Modulation is a scheme that allows some signal, which exists in frequencies and power levels not suitable for transmission over long distances, to be transmitted to different receivers. There are different types of modulation employed in today’s communications equipment. For analog modulation, there are three basic types: amplitude modulation, frequency modulation and phase modulation.

Amplitude modulation stores the information(amplitude of the modulating signal) in the amplitude variations of the modulated signal. It manages this by using a mixer that results in the product of the modulating signal and carrier signal, which is usually at a higher frequency. Mixer outputs a carrier signal and two sidebands. A single sideband contains the information which is why some schemes usually involve filtering the other sideband and the carrier signal to save transmit power. Modulation can be characterized by the modulation index which is the ratio of the modulation amplitude to the carrier amplitude. Amplitude modulation indices range from 0(no modulation) to 1(100% modulation).

Phase modulation store information in the phase deviations that the modulated signals possess. Thus the modulating signal will appear inside the sinusoidal waveform function of the modulated signal to represent the phase difference. The modulation index of phase modulation is equal to the phase deviations of the modulated signal. Phase modulated signals are generated by using the Armstrong system, which makes use of a comparator whose inputs are the carrier signal entered into a phase shifter and the output of the mixer of the baseband and carrier signal.