JPS6439807A - Filter frequency controller - Google Patents

Abstract

PURPOSE:To maintain a filter frequency of a resonance means at a prescribed value by generating two phase-shifting signals by bringing an input AC signal to a phase shift by the resonance means, and generating a control signal corresponding to a shift quantity and controlling a circuit constant of the resonance means, when a phase difference has been shifted from a prescribed phase difference. CONSTITUTION:Phase-shiftng signals outputted from output terminals A, B in a second resonance circuit 2 set a phase difference of 90 deg. to 70 deg. each other, when a resonance frequency filter of the resonance circuit 2 coincides with a specific frequency in an AC signal, and on the contrary, when it is shifted from a resonance state, a phase difference of two phase-shifting signals is shifted from 90 deg.. These phase-shifting signals are brought to an amplitude limit by first and second limiter amplifying circuits 3, 4, respectively, applied to a detecting circuit 5, and by multiplying two phase-shifting signals in a period in which a pulse signal 8 is applied, a signal corresponding to the shift quantity from 90 deg. of the phase difference is outputted. This detecting output is smoothed by a smoothing circuit 6, provided on a control circuit 7, a control signal from the control circuit 7 is applied to the resonance circuit 2, a value of a coil L or a capacitor C is varied, and a control is executed so that the phase difference of two phase-shifting signals always maintains 90 deg..