Sideband
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Key: fc is the carrier frequency, fm is the maximum modulation frequency
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing energy as a result of the modulation process. While all forms of modulation have sidebands by definition, it is most commonly discussed in amplitude modulation (AM).
Amplitude modulation of a carrier wave normally results in two mirror-image sidebands. The signal components above the carrier frequency constitute the upper sideband (USB) and those below the carrier frequency constitute the lower sideband (LSB). In conventional AM transmission, the carrier and both sidebands are present, sometimes called double sideband.
In some forms of AM the carrier may be removed, producing double sideband with suppressed carrier (DSB-SC). An example is the stereophonic difference (L-R) information transmitted in FM stereo broadcasting on a 38 kHz subcarrier. In this case the receiver locally regenerates the subcarrier by doubling a special 19 kHz pilot tone, but in other DSB-SC systems the carrier may be regenerated directly from the sidebands by a Costas loop or squaring loop. This is common in digital transmission systems such as BPSK where signal energy is continually present.
If part of one sideband and all of the other remain, it is called vestigial sideband, used mostly with television broadcasting, which would otherwise take up an unacceptable amount of bandwidth. Transmission in which only one sideband is transmitted is called single-sideband transmission or SSB. SSB is the predominant voice mode of HF Radio, particularly USB and sometimes LSB , (though LSB is used traditionally in amateur radio.)
In the case of SSB, a suppressed carrier is used, significantly reducing the amount of electrical power used (by up to 12 times), while still leaving the audio or other information present in the sideband. This makes for more efficient use of transmitter power and bandwidth with the tradeoff being that a beat frequency oscillator must be used at the receiver to reconstitute the carrier so the signal may be heard.
Sidebands are also what cause interference to adjacent channels, and therefore they must be suppressed by filters, either before or after modulation (or often both). In frequency modulation (FM), subcarriers above 75 kHz are suppressed to a small percentage of modulation, and are prohibited above 99 kHz altogether, in order to protect the ±75 kHz normal deviation and ±100 kHz channel boundaries.
[edit] See also
- Single-sideband modulation for more technical information about sideband modulation
[edit] References
- partly from Federal Standard 1037C in support of MIL-STD-188
- partly from Department of The Army Technical Manual TM 11-685 "Fundamentals of Single Sideband Communications"
