I have a question, why don't we use XTAL (crystal oscillators) instead of RC tank circuit?
(lets say I don' wanna tune)
If a XTAL is used shall we get a stable and better output?

 

Crystals can't be adjusted very much, which is why they are more stable. An LC circuit can be adjusted all over the place, which also has it's uses.

LC circuits have simplier characteristics than crystals overall.

 

When you place your unrelated question into an existing thread, it can be confusing. We ask that you not hijack threads, but always start a new one.

This is the original - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

How much energy can you pull off a crystal, and how much out of a tank circuit?

 

Tank circuits are used in Class C amplifiers to complete the rest of the R.F. cycle. A Class C tube or transistor may only conduct between 10 or 20% of the R.F. cycle, the tank circuit resonates so you get something resembling a sine wave out of the circuit. Tank circuits are also used for impedance matching in most transmitters, regardless of the class of operation.

eric