Tone signal detecting circuit

Abstract

A tone signal detecting circuit for use in a paging receiver or the like employs an N-path filter in place of an active filter to facilitate circuit integration. The circuit includes a clock signal generating circuit responsive to a first control signal for selecting and reading out one of plural clock signals of a frequency equal to N (N being an integer equal to or greater than 2) times the frequency of the plural tone signals. The N-path filter has a prescribed rise and fall time and selects and responds to a tone signal in accordance with a clock signal from the clock signal generating circuit. A low-pass filter having prescribed frequency characteristics removes harmonic signals of the tone signals contained in the output of the N-path filter. A tone designating circuit responsive to a second control signal is connected to the low-pass filter and changes its frequency characteristics. A detector is connected to the output of the low-pass filter, and control circuits are responsive to the output of the detector to generate the first and second control signals.