Crystal oscillators vs LC circuits

Question

I have 3 questions about crystal oscillators, their relation with an RF coil and their usage in electric circuits.

1. This image of a crystal oscillator is all over google and youtube, but no one actually shows what it does. When you apply a voltage and briefly disconnect it, does the disk wobble in place or hit each other, or both? Does this action work like an on/off switch and connect/disconnect the two discs OR do wobbling discs produce alternating electricity? If the latter, then where does this electricity come from, does the crystal create it out of thin air? I wish there was a slow motion video of what it actually does.
2. I wanted to compare crystals with the combination of a RF coil and a capacitor in an LC circuit. Does the crystal oscillator pretty much do the same thing and create alternating current in the circuit?
3. My last question is about the crystal's frequency and how it is used in electric circuits. A computer clock consists of a high voltage and a low voltage (i.e 5V and 0V,) but the crystal oscillator is alternating current with a sine graph correct? It does't just go from 1 to 0 in an instant? It works like an analogue signal. If so then how id its current converted to 0V/5V or a square graph?

Answer 1

When you apply a voltage and briefly disconnect it, does the disk wobble in place or hit each other, or both? does this action work like an on/off switch and connect/disconnect the two discs OR do wobbling discs produce alternating electricity?

The disk doesn't so much wobble as it expands and contracts due to its piezoelectric nature. The Wikipedia article on crystal oscillators has an illustration showing the various modes a crystal might vibrate in. Likely these vibrations are too small to be seen with the naked eye.

if the latter then where does this electricity come from, does the crystal create it out of thin air?

The crystal is piezoelectric so in principle you could apply a mechanical force to it and it would produce a small voltage in response.

Practically, we build the crystal into an oscillator circuit and provide power to it with a battery or power supply, and this is what provides the energy to vibrate the crystal and to produce the oscillator's output signal. In this case, any piezoelectric response to mechanical stimulus (called "microphonics") is undesirable as it produces noise in the oscillator output.

I wanted to compare crystals with the combination of a RF coil and a Capacitor in an LC circuit. Does the crystal oscillator pretty much do the same thing ..

Yes, electrically, the crystal behaves very much like an inductor and capacitor in series.

Its advantage is that it has a higher "Q" factor than can be realized with practical discrete inductors and capacitors, so it provides a sharper resonant peak than an LC circuit can.

and create alternating current in the circuit?

No, this is not what a crystal does and it is not what an inductor-capacitor filter circuit does.

To use a crystal (or LC filter) to produce an alternating current, you must build an oscillator circuit around the resonant element.

There are many such circuits, which apply the resonant element in different ways. One of the simplest conceptually is the Pierce oscillator.

A computer clock consists of a high voltage and a low voltage (i.e 5V and 0V). But the crystal oscillator is alternating current with a sine graph correct? it does't just go from 1 to 0 in an instant? it works like an analogue signal. If so then how does its current get converted to 0V/5V or a square graph?

You can use a comparator or a high gain digital buffer circuit to convert a sinusoidal signal to a digital square wave.

In the case of the Pierce oscillator referenced above, since the gain element is actually a digital inverter, the output is digital (square wave rather than sine wave) and no conversion is required.

Answer 2

Neither a picture nor a normal video will show what the resonator does, because its mechanical vibrations are tiny. You don’t see it doing anything at all. The power levels involved are microwatts. A crystal resonator attached to a working oscillator circuit appears just as “dead” as if the battery ran out.

As for how can a crystal resonator work: by transduction. The transducer action couples the mechanical and electrical domains. In fact, the main reason we use mechanical resonators is that it’s not feasible to produce equivalently good lumped electrical components. So instead, we couple the electrical voltage to mechanical strain, and can use the mechanical system to implement an equivalent LC tank that would be impossible to make as electrical coil and capacitor. But we can describe the mechanical resonator’s behavior, as seen from electrical domain, very accurately by using the idealized components that you find in circuit modeling.

Same thing happens with speakers. You can couple a speaker to an acoustic resonator, use it as a transducer, and have that resonator put into an electrical oscillator circuit. Voilà, an electro-acoustic oscillator! And by the way, yes, you could replace the speaker+resonator with a purely electrical model.

Same happens with electrical circuits driving heaters and heat propagating through solids, etc. You can replace electrical circuits with physical equivalents in other domains. Sometimes those other domains make the problem much easier. For example the high-q resonators can often be mechanical rather than electrical. If you need to delay electrical signals, using a mechanical transmission line as a delay element for “low” frequency signals is much cheaper and easier than using an equivalent electrical transmission line, even one built from lots of lumped elements.

From electrical circuit’s perspective, the crystal resonator appears exactly like a very efficient high Q electrical tank circuit would look. The reason that this “appearance” works is by coupling mechanical and electrical domains via transduction that uses some effect that links the domains. E.g. crystal resonators use piezoelectric effect, electroacoustic resonators and delay lines may use Faraday forces (say a speaker coil), magnetostriction is used in nickel actuators in wire memories (a real thing, used in some electronic calculators from the early 70s!), and piezoelectricity is used in those annoying buzzers, but also in sonography heads, etc.