**5.2 Frequency shift keying**

The principle of frequency shift keying is that the frequency of the carrier wave is modulated in accordance with the digital message signal, i.e., FSK signal represents the binary data in the form of variations in the frequency of the carrier signal. When an ON condition of digital pulse exists, then carrier will be switched to one frequency, and when an OFF condition encounters the carrier, it will be switched to another frequency, i.e., in "frequency shift keying," the frequency of a sinusoidal carrier is shifted between two discrete values (**Figure 21**).
