PURPOSE: To attain no adjustment and to obtain a wide band demodulation characteristic by using a ladder filter comprising the 1st piezoelectric resonator connected in series and the 2nd piezoelectric resonator connected in parallel with a phase shift circuit and an impedance circuit included in a quadrature circuit.
CONSTITUTION: A circuit 7 is provided in the quadrature circuit as a phase shifter 4. The circuit 7 comprises a phase shift capacitor connected in series between an output terminal of an intermediate frequency amplifier 2 and a multiplier 3, the 1st piezoelectric resonator 72, the 2nd piezoelectric resonator 73 connected in parallel between the output of the 1st piezoelectric resonator 72 and ground and a matching resistor 74. As the piezoelectric resonators 72, 73, piezoelectric ceramic resonators are used. The operating band is selected freely by combining a difference ΔF1 between the resonance frequency and the anti-resonance frequency of the 1st piezoelectric resonator 72 and a difference ΔF2 between the resonance frequency and the anti-resonance frequency of the 2nd piezoelectric resonator 73 properly.
SAWAHARA SHINICHI
AWATA SATOSHI
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