Fast Circuit Boards

4Interference

4.1 Introduction

Radiated interference can be roughly divided into two parts: energy that couples within a circuit and energy that enters from outside sources. Entering interference can come from radiation or cable connections. We discuss lightning and electrostatic discharge (ESD) separately.

This chapter deals with interference of all types and ways to limit its impact. Errors can occur in the sampling process. Utility power can be a source of interference when there are voltage surges, voltage dropouts, neutral voltage drops, and capacitive coupling. There can be instabilities in active circuits. Logic errors can occur when there are reflections, insufficient signal, or a lack of energy to carry the logic.

High‐speed logic requires greater bandwidth and this adds to the radiated levels. Reducing operating voltages limits radiation but makes circuits more susceptible to interference. The use of hard wires to carry signals through any distance has always been a problem. One difficulty is the coupling to interfering fields. The use of fiber optics and high‐frequency carrier links has eliminated many problems but this comes at a price. The data must first be put into a form where it can be transmitted.

We have already discussed two sources of interference generated on a circuit board: radiation from traces at the leading edges of waves and board edge radiation caused if fields are not provided a controlled space to use. The cross‐coupling of logic signals is another ...