A functional diagram and an equivalent circuit diagram of an autodyne oscillator are presented in the paper. Features of the formation of autodyne and noise characteristics are discussed taking into account the nonlinearity of autodyne phase characteristics. Analytical expressions for the noise process are given for the case of simultaneous influence of the autodyne’s reflected self-radiation and self-noise on the autodyne. The positive influence of the use of an external high-Q resonator on the formation of low-frequency data signals is shown. It consists in the increase of the dynamic range, reduction of autodyne frequency deviation, narrowing of microwave radiation spectrum. The results of experimental research of conventional autodynes and those stabilized by an external high-Q resonator are presented in the paper.