Quartz crystals and ceramic resonators are made from slices of quartz or slabs of ceramic material, with electrodes plated on the sides. They exploit the piezoelectric effect, in which some materials move when subjected to a voltage and also generate a voltage when mechanically flexed. Crystals are always found encased in metal, while ceramic resonators are usually in yellow or orange plastic. Crystals have two leads, and ceramics may have two or three.
Symbols Crystals and resonators use the same symbol unless the resonator features internal capacitors and has three leads.
Crystals and resonators are marked with their frequencies. For crystals, assume the number means MHz. On resonators, assume hundreds of kHz, though some may be in MHz as well. Crystals can sport other numbers indicating the type of cut used, which is quite a complicated topic. It’s not a concern in most service work, though. Either the crystal oscillates or it doesn't.
Quartz crystals are used as frequency-determining elements in oscillators, and sometimes as tuned filters in radio applications. Ceramic resonators are used the same way, but in applications requiring less stability and accuracy, and usually at lower frequencies. You are more likely to find a quartz crystal in the clock oscillator running a digital device like a laptop, DVD player or MP3 player, with a ceramic resonator lurking in a remote control or some radio circuit stages. Crystals and resonators are mechanical. They actually move on a microscopic level, vibrating at their resonant frequency. They are also made of crystalline material, so they’re somewhat brittle. Heat and vibration can crack them, as can a drop to the floor. Quartz crystals, especially, can develop tiny internal fractures and just quit on their own, with no apparent cause.
Some flaws don’t stop them outright; they become finicky and unpredictable. Touching their terminals may cause them to stop or start oscillating. Also, crystals drift in frequency as they age, sometimes drifting past the point at which the circuit will operate properly.
Without a crystal checker, which is simply an oscillator with an indicator light, there’s no way to tell whether a crystal works without scoping its signal in an operating circuit. Even a crystal checker may lie to you, indicating a good crystal that still won’t start in the circuit for which it’s intended.
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