crystal osc

[amitpj]

y two capacitor is add at the two end of crystal like below photo

 

[Diver300]

The input to the 4060, pin 11, is very high impedance, so no current flows in or out. If you don't have capacitors, and change of voltage on the output, pin 10, also happens on the input due to the capacitance of the crystal. The static capacitance of the crystal, (C0 in https://upload.wikimedia.org/wikipedia/commons/thumb/c/cb/Crystal_oscillator.svg/500px-Crystal_oscillator.svg.png) will then be the easiest path for a signal to get though the crystal, and the circuit won't oscillate, or will oscillate as an RC oscillator.

Having a load capacitance on the input means that much more charge is needed to change the input state of the 4060, and C0 cannot provide enough charge, so the circuit will be stable and the input and output voltages will be at about half the supply voltage, where the 4060 has most gain. The resistor's function is to make sure that the voltages are the same at start up. (Most circuits like this will continue to run if the resistor is removed after the oscillator has started).

Then any thermal or other noise will be amplified, and the crystal will only let signals at its resonant frequency pass through. This is amplified by the 4060 and so the amplitude builds up over thousands of cycles until it is limited by the supply voltage. It will take 0.1 - 0.5 mS for a circuit like that to get to maximum amplitude.

The quartz crystal vibrates, and that movement, along with the properties of the quartz, allows much more charge though the crystal than could be provided by C0, so the input capacitance can be charged and discharged at the resonant frequency.

The output capacitor provides a different function. The output of the 4060 is quiet low impedance, so the current in the crystal could easily travel though the 4060 to ground or +ve. The output capacitor helps to slow the rise and fall of the 4060 and makes unwanted oscillation less likely. Some designs put a small resistor between the output of the 4060 and the common point of the crystal, the resistor and the output capacitor, in order to slow the rise and fall more.

Crystals are adjusted to expect capacitors like that, so you also need the capacitors to get the correct frequency. The frequency can be adjusted by altering the capacitors, but the input capacitor has far more effect on frequency than the output capacitor, especially if there is no small resistor on the output.

 

[MikeMl]

Read:

Colpits Oscillator

**broken link removed**