Which modulation changes phase to represent digital data?

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

Answer: (B)

Phase shifts carry the digital information. In Phase Shift Keying, the carrier’s phase is shifted to represent bits, while the amplitude stays essentially constant. Each symbol maps to a specific phase angle, so data is conveyed by where the phase sits. For example, two phases can encode a binary digit, while more phases can encode more bits per symbol. This focus on altering phase distinguishes PSK from other methods.

The other options change amplitude or frequency instead of phase: Amplitude Modulation and Amplitude Shift Keying vary the signal’s strength, and Frequency Modulation changes the carrier’s frequency. Those approaches encode data in amplitude or frequency, not in phase.