Tuning circuit

Claim 1

In a transmitter-receiver having a receiving circuit for converting a received carrier frequency into an intermediate frequency by mixing it with a local oscillation frequency which is the controlled output frequency of a voltage-controlled oscillator of a phase locked loop, and a transmitting circuit connected to said receiving circuit through a transmission-and-reception change-over switch; the improvement which comprises a tuning circuit having first and second variable-capacity elements provided in parallel with an oscillation inductor of said voltage-controlled oscillator, said first element receiving a DC voltage corresponding to said intermediate frequency and simultaneously receiving a DC voltage necessary for frequency modulation using transmitting audio signals while interlocking with the switching action of a press-to-talk switch for reception and transmission, said second variable-capacity element receiving voltages ranging within the controllable limits of said voltage-controlled oscillator from said phase locked loop so that the follow-up control of said phase locked loop may be achieved, and DC power supply means for said first variable-capacity element including a first and a second variable-resistance means, means for connecting said first variable-resistance means across the terminals of an external DC power supply, means for connecting said second variable-resistance means across the terminals of the external DC power supply, and a switching means for selectively applying divided voltages produced by said first and second variable-resistance means to said first variable capacity element according to the position of said press-to-talk switch, whereby frequency phase synchronization is achieved by one phase locked loop.