PURPOSE: To suppress variations in oscillation frequency and oscillation voltage due to internal resistance variations of a varactor diode, by connecting a resistance in series to the varactor diode.

CONSTITUTION: When a voltage E applied to a varactor diode D is varied, the electrostatic capacity of the varactor diode D varies and an oscillation frequency varies. Further, the value of a resistance R6 connected in series to the varactor diode D is set much greater than the internal resistance of the varactor diode D to suppress the influence of variations in internal resistance upon an oscillation voltage and an oscillation frequency. When the value of the resistance R6 is set too greater, a quarts oscillator X is disabled to be energized, so the value should be less than the internal resistance of the quartz oscillator X. Therefore, the value of the resistance R6 is set favorably to 10∼100Ω in general.