Electronic – How to use a quartz crystal in an oscillator

1. Crystal, or quartz crystal. Crystal resonator is not at all incorrect but they have been called just crystals for over 50 years to no harm...

2. Ceramic resonator; maybe just resonator, but certainly not crystal or oscillator.

3. Basically yes. (The middle terminal will be ground). However there are also ceramic filters with this configuration, so be careful and double check the datasheet. (Especially if they are marked with common radio IF frequencies, 455 kHz or 10.7 MHz!)

4. I don't know; most likely no.2 in different packaging, I have never seen quartz crystals packaged this way.

5. Yes.

6. No, though connected to the right pins on an IC it might be. A crystal oscillator is either:

   a. something like package (1) with 4 pins; it takes 3.3V or 5V in, and puts out a clean signal (making it dead easy to use), or

   b. a circuit involving these components, possibly a resistor, and some kind of amplifier (inside the IC, or a separate transistor) or a misused logic gate, to drive the crystal. In the latter case you can usually substitute a ceramic resonator, but you may have to change the capacitors and a gain setting resistor (or drive limiting resistor) to make it work at all, or work reliably.

Quartz doesn't generate current at all. Applying strain to a piezoelectric material causes an electric field to appear across it. (And vice-versa — if you apply an electric field, a strain will appear.) That field can drive current through an external circuit, but since the source impedance is effectively infinite, the total charge is limited basically by the capacitance of the electrodes. When enough charge has moved, the resulting electric field across the electrodes cancels the original field produced by the crystal. In other words, for each step change in strain applied to the crystal (either increasing or decreasing), you'll see a corresponding narrow pulse of current in the external circuit.

Electrical resonance is caused by the exchange of energy between the electrical circuit and the mechanical flexing of the crystal. The crystal is shaped so that it has a single well-defined frequency at which it "rings". The mass, stiffness and dimensions of the crystal determine the frequency and how sharply-defined it is ("Q", or quality factor). This mechanical resonance can be shown to be equivalent to an electrical circuit consisting of a coil and a capacitor in series (ignoring parasitic effects for the moment). Because of the high stiffness of quartz, the effective inductance typically is very high — hundreds or even thousands of Henries — and the effective capacitance is very low — a few femto-Farads. It would be impossible to construct a circuit with these characteristics by any other means.

When you use a crystal as the resonant element in an oscillator circuit, the circuit will strongly prefer to oscillate at the resonant frequency of the crystal, because feedback at all other frequencies will be be strongly attenuated by the off-resonance impedance of the crystal.