Analog modulation: The carrier signal sent in analog radio transmission is simply a continuous electrical signal. It carries no information and is referred to as a CW. Only when the CW is modulated is it called a carrier. Analog modulation is the representation of analog information by an analog signal. There are three types of modulation that can be applied to a analog signal to enable it to carry information. The height of the signal, the frequency of the signal and the relative starting point, or phase of the signal.
Amplitude Modulation (AM): the height of a wave, know as amplitude, can be measured in volts (electrical pressure) . In Amplitude modulation (AM) the height of the wave is changed in accordance with the height of another signal, called the modulating signal. AM is very susceptible to interference from outside sources such as lighting , it is general not used for data transmissions.
Figure: Amplitude of a signal
Figure: Amplitude modulation(AM)
Frequency Modulation (FM): In Frequency modulation (FM) , the number of waves that occur during one second undergoes change based on the amplitude of the modulating signal while the amplitude and the phase of the carrier remain constant.
FM is not as susceptible to interference from outside sources and is most commonly used to broadcast radios programs. An FM carrier has a wider bandwidth, which allows it to carry Hi-Fi as well as stereophonic signals, with two separate sound channels.
Figure: Frequency modulation (FM)
Phase Modulation: In contrast to AM, which changes the height of the wave, and FM which increase the number of waves per cycle, phase modulation (PM) changes the starting point of the cycle, while the amplitude and frequency of the carrier remain constant. Phase modulation is not generally used to represent analog signals.
A signal composed of sine waves has a phase associated with it. The phase is measured in degrees and one complete wave cycle covers 360 degrees. A phase change is always measured with reference to some other signal.
J. L. Olenewa (2014). Guide to Wireless Communications, ( Third Edition). Boston:CENGAGE Learning