FSK stands for Frequency Shift Keying; what does it change?

• A. Amplitude Modulation; it changes the amplitude
• B. Phase Shift Keying; it changes the phase
• C. Frequency Modulation; it changes the frequency
• D. Frequency Shift Keying; it changes the frequency to represent digital data

Answer: (D)

Frequency shift keying encodes digital data by switching the carrier between distinct frequencies. In the common binary form, two frequencies are used: one represents a 0 and the other a 1. The transmitter hops between these frequencies according to the data stream while keeping the amplitude and most often the phase steady, so the information is carried by the frequency state rather than the wave’s strength or timing of phase. The receiver detects which frequency is present to recover the bits.

This isn’t about changing amplitude (that would be amplitude modulation), nor about changing phase (that would be phase shift keying), and it isn’t the continuous frequency variation of frequency modulation. FSK deliberately uses discrete frequency steps to represent digital symbols.