The Crystal-based astable multivibrators are used for every precise clock application. Crystal Oscillator are used as the clocks in microprocessors and microcomputers.
The above figure shows a clock generator using NAND gates in which crystal is the controlling element. This is a modification of the astable multivibrator using NAND gates. In the figure, two NAND gates are connected through RC coupling and they act as high-gain RC-coupled amplifiers. The feedback is provided through the crystal which acts as an RLC circuit. Under resonance conditions, the capacitive and inductive reactances of the crystal cancel each other so that the crystal produces square waveforms at the resonant frequency of the crystal. This frequency of oscillation is highly stable.