Difference between ASK, FSK & PSK:

This page on ASK vs FSK vs PSK provides difference between ASK, FSK, PSK modulation types. All these are digital modulation techniques. Unlike Analog modulation, here input is in digital binary form. The other input is the RF carrier. Input binary data is referred as modulating signal and output is referred as modulated signal.

ASK:

The short form of Amplitude Shift Keying is referred as ASK. It is the digital modulation technique. In this technique, amplitude of the RF carrier is varied in accordance with baseband digital input signal. The figure depicts operation of ASK modulation. As shown in the figure, binary 1 will be represented by carrier signal with some amplitude while binary 0 will be represented by carrier of zero amplitude(i.e. no carrier).

Fig.1 ASK

ASK modulation can be represented by following equation: 
s(t) = A* cos(2*π*fc*t) for Binary 1
s(t) = 0 for Binary 0

FSK:

The short form of Frequency Shift Keying is referred as FSK. It is also digital modulation technique. In this technique, frequency of the RF carrier is varied in accodance with baseband digital input. The figure depicts the FSK modulation. As shown, binary 1 and 0 is represented by two different carrier frequencies. Figure depicts that binary 1 is represented by high frequency 'f1' and binary 0 is represented by low frequency 'f2'.

Fig.2 FSK

Binary FSK can be represented by following equation: 
s(t) = A* cos(2*π*f1*t) for Binary 1 
s(t) = A* cos(2*π*f2*t) for Binary 0 

PSK:

The short form of Phase Shift Keying is referred as PSK. It is digital modulation technique where in phase of the RF carrier is changed based on digital input. Figure depicts Binary Phase Shift Keying modulation type of PSK. As shown in the figure, Binary 1 is represented by 180 degree phase of the carrier and binary 0 is represented by 0 degree phase of the RF carrier.

Fig.3 PSK

Binary PSK can be represented by following equation : 
If s(t) = A*cos(2*π*fc*t) for Binary 1 than 
s(t) = A*cos(2*π*fc*t + π) for Binary 0