Radio receiver station-seeker stop-signal generator

摘要

Radio receivers which scan through a predetermined band of the radio spectrum, in a station-seeking mode, must independently detect whether the currently tuned frequency is the central frequency of the transmitting station and whether this transmitter provides a signal whose strength is sufficient, i.e. whose strength exceeds a predetermined threshold value. Once these two conditions are satisfied, the radio generates a "stop" signal which terminates the station-seeking mode, i.e. keeps the tuning circuit set to the frequency which provided the sufficiently strong signal. The present invention provides an unusually simple way to determine whether these two conditions are satisfied, by using a Finite-Impulse-Response digital filter (2) to extract a d.c. component of the demodulated signal (MPX), for comparison (6) with a predetermined maximum value, and by filtering (10) a field strength signal (FST), for comparison (11) with a predetermined minimum value. When both conditions are satisfied, a simple AND-gate (8) generates the stop-signal.