Abstract: A modulator employs a pair of variable RF attenuators to produce a double sideband, suppressed carrier output. The attenuator devices are PIN diodes which are rendered alternately conductive in synchronism with the recurring half cycles of the audio frequency input signal which is to be impressed upon the radio frequency signal as modulation. As each diode conducts the RF signal, its attenuation is varied by a control signal in a manner to provide the conducted RF signal with a modulation envelope that follows the amplitude of the audio input. The modulated signals from both diodes are combined at the modulator output, after a phase reversal of the modulated signal from one of the diodes, in order to thereby provide a double sideband, suppressed carrier output signal. The control signal for each conducting diode is produced by a function generator operating within a closed loop feedback system.

Abstract: In an instrument landing system for aircraft, the transmitter is required to produce an amplitude modulated signal comprising a carrier wave having sidebands containing the sum of a 90 Hz and a 150 Hz signal as modulation, and to also produce a double sideband, suppressed carrier signal in which the modulation is the difference between the 90 Hz and 150 Hz signals. In the present invention, the 90 Hz and 150 Hz signals are summed to provide a composite modulating signal that is employed to directly modulate the radio frequency carrier. To provide the double sideband signal the carrier is fed to a 180.degree. phase shift network, and the carrier and the network output are alternately delivered to a modulated amplifier stage by an RF switch which is operated with each change in the sign of a difference signal derived from the 90 Hz and 150 Hz signals. Accordingly, the output from the RF switch is a phasechanging carrier that undergoes a phase reversal with each change in the polarity of the difference signal.