Abstract: The disclosure relates to the changing of frequencies. The mixing of a signal at a frequency Fe and of the signal of a local oscillator gives a resultant signal that comprises a first component at the useful frequency, a second component at the frequency of the oscillator and a third component which is an image of the first component with respect to the second one. A filtering highly attenuates the third component. A servo-control makes a part of the signal from the oscillator undergo variations in amplitude and phase before adding it to the resultant signal to be filtered; the variations are made so as to obtain a minimum energy from the second component before filtering. Application to the changing of frequencies with suppression of the component at the frequency of the local oscillator.

Abstract: Servo-controlling position in a modulator having 2.sup.n phase and amplitude states (used for e.g. 16-QAM radio beam transmission) is achieved by monitoring the amplitude of the signal output from the modulator. In a first or inner loop, any constellation centering error in the modulation states is corrected; in a second or middle loop, any quadrature error between the modulated carriers is corrected; and in a third or outer loop, any amplitude difference between the X and Y channels input to the modulator is corrected.

Abstract: A very low loss microwave modulator having two phase states: O, .pi.; the modulator comprising the following between an input (E) which receives a pure carrier at a frequency f.sub.O, and an output which delivers a carrier modulated with two phases states: a first switching element (17); in parallel therewith, a second switching element (18) connected in series with a main transmission line (19); and a control system (20) for causing a selected one of the first and second switching elements (17, 18) to be conductive and for causing the other one to be non-conductive. The invention is particularly suitable for microwave transmission.