Method of controlling filter time constant and filter circuit having the time constant control function based on the method

A method of controlling the time constant of a filter for use in a radio receiver which receives a signal, transmitted from a transmitting station at a predetermined period, intermittently at the predetermined period and demodulates and delivers the received signal, a filter circuit having the time constant control function based on the method, and a radio receiver having the filter circuit. Preferably, the filter has its time constant switchable stepwise and specifically, parallel connection of capacitors or shortcircuiting of resistors is controlled by turning on/off switches. A controller within the filter circuit or the radio receiver performs control for applying a predetermined periodical signal (or standard pulse signal) to the filter and controlling the time constant of the filter in accordance with an output signal (or triangular pulse) during a first time zone within each operation interval for the intermittent reception and for filtering the signal from the transmitting station and causing a demodulating circuit to demodulate and deliver a filtered signal.