Need help with Relaxation Oscillator with Parasitic Capacitance

Here is an engineers and not a physics forum, so I will try to give a simple answer.

A radio (electrical) oscillator working like a fast switch opening and closing a circuit those producing a series of pulses (very simplified). It does not charge atoms to change their energy level and then to release photons that have energy related with their frequency. So in radio oscillator it is not necessary to add power as the frequency increase. Few meters from an ordinary RF antenna you will have EM radiation or photons with very low energy (not intensity!) hundred thousand lower than a UV photon.

But in –let’s say- X-ray tube yes you have to increase the voltage (even DC) to accelerate electrons and to obtain high-energy photons from specific metals (i.e. 100kV for 100eV photon).

It is clearly different mechanism.

In any case don’t mix-up classical electrodynamics and quantum mechanics.

The problem with varying power supply is that you'll vary amplitude as well, which will create a big deal of intermodulation and parasitic frequency modulation; an undesirable heterodyne effect. So, therefore a good VCO should have the amplitude independent from the controlling voltage.