Also known as: carrier wave, carrier
A carrier wave is a steady radio-frequency signal at a single frequency that conveys no information on its own.1 It becomes useful only when modulation varies one of its properties in step with a message.
How it works
An unmodulated carrier appears on a spectrum display as a single narrow spike. Modulation spreads energy into sidebands around it, and the width of those sidebands is essentially the signal’s bandwidth.
Relevance to SDR
Receivers tune to a carrier’s frequency and then demodulate the variations around it. A residual carrier at zero frequency after downconversion is the familiar “DC spike” seen on SDR spectra.
Sources
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Carrier wave — Wikipedia, on the steady reference signal modulated to convey information. ↩