Single-Sideband Modulation(redirected from Quadrature modulator)
single-sideband modulation[′siŋ·gəl ¦sīd‚band ‚mäj·ə′lā·shən]
a method of controlling electrical waves by transmitting an information signal on only one sideband of frequencies. This process is used mainly in single-sideband communication, radio-telemetry, radio control, and television.
With amplitude modulation, information is ordinarily contained in each of two sidebands:
ωc – (Ωl/Ωu) and ωc + (Ωl/Ωu)
With single-sideband modulation, however, the carrier frequency (carrier wave) and the frequencies of one sideband are usually suppressed (see Figure 1). As a result, the frequency band filled by the signal is cut to approximately half its original width, and twice the number of communication channels can be allocated to the same frequency range.
Single-sideband modulation can be achieved by suppressing the carrier wave with a balanced modulator and then isolating either the upper or the lower sideband with a band-pass filter. A second method is the phase-compensation method, in which
Figure 1. Spectral composition of electrical waves transmitted by a single-sideband modulation: (U) wave voltage, (Ωi) lower-sideband frequency, (Ωu) upper-sideband frequency, (ωc) carrier frequency. The suppressed waves are indicated by broken lines.
corresponding waves of the modulated (high-frequency) spectrum are balanced out in a nonlinear conversion of the spectrum. The phase-filter method, a combination of these two principles, is also used.
REFERENCEVerzunov, M. V. Odnopolosnaia moduliatsiia ν radiosviazi. Moscow, 1972.
V. M. TIMOFEEV