31.1 Introduction

Digital circuits, switches, transmission systems, and storage devices are currently some of the highest growth areas in electronics. Fueled by the almost limitless bandwidth of fiberoptic cables, digital telecommunications is transforming the telephone industry and creating the rapid convergence of voice, data, and video communications. In the space of a few years in the 1990s, the speed of digital systems has increased from hundreds of megabits per second (Mbits/s) to tens of gigabits per second (Gbits/s).

The fundamental measure of performance or quality in these new digital systems is the probability of any stored or transmitted bit being received in error. With the latest equipment, the probabilities are very low, being on the order of 10 ^?12 or less. However, it is still necessary to measure the performance of these systems and in particular to analyze the margins of safety available and to explore potential weaknesses which could lead to degraded performance later. This is the purpose of digital pattern generators and error detectors, sometimes referred to as bit error rate test sets, or BERTS.

31.2 Sources of Errors

Errors in digital systems arise as a result of several distinct practical effects. When viewing a random digital signal on an oscilloscope, the common eye diagram shown in Fig. 31.1 is displayed. To obtain the eye diagram display on an oscilloscope, the sweep is triggered using the data clock source, and the time base is adjusted so that, say, two...