Tone signal detector

Abstract

A digital tone signal detector having N-path filters eliminates the need for a variable-frequency active filter and makes possible the use of integrated circuits in paging receivers, transceivers, mobile radio equipment and so forth. The tone signal detector comprises a clock pulse generator, an N-path filter (where N is an integer equal to or greater than 2), a low-pass filter, a wave shaping circuit, a gate circuit, a counter and detection circuitry. The clock pulse generator generates clock pulses of a predetermined frequency Fc equal to N times the frequency fo of tone signals. The N-path filter samples and selects the tone signal of the frequency fo according to the output of the clock pulse generator. The low-pass filter converts the sampled tone signal including the clock frequency component into a form close to sine wave to eliminate the clock frequency component. The wave shaping circuit shapes the output of the low-pass filter to produce pulses, and the gate circuit passes the clock pulses in accordance with the output of the wave shaping circuit. The counter is connected to the output of said gate circuit to count the number of the output pulses of the gate circuit. The detection circuitry is connected to the counter to detect whether or not the received tone signals are the designated tone signals by the count accumulated by the counter in a prescribed period of time.

Dependencies are inferred from the claim text.