Analog modulation

Modulation is the process of varying a property of a high-frequency carrier wave in step with a lower-frequency information signal, so that the information can be carried efficiently over a radio channel. A bare audio or data signal occupies low frequencies that radiate poorly and cannot share the spectrum with other transmissions; by shifting it up to a chosen carrier frequency, many signals can coexist and antennas of practical size can be used. At the receiver the reverse process, demodulation, recovers the original information.

The two classic analog schemes are amplitude modulation and frequency modulation. In amplitude modulation (AM) the strength of the carrier is varied in proportion to the message, producing sidebands above and below the carrier frequency. Variants such as single-sideband (SSB) and double-sideband (DSB) suppress part of this signal to save power or bandwidth. In frequency modulation (FM) the carrier amplitude stays constant while its instantaneous frequency is shifted by the message; this makes FM more resistant to amplitude noise, which is why it is favoured for high-fidelity broadcasting.

Practical transmitters and receivers build these schemes from a few key blocks. A mixer multiplies signals to translate them between frequencies, an intermediate-frequency (IF) amplifier provides selective gain at a fixed frequency, and quadrature or other demodulators extract the message. The same modulation principles underpin radio broadcasting, two-way radio, radar and many other systems that send information through the electromagnetic spectrum.

Frequently asked questions

What is the difference between AM and FM?

AM varies the amplitude of the carrier with the message while keeping frequency fixed; FM varies the carrier frequency while keeping amplitude constant. FM is generally more immune to amplitude noise.

Why is single-sideband used?

An AM signal carries the same information in both sidebands. Transmitting only one sideband (SSB) saves transmitter power and uses roughly half the bandwidth, which is valuable for long-distance voice communication.

What does a mixer do in a radio?

A mixer multiplies the incoming signal with a local oscillator to shift it to a fixed intermediate frequency, where it can be filtered and amplified efficiently before demodulation.