constantamplitude

Constant amplitude refers to a signal whose instantaneous magnitude remains fixed over time. In the common complex (baseband) representation, a constant-amplitude signal can be written as x(t) = A e^{jφ(t)}, where A is a fixed amplitude and φ(t) is the instantaneous phase. For real-valued signals, constant amplitude means the absolute value |s(t)| is constant, for example s(t) = A cos(ωt + φ) has amplitude A.

In communications and signal processing, constant amplitude is closely related to constant envelope signals. Such signals maintain the same magnitude while allowing the phase to vary, which is advantageous for use with nonlinear RF power amplifiers because it reduces amplitude distortion and improves efficiency. Modulation schemes that preserve constant envelope include phase-shift keying (PSK) variants, such as BPSK and QPSK, where information is encoded in the phase while the amplitude remains fixed. Some frequency-modulated signals and other modulation formats can also be designed to approximate a constant envelope.

The concept is contrasted with amplitude modulation (AM), where the signal’s magnitude carries information and the envelope varies in time. Constant-amplitude approaches trade potential increases in data rate or spectral efficiency for robustness to nonlinear amplification and simpler envelope handling.

Related ideas include constant-envelope signaling, modulus constraints in optimization, and envelope detection, all of which concern maintaining or exploiting a fixed signal magnitude in various contexts.