Introduction:
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, to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters.
Piezoelectricity was discovered by Jacques and Pierre Curie in 1880. Paul Langevin first investigated quartz resonators for use in sonar during World War I. The first crystal controlled oscillator, using a crystal of Rochelle salt, was built in 1917 and patented in 1918 by Alexander M. Nicholson at Bell Telephone Laboratories, although his priority was disputed by Walter Guyton Cady. Cady built the first quartz crystal oscillator in 1921
A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions.
Almost any object made of an elastic material could be used like a crystal, with appropriate transducers, since all objects have natural resonant frequencies of vibration. For example, steel is very elastic and has a high speed of sound. It was often used in mechanical filters before quartz. The resonant frequency depends on size, shape, elasticity, and the speed of sound in the material. High-frequency crystals are typically cut in the shape of a simple, rectangular plate. Low-frequency crystals, such as those used in digital watches, are typically cut in the shape of a tuning fork. For applications not needing very precise timing, a low-cost ceramic resonator is often used in place of a quartz crystal.
When a crystal of quartz is properly cut and mounted, it can be made to distort in an electric field by applying a voltage to an electrode near or on the crystal. This property is known as piezoelectricity. When the field is removed, the quartz will generate an electric field as it returns to its previous shape, and this can generate a voltage. The result is that a quartz crystal behaves like a circuit composed of an inductor, capacitor and resistor, with a precise resonant frequency.
Quartz has the further advantage that its elastic constants and its size change in such a way that the frequency dependence on temperature can be very low. The specific characteristics will depend on the mode of vibration and the angle at which the quartz is cut (relative to its crystallographic axes). Therefore, the resonant frequency of the plate, which depends on its size, will not change much, either. This means that a quartz clock, filter or oscillator will remain accurate. For critical applications the quartz oscillator is mounted in a temperature-controlled container, called a crystal oven, and can also be mounted on shock absorbers to prevent perturbation by external mechanical vibrations.
Quartz timing crystals are manufactured for frequencies from a few tens of kilohertz to tens of megahertz. More than two billion (2×109) crystals are manufactured annually. Most are small devices for consumer devices such as wristwatches, clocks, radios, computers, and cellophanes. Quartz crystals are also found inside test and measurement equipment, such as counters, signal generators, and oscilloscopes.
Crystal modeling:
A quartz crystal can be modeled as an electrical network with low impedance (series) and a high impedance (parallel) resonance point spaced closely together. Mathematically the impedance of this network can be written as:
or,
where s is the complex frequency (s = jω), ωs is the series resonant frequency in radians per second and ωp is the parallel resonant frequency in radians per second.
Adding additional capacitance across a crystal will cause the parallel resonance to shift downward. This can be used to adjust the frequency that a crystal oscillator oscillates at. Crystal
Schematic symbol and equivalent circuit for a quartz crystal in an oscillator
A crystal used in hobby radio control equipment to select frequency.
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 many oscillators, any spectral energy at the resonant frequency will be amplified by the oscillator, resulting in a collection of tones at different phases. In a crystal oscillator, the crystal mostly vibrates in one axis. Therefore, only one phase is dominant. This property of low phase noise 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 megahertz.
