INTERMEDIATE FREQUENCY PULL-IN METHOD

摘要

PURPOSE:To stop frequency sweep of an intermediate frequency signal at an operation stable point of an automatic frequency control circuit by stopping the sweeping of an oscillating frequency from a local oscillation light source at stable region of a real band or an image band. CONSTITUTION:The frequency is swept while applying small amplitude FM modulation to a radiation light 5 of a local oscillation light source 4, and a frequency discriminator 10 installed in the reception system demodulates the intermediate frequency signal subject to FM modulation. The demodulation signal and an FM modulation signal before being inputted to the local oscillation light source 4 are multiplied. The time when the multiplied value is at a high or a low level is measured while the oscillating frequency is swept. The band when the time is longer than a predetermined time is discriminated to be a stable region of a real band or an image band. Then the sweeping of the oscillation frequency of the local oscillation light source 4 is stopped at either of the bands.