This diagram shows three digitally modulated waveforms labelled A to C. Time periods are represented by a series of equidistant dotted vertical lines. The X or horizontal axes are labelled as time, measured in seconds. The Y or vertical axes are labelled as voltage, measured in volts.

In part A, a sinusoidal signal of amplitude 1 volt is shown for three time periods, then the phase changes for one time period, before reverting back to the original phase for one time period. The sinusoid then changes phase again for three time periods.

In part B, the sinusoidal signal has a small amplitude of about 0.5 volt for four time periods, then rises to an amplitude of 1 volt for two time periods. It then reverts to the smaller amplitude for one time period, and back to the higher amplitude for one time period.

In part C, a sinusoidal signal of amplitude 1 volt has a relatively high frequency for three time periods. Then the frequency lowers for two time periods, before reverting back to the original frequency for one time period. The sinusoid then changes to the lower frequency for a single time period, then back to the higher frequency for a single time period.

1.6 Phase-shift keying (PSK)