Principal of Oscillatior

Oscillator are these active Circuit's which produces high frequency signal ! 

Such that : 

-Audio frequency oscillator are use to produce few Hz to 20KHz frequency signal 

- Radio frequency oscillator are use to produce 10⁴ KHz to 10⁷ MHz frequency and more oscillator . 

Oscillator are self excited active Circuit's because there is no use of any external signal and it's transform DC signal to AC signal , we can say that finally oscillator are use to provide high frequency signal ! 

" Let we consider a circuit , in this circuit two elements one inductor L and one capacitor C are connected together in parallel , this circuit are known as TANK CIRCUIT ! 

If , we charged capacitor C by a source then electric energy stored in capacitor C . And current flow in circuit , then charged capacitor C charged by inductor L , the current flow in Inductor L increases with time and electrical energy in capacitor C are changed in maganatic field energy by Inductor L ! So changed in maganatic flux in Inductor L produce induce electro motive force ( emf ) according to Faraday and Lenz law. Due to this emf capacitor are charged in opposite direction and again capacitor C charged by inductor L by induce emf. 

If any energy losses doesn't have in this process then this process are going to infinity time. But due to resistance of circuit , the amplitude to oscillation are decreases with time . If doesn't have any energy loss in circut then the amplitude are being constant.

For recive constant amplitude of output signal we use feedback in oscillator.