Introduction

Noise generally refers to unpredictable short-term deviations of a signal from its nominal value. Although noise is not nearly random in digital circuits, the same word is nevertheless used. To comprehend the impact, noise generation as well as a circuit's tolerance to noise must be studied. This chapter aims at understanding potential failure mechanisms, at quantifying their repercussions, and at learning how to keep switching noise below critical levels.

How does noise enter electronic circuits?

One can distinguish four mechanisms that convey noise from a source to a receptor, see fig.10.1.

Conductive coupling develops when a wire collects noise outside an electronic circuit and brings it to sensitive nodes there. A power rail corrupted by spikes or ripple from a poorly filtered power supply is a classic example. A data cable that picks up electromagnetic radiation from a nearby motor, chopper circuit, or RF transmitter — effectively acting like an antenna — also falls into this category.

Electromagnetic coupling occurs when noise sources impinge upon a circuit by the immediate effects of the electromagnetic field. No external line acts as conveyor in this case, rather, the receiving antenna is within the victim itself. The tiny dimensions of microelectronic circuits tend to render them relatively immune to externally generated fields.

Crosstalk is a particular form of electromagnetic coupling. Polluter and victim sit close to each other on the same die, package, or printed circuit board (PCB). Crosstalk effects are typically modelled in terms of lumped elements such as coupling capacitances and mutual inductances.