Quartz Crystals, Xtals Tutorial Includes:
Quartz crystals: xtals What is quartz How a crystal works Crystal overtone operation Quartz crystal frequency pulling Quartz crystal cuts Quartz ageing Crystal resonator manufacture How to specify a quartz crystal VCXO TCXO OCXO Crystal filter Monolithic crystal filter
Quartz crystals are often referred to as xtals and they are used to provide very high Q resonant elements for oscillators and filters.
Quartz crystals or xtals can be cheap to produce even though they offer exceptional performance and can be used for everything from clock oscillators for microprocessors to high performance filters and highly stable oven controlled oscillators.
As the name implies quartz crystal resonators or xtals are made from quartz which is is a naturally occurring form of silicon. However most of the quartz used for the electronics industry is manufactured synthetically.
Quartz crystal, xtal resonators are available in many sizes and formats to suit the requirements of most applications. From small surface mount devices right through to larger through hole mounted crystals as well as those for sockets, there are many sizes and formats available.
Quartz crystal, xtals: the basics
Quartz crystal resonator technology relies on the remarkable properties of quartz for its operation. When placed into an electronic circuit a quartz crystal acts as a tuned circuit. However it has an exceptionally high Q. Ordinary LC tuned circuits may exhibit values of a few hundred if carefully designed and constructed, but quartz crystals exhibit values of up to 100 000.
Apart from their Q, crystal technology also has a number of other advantages. They are very stable with respect to temperature and time. In fact most crystals will have these figures specified and they might typically be ±5 ppm (parts per million) per year for the ageing and ±30 ppm over a temperature range of 0 to 60 °C.
In operation the quartz crystal or xtal uses the piezo electric effect to convert the electrical signals to mechanical vibrations. These cause the xtal to vibrate and the mechanical resonances of the crystal then act on the mechanical vibrations. The piezo-electric effect then links back to the electrical domain and the signals are converted back having been affected by the mechanical resonances.
The overall effect is that the quartz crystal links the very high Q mechanical resonances to the electrical domain, enabling very highly stable and high Q resonances to affect electrical signals.
. . . . . Read more about quartz as a material.
Quartz crystal circuit symbol
The circuit symbol for a quartz crystal resonator or xtal is straightforward. The quartz crystal symbol shows the two plates either side of the main quartz element. It has two lines, one top and the other at the bottom with a central rectangle.
Quartz crystal uses
Quartz crystals or xtals are used in two main forms of application: as the resonant element in oscillators, and in filters. In both applications the very high Q of the xtal resonator enables very high performance levels to be achieved.
Some of the uses are outlined in more detail below:
- Oscillators: The high Q of the xtal or quartz crystal means that oscillators using are able to offer very high levels of accuracy and stability. There are several options for the ways in which quartz resonators can be used depending upon the performance requirements and the cost restraints.
- Basic quartz crystal oscillator: Quartz resonators can be used very simply within a straightforward oscillator circuit. As basic quartz resonators are relatively inexpensive, they are often be used as the resonator for applications where they are the resonator within a clock oscillator for a microprocessor, for example. Generally the requirements for accuracy these oscillators are not excessively high and therefore costs can be kept to a minimum by using a quartz crystal. When used in these applications, quartz crystals are cheaper than many other solutions that would not perform as well. Obviously straightforward crystal oscillators are used in many other areas as well.
- VCXO: For some applications a small degree of change of the oscillator frequency may be needed. A VCXO or voltage controlled xtal oscillator is relatively easy to construct. The circuits are straightforward and generally involve using a variable voltage to drive a varactor diode in the crystal circuit. The change in reactance of the varactor changes the overall resonant frequency of the crystal and its associated circuitry. However in view of the high Q of the crystal resonator, only relatively small changes in frequency are possible. These circuits can be built, or they are available as commercial modules.
- TCXO: One of the main causes of frequency change of a crystal oscillator is temperature change. Where more frequency stability is required than can be supplied by a standard oscillator, then a TCXO, temperature compensated xtal oscillator is an option. As the name implies, this form of oscillator applies temperature compensation.
- OCXO: Where the very highest levels of frequency stability are required, the best option is an oven controlled crystal oscillator. This form of crystal oscillator keeps the crystal and its associated circuitry in a temperature controlled 'oven'. This runs at a temperature above the ambient and is maintained at a constant temperature while there oscillator is running. In this way any changes resulting from temperatures changes are minimised.
- Filters : The other main application for quartz crystal resonators is within filters. Here the resonator is used in a circuit which is used to accept wanted signals and reject unwanted ones. The very high Q levels attainable using quartz mean that these filters are very high performance.
The quartz crystal filters may consist of a single crystal, but more sophisticated filters offering a much higher level of performance may be made using six or even eight crystals. In view of the fact that these filters involve experience and advanced design, they are often obtained as filter modules, although many are manufactured by the final manufacturers / designers themselves.
Quartz crystal advantages & disadvantages
Quartz crystal technology offers very many advantages, but against this there are also some other points to be placed into the equation when considering their use:
Advantages of quartz crystal resonators:
- Very high Q resonator: The Q of a quartz crystal is very high. This in turn reflects in terms of several advantages:
- Very stable signal when used in an oscillator.
- Low levels of phase noise when used in an oscillator.
- When used in a filter it is possible to achieve very high levels of selectivity. Crystal filters are able to provide excellent performance and provide some of the best options for sharp filters within a variety of applications.
- Low cost: Basic crystals are available at very reasonable costs. Their use can often result in a cheaper clock or other source when used as the resonator. Highly specified quartz crystal resonators obviously cost more.
Disadvantages of quartz crystal resonators:
- Size: A crystal relies on mechanical vibrations for its resonant behaviour. As a result size cannot be reduced easily and they may be large when compared to other SMT components. That said, new surface mount technology crystals are available in very small packages now.
- Soldering: In view of their performance soldering needs to be undertaken with care observing maximum temperatures and soldering times.
- Fixed frequency: Although this can be an advantage as well, a crystal has its own natural resonant frequencies. Once chosen and manufactured these cannot be altered, although it is possible to 'pull' the frequency of an oscillator by a small amount.
Quartz crystal and oscillators time line
Since the first signs of the piezo electric effect and the action of quartz crystals, it has taken many years for their development to be taken to the stage where it is now.
Early investigations demonstrated the effect, and it was some years before radio technology was developed and the action of quartz crystal resonators or xtals could be demonstrated and then refined.
. . . . Read more about the quartz crystal resonator development timeline & history.
How quartz crystal resonators are made
Quartz crystal resonators or xtals are manufactured in vast quantities. The manufacturing process starts with the raw silicon which is converted into synthetic quartz and then the individual quartz crystal resonators are manufactured from there. Once the basic xtals have been manufactured they are trimmed and then encapsulated.
Specifying quartz crystal resonators
When choosing a quartz crystal resonator there are many parameters that need to be selected. Typically manufacturers will require a number of parameters, often set out on a specific form before they are able to manufacture and supply the required xtal element.
Quartz crystal resonators are widely used within the electronics industry. They can be used in quartz crystal oscillators and crystal filters where they provide exceptionally high levels of performance. In addition to this, low cost elements with lower tolerance specifications are widely used in crystal oscillators for microprocessor board clocks where they are used as cheap resonator elements. Whatever its use a quartz crystal resonator provides an exceptionally high level of performance for the cost of its production.
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