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 (2) 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 (211 to 213, 221 to 223) or shortcircuiting of resistors (230 to 232, 240 to 242) is controlled by turning on/off switches (311 to 313, 321 to 323; 330 to 332, 340 to 342). A controller (3; 30) within the filter circuit or the radio receiver performs control for applying a predetermined periodical signal (or standard pusle signal) (PL) to the filter and controlling the time constant of the filter in accordance with an output signal (or triangular pulse) (FOUT) 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 demodualting circuit (6) to demodulate and deliver a filtered signal.