Phase Modulation

Phase Modulation

It is very similar to FM except that the phase of the carrier is varied and not the frequency. As the modulating signal goes positive, the amount of phase lag increases with an increase in the amplitude of the modulating signal. The effect is that the carrier signal is stretched or its frequency is reduced. When the modulating signal goes negative, the phase shift becomes leading. This causes the carrier to be compressed and hence the frequency increases. Thus phase modulation produces frequency modulation. The below fig.(a) shows the modulating signal fig.(b) shows FM and fig.(c) shows PM.




PSK: Phase Shift Keying
Comparison of AM, FM, PM


                        FM 
                      AM
1.
Amplitude of FM wave is constant. It is independent of the modulation index.
Amplitude of AM wave will change with the modulating voltage.
2.
Transmitted power remains constant and is independent of modulation index.
Transmitted power is dependent on modulation index.
3.
All of transmitted power is useful.
Power of Carrier and one sideband is of no use.
4.
Receivers are immune to noise.
Receivers are susceptible to noise.
5.
Bandwidth depends on modulation index.
Bandwidth is independent of modulation index.
6.
Space Wave is used for propagation. So radius of transmission is limited to line of sight.
Large area is covered as ground Wave and Sky Wave propagation is used.
7.
It is possible to operate several transmitters on the same frequency.
Not possible to operate multiple channels on same frequency.
8.
Number of sidebands having significant amplitudes depends on modulation index.
Total two sidebands present.

Sr No
                              FM
                                    PM
1.
Frequency deviation is proportional to the modulating voltage.
Phase deviation is proportional to the modulating voltage.
2.
Modulation index is proportional to modulating voltage and modulating frequency.
Modulation index is proportional to modulating voltage only.
3.
SNR is better than that of PM.
SNR is inferior than that of FM.
4.
Noise Immunity better than in AM and PM.
Noise immunity is better than AM but worse than FM.
5.
Frequency deviation is proportional to modulating voltage only.
Frequency deviation  is proportional to modulating voltage and modulating frequency.


Application of Analog Modulation



Abbreviations used above: AM(Amplitude Modulation), FM(Frequency Modulation),DSB(Double Sideband),VSB(Vestigial Sideband),FSK(Frequency Shift Keying),PSK(Phase Shift Keying),SSB(Single Sideband),QAM(Quadrature Amplitude Modulation)


 FM Radio: FM Technique was invented by Mr Edwin Armstrong in 1933 and the first station was setup by him in 1939 in New Jersey (USA) using a 35kW transmitter. It was first introduced in India in 1977 by installing a 10kW transmitter at Chennai. FM Broadcasting Industry was made open to private players in India in year 2000. The FM radio band is from 88 to 108 MHz .




Ref :- https://en.wikipedia.org/wiki/FM_broadcasting#/media/File:Armstrong_FM_radio_station_W2XMN_Alpine_NJ_1940.jpg


AM Radio : The Amplitude Modulated (AM radio) carrier frequencies are in the frequency range 535-1605 kHz. Carrier frequencies of 540 to 1600 kHz are assigned at 10 kHz intervals.
TV Broadcasting: C3F is used for transmission (VSB).A vestige which is a trace of the unwanted sideband is transmitted usually with a full carrier. Video signals require large bandwidth if they are transmitted using DSBSC hence VSB is used.
Navigation: GPS uses the Binary Phase Shift Keying (BPSK) to modulate data onto the carrier signal.


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