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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.

The oscillator circuit of a quartz crystal timepiece is provided with a tuning capacitor device consisting of a stepped variable capacitor and a continuously variable capacitor connected in parallel with each other and in series with said quartz crystal.

In a circuit particularly suited for producing oscillating signals and including a parallel coupled continuous variable capacitor and a step variable capacitor, the improvement comprising said step variable capacitor, and continuously variable capacitor being mounted to a base plate, first and second lower electrodes mounted on said base plate, and means mounted on said base plate electrically connecting said lower electrodes, said step variable capacitor being defined by a fixed dielectric mounted on said first lower electrode, and a plurality of upper electrodes mounted on said fixed dielectric, each of said electrodes having a different area overlapping said lower electrodes, a rotatable electrically conductive first switch contact member selectively positionable in electrical engagement with one of said upper electrodes, and said continuously variable capacitor being defined by an electrically conductive shaft rotatably mounted on said base plate out of electrical engagement with said first and second lower electrodes; a rotatable dielectric mounted for rotation with said shaft partly overlapping said second lower electrodes; a rotatable upper electrode supported by said rotatable dielectric in variable overlapping relation to said second lower electrode and electrically connected to shaft; said switch contact being supported by and electrically connected to said electrically conductive shaft.

Variable Electrical parameters are those that the Design Engineer must specify when selecting a Quartz Crystal device for any particular application.

1. Package Type: Will the package be Through Hole or Surface Mount, and are there size constraints with either.

2. Frequency

3. Load Capacitance: The specified Load Capacitance is dependent on the Resonance Mode required in the application . For Series resonance, no Load Capacitance is required. For Parallel Resonance, the Load Capacitance specified by the Design Engineer will be used to calibrate the Quartz Crystal thereby effecting the major operating characteristics of the device, including initial Frequency Tolerance.

4. Frequency Tolerance: At 25 degree Celsius, an amount of initial frequency deviation acceptable for the application is required. Tighter specifications of Frequency Tolerance lower yield in Quartz Crystal Blank production thereby serving to increase production costs.

5. Stability: Over the Operating Temperature Range, an amount of total deviation acceptable for the application. Tighter specifications of Stability lower yield in Quartz Crystal Blank production thereby serving to increase production costs.

6. Operating Temperature: Standard Operating Temperature ranges are generally considered as -20-+70 degrees Celsius (considered "commercial" Operating Temperature), and -40-+85 degrees Celsius (considered "Industrial" Operating Temperature) Other Operating temperature ranges are available and should be specified.

To analyse the electrical response of a quartz crystal resonator, it is very often useful to depict it as the equivalent electrical components that would be needed to replace it. This equivalent circuit is can then be used to analyse its response and predict its performance. The basic equivalent circuit of a crystal is shown below. In this circuit C1 represents the capacitance between the electrodes. L, C, and R represent the vibrational characteristics of the crystal. The inductance results from the mass of the material, C from the compliance, and R arises from the losses of which the greatest contributor is frictional losses.

Looking at this circuit it can be seen that there are two ways in which the circuit can resonate. One is from the resonance of L and C which provides a series resonance, giving a very low value of impedance at resonance. This is determined by the value of the resistance R. In this mode the external circuit has very little effect on the crystal resonance.

Limited resources of natural quartz crystal suitable for direct electronic or optical use are available throughout the world. World dependence on these resources will continue to decline because of the increased acceptance of cultured quartz crystal as an alternative material; however, use of cultured quartz crystal will mean an increased dependence on lascas for growing cultured quartz.

Trends indicate that demand for quartz crystal devices should continue to grow, and consequently, quartz crystal production should remain strong well into the future. Growth of the consumer electronics market (for products such as personal computers, electronic games, and cellular telephones) will continue to drive global production. The growing global electronics market may require additional production capacity worldwide.

In the past several years, cultured quartz crystal was increasingly produced in the world, primarily in Asia. Electronic applications accounted for most industrial uses of quartz crystal; other uses included special optical applications.Virtually all quartz crystal used for electronics was cultured rather than natural crystal. Electronic-grade quartz crystal was essential for making filters, frequency controls, and timers in electronic circuits employed for a wide range of products, such as communications equipment, computers, and many consumer goods, such as electronic games and television receivers.

Quartz crystal is the best material for frequency-control oscillators and frequency filters in electronic circuits.

Kingtronics offers leaded quartz crystal HC49U, HC49S and SMD quartz crystal HC49SMD.If you want more details ,please visit website www.kingtronics.com for full specification.

