A variable capacitance diode termed as a varicap diode or a varactor diode is often used to vary a capacitance by controlling a voltage applied to the diode. When a reverse bias voltage is applied, the variable capacitance diode is able to vary the junction capacitance by varying the thickness of the depletion layer in the p-n junction. The variable capacitance diode is highly reliable because the variable capacitance diode includes no mechanical elements. For this reason, variable capacitance diodes are widely used in voltage-controlled oscillators and voltage-controlled variable filters. Variable capacitance diodes are components essential for television receiver sets, cellular phones and the like.
A variable capacitance diode, however, has a low Q factor (quality factor), and the Q factor largely varies depending on a reverse bias voltage applied to the variable capacitance diode. The Q factor is decreased depending on internal resistance of the variable capacitance diode.
In addition, a variable frequency resonator generally uses a variable capacitance diode as a capacitor of the resonator which includes an inductor and the capacitor. Decrease in the Q factor of the variable capacitance diode results in decrease in the Q factor of the resonator. When a resonator is used in an oscillator, the resonator with a lower Q factor deteriorates the phase noise characteristic of an oscillated signal. For this reason, resonators in use for measurement and communications, for instance, are required to have a higher Q factor.
Two open-end strip line resonators having different electrical lengths are known to be connected in parallel to a bias supply point of a variable capacitance diode in order to achieve satisfactory phase noise characteristic, for example (see Japanese Patent Application Publication No. 2008-277938).