Which modulation technique refers to changes in phase?

Phase Shift Keying (PSK) is the correct modulation technique that refers to changes in phase. In PSK, the phase of a carrier signal is varied to represent data. In digital communications, this technique is commonly used to transmit binary data, where different phases correspond to different bit values. For example, a phase of 0 degrees might represent a binary '1', while a phase of 180 degrees might represent a binary '0'.

The effectiveness of PSK lies in its ability to utilize the phase of the signal to encode information, which can allow for more efficient use of bandwidth and greater resistance to noise compared to other modulation techniques. This makes it particularly valuable in varied communication systems, including satellite communication and wireless networks.

In contrast, Frequency Shift Keying modulates the frequency of the carrier signal, Amplitude Shift Keying modulates the amplitude, and Time Shift Keying is not a widely recognized standard for modulation in the context of phase changes. Therefore, the other options do not align with the definition of modulation based on phase variations.