The Zero-Beat Method of Frequency Discrimination

A method of frequency discrimination in which the frequency of balance is determined solely by the frequency of the controlling oscillator is explained. The method utilizes the phase turnover, which occurs at zero beat, between two beat sources when one of the beating signals is dephased by 90 degrees before it is applied to one of the beat detectors. A network is inserted in one of the beat sources which shifts its phase an additional 90 degrees and makes its output a direction function of the frequency. The beats are then amplified and supplied to a balanced rectifier, from which a direct voltage is available which changes polarity with the direction of frequency deviation.