Electronic-grade quartz crystal is single-crystal silica that is free from all visible defects and has piezoelectric properties that permit its use in electronic circuits for accurate frequency control, timing, and filtration. These uses generate practically all the demand for electronic-grade quartz crystal. A smaller amount of optical-grade quartz crystal is used as windows and lenses in specialized devices including some lasers.

More natural quartz crystal was consumed in electronic and optical applications until 1971, when cultured quartz crystal took the lead. Since that time cultured (synthetic) quartz has replaced natural crystal in practically all these applications. The use of natural quartz crystals for carvings and other gemstone applications continued; quartz crystals for this application are discussed in the Gemstones Annual Report of the U.S. Bureau of Mines.

A quartz crystal resonator depends on the piezo-electric effect to work. This effect converts a mechanical stress in a crystal to a voltage and vice versa. In this way the piezo-electric effect converts the electrical impulses to mechanical stress which is subject to the very high Q mechanical resonances of the crystal, and this is in turn linked back into the electrical circuit.

The quartz crystal can vibrate in several different ways, and this means that it has several resonances, all on different frequencies. Fortunately the way in which the quartz crystal blank is cut from the original crystal itself can very significantly reduce this. In fact the angle of the faces relative to the original crystal axes determines many of its properties from the way it vibrates to its activity, Q, and its temperature co-efficient. There are three main ways in which a crystal can vibrate: longitudinal mode, low frequency face shear mode, and high frequency shear. A cut known as the AT cut used for most crystals used in traditional radio and electronics circuits uses the high frequency shear mode.

Vibrational modes of a quartz crystal resonator
(For the sake of clarity, the movements have been greatly exaggerated)

The equivalent series resistance is the resistive element (R1) of the quartz crystal equivalent circuit. (see Equivalent Circuit below) This resistance represents the equivalent impedance of the crystal at natural resonant frequency (series resonance) ESR is measured by a Crystal Impedance (CI) meter.

ESR values are generally stated as maximum values expressed in ohms. The ESR values vary with frequency, mode of operation, holder type, crystal plate size, electrode size, and mounting structure.

It is worth noting that the ESR value at a given frequency for an AT- strip crystal design is generally higher than that of the standard (round blank) design. This becomes more significant at lower frequencies. When transitioning from a series resonant through-hole HC-49/U type crystal to a smaller surface mount type utilizing an AT-strip crystal, some consideration may be given to the difference in the ESR values produced by different cuts.

The ESR becomes critical when resistance values reach a point were the oscillator circuit cannot adequately drive the crystal. Sluggish start-up or unwanted modes of operation may result.

Equivalent Circuit:

The equivalent circuit (shown in Figure A) depicts electrical activity of a quartz crystal unit operating at its natural resonant frequency. The shunt capacitance (Co), represents the capacitance of the crystal electrodes plus the capacitance of the holder and leads. R1, C1, and L1 compose the "motional arm" of the crystal, and are referred to as the motional parameters. The motional inductance (L1) represents the vibrating mass of the crystal unit. The motional capacitance (C1) represents the elasticity of the quartz, and the resistance (R1), represents bulk losses occurring within the quartz.

Quartz crystals used in filters and oscillators in electronic circuits are renowned for their performance, stability, frequency tolerance and their high Q. Yet they do change their frequency very slightly with time in a process known as ageing. Although the frequency variations are small by many standards, they are permanent and may have an effect in some applications where the frequency is of great importance. As a result manufacturing techniques take account of this to reduce the effects of ageing in these crystals as far as possible.

Ageing is caused by a number of interrelated factors. These include internal contamination, excessive drive level, surface change of the crystal, various thermal effects, wire fatigue and frictional wear. The level of ageing can be minimised in a number of ways. During manufacture they should be encapsulated in an inert gas environment, the ensuring should have a good seal so that other gases do not enter. Also the final stages of the preparation of the crystal blank must be prepared as finely as possible. Rather than lapping the blank to bring it to the right dimensions, chemical etching is used. In this way the minimum disruption is caused to the crystal lattice, and this reduces the ingress of contaminants over time that will cause ageing.

The design of the circuit in which the crystal will be used also has an effect. By keeping the drive levels low again the crystal ageing will be less.

The frequency of the quartz crystal is obviously a fundamental specification. It is normally expressed to as many significant figures as demanded by the frequency tolerance, although seven figures is normally the maximum. It is wise to express the frequency to the right number of significant figures to avoid misunderstandings in this area of the quartz crystal specification.

Kingtronica Quartz Crystal series are popular in the word market , Quartz Crystal include HC49U, HC49S and SMD quartz crystal HC49SMD .The HC49U is available in tight tolerances and stabilities for demanding applications. With a broad frequency range from 1.8432MHz to 200.000MHz, the HC49U can serve as a low cost solution in virtually any application or market.