TABLE OF CONTENTS In this chapter, we will investigate distortion, but begin by looking at the fundamentals of distortion measurement. Even designers of test equipment would probably concede that this is not the sexiest of topics, but unless we understand how errors can creep into our measurements, we will not have the confidence to compare the results of one measurement with another, leaving us unable to test and improve our designs.
Although we may glibly use the word distortion when talking about amplifiers, there are actually two distinct types of distortion.
Linear distortions do not change with amplitude. If we consider the transfer characteristic of a device producing linear distortion, it is a straight line hence the term linear distortion. See Fig. 3.1a.
Although a device causing linear distortion changes the shape of the waveform, there are no additional frequencies at the output of the device. Linear distortion typically causes errors in the amplitude versus frequency response and this is the way that it is usually assessed. However, it is perfectly possible to change the shape of a waveform without changing the amplitude vs frequency response by distorting the time at which frequencies arrive. Loudspeaker crossover systems and analogue tape machines are particularly good at this feat. The shape of a square wave s leading edge is particularly sensitive to timing errors, so an oscilloscope quickly reveals problems. Alternatively, timing errors between sine waves of differing frequencies can be assessed by plotting...
