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Chapter 9: Carrier Modulation

When a signal is to be transmitted over a transmission medium, the signal is superimposed on a carrier, which is a high-frequency sine wave. This is known as carrier modulation. In this chapter, we will study the various analog and digital carrier modulation techniques. We also will discuss the various criteria based on which a particular modulation technique has to be chosen.

9.1 WHAT IS MODULATION?

Modulation can be defined as superimposition of the signal containing the information on a high-frequency carrier. If we have to transmit voice that contains frequency components up to 4kHz, we superimpose the voice signal on a carrier of, say, 140MHz. The input voice signal is called the modulating signal. The transformation of superimposition is called the modulation. The hardware that carries out this transformation is called the modulator. The output of the modulator is called the modulated signal. The modulated carrier is sent through the transmission medium, carrying out any other operations required on the modulated signal such as filtering. At the receiving end, the modulated signal is passed through a demodulator, which does the reverse operation of the modulator and gives out the modulating signal, which contains the original information. This process is depicted in Figure 9.1. The modulating signal is also called the baseband signal. In a communication system, both ends should have the capability to transmit and receive, and therefore the modulator and the demodulator should be present at both ends. The modulator and demodulator together are called the modem.

Figure 9.1: Modulation and demodulation.

Modulation is the superimposition of a signal containing the information on a high-frequency carrier. The signal carrying the information is called the modulating signal, and the output of the modulator is called the modulated signal.