Given that the frequency of the source is very high than the resonant frequency, What happens when we withdraw the source supply from a parallel LC network ?
Will the frequency come down to resonant frequency and the LC network resonate?

 

Suddenly starting or stopping a sinusoidal signal generates a wide spectrum. Even though the frequency of that sinusoid is far removed from the resonance of the LC circuit, some of the sideband energy caused by a high dv/dt will cause the LC tuned circuit to ring a little bit, especially if it has a high Q. (Hams call this key clicks).

The circuit would be excited by any high dv/dt signal, regardless if it is a sharply keyed sinusoidal signal or something else.

 

After we remove the source , still there will be some energy stored in the inductor or capacitor. So using this energy and the frequency same as the source , the inductor and capacitor will exchange energy among themselves. So i want to know , will the frequency at which the circuit "RINGS" will decrease with time, even though the frequency is not the resonant frequency ?

 

No, look at this simple example: Sinosoidal excitation at 20kHz V(in) with sudden turn on and sudden turn off. Excitation lasts 250 cycles. The resonant frequency of the LC network is 1.6kHz. During turn on, V(out) has frequency components of both 1.6kHz and 20kHz. After turn off, the LC network continues ringing, but the only frequency component is due to its own resonance.



Second set of plots shows what happens as the excitation is suddenly shut off.



The third set shows where the ringing comes from...

 

Thanks Mike