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 lines that connect to a board from external sources can transfer interference into and out of the board. Interference can be rejected by using balanced transmission techniques and, in some cases, passive filters. Interfacing cables with circuit boards is discussed in later chapters.
The energy that is transported on any transmission line is in the field between the conductors. When one of the conductors is a conducting plane, the fields associated with the energy are confined to the area immediately under that trace.3 The current that flows is confined to the trace and the conducting surfaces directly under the trace. The current path follows the surface areas where the field patterns terminate. The current path in the plane can be likened to a river under the shadow of the trace. Even though one conducting plane may be shared by many traces, there is no cross coupling, unless the fields associated with transmission share the same volume in space. A simple right-angle crossing of traces is acceptable. The rivers of current (fields) will know what to do.
Transmission lines formed by traces over a conducting plane are called microstrips. If a dielectric surrounds the trace, the term embedded microstrip is used. This type of transmission line is usually found on the outer layers of a PCB (printed circuit board). Sometimes the embedding dielectric can be a conformal coating or a screening material. See later discussions on characteristic impedance. Transmission lines formed by traces between two conducting planes are called stripline.