How do quartz crystals work?

I interested in understanding the operation manner of quartz crystal, and broadly, a clock.

I read that the crystal is formed in such a way that it produces a frequency of 32768Hz.

1. Does that mean that if you power the crystal with DC voltage it will oscillate in the above frequency or is it some kind of feedback circuit?

2. Why is an electro-magnet needed inside clock apparatus?

A 32.768kHz quartz crystal is (mechanically) resonant at that frequency. You need an appropriate circuit connected to it in order to get it to oscillate, such as a Pierce oscillator circuit. Applying a voltage causes it to change shape slightly, which, in turn generates a voltage. By using an amplifier the entire circuit can be made to oscillate at a frequency close to the mechanical resonant frequency of the crystal.

An electromagnet is not required to make a clock. Digital clocks generally do not have electromagnets. If you need to make clock hands move then you may use a mechanism to achieve that goal via impulses fed to an electromagnet. If you want to research it further, the most common type is a single-phase stepping motor called a Lavet-type stepping motor.

• I remember an old battery-driven clock that indeed contained an electromagnet rather than a motor. The clockwork was purely mechanical and the electromagnet engaged regularly (order of magnitude was like hourly) to retension the spring used as mechanical energy source. Not what the question was about, though... Commented Oct 22, 2023 at 15:05

Many 32768 Hz. crystals have a similar construction to a tuning fork...its two tines move in a similar way. And like a tuning fork, constant pressure (as DC voltage would supply) does not excite the resonant vibrations of tine movement. Constant pressure might move the tines to a new position, but when slowly released, the tines simply move slowly back.

A very quick "rap" pulse does excite resonant tine movement, which rings like a bell, and slowly dies away much later. A quick pulse of voltage would do the same to a crystal resonator, via its piezoelectric property.

An oscillator excites the 32768 Hz crystal regularly: since it provides a pulse every cycle, very little pulse power is needed to keep it vibrating. The oscillator works by sensing tine motion, and providing pulse power to reinforce tine motion.
These oscillators sense tine motion by using the quartz piezoelectric property to convert mechanical motion to electrical voltage. The oscillator boosts the sensed voltage and then feeds back enough power (microwatts) to keep oscillations going.

The oscillator circuit is powered by a DC source. It is the oscillator's job to extract some power from this DC source to provide AC power at the crystal's resonant frequency (32768 Hz) to keep it going.

The electromagnet you might find inside a watch or clock provides a means of advancing the clock's hands each second.

• Great explanation! The crystal has 2 terminals. So the 2 of them are utilized for: feeding voltage , and also as means for measuring the voltage exerted by the crystal? Commented Oct 22, 2023 at 21:21
• How an oscillator circuit can replenish AC power lost to a crystal's resonant flexing is a complex topic that John Doty has touched on in his answer. Spehro too. Commented Oct 22, 2023 at 21:58
• Can I assume that one terminal of the crystal is connected to the power supply, and the other terminal provides the output signal? Commented Oct 23, 2023 at 6:23
• No - not even close. A transistor oscillator circuit is required to provide an output signal. This circuit is powered by the DC power supply. Perhaps you're looking for a clock oscillator, which has more than two terminals: these modules might include a crystal and oscillator circuit inside. Most have four pins, none have only two. Commented Oct 23, 2023 at 12:56

Quartz is a piezoelectric material. Apply a voltage to it, it deforms, deform it and it makes a voltage. This means it can reflect an applied wave back into the circuit driving it.

It's very stiff, so if excited, it vibrates very rapidly. It functions as a a resonator.

A crystal oscillator works by sensing the reflected wave and feeding it back to the crystal, thus increasing the vibration. By design, it oscillates at a resonant frequency of the crystal because the phase of the feedback is arranged to add to the vibration at that frequency.

• But if you can design a resonant circuit, why you need explicitly 32.768kHz quartz crystal? In other words, doesn't the circuit itself determines the frequency? Commented Oct 23, 2023 at 10:25
• @YonatanElizarov The oscillator circuit just feeds an amplified version of the reflected wave from the crystal back at it. It doesn't determine the frequency: the resonant vibration of the crystal does. Commented Oct 23, 2023 at 12:22
• So it doesn't vibrate like tuning fork - the primary voltage pulse doesn't caused it to vibrate in 32.768kHz, but each movement is excited by the Oscillator circuit in some kind of feedback loop Commented Oct 23, 2023 at 19:01
• @YonatanElizarov It vibrates like a tuning fork, but the usual excitation is different. An impulsive excitation doesn't make a sustained oscillation. Commented Oct 23, 2023 at 20:02