unmodulated

Unmodulated is an adjective used to describe a signal, device, or system that carries no modulation. In communications, it refers to a signal whose amplitude, frequency, or phase remains constant, so no information is encoded on the carrier. The term contrasts with modulated signals in which information is conveyed by varying one or more parameters of the carrier.

Radio engineering: an unmodulated carrier is a pure, continuous wave that can be transmitted or generated for testing purposes. It serves as a reference for calibrating receivers, measuring channel effects, and evaluating receiver linearity and noise performance. Transmitters may emit an unmodulated carrier during maintenance or when the modulator is disabled.

In measurement and testing, unmodulated signals provide a baseline against which modulated signals are compared. They help characterize equipment response to a known, constant input and isolate distortion or interference introduced by modulation stages.

In optical communications, lasers can emit an unmodulated optical carrier when not sending data. Such a carrier is used as a stable reference for dispersion testing, channel allocation in wavelength-division multiplexing, or as a local oscillator in coherent detection.

In music and acoustics, the term unmodulated can describe a work or passage that remains in a single key without modulation to a different key.

Other uses include instrumentation and control, where an unmodulated signal serves as a baseline or DC reference, helping to identify offsets, drifts, and static errors in sensors and measurement chains.