We already discussed modulation and in particular- the amplitude modulation. but when it comes to practical applications it is frequency modulation, which is preferred. However, the theory and generation of FM is not as simple as AM. FM is a complex process because it involves minute frequency variations of the carrier whereas in AM we produce large-scale amplitude variations. There are some more differences between the two which we shall discuss after going in details of FM.

In frequency modulation, only the instantaneous frequency of carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal, however the amplitude of the carrier remains constant. The amount of deviation in frequency is proportional to the amplitude of the signal while the rate is determined by the frequency of modulation source. The frequency of modulated carrier increases as the signal amplitude increases but decreases as the signal amplitude decreases.

When the signal amplitude is zero, the carrier frequency is at its normal frequency. for example, when the signal approaches its positive peaks at band H, the F-M carrier frequency increases to maximum (High) as shown by closely spaced lines. On the other hand, during the negative peak, of signal as at L Band, the F-M carrier frequency reduces to minimum (Low) as shown by widely spaced lines.

(1) ADVANTAGES OF FREQUENCY MODULATION

(a) It gives high-fidelity reception.

(b) It gives gives noiseless reception. As discussed before, noise is a form of amplitude variations and a FM receiver will reject such signals.

(c) The efficiency of transmission is very high.

(d) The operating range is quite large.

                             LIMITATION OF FREQUENCY MODULATION

(a) Signal reception is very small.

(b) Implementation of FM set up cost is very high.

(c) It requires much wider channel.

Amplitude modulation is defined as a system of modulation in which the amplitude of carrier is made proportional to the instantaneous amplitude of modulating voltage.

Hence, in AM the instantaneous modulating voltage variations are superimposed on the carrier amplitude. as shown in fig b, when there is temporarily no modulation, the amplitude of carrier is equal to its un-modulated value.

(a)

(b)

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the amplitude of carrier is varied by the instantaneous value when the modulation is present. the amplitude of “ the top envelope “ of the AM wave is given by the relation

A = V_c + V_m SIN W_m*T.

Similarly the maximum negative amplitude of the bottom envelope is given by

–A = –(V_c + V_m SIN W_m*T).

This is shown in fig c where one cycle of the modulating sine-wave is shown, it extends between two limiting envelopes and has a repetition rate equal to the un-modulated carrier frequency. It is mention that the amplitude of both positive and negative half cycles of carrier wave are changed in accordance with signal. for the instant when the signal is increasing in positive sense, the amplitude of carrier wave also increases. On the other hand,during negative half cycle of the signal, amplitude of carrier wave decreases. amplitude modulation is done by an electronic equipment called modulator. In short we can say that;

1. According to the intensity of the signal, the amplitude of carrier wave changes.
2. Modulating wave or signal frequency f_m governs the amplitude variations of the carrier wave.
3. the frequency of amplitude modulated wave remains same to the carrier frequency.

The following factors responsible for the need of modulation are:

(a) PRACTICAL ANTENNA LENGTH: We know that lingth (l) of transmitting and receiving antenna is given by

                   l =(1/4) wave – length = [ 1/4 * velocity/frequency(in hertz) ] meters

Now, for audio frequency range (20hz to 20khz) antenna length can be calculated as

                   l = 1/4 * (3*10⁸)/(20*10³) = 3750 meters

Hence to transmit an audio frequency of 20khz an antenna of 3750 meters is required which is practically impossible. Therefore, if a carrier wave of say 1Mhz is used to carry the signal, we need an antenna of about 75 meters that can be easily constructed.

(b) OPERATING RANGE: The energy of any wave depends on its frequency. so if the frequency is more, the energy possessed by it is also more. the audio signal with high frequency can be transmitted over large distances. therefore, a high frequency wave as the carrier wave is needed.

(c) To remove interference and reduction of noise: when a signal is sent using high frequency carrier, there is less distortion of the signal and less noise in the received signal. also using several modulation techniques, the effect of noise can be minimized.

(d) NO MIXING OF AUDIO SIGNALS: In modulation technique each audio signal is carried by a separate high frequency carrier wave and therefore no mixing of audio signals occurs.

The process of varying one of the characteristics of a high frequency wave (carrier) in accordance with the instantaneous value of a modulating wave (low frequency signal) is known as modulation.

the audio frequency signal is also called modulating wave and resultant wave produced is called modulated wave.

let the carrier wave expressed as

e =  E_msin(wt+Ø)

here, e is the instantaneous value of carrier wave

E_m is the maximum value of the carrier wave

Ω is the angular frequency

Ø is the phase relation with respect to some reference.

Since there are three variables in the equation, therefore three types of modulation possible which are described below.

TYPES OF MODULATION : Different types of modulation are as follows:

(a) AMPLITUDE MODULATION

The process in which the amplitude of sinusoidal carrier wave is varied in accordance with the modulating signal, keeping the frequency and phase of the carrier wave constant, is known as amplitude modulation.

(b) FREQUENCY MODULATION

The process in which the frequency of carrier wave is varied in accordance with the modulating, signal, keeping amplitude & phase of the carrier wave constant is known as frequency modulation.

(c) PHASE MODULATION

the process in which the phase (Ø) of carrier wave is varied in accordance with the modulating signal, keeping the frequency & amplitude of carrier wave constant is known as phase modulation.

In India, in T.V. transmission, frequency modulation is used for sound signal and amplitude modulation for picture signal. However, amplitude modulation is used in radio transmission.