Coupling of electric, magnetic, and electromagnetic fields from one part of a circuit to another is usually undesirable and can lead to many problems, such as noise injection, interference, instability through parasitic feedback, and transient ringing. Ultimately this leads to reduced sensitivity in a system such as a radio receiver or a fast digital circuit. In this chapter we explore various coupling mechanisms and techniques for isolation. Entire books have been dedicated to this subject, such as the excellent book by Ott. Much of the material for this chapter has been inspired from this source and the interested reader is highly encouraged to read Ott for further details.

In practice coupling problems are very difficult to solve, since the source of the coupling is often difficult to identify. The techniques of this chapter should give the necessary tools to properly diagnose and solve such coupling problems. As we shall see, there is no straightforward way to do this, but applying fundamental principles of electromagnetics, with the right approximations, can lead to tractable solutions.

Electrical coupling

Electrical coupling occurs due to stray capacitance in a circuit. A typical scenario is shown in Fig. 13.1. In this example the source of interference is from the first circuit, and the receptor circuit picks up the interference or noise due to the coupling capacitor Cc.