9.5 ANALOG MODULATION TECHNIQUES

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9.5 ANALOG MODULATION TECHNIQUES

Analog modulation is used extensively in broadcasting of audio and video programs, and many old telecommunication systems are also based on analog modulation. All newly developed systems use digital modulation. For broadcasting, analog modulation continues to play a vital role.

9.5.1 Amplitude Modulation

In AM, the amplitude of the carrier is proportional to the instantaneous amplitude of the modulating signal. The frequency of the carrier is not changed.

In Figure 9.2(a), the modulating signal (a sine wave) is shown. Figure 9.2(b) shows the amplitude-modulated signal. It is evident from this figure that the carrier's amplitude contains the information of the modulating signal. It can also be seen that both the upper portion and the lower portion of the carrier amplitude contain the information of the modulating signal.

Figure 9.2: (a) Modulating signal.

Figure 9.2: (b) Amplitude modulated signal.

If f_c is the frequency of the carrier, the carrier can be represented mathematically as

A sin(2πf_ct)

If f_m is the frequency of the modulating signal, the modulating signal is represented by

B cos(2πf_mt)

The amplitude-modulated wave is represented by

(A + B cos2πf_mt) sin(2πf_ct) or

A{1 + (B/A) cos(2πf_mt)} sin(2πf_ct)

The value of B/A denoted by m is called the modulation index. The value of (m × 100) is the modulation index, represented as a percentage.

On expanding the above equation, we get three terms: one term with f_c, the second term with (f_c + f_m), and the third term with (f_c − f_m).

The terms with (f_c + f_m) and (f_c − f_m) represent sidebands. Both the sidebands contain the information of the modulating signal. The frequency component (f_c + f_m) is called the upper sideband, and the frequency component (f_c + f_m) is called the lower sideband. Only the upper sideband or lower sideband can be transmitted and demodulated at the receiving end.

The bandwidth required for AM is twice the highest modulating frequency. If the modulating frequency has a bandwidth of 15kHz (the bandwidth used in audio broadcasting), the amplitude-modulated signal requires 30kHz.

It is very easy to implement the modulator and demodulator for AM. However, as the amplitude-modulated signal travels on the medium, noise gets added, and the amplitude changes; hence, the demodulated signal is not an exact replica of the modulating signal; i.e., AM is not immune to noise. Another problem with AM is that when the AM waves are transmitted, the carrier will take up most of the transmitted power, though the carrier does not contain any information; the information is present only in the sidebands.

Amplitude modulation is used in audio broadcasting. Different audio programs are amplitude modulated, frequency division multiplexed, and transmitted over the radio medium. In video broadcasting, a single sideband is transmitted.

The bandwidth required for AM is twice the highest modulating frequency. In AM radio broadcasting, the modulating signal has bandwidth of 15kHz, and hence the bandwidth of an amplitude-modulated signal is 30kHz.

9.5.2 Frequency Modulation

In frequency modulation (FM), the frequency of the carrier is varied according to the amplitude of the modulating signal. The frequency deviation is proportional to the amplitude of the modulating signal. The amplitude of the carrier is kept constant.

Figure 9.2(c) shows the frequency-modulated signal when the modulating signal is a sine wave as shown in Figure 9.2(a).

Figure 9.2: (c) Frequency modulated signal.

If the carrier is represented by

and the modulating signal is represented by

the instantaneous frequency of the frequency-modulated carrier is given by

where k is a proportionality constant.

In frequency modulation, the frequency deviation of the carrier is proportional to the amplitude of the modulating signal.

According to Carlson's rule, the bandwidth of an FM signal is twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is 180kHz.

As compared to AM, FM implementation is complicated and occupies more bandwidth. Because the amplitude remains constant, FM is immune to noise.

Many audio broadcasting stations now use FM. In FM radio, the peak frequency deviation is 75kHz, and the peak modulating frequency is 15kHz. Hence the FM bandwidth requirement is 180kHz. The channels in FM band are separated by 200kHz. If we compare the quality of audio in AM radio and FM radio, we can easily make out that FM radio gives much better quality. This is because FM is more immune to noise as compared to AM. FM also is used to modulate the audio signal in TV broadcasting.

The bandwidth of the frequency-modulated signal is twice the sum of the modulating signal frequency and the frequency deviation.

Note 

In FM radio broadcasting, the peak frequency deviation is 75kHz, and the peak modulating signal frequency is 15kHz. Hence the bandwidth of the modulated signal is 180kHz.

9.5.3 Phase Modulation

In phase modulation, the phase deviation of the carrier is proportional to the instantaneous amplitude of the modulating signal. It is possible to obtain frequency modulation from phase modulation. The phase modulated signal for the modulating signal shown in Figure 9.2(d) looks exactly same as Figure 9.2(c).

Figure 9.2: (d) Phase modulated signal.

In phase modulation, the phase deviation of the carrier is proportional to the instantaneous amplitude of the modulating signal. No practical systems use phase modulation.

No practical systems use phase modulation.

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