PURPOSE: To set a center frequency of a BPF automatically to a prescribed setting frequency by controlling a center frequency of the BPF so as to minimize a signal level of a DC component based on a signal of the DC component subject to low pass filtering.
CONSTITUTION: When a unipolar pulse motor drive signal is inputted to a stepping motor 33, a dielectric tuning element in a BPF 30 is moved to change a static capacitance of a variable capacitor in an equivalent circuit thereby changing a resonance frequency f0 of a dielectric resonator 31. Thus, a center frequency fc of the BPF 30 almost equal to the frequency f0 is changed. Then a control circuit 50 drives the stepping motor 30 to minimize a level of a signal of the DC component outputted from an LPF 61 to change the resonance frequency of the dielectric resonator 31 in the BPF 30. Thus, the center frequency fc of the BPF 30 almost equal to the frequency f0 is nearly made coincident with a frequency fs of the reference signal generated by a signal generator 71.
WADA SHUICHI
YAMASHITA SADAO
KUBO HIROYUKI
