chapter  7
18 Pages

1 Television Performance Measurements

A complicating factor is return loss. When the source impedance is not equal to the input impedance of an equipment, or the output impedance of the equipment is not equal to the terminating impedance, then part of the signal energy is reflected in the form of an echo, so that the signal amplitude passed on is less than it should be. Furthermore, since these impedances may be a function of frequency, amplitude under ideal matching to that of the reflection, expressed in decibels, is the return loss.6-8

Another very frequently overlooked factor is the behaviour of coaxial connecting cables. It is usually assumed that these always have an impedance of 75 ohms, or whatever the nominal value may be. In fact, this value is only approached with high-quality double-sheathed flexible cables at frequencies of 1MHz and above. Moreover, inevitable variations in the cable constants can result in only a very few metres of cable having an intrinsic return loss of as little as 30 dB. Some useful information is given in Whalley.8