With the permeability increase of renewable energy, the interaction between power electronic converters and power grid causes oscillation accidents frequently. In order to study the characteristics of oscillation induced by grid-connected new energy sources, this paper explains the mechanism that a single-frequency disturbance voltage as input to a single-phase inverter considering the PLL effect leads to double-frequency disturbance current as output, through the relationship of real number signal and complex vector. The mechanism of frequency coupling in weak power grid is further explained. The models of self-admittance and mutual-admittance are established, and the equivalent output admittance of inverter is then derived. Finally, the correctness of the self-admittance and mutual-admittance models are verified by MATLAB/Simulink. The equivalent output impedance of inverter can be utilized to judge the stability of an interconnected system according to the Nyquist stability criterion.