This article presents the homodyne detection in a self-oscillation system, which represented by a short-range radar (SRR) circuit, that is analysed using a multi-time forced oscillator (MTFO) model. The MTFO model is based on a forced oscillation perspective with the signal and system theory, a second-order differential equation, and the multiple time variable technique. This model can also apply to analyse the homodyne phenomenon in a difference kind of the oscillation system under same method such as the self-oscillation system, and the natural oscillation system with external forced. In a free oscillation system, which forced by the external source is represented by a pendulum with an oscillating support experiment, and a modified Colpitts oscillator circuit in the UHF band with input as a Doppler signal is a representative of self-oscillation system. The MTFO model is verified with the experimental result, which well in line with the theoretical analysis.

In this article, a novel coherent amplitude demodulation based on time-varying force function of secondorder oscillator circuit and injection locking Phenomenon is proposed. The compactness of demodulation circuit is obtained because amplitude demodulation mechanism is inherent in one oscillator circuit. With injection locking property, frequency and phase of the oscillator is identical to that of the transmitted carrier. Based on time varying force function which is an AM signal, coherent demodulation is taken place. In laboratory experiment, the Colpitts oscillator at UHF band is employed where experimental results agree well with the proposed principle.

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