Types of Modulation

Amplitude Modulation (AM)

In Amplitude modulation (AM), the information signal is mixed with the carrier signal in such a way as to cause the AMPLITUDE of the carrier to vary at the frequency of the information signal.

Figure 6: Amplitude Modulation

Frequency Modulation (FM)

With frequency modulation, the modulating signal and the carrier are combined in such a way that causes the carrier FREQUENCY(fc) to vary above and below its normal(idling) frequency. The amplitude of the carrier remains constant as shown in figure below.

Figure 7: Frequency Modulation

As the voltage of the modulating signal increases in the positive direction from A to B, the frequency of the carrier is increased in proportion to the to the modulating voltage. As the voltage reduces from B to C, the frequency of the carrier decreases until it is back to the original value at C. the modulating voltage increases in the negative direction from C to , hence the carrier frequency decreases to a minimum value at D. It will start increasing again from D to E until it reaches its original value again. Thus in one cycle, the frequency has gone up and down (i.e. above and below the idling frequency) as mentioned earlier.

Throughout this process the amplitude of the carrier is not affected. The carrier frequency change above and below that of the unmodulated condition is proportional to sign and amplitude of the modulating signal.

Phase Modulation (PM or Indirect FM)

Phase modulation is a type of frequency modulation. Here, the amount of the carrier frequency shift is proportional to both the amplitude and frequency of the modulating signal.

The phase of the carrier is changed by the change in amplitude of the modulating signal as shown in Figure 8 below.

Figure 8: Phase Modulation

The modulated carrier wave is lagging the carrier wave when the modulating frequency is positive. This can be clearly seen if we concentrate on the peak amplitude along line AB (previous slide). When the modulating frequency is negative, the modulated carrier wave is leading the carrier wave. This can be seen clearly by looking at the peak amplitude along line CD (previous slide).