Commonly used crystal frequencies
Frequency (MHz) | Primary uses |
32.768 kHz | Real-time clocks, allows binary division to 1 Hz signal (215 × 1 Hz); also often used in low-speed low-power circuits |
1.8432 | UART clock; allows integer division to common baud rates |
2.4576 | UART clock; allows integer division to common baud rates up to 38400 |
3.2768 | Allows binary division to 100 Hz (32768 × 100 Hz, or 215 × 100 Hz) |
3.575611 | PAL M color sub carrier |
3.579545 | NTSC M color sub carrier; very common and inexpensive, used in many other applications, eg. DTMF generators |
3.582056 | PAL N color subcarrier |
3.686400 | UART clock (2 × 1.8432 MHz); allows integer division to common baud rates |
4.096000 | Allows binary division to 1 kHz (212 × 1 kHz) |
4.194304 | Real-time clocks, clearly divides to 1 Hz signal (222 × 1 Hz) |
4.43361875 | PAL B/D/G/H/I and NTSC M4.43 color subcarrier |
4.9152 | Used in CDMA systems; divided to 1.2288 MHz baseband frequency as specified by J-STD-008 |
5.068 | Used in radio transceivers as an IF source |
6.144 | Digital audio systems - DAT, Minidisk, sound cards; 128 × 48 kHz (27 × 48 kHz). Also allows integer division to common UART baud rates up to 38400. |
6.5536 | Allows binary division to 100 Hz (65536 × 100 Hz, or 216 × 100 Hz); used also in red boxes |
7.15909 | NTSC M color sub carrier (2 × 3.579545 MHz) |
7.3728 | UART clock (4 × 1.8432 MHz); allows integer division to common baud rates |
8.86724 | PAL B/G/H color subcarrier (2 × 4.433618 MHz) |
9.216 | Allows integer division to 1024 kHz and its halves (16 kHz, 32 kHz, 64 kHz...) |
9.83040 | Used in CDMA systems (2 × 4.9152); divided to 1.2288 MHz baseband frequency |
10.245 | Used in radio transceivers; mixes with 10.7 MHz sub carrier yielding 455 kHz signal, a common second IF for FM radio and first IF for AM radio[1] |
10.700 | Used in radio transceivers as an IF source |
11.0592 | UART clock (6 × 1.8432 MHz); allows integer division to common baud rates |
11.2896 | Used in compact disc digital audio systems and CDROM drives; allows binary division to 44.1 kHz (256 × 44.1 kHz), 22.05 kHz, and 11.025 kHz |
12.288 | Digital audio systems - DAT, Minidisk, sound cards; 256 × 48 kHz (28 × 48 kHz). Also allows integer division to common UART baud rates up to 38400. |
13.500 | Master clock for PAL/NTSC DVD players, Digital TV receivers etc. (13.5 MHz is an exact multiple of the PAL and NTSC line frequencies) |
13.875 | Used in some teletext circuits; 2 × 6.9375 MHz (clock frequency of PAL B teletext; SECAM uses 6.203125 MHz, NTSC M uses 5.727272 MHz, PAL G uses 6.2031 MHz, and PAL I uses 4.4375 MHz clock) |
14.3182 | NTSC M color sub carrier (4 × 3.579545 MHz). Also common on VGA cards. |
14.7456 | UART clock (8 × 1.8432 MHz); allows integer division to common baud rates |
16.368 | Commonly used for down-conversion and sampling in GPS-receivers. Generates intermediate frequency signal at +4.092 MHz. 16.3676 or 16.367667 MHz are sometimes used to avoid perfect lineup between sampling frequency and GPS spreading code. |
16.9344 | Used in compact disc digital audio systems and CDROM drives; allows integer division to 44.1 kHz (384 × 44.1 kHz), 22.05 kHz, and 11.025 kHz. Also allows integer division to common UART baud rates. |
17.734475 | PAL B/G/H color sub carrier (4 × 4.433618 MHz) |
18.432 | UART clock (10 × 1.8432 MHz); allows integer division to common baud rates. Also allows integer division to 48 kHz (384 × 48 kHz), 96 kHz, and 192 kHz sample rates used in high-end digital audio. |
19.6608 | Used in CDMA systems (4 × 4.9152); divided to 1.2288 MHz baseband frequency |
24.576 | Digital audio systems - DAT, Minidisk, sound cards; 512 × 48 kHz (29 × 48 kHz) |
27.000 | Master clock for PAL/NTSC DVD players, Digital TV receivers etc. (27 MHz is an exact multiple of the PAL and NTSC line frequencies) |
29.4912 | UART clock (16 × 1.8432 MHz); allows integer division to common baud rates |
