Abstract: A duplex transmission and reception system wherein a local oscillator of the receiver is dispensed with by using the output of a modulated transmission oscillator, with its modulation cancelled, in lieu of a receiver local oscillator. The transmitter modulation signal is adjusted by appropriate level correction and phase inversion. The modulated output of the transmission oscillator is itself applied to a modulator, where it is modulated by the adjusted modulation signal. This results in substantially cancelling out the modulation component of the transmission oscillator output. This modulation-cancelled signal is supplied to the mixer of a superheterodyne receiver as a local oscillator signal.