The Quartz Crystal Oscillator
In our article “What is the Stepper Motor in a Quartz Watch?”, I showed you how the Stepper motor converts pulses into rotary motion. The speed of the rotation is determined by the frequency of the current pulses. This is where the quartz crystal oscillator come in to play.
All modern mechanical timekeepers use a resonant oscillator as their heart. It could be either a pendulum or a balance. Quartz watches use something similar, but much smaller in size. Quartz watches use a tiny piece of quartz shaped like a tuning fork.
If you could “ping” a quartz tuning fork with something, you would find that I will vibrate at a constant frequency. You would also find that it takes a long time for them to stop. In this sense, a quartz crystal is like a tiny pendulum, although the elasticity of the quartz itself provides the restoring force, not gravity like for the pendulum.
Special Properties of Quartz Oscillators’
Quartz has a few very usual properties and advantages. First, it is far less sensitive to temperature changes than a balance wheel or pendulum. This gives it an extremely low temperature error.
Second, the quartz tuning fork is so small and light weight that it is unaffected by gravity. This means that it will vibrate at the same frequency no matter what the position of the watch is. This is a huge advantage over mechanical watches whose rates change depending on whether they are in the horizonal or vertical positions.
Finally, the fact that they are ridiculously small, make it quite easy to encase a quartz crystal in a sealed container, which makes it insensitive to barometric changes or pressure. A quartz oscillator has no barometric error.
If you could find a way to detect the vibrations of the quartz crystal, and provide impulses to keep it going, you would have the basis for a highly accurate timekeeper. This is where the piezo-electric effect comes in to play.
The Piezo-Electric Effect
If you bend a piece of quartz, it will generate a voltage across it. When a quartz crystal vibrates it produces an alternating voltage across it, in time with the vibrations. Therefore, you can use this voltage to detect the vibrations of the quartz crystal. This is exactly what a quartz watch does.
But what happens when you apply an impulse to it? There is another property involved. We have already said that when you bend a piece of quartz it causes a voltage to appear across it. Well, the converse happens as well. If you apply a voltage to a piece of quartz it will bend.
We in the watchmaking world call these two phenomena, the Piezo-Electric Effect.
How the Piezo-Electric Effect works
We can apply a voltage to a quartz crystal to set it vibrating, and then measure the alternating voltage it produces while it vibrates, this would be like giving a pendulum a push and then leaving it to swing from side to side.
In a real pendulum clock, we give the pendulum a tiny push on every swing, at just the right moment. We let the pendulum determine the right moment, as it moves the pallets along the teeth of the escape wheel.
Quartz oscillators work in the same way. Since the crystal is given a tiny push with every cycle, which sustains the vibration. An electric circuit follows the vibration using the alternating voltage from the crystal and generates each push at just the right time.
So, when a quartz crystal is combined with its electrical circuit, it oscillates continuously at a fixed frequency, which can be used as a timing source. I will go into more detail on how the electronic circuit makes use of quartz crystals vibration in future articles.