Abstract: A variable frequency passband circuit comprising two passband filters and three mixers (front, intermediate and output mixers) connected in a cascade fashion disposed in a signal path. Two VCOs inject frequency signals into the front mixer and the output mixer. The two VCOs are respectively controlled by control signals at two potentiometers through transfer switching means so that the synthetic passband of the filter may be varied. One potentiometer suppresses the higher frequency region of a baseband signal and the other potentiometer suppresses the lower frequency region of the base band signal, irrespective of USB or LSB receiving mode.

Abstract: An improvement in a transceiver having first and second carrier frequency oscillators for demodulation, a change-over device for making change-over of oscillation outputs between the first and second carrier frequency oscillators, a mixer for mixing the oscillation outputs from the first and second carrier frequency oscillators, and a switching circuit for connecting and disconnecting the input signal into, or output signal from, said mixer by on-off operations of the telegraphic transmission key to apply the signal to a low frequency amplifier of the transceiver.