(a) Time periodic capacitor connected to the inductor in parallel. The capacitor varies between two values C₁ and C₂ with T_m period, as shown as an inset. (b) Real and (c) imaginary parts of resonanse frequency evolution varying modulation frequency f_m. (b) The time domain signal revealing the second order EPD due to the capacitor’s voltage linear growth, with initial condition of v_c (0⁻) = 50mV, and f_m = 63.95kHz.