# What makes quartz oscillators vibrate?

I understand that the principle behind it is piezoelectricity and electrostriction (inverse piezoelectricity), but how does one make the crystal vibrate?

The only thing I can think of is using an alternative tension source, but wouldn't that make the crystal vibrate at the frequency of the source? Also, for example quartz wristwatches use a battery as a power source, which are DC.

From what I understand, it seems like it is sort of cyclical system where the where an initial tension create a strain via electrostriction, and that strain causes a piezoelectric effect which creates an electric field, which creates electrostriction... In that case, how would the battery help the system?

I answer you without knowing the details of quartz oscillators for clocks, but on general principles and having seen some quartz oscillators.

You indeed use an oscillating tension source, and then you measure the oscillations that you induce (I think this is made by measuring the current). The oscillator is strongly resonant at its natural frequency, so you can look for the frequency at which you have the strongest response (that is, the highest value of the current). In this way you determine the frequency.

This is also a general way to use oscillators to mark time: excite them with an oscillating input, find the frequency at which their oscillations are largest. Take a look at this Wikipedia article for a general idea.

I do not have in mind the details on how energy is dissipated in the cyclical system in which the tension is transformed into a mechanical oscillation and that back into tension, but there must be some loss somewhere, and the battery will fill in the dissipated energy.

• In quartz watches where the only power source is a DC battery, how can I send an oscillating signal? The only circuitery on those watches seem to be for counting the frequency and converting them to seconds. – sebasket Apr 11 at 0:46
• I do think you need to supply to the quartz oscillator an oscillating voltage to make it oscillate. I do not know the details of how this works, that is, what sort of circuit is use to generate the oscillating voltage. Maybe someone else can fill them in (@hdhondt in his answer writes "electronic oscillator circuit ") – JTS Apr 11 at 0:55
• The power source is DC, but the circuit is an oscillator, which converts DC into AC. – hdhondt Apr 11 at 1:14
• Awesome, thank you very much! – sebasket Apr 11 at 1:29

Electrostriction is the property of insulators that makes them change their shape when an electric field is applied. Piezo-electricity is the reverse: it makes the material generate an electric charge when force is applied.

Suppose we apply a varying electrostatic field to the crystal. As the voltage goes up and down, the crystal will compress and expand. This change in shape in its turn generates an electric field, which interferes with the applied field. When the two are in sync the output from the crystal will be maximum, and the crystal will be vibrating with its maximum amplitude.

The electronic oscillator circuit that generates the varying field is designed to use feedback to adjust itself to the frequency that generates the maximum output, and as a result its frequency is locked to that of the crystal. For this to work, the starting frequency needs to be reasonably close to the resonant frequency of the crystal, but that does not present any real problem.

• So from what I understand, I could make a crystal vibrate at any frequency I want, it is just that at it's resonant frequency it needs less exterior energy to keep vribrating, is that right? Or the crystal always vibrates at its resonant frequenecy of the crystal, but the amplitude changes? – sebasket Apr 11 at 1:07
• You can force it to vibrate at any frequency, but the output will be very low, except at the resonant frequency, because the crystal has a very high Q factor. – hdhondt Apr 11 at 1:12