**5.1 Amplitude shift keying**

The principle of amplitude shift keying is that the amplitude of the carrier wave is modulated in accordance with the digital message signal, i.e., ASK signal represents the binary data in the form of variations in the amplitude of the carrier signal. When an ON condition of digital pulse exists, then carrier will be switched ON, and when an OFF condition encounters, the carrier will be switched OFF. The time period for which the carrier is present or absent depends on the time interval for which the unipolar pulses are present.

Here the amplitude of carrier signal is varied to represent binary 1 and binary 0 data inputs, while the frequency and phase of the carrier signal remain constant. Voltage levels are left to designers of the modulation system (**Figure 20**).
