Gateways Into Electronics

11

TRANSMISSION LINES

11-1 INTRODUCTION

A signal with a rise time of 1 microsecond can be transmitted with negligible distortion over a distance of 1 meter if minimal precautions are taken. As we will see, this is so because the rise time is much larger than the propagation time, that is, the distance divided by the speed of light, 10 nanoseconds at most in this case. In a familiar example, audio signals are transmitted to speakers over a comparable distance, and little thought is given to precisely how this is done: almost any available pair of wires will do the job.

In contrast, a signal with a rise time of order 1 nanosecond can be severely distorted when transmitted over 1 meter. ECL circuits, for example, are characterized by such rise times, and transitions between logic levels can exhibit large overshoots and undershoots at the receiving end if due care is not taken. That such behavior must be avoided can be seen by observing that a 100% overshoot in a LOW-to-HIGH transition will saturate the input transistor of the receiving circuit, whereas a 50% undershoot can be misinterpreted as a return to a LOW level.

ECL circuits generate the type of wideband pulsed signals that motivate many of the developments in this chapter; in addition, they tolerate a certain amount of distortion in the form of overshoots and undershoots and thus offer a setting in which calculations of imperfect responses make sense. For this reason, we will choose our examples with ECL circuits preferably ...