits transmission line effects.

• Transmission line effects impact signal integrity and require consideration of characteristic impedance and proper termination.

• The highest spectral content of a signal is often determined by its rise time (T_rise), and a formula like F = 0.35/Tr can estimate the highest frequency.

Further Reading

For more detailed information on the relationship between a signal's rise time and bandwidth, you can refer to the provided reference and further reading link: "Bandwidth of a signal from its rise time" available at https://www.edn.com/rule-of-thumb-1-bandwidth-of-a-signal-from-its-rise-time/.