Phase and Frequency Modulation

Abstract

Amplitude modulation is often called linear modulation, since superposition applies for AMDSB-SC systems. For instance, in AMDSB-SC if a message signal m₁(t) generates a modulated waveform s₂(t), and message m₂(t) generates the waveform s₂(t), the sum of the messages, m₁(t) + m₂(t), will produce the modulated waveform s₁(t) + s₂(t). If we ignore the constant added to the message signal, AMDSB-TC, or conventional AM, also has this property. In this chapter we turn our attention to angle modulation, where instead of varying the carrier amplitude in proportion to the message, we vary the carrier phase or frequency in relation to the message signal. Since the phase and frequency are in the argument of a sine or cosine function, angle modulation does not obey superposition, and hence it is sometimes called nonlinear modulation. This nonlinearity will become clear as the development progresses.