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Phase Modulation

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Phase Modulation
Phase Modulation “is used in many applications to carry both analogue and digital signals. Keeping the amplitude of the signal constant, the phase is varied to carry the required information or signal” (Poole, Radio-Electronics.com). An advantage of phase modulation is that it is easier compared to Frequency modulation. A Phase modulator is used in determining velocity of moving targets by extracting Doppler information. Doppler information needs a stable carrier which is possible in phase modulation but not in frequency modulation. A disadvantage of Phase modulation is that a frequency multiplier is needed in order to increase Phase modulation.
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation or QAM is a form of modulation which is widely used for modulating data signals onto a carrier used for radio communications. It is widely used because it offers advantages over other forms of data modulation such as PSK, although many forms of data modulation operate alongside each other.
“QAM is a signal in which two carriers shifted in phase by 90 degrees are modulated and the resultant output consists of both amplitude and phase variations. In view of the fact that both amplitude and phase variations are present; it may also be considered as a mixture of amplitude and phase modulation” (Poole, Radio-Electronics.com). There are more disadvantages with QAM then there are advantages. One disadvantage is it is more susceptible to noise because the states are closer together so that a lower level of noise is needed to move the signal to a different decision point. Also, “when a phase or frequency modulated signal is amplified in a radio transmitter, there is no need to use linear amplifiers, whereas when using QAM that contains an amplitude component, linearity must be maintained. Unfortunately linear amplifiers are less efficient and consume more power, and this makes them less attractive for mobile applications” (Poole, Radio-Electronics.com).
Reference
Poole, Ian. (203). QAM-What is Quadrature Amplitude Modulation. Retrieved from http://www.radio-electronics.com

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