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In this paper, the envelope-transient method is applied to the analysis of intermodulation distortion in self-oscillating mixers (SOM). A two-tone Fourier-series expansion of the circuit variables with time-varying harmonic components is used with a new initialization technique of the oscillation to avoid convergence toward unstable forced solutions. The two cases of an autonomous oscillation and a sub-synchronized oscillation are studied and compared. In the sub-synchronized SOM, the ranges of sub-synchronized operation in terms of the sub-synchronization generator power and frequency are determined through harmonic balance. The techniques are applied to a SOM with 5.5-GHz input frequency and 0.5-GHz IF. In the case of an autonomous oscillation, two different values of the quality factor of the load circuit are considered. For sub-synchronized operation, a generator is introduced at approximately one-third the self-oscillation frequency. In order to validate the analysis techniques, the circuit has been experimentally characterized in both autonomous and sub-synchronized operation, obtaining very good agreement with the simulation results.