Can you change the frequency a crystal resonates at by applying a very high voltage across it(through a high value resistor) so causing it to be compressed somewhat due to the electrostatic force. What impact does this have on Q etc? Regards, Sean O'Connor
If that worked, you likely can't rely on the frequency shift, or even the state of the crystal.
Decades and decades ago, people were pretty careless with crystals, running them in power oscillators and with high voltage directly on the crystals, but then the crystals were considerably bigger back then.
You can add capacitance to the circuit to load down the crystal, and shift it a bit downward. Indeed any circuit that needs a crystal to be on an exact frequency would include a trimmer capacitor for such an adjustment.
You can add inductance in series with the crystal to raise its frequency, but don't try to shift it too much or else it will become unstable as the coil becomes a more important component.
Combine capacitance and inductance to provide a wider shift.
But the real method is to open up the crystal case and grind it. That will raise the frequency, permanently. This was done years ago when there were all kinds of surplus crystals around, and they needed to be shifted a bit. It was also easier when crystal holders were held together with screws, so it was easy to open up the case. And the actual crystal blanks were held in place with pressure. Now, you'd have to unsolder the case, extract the blank, and then put it all back together. You can shift a crystal frequency downward by adding something to the blank like some pencil lead, but again not by much. Load it down by much, and it may stop oscilalting.
Keep in mind that crystals are ground to frequency, that's how they come from the factory.
Michael
You can lower the frequency by making a line or two on the crystal with a pencil ... not all that reliable or predictable, but it does work. It lowers the Q.
Kilovolts across a crystal???!!!???? Don't be surprised when the crystal fractures and becomes quite unusable.
-- Dave M MasonDG44 at comcast dot net (Just substitute the appropriate characters in the address) Some days you're the dog, some days the hydrant.
I just tried it since you triggered my curiosity. Not kV, but 30V, with a garden variety 4MHz crystal in a Colpitts.
With 0.1Hz resolution, the supposed effect is totally indiscernible from thermal noise and is certainly well under 0.5Hz, which is 0.1ppm
You'd have to use gigavolt level to achieve something, hem, useful.
-- Thanks, Fred.
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