Modulation: For long distance transmission, the low frequency baseband signal input signal) is superimposed onto a high frequency radio signal by a process called modulation There are 3 types of modulation based on which parameter is modified. They are (1) Amplitude modulation, (ii) Frequency modulation, and (iii) Phase modulation.
(i) Amplitude Modulation (AM): If the amplitude of the carrier signal is modified according to the instantaneous amplitude of the baseband signal, then it is called amplitude modulation. Here the frequency and the phase of the carrier signal remain constant. Amplitude modulation is used in radio and TV broadcasting.
The signal shown in figure (a) is the me ation figure (b) shows the high frequency carrier signal and figure (c) amplitude modulated sional We can see clearly that the carrier Wave is modifi proportion to the amplitude of the baseband signal. information figure (b) show amplitude modulated signal
(a) Amplitude
(ii) Frequency Modulation (FM): The frequency of the carrier signal is modifiedFrequency Modulation according to the instantaneous amplitude of the baseband signal in frequency modulation. A Baseband Signal C (a) Here the amplitude and the phase of the carrier signal remain constant. Increase in the amplitude of the baseband signal increases the frequency of the carrier signal and vice versa. Amplitude This leads to compressions and rarefactions in the frequency spectrum of the modulated wave. Louder signal leads to compressions and relatively weaker signals to rarefactions. When the amplitude of the baseband signal is zero in Figure (a), the frequency of the modulated signal is the same as the carrier signal. The frequency of the modulated wave increases when the amplitude of the baseband signal increases in the positive direction (a) baseband signal (bittet signal A. C). The increase in amplitude in the c) frequency modulated signal ative half cycle (B, D) reduces the frequency of the modulated wave (Figure(c)).
(iii) Phase Modulation (PM)
The instantaneous amplitude of the baseband signal modifies the phase of the carrier signal keeping the amplitude and frequency constant is called phase modulation. This modulation is used to generate frequency modulated signals. It is similar to frequency modulation except that the phase of the carrier is varied instead of varying frequency
The carrier phase changes according to increase or decrease in the amplitude of the baseband signal. When the modulating signal goes positive, the amount of phase lead increases with the amplitude of the modulating signal. Due to this, the carrier signal is compressed or its frequency is increased.
On the other hand, the negative half cycle of the baseband signal produces a phase lag in the carrier signal. This appears to have stretched the frequency of the carrier wave. Hence similar to frequency modulated wave, phase modulated wave also comprises of compressions and rarefactions. When the signal voltage is zero (A, C and E) the carrier frequency is unchanged.
