Until a simulation was attempted (in LTSpice), this circuit seemed a simple one to excite.
Discovered some tricky interaction between simulation time-step and resonance that affected results. First, here's the circuit. It starts off at t=0 with DC voltage source at zero volts. The RC time constant (R=1MEG, C=1n) rises to +20 volts in about five milliseconds. This transient alone is enough to excite resonance. During this time, varactor capacitance decreases, which increases resonant frequency from about 52 kHz to 400 kHz.:
A transient simulation shows the resonant voltage across the inductor rising for a few milliseconds while resonant frequency changes, then decays slowly as resonant energy dissipates via R1's damping effect, and possibly by dissipative resistances in the Varactor diode D2 model. Note the fine timestep of 5 nanoseconds - this simulation seems reasonable:
The same circuit with only the timestep changed to 150 nanseconds yielded a different simulation result, showing no resonant decay, and a distorted envelope rise - this simulation seems to have run afoul:
These plots don't show resonant frequency change (one must zoom in very far to see cycle-by-cycle period). One would think that a 150ns step size should yield a decent simulation, but it seems that an upper limit of 5ns does a far better job. Estimates of resonant frequency benefit from small step sizes. Be sure to limit your simulator to a small time-step.