Quartz Crystals & Quartz Crystal Resonator Tutorial

- quartz crystals are resonators that are widely used within the electronic industry as a cheap technology providing highly effective performance for use within filters to oscillators.

Quartz crystals are widely used in today's electronics circuits as high quality tuned circuits or resonators.

Despite their high performance, quartz crystals are cheap to produce and they find many uses in applications from oscillator clock circuits in microprocessor boards, the timing element in digital watches as well as their more traditional applications in radio frequency applications where they may be used as the resonators in highly stable quartz crystal oscillators of high performance crystal filters.

As the name implies quartz crystal resonators are made from quartz, a naturally occurring form of silicon, although most of that used for electronics applications is manufactured synthetically these days.

Today quartz crystal 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 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 degrees Celsius.

Natural quartz crystal
A crystal of naturally occurring quartz

Quartz crystal applications

Quartz crystals are used in two main forms of applications, namely as the resonant element in oscillators, and they are also used in filters. In both applications the very high Q of the resonator enables very high performance levels to be achieved.

  • Oscillators:   The high Q of the quartz crystal means that oscillators using them as the resonant element are able to offer very high levels of 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 variable frequency crystal 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. Read more about the VCXO
    • 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 crystal oscillator is an option. As the name implies, this form of oscillator applies temperature compensation. Read more about the TCXO
    • 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. Read more about the OCXO
  • 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. Read more about the Crystal filter

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:

    • 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.
    • 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.
  • Disadvantages:

    • 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 SMYT 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.

How quartz crystal resonators are made

The individual quartz crystal resonators are manufactured from large man-made crystals that are generally several inches long. They are around two inches in diameter and have a hexagonal cross section. The individual quartz crystals are cut from the large crystal using diamond wheels. These are required in view of the hardness of the material. The angle of the cut to the axes of the original crystal is determined very accurately to ensure the final crystal has the right properties. The blanks that are created from the cutting process are in the form of discs, often about the size of a small coin, although this varies according to their final frequency of operation. Once the blanks have been cut they are lapped using a very fine paste to bring them to nearly the right size. The lapping paste normally consists of very fine silicon carbide or aluminium oxide. The final stage of preparation usually involves chemical etching, because this process enables the required very fine finish to be obtained.

The next stage in manufacture involves mounting the quartz crystal. Silver or gold contacts are chemically deposited onto both sides of the blank. The amount of metal that is used in this process can be used to trim the operating frequency of the crystal to its final value. Finally the crystal is mounted into its can or glass envelope. This is either evacuated or filled with an inert gas to minimise ageing.

Specifying quartz crystal resonators

When choosing a quartz crystal resonator there are many parameters that need to be selected. Many are fairly simple like the tolerance figures. However a few of the others need a little extra explanation. One is the type of resonance. Like any tuned circuit a crystal can have a parallel or series form of resonance as shown. This will have to be specified. If the crystal is to have a parallel resonance then a load capacitance will have to be chosen. This is required because any capacitance across the crystal will alter its resonance slightly. Typically this might be 30 pF, but it will be dependent upon the circuit to be used. Also the tolerance required must be specified. The closer the tolerance, the more expensive the crystal will be, so it is wise not to over-specify the item.

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.

By Ian Poole

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