Automatic frequency control circuit for coherent diversity reception

Abstract

In an optical heterodyne receiver having a local oscillating light source controlled by a driver circuit and which uses a polarity diversity technique thereby producing first and second intermediate frequency signals, an automatic frequency control circuit (AFC) controls the driver circuit by generating an error signal. The AFC includes two power regulators for generating respective fixed power signals based on the two intermediate frequency signals. The AFC also includes two detectors for converting the fixed power signals to corresponding voltage signals based on their frequency; two control circuits for providing respective logic signals based on the fixed power signals; first and second switching circuits which enable/disable a respective detector in response to the logic signal from the respective control circuits; a third control circuit which generates an AND signal based on the logic signals from the control circuits; and an amplifier for generating the error signal by comparing a reference voltage and the voltage signals from the detectors, the amplifier changing an amplification factor in response to the AND signal from the third control circuit.