Loading caps on crystals.

[hamster_nz]

I have a newbie building a digital clock tomorrow, and am looking for a simple explination for the loading caps on the 32.768kHz crystal. It is the only part that I don't have a simple explination for.

The best I can come up with is:

They act as the electrical equivilent of springs at either end of a beam, encouraging the crystal to vibrate correctly.

If these `springs` are too soft or too firm it dampens the oscillation rather than helping it.

Does this sound about right/simple enough? Anybody got anything simpler?

[Rerouter]

the oscillator for a crystal is built to be as efficient as possible in order to accomplish this its built to work best in only a certain narrow capacitive loading range, The capacitors are used to trim this loading for all different crystal capacitance values.

What it is specifically doing is adjusting the phase shift on the inverter so that it is very close to 180 degrees.

[hamster_nz]

I would say it adds weight to the beam.

Yes, much better!

[tooki]

the oscillator for a crystal is built to be as efficient as possible in order to accomplish this its built to work best in only a certain narrow capacitive loading range, The capacitors are used to trim this loading for all different crystal capacitance values.

What it is specifically doing is adjusting the phase shift on the inverter so that it is very close to 180 degrees.

This is a great example of a reply that might be technically correct*, but is completely and utterly useless to a beginner (we are in the Beginners forum!). The OP made it clear they were trying to understand an analogy.

Also, punctuation helps. Please use it.


*I can't judge it, since I don't understand crystal oscillators, either.

[wilfred]

I found the App note from Freescale. It might be of interest.

https://www.nxp.com/docs/en/application-note/AN2606.pdf  "Practical Considerations for Working With Low-Frequency Oscillators"

[magic]

If these `springs` are too soft or too firm it dampens the oscillation rather than helping it.

Not exactly, it shifts the resonant frequency a bit.
Load capacitance needs to be as specified in order to obtain exact frequency.