Abstract: A single sideband modulator provides a means of translating low frequency baseband signals directly to radio frequency in a single stage. Such modulators, providing suppressed carrier and one or two of the sidebands, facilitates the transmission of intelligence with significantly increased gain over AM transmission. Control signals are continuously generated to keep the local oscillator breakthrough and image sidebands down to an insignificantly low level. This is achieved by monitoring amplitude of the RF output of the single sideband modulator, and comparing this with the baseband signals. By adjusting the d.c. offsets at the baseband inputs to the balanced modulators, carrier breakthrough is cancelled. By adjusting the relative phases of the baseband signals, deviations from the 90.degree. split are compensated. By changing the amplitude of one of the baseband signals, the level of one of the RF paths is adjusted to achieve amplitude balance.