Sideband

Lore & Background

The creation of sidebands can be illustrated using the trigonometric identity cos(A)·cos(B) ≡ ½ cos(A+B) + ½ cos(A−B). Adding cos(A) to both sides and substituting A ≜ 1000·t and B ≜ 100·t shows that amplitude modulation of a carrier wave produces an upper sideband at ½ cos(1100 t), the carrier at cos(1000 t), and a lower sideband at ½ cos(900 t). Adding more complexity and time-variation to the amplitude modulation widens the sidebands and changes them over time, as the sidebands carry the information content of the signal.

In conventional AM transmission, as used by broadcast band AM stations, the original audio signal can be recovered by synchronous detector circuits or simple envelope detectors because the carrier and both sidebands are present. This is sometimes called double sideband amplitude modulation (DSB-AM). In some forms of AM, the carrier may be reduced or removed completely, producing double sideband with suppressed carrier (DSB-SC). Suppressed carrier systems require more sophisticated receiver circuits and methods to deduce the original carrier frequency, such as a Costas loop or squaring loop.

If part of one sideband and all of the other remain, it is called vestigial sideband, used mostly with television broadcasting. Transmission in which only one sideband is transmitted is called single-sideband modulation (SSB). SSB is the predominant voice mode on shortwave radio other than shortwave broadcasting. In SSB, the carrier is suppressed, significantly reducing electrical power by up to 12 dB without affecting the information in the sideband, but a beat frequency oscillator must be used at the receiver to reconstitute the carrier.

Reader's Guide

Sidebands are fundamental to all forms of modulation, as they carry the transmitted information. How amplitude modulation of a carrier normally results in two mirror-image sidebands, and the bandwidth required for transmission is directly related to the audio bandwidth—for example, a 7 kHz audio signal requires a 14 kHz radio spectrum bandwidth. The ability to reduce or suppress the carrier in double sideband systems allows for power savings, though it requires more complex receiver circuits such as Costas loops or squaring loops to regenerate the carrier. Single-sideband modulation, which transmits only one sideband and suppresses the carrier, is particularly significant for shortwave voice communication because it makes efficient use of transmitter power and RF bandwidth. In SSB, if the reconstituted carrier frequency at the receiver is wrong, the output will have wrong frequencies, but for speech small frequency errors do not affect intelligibility. Vestigial sideband is used in television broadcasting to reduce bandwidth usage. The sideband concept is also applied in stereo FM broadcasting, where a 38 kHz subcarrier carries stereophonic difference information, and a 19 kHz pilot tone is used to regenerate the subcarrier at the receiver.

Did You Know?

• All forms of modulation produce sidebands.
• In amplitude modulation, a 900 kHz carrier modulated by a 1 kHz audio signal produces components at 899 kHz and 901 kHz.
• Single-sideband modulation (SSB) reduces electrical power by up to 12 dB by suppressing the carrier.
• Vestigial sideband is used mostly with television broadcasting.