Digital Circuit Boards: Mach 1 GHz

2.3 Transmission Line Types

The transmission lines on a typical circuit board are traces over a ground or power plane or traces placed between conducting planes. Parallel traces can also be a transmission line carrying a balanced signal (odd-mode transmission). This transmission line arrangement allows circuitry to reject interference, which is common-mode in character (even-mode transmissions). Parallel traces can be isolated, over a ground plane or in a conducting sheath.

A coaxial cable can be a transmission line. The conducting sheath is the return line. In some cases, two carefully positioned conductors in a shielded enclosure can serve as a balanced transmission line. A balanced structure means that any interference couples to each line equally. It is possible to reject this common signal in the receiving electronics.

Traces between conducting planes are also transmission lines with fields that are confined by the planes. These planes reduce radiation and limit coupling from external fields. Any pair of conductors that form a parallel path for signals can be considered a transmission line, although the pair may not be practical for gigahertz transmissions.

All conductor pairs are capable of transporting electromagnetic energy in both directions. Pairs include conductors in a cable, shield-to-shield, cables-to-conduit, cables over rack surfaces, power lines over the earth, etc. In this book, we direct our efforts toward traces and conducting planes on circuit boards. Transmission ...