Modulacion

Principles of Communication

Prof. V. Venkata Rao

4

CHAPTER 4

Linear Modulation

4.1 Introduction
We use the word modulation to mean the systematic alteration of one waveform, called the carrier, according to the characteristic of another waveform, the modulating signal or the message. In Continuous Wave (CW) modulation schemes, the carrier is a sinusoid. We use c ( t ) and m ( t ), to denote the carrier and the message waveforms respectively. The three parameters of a sinusoidal carrier that can be varied are: amplitude, phase and frequency. A given modulation scheme can result in the variation of one or more of these parameters. Before we look into the details of various linear modulation schemes, let us understand the need for modulation. Three basic blocks in anycommunication system are: 1) transmitter 2) Channel and 3) Receiver (Fig. 4.1).

Fig. 4.1: A basic communication system The transmitter puts the information from the source (meant for the receiver) onto the channel. The channel is the medium connecting the transmitter and the receiver and the transmitted information travels on this channel until it reaches the destination. Channels can be of twotypes: i) wired channels or ii) wireless channels. Examples of the first type include: twisted pair telephone

4.1
Indian Institute of Technology Madras

Principles of Communication

Prof. V. Venkata Rao

channels, coaxial cables, fiber optic cable etc. Under the wireless category, we have the following examples: earth’s atmosphere (enabling the propagation of ground wave and sky wave),satellite channel, sea water etc. The main disadvantage of wired channels is that they require a man-made medium to be present between the transmitter and the receiver. Though wired channels have been put to extensive use, wireless channels are equally (if not more) important and have found a large number of applications. In order to make use of the wireless channels, the information is to be convertedinto a suitable form, say electromagnetic waves. This is accomplished with the help of a transmitting antenna. The antenna at the receiver (called the receiving antenna) converts the received electromagnetic energy to an electrical signal which is processed by the receiver. The question is: can we radiate the baseband1 information bearing signal directly on to the channel? For efficientradiation, the size of the antenna should be λ 10 or more (preferably around λ 4 ), where λ is the wavelength of the signal to be radiated. Take the case of audio, which has spectral components almost from DC upto 20 kHz. Assume that we are designing the antenna for the mid frequency; that is,10 kHz. Then the length of the antenna that is required, even for the λ 10 situation is,

3 × 108 c = = 3 × 103meters, c being the velocity of light. 4 10 ⋅ f 10 × 10

1

Baseband signals have significant spectral content around DC. Some of the baseband signals

that are of interest to us are: a) Speech b) music and c) video (TV signals). Approximate spectral widths of these signals are: Speech: 5 kHz, Audio : 20 kHz, Video : 5 MHz

4.2
Indian Institute of Technology Madras

Principles ofCommunication

Prof. V. Venkata Rao

Even an antenna of the size of 3 km, will not be able to take care of the entire spectrum of the signal because for the frequency components around 1 kHz, the length of the antenna would be λ 100 . Hence, what is required from the point of view of efficient radiation is the conversion of the baseband signal into a narrowband, bandpass signal. Modulation processhelps us to accomplish this; besides, modulation gives rise to some other features which can be exploited for the purpose of efficient communication. We describe below the advantages of modulation.

1. Modulation for ease of radiation

Consider again transmission of good quality audio. Assume we choose the carrier frequency to be 1 MHz. The linear modulation schemes that would be discussed...