See modulation and carrier. Vary the Amplitude In AM modulation, the voltage (amplitude) of the carrier is varied by the incoming signal. In this example, the modulating wave implies an analog signal.

Our 4-bit example results in a 16-QAM signal “constellation” of 16 symbols (16 = 24), forming a 16-point grid when viewed on a modulation analyzer. Typical digital transmission systems use a 16-QAM, ...

For example, a standard broadcast amplitude modulation signal looks like Figure 18 where the high frequency carrier signal always has an envelope that looks like the low frequency modulating signal.