If you have a resonant circuit, oscillating at its self-resonant frequency, how would you extract the excess power without draining it fully? A high Q resonant transformer impedance matched to the circuit perhaps? Is there another way? I actually need DC output so perhaps rectifying before extraction is a good strategy?
One definition of Q is energy_stored/energy_lost_per_cycle.
Increase the losses, to your pickoff, and you necessarily decrease the Q.
If you have a parallel LC at some point in your oscillator, then a series diode to an output is a good way of getting a DC voltage out, while only loading the circuit at the conduction instants, and leaving it unloaded throughout the rest of the cycle.
Note that I've included the sustaining circuit, as resonant circuits don't just resonate by themselves, especially if you skim off the 'excess power'. I've also included its power supply, just in case anybody is so entranced by the idea of resonance they think they can get free energy out of it.