Audio Engineering

214 Chapter 7

7.16.2 Transient Defects

A more insidious group of signal distortions may occur when brief signals of a transient
nature, or sudden step type changes in base level, are superimposed on the more
continuous components of the program signal. These defects can take the form of
slew-rate distortions, usually associated with a loss of signal during the period of the
slew-rate saturation of the amplifi er—often referred to as transient intermodulation
distortion or TID.

This defect is illustrated in Figure 7.45 and arises particularly in amplifi er systems
employing substantial amounts of negative feedback when there is some slew-rate
limiting component within the amplifi er, as shown in Figure 7.46.

A further problem is that due to “ overshoot, ” or “ ringing, ” on a transient input, as
illustrated in Figure 7.47. This arises particularly in feedback amplifi ers if there is
an inadequate stability margin in the feedback loop, particularly under reactive load

HF

oscillator

High-pass

filter Demodulator

Low-pass

filter

Amplifier

(or other device)

under test

Mixer

mV

LF

oscillator

Low-pass

filter Demodulator

High-pass

filter

mV

Figure 7.44 : Two-tone intermodulation distortion test rig.

Input

Output

Figure 7.45 : Effect of amplifi er slew-rate saturation or transient intermodulation distortion.