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# Will a quartz crystal block DC?

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#### neazoi

##### Advanced Member level 6
Will a quartz crystal block DC?
Also, what will happen if I apply DC in a crystal that resonates in an oscillator? Will it's Q be affected signifficantly so that it affects the oscillation?

You do not "apply" DC to the crystal in an oscillator, it simply has the circuit's voltage across it and since it does not conduct DC current then the voltage it gets does not matter.
Here is a standard transistor crystal oscillator that has the base voltage of the transistor across the crystal:

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neazoi

### neazoi

Points: 2
You do not "apply" DC to the crystal in an oscillator, it simply has the circuit's voltage across it and since it does not conduct DC current then the voltage it gets does not matter.
Here is a standard transistor crystal oscillator that has the base voltage of the transistor across the crystal:

As far as I can understand the crystal won't pass DC and won't be affected if an external DC is present at it's terminals in an oscillator circuit.

In my experiments a crystal produces a small spike at the opposite end from where you apply a transition.

The small spike is delayed slightly. It does not convey DC. It's at high impedance. The small spike is easily drowned out if noise or signals reach it through a low impedance.

neazoi

### neazoi

Points: 2
Wikipedia shows it very well:

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### neazoi

Points: 2
In my experiments a crystal produces a small spike at the opposite end from where you apply a transition.

The small spike is delayed slightly. It does not convey DC. It's at high impedance. The small spike is easily drowned out if noise or signals reach it through a low impedance.

I think the spike you notice is from the 0 to VCC transition and vice versa. Ie the spike caused by the DC pulse transition.

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