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It is known that if the capacitance in an LC tank changes periodically with frequency twice the resonant frequency of the LC tank, the amplitude of the oscillations in the tank grows continuously. This is known as "parametric amplification" or "parametric generation". The very same principle you use when swinging on a swing without help from another person (you change body position twice per swing period).
In electronics, varactor diodes are used in parametric amplifiers and generators. Unfortunately, the diode capacitance in EC cannot be controlled with voltage, therefore, here I use an opamp-based circuit which models a capacitance controlled by a pulse voltage source. The input of this circuit is connected to an inductance thus forming an LC tank. The tank is tuned at 10 kHz, and the pulse frequency is set to 2*10 = 20 kHz.
To start oscillations press and release the push button (this is much like the initial push on the swing), and observe the process.
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