how does a crystal oscillator work?

Answer 1

A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters.

Answer 2

The crystal oscillator circuit sustains oscillation by taking a voltage signal from the quartz resonator, amplifying it, and feeding it back to the resonator. The rate of expansion and contraction of the quartz is the resonant frequency, and is determined by the cut and size of the crystal.

A regular timing crystal contains two electrically conductive plates, with a slice or tuning fork of quartz crystal sandwiched between them. During startup, the circuit around the crystal applies a random noise AC signal to it, and purely by chance, a tiny fraction of the noise will be at the resonant frequency of the crystal. The crystal will therefore start oscillating in synchrony with that signal. As the oscillator amplifies the signals coming out of the crystal, the crystal's frequency will become stronger, eventually dominating the output of the oscillator. Natural resistance in the circuit and in the quartz crystal filter out all the unwanted frequencies.

One of the most important traits of quartz crystal oscillators is that they can exhibit very low phase noise. In other words, the signal they produce is a pure tone. This makes them particularly useful in telecommunications where stable signals are needed, and in scientific equipment where very precise time references are needed.

The output frequency of a quartz oscillator is either the fundamental resonance or a multiple of the resonance, called an overtone frequency.

A typical Q for a quartz oscillator ranges from 104 to 106. The maximum Q for a high stability quartz oscillator can be estimated as Q = 1.6 × 107/f, where f is the resonance frequency in MHz.

Environmental changes of temperature, humidity, pressure, and vibration can change the resonant frequency of a quartz crystal, but there are several designs that reduce these environmental effects. These include the TCXO, MCXO, and OCXO (defined below). These designs (particularly the OCXO) often produce devices with excellent short-term stability. The limitations in short-term stability are due mainly to noise from electronic components in the oscillator circuits. Long term stability is limited by aging of the crystal.

Due to aging and environmental factors such as temperature and vibration, it is hard to keep even the best quartz oscillators within one part in 10−10 of their nominal frequency without constant adjustment. For this reason, atomic oscillators are used for applications that require better long-term stability and accuracy.

Although crystals can be fabricated for any desired resonant frequency, within technological limits, in actual practice today engineers design crystal oscillator circuits around relatively few standard frequencies, such as 10 MHz, 20 MHz and 40 MHz. Using frequency dividers, frequency multipliers and phase locked loop circuits, it is possible to synthesize any desired frequency from the reference frequency.

Care must be taken to use only one crystal oscillator source when designing circuits to avoid subtle failure modes of metastability in electronics. If this is not possible, the number of distinct crystal oscillators, PLLs, and their associated clock domains should be rigorously minimized, through techniques such as using a subdivision of an existing clock instead of a new crystal source. Each new distinct crystal source needs to be rigorously justified, since each one introduces new, difficult to debug probabilistic failure modes, due to multiple crystal interactions, into equipment.

Answer 3

Simply put, all crystals have a natural oscillation. It is very dependant on the mineral used and does not depend on size or mass of material. Using electrical circuits you can harness the oscillation and amplify it and/or decimate it (count fractions or multiples of vibrations). It has many applications.

Answer 4

A particular crystal`s output frequency(s) may be tuned over a narrow range of frequencies by varying the Voltage applied across the crystal.. Changing the ambient temperature may also cause a change in the crystal`s output frequency over a limited range of frequencies. Tuned circuits may be used in conjunction with a crystal to narrow the band of frequencies present on the oscillator`s output.