Analog Modulation Schemes

Analog Modulation Schemes

As discussed in the last post that the basic idea of analog modulation is to superimpose / ride an analog message signal over a periodic usually a sinusoidal signal to transmit the data/ information over longer distances. Modulator takes two inputs

1. a carrier
2. and a message signal

Then it generates a modulated signal at output. see the figure below

Mathematically it is expressed as
 carrier signal  

the carrier signal can be written in one of the forms shown above. In essence, a sender must change one of the fundamental characteristics of the wave to produce the modulated signal. 

There are three primary techniques that modulate an electromagnetic carrier according to a signal: 



Amplitude Modulation ,Frequency Modulation, Phase Modulation .The first two methods of modulation are the most familiar and have been used extensively

Amplitude Modulation (AM)

AM varies the amplitude of a carrier in proportion to the information being sent (i.e., according to a signal).The carrier continues oscillating at a fixed frequency, but the amplitude of the wave varies
Figure illustrates

1. an unmodulated carrier wave
2. an analog information signal

and the resulting signal is amplitude modulated (AM) signal

As it is seen from the figure only the amplitude (i.e., magnitude) of the sine wave is modified
a time-domain graph of a modulated carrier has a shape similar to the information/data signal that was used
imagine an envelope consisting of a curve that connects the peaks of the sine wave in Figure part c
the resulting curve has the same shape as the signal in Figure part b

A

Frequency Modulation (FM)

In FM, the amplitude of the carrier remains fixed but the frequency changes according to the information carrying message signal.When the signal is stronger, the carrier frequency increases slightly,and when the signal is weaker, the carrier frequency decreases slightly.

Figure clearly illustrates an example of Frequency Modulated signal for an information signal
FM is more difficult to visualize because slight changes in frequency are not as clearly visible
However, one can notice that the modulated wave has higher frequencies when the signal used for modulation is stronger, and different varying frequencies are clearly visible in the given figure to give you an idea how FM signal looks like.

Phase Modulation (PM)

One of the property of a sine wave is its phase, the offset from a reference time at which the sine wave begins. It is possible to use changes in phase to represent a  data signal.We use the term phase shift to characterize such changes.If phase changes after cycle k, the next sine wave will start slightly later than the time at which cycle k completes.A slight delay resembles a change in frequency .PM can be thought of as a special form of frequency modulation.As discussed previously both FM and PM are different types of angle modulation.

However, phase shifts are important when a digital signal is used to modulate a carrier