There is known, for example, a high frequency oscillator having a function of subjecting a transmission carrier wave to FM modulation, as shown in FIG. 2, in which a reference character Q designates a transistor for oscillation. The basic structure of the high frequency oscillator is composed of a transistor Q; a resonant strip line unit 3 consisting of a main strip line member 1 and an auxiliary strip line member 2; two terminal capacitors C.sub.1, C.sub.2 connected to the terminals of the strip line unit 3; a varactor diode VD connected to the auxiliary strip line member 2; and a switch diode for switching the characteristic impedance of the strip line unit 3. Of the auxiliary strip line member 2, one terminal is connected exclusively for passing high frequency waves (referred to as "high-pass-connected" hereinafter) to the main strip line member 1 through a capacitor C.sub.3, and the other terminal through the switch diode "D". The cathode of the switch diode D is connected to a supply power input terminal 4 through a preset negative bias introducing component consisting of resistors R.sub.1, R.sub.2. The anode of the switch diode D is connected to the positive-bias switchable voltage input terminal 5 through a choke L.sub.1. To the varactor diode VD, a regulation capacitor C.sub.4 is connected in series. The series connection line is connected in parallel to a terminal capacitor C.sub.2. Of the varactor diode VD, the cathode is connected to the control voltage input terminal 6 through a choke L.sub.2 and the anode to a modulating-signal input terminal 7 through a modulation resistor R.sub.3 and another capacitor C.sub.5. The anode of the varactor diode VD is grounded through a parallel circuit of another resistor R.sub.4 and a bypass capacitor C.sub.6. Reference characters C.sub.7, C.sub.8, C.sub.9 and C.sub.10 designate bypass capacitors.
Of the main strip line number 1, one terminal is connected to the collector of the transistor Q through a capacitor C.sub.11 and the other terminal to the output terminal 8 through a coupling capacitor C.sub.12. Reference characters designate respectively: R.sub.5, R.sub.6, bias resistors of the transistor Q; C.sub.13, a bypass capacitor; R.sub.7, an emitter resistor; C.sub.14, C.sub.15, feedback capacitors; and R.sub.8, a damper resistor.
Upon application of a H-level voltage to the switch voltage input terminal 5, the switch diode D is positively biased to turn "on", and thus the auxiliary strip line member 2 becomes connected in parallel to the main strip line member 1, leading to reduction of the characteristic impedance of the strip line unit 3, and an increase in oscillation frequency, and as the result a receiving carrier wave which is a first frequency signal is output from the output terminal 8.
On the other hand, upon application of a L-level voltage to the switchable voltage input terminal 5, the auxiliary strip line member 2 is isolated from the main strip line member 1, by the action of the negative bias of the switch diode D caused by +B voltage, which is applied to the supply power input terminal 4. In this way, oscillation of a diminished frequency is caused exclusively through the characteristic impedance of the main strip line member 1, and so a second-frequency signal or transmission carrier wave is generated from the output terminal 8.
When a reception or transmission carrier wave is being output, upon application of a control voltage to terminal 6, the capacitance of the varactor diode VD is changed and as the result, channel switching is caused during reception/transmission. Besides, during transmission, a modulating signal is input to the modulating signal input terminal 7, and the capacitance of the varactor diode VD is caused to change in accordance with the modulating signal to subject directly the transmission carrier wave to FM modulation.
With the prior art high frequency oscillator, for the purpose of channel switching, capacitance values of the varactor diode VD are separately set by the application of control voltages of desired values to the varactor diode VD. In addition, during transmission, a modulating signal is applied to the varactor diode VD and the capacitance is caused to change in response to a modulating signal in order to subject the transmission carrier wave directly to FM modulation.
Now, the characteristic of the control voltage to capacitance of the varactor diode is nonlinear. This brings the problem that though the level of a modulating signal applied at the modulating-signal input terminal 7 is constant, change in capacitance responsive to the modulating signal is dependent on the value of control voltage applied to the varactor diode VD, resulting in change in FM modulation degree. Another problem is that owing to application of a modulating signal after it has been superimposed on a control voltage, the circuit section to which the modulating signal is to be applied is required to include circuit elements such as capacitors C.sub.5, C.sub.6 and a resistor R.sub.4. a high frequency oscillator circuit of this type needs to be miniaturized and compact, and therefore the increase in the number of parts and in turn the increased difficulty of construction is disadvantageous.