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 — amplitude, frequency, or phase — in step with a message. The carrier is the vehicle; the message rides on it. Its frequency is what you tune a receiver to, and it is why “the frequency of a station” and “its carrier” mean nearly the same thing.
How it works
A pure, unmodulated carrier is a single sinusoid, so in the frequency domain it appears as one narrow spike — all of its energy at exactly one frequency. The moment it is modulated, that energy spreads: the message frequencies mix with the carrier and produce sidebands, mirror-image bands of energy above and below the carrier frequency. The total span the sidebands occupy is essentially the signal’s bandwidth (more precisely its occupied bandwidth), and it grows with the rate and depth of the modulation. A voice AM signal spreads a few kilohertz either side of its carrier; a wideband FM broadcast, hundreds of kilohertz.
Why use a carrier at all? Two reasons. First, an antenna is only efficient at a size comparable to the wavelength, and baseband audio (a few kilohertz) has wavelengths of tens of kilometres — impractical to radiate. Shifting the message up onto a high-frequency carrier makes it radiatable from a reasonable antenna. Second, carriers let many stations share the spectrum: each occupies its own carrier frequency with a guard band between them, and a receiver picks one out by tuning. Some systems suppress the carrier itself to save power (single sideband transmits only one sideband and no carrier), trading receiver complexity for efficiency.
In practice
- Spotting a carrier. On a waterfall or spectrum analyzer, an idle transmitter shows a thin bright line; keying up blooms it into a modulated band.
- Carrier squelch. The simplest squelch opens the audio when a carrier of sufficient strength is present, regardless of content.
- Subcarriers. A modulated carrier can itself carry a second, lower-frequency subcarrier — how FM broadcast fits stereo and RDS data alongside the main audio.
Relevance to SDR
After an SDR mixes a chosen slice of spectrum down toward baseband, the tuned carrier ends up near zero frequency. A residual carrier or DC offset sitting at exactly zero produces the familiar bright “DC spike” in the centre of an SDR’s spectrum — an artifact, not a real station. GopherTrunk locks onto each channel’s carrier to establish a phase and frequency reference, then demodulates the variations around it; the digital voice systems it decodes keep the carrier present and modulate its phase, so accurate carrier tracking is the foundation of the whole decode chain.
Sources
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Carrier wave — Wikipedia, on the steady reference signal modulated to convey information. ↩