Quartz crystals find their applications in virtually every piece of modern communication equipment such as transceivers, telephone, fax, data transmitting digital equipment, radio and TV transmitters, radar and sonar equipment, as well as data processing equipment and time pieces, not least of all the precise timing of microprocessors.

For all these applications, the crystal's inherent property to oscillate at an extremely constant frequency under certain conditions is utilized. For example, a quartz crystal in a watch may oscillate precisely at 32.768Hz (cycles per second). By means of electronic circuitry, these oscillations control, in the case of the watch, very accurately the display mechanism.

The underlying principle which is utilized by a quartz crystal is its inherent piezoelectric property.

Piezoelectricity is derived from a crystal such as quartz upon the mechanical stress is applied. The opposite sides of the crystal become electrically charged and the difference of potential is proportional to the amount of stress used. Reversal of the stress reverses the electrical charges as well. Moreover, the piezoelectrical effect is reversible. A changing electrical potential will be experienced by the crystal as a mechanical strain, as it starts vibrating in proportion to the varying voltage applied.

A modern oscillator crystal is based on the principle that a properly mounted and supported quartz crystal will vibrate mechanically at a very definite frequency, if an alternating current is applied to its surfaces. The frequency depends on the dimensions of the crystal: For AT Type cut blanks the thinner the crystal, the higher the vibrating frequency. The vibrating crystal consumes power at its resonant frequency. Consequently, it can be used as the frequency controlling element in an oscillator circuit.

The growing of quartz crystals simply involves dissolving quartz from small chips and allowing the quartz to grow on prepared seeds. These encompass a batch process that requires about 21 days to result in the desired size crystals. Approximately one half of each day is required to bring the charge and the equipment to operating temperature.

Employing a hydrothermal process, the quartz chips are dissolved in sodium hydroxide solution. Temperatures are maintained above the critical temperature of the solution. The growth process of the quartz is controlled by a two-zone temperature system such that the higher temperature exists in the dissolving zone and the lower in the growth zone.

In the actual manufacturing process, the quartz chips or nutrients are placed in the bottom of a long vertical steel autoclave which is specifically designed to withstand high temperatures and pressures. The quartz wafers, sliced along the basal plane, are suspended in the top zone of the vessel of seeds. Calculated amounts of deionized water and sodium hydroxide pellets are added and the vessel is sealed. External heat is now applied to achieve the two isothermal zones. Special insulation and a careful controlled pattern of heat application are important to obtain the proper results; the small quartz chips, the raw material, are dissolved by the caustic solution. This solution is now carried by convection currents to the cooler growing zone where it becomes supersaturated and growth begins on the seed plates.

Of paramount importance is temperature control, because the temperature affects the dissolution rate of the nutrient, the deposition of quartz on the seeds and the convective transfer of nutrients between zones. For example, a high temperature differential between the zones leads to rapid growth, however, seed faulting will occur if the temperature difference is too high. Through practical experience, a workable differential of about 38 degree Centigrade has been established.

The manufacture of quartz crystals can be divided into four sections: Cutting , Lapping, Finishing and Quality Control.

A.  Cutting

The cutting operation is the first process performed on the quartz bar. The bar is sliced with special wafering machines into small square dice of dimensions 1.27mm x 1.27mm x 0.04mm. The angle at which the dice are cut is very important for the overall performance of the finished crystals. Special X-ray units are used to assure the proper angle of cut in relation to the atomic planes.

B. Lapping

The quartz dice, called "blanks," cut from the "mother stone," now undergo lapping on precision lapping machines. A progressively finer finish on the major surfaces of the blanks is achieved as the crystals are lapped, first on one machine, then onto another. As the lapping operation reduces the thickness of the blanks, the frequency of the crystals is increased. Proper control of the lapping machines will result in the production of crystals with extremely accurate frequencies.

C. Finishing

After the quartz blanks have been lapped to a thickness which will yield the desired frequencies, they are thoroughly cleaned and metal electrodes are vacuum deposited on their two major faces. The electrodes pick up the electrical impulses which exist on the crystal surface and direct them to springs. The springs, in turn, pick up the electrical impulses and, in addition, help support the crystal to its mounting base.

A final frequency adjustment is made after the crystal has been mounted. Additional metal is vacuum deposited on each crystal. The final step is to seal the crystal hermetically by welding a metal can to its base, in order to protect the fragile blank from damage by moisture, air, handling, etc.

D.  Quality Control

In process quality control, at various manufacturing steps, ascertains favorable production yields. Before the finished units are released, they are thoroughly tested at a Quality control station where paramount attention is given to stable crystal frequency over a temperature range from as low as -55 degree Centigrade to 125 degree Centigrade. The "activity" of the crystal is checked as it indicates how strongly the crystal is vibrating, and a "leak test" is performed in order to assure that the crystal is hermetically sealed from its environment in order to preclude degradation of the unit.

The mode of vibration and orientation angle usable for the desired frequency are determined from this Table.

TABLE 1

*n: overtone order t-thickness l-length w-width

CONSIDERATIONS FOR OSCILLATOR DESIGNS

The design engineer in selecting his crystal oscillator circuit should give careful consideration to the following factors.

(A) AMBIENT CHANGES

Crystals are temperature responsive. Avoid placing crystals near hot spots or cold spots. Where stability is critical, a temperature controlled oscillator or oven may be necessary.

(B) DRIVE LEVEL

The frequency of all crystal units will change to some degree with variations of drive level. Therefore, it is necessary that the drive level specified is that actually being used in the equipment.

GED adjusts at the maximum drive level indicated for the various cuts. Where reduced drive level are desired, please specify along with the reference oscillator.

In some instances, the customer's oscillator circuit may be required.

( C ) OPERATING MODE

Oscillator circuits should be designed to operate in series or parallel resonant mode.

Crystals maintain stable oscillation between series resonance and load resonance.

Oscillator circuits operating inductively (below series resonance) are often unstable.

The value of the (C) can be changed for a particular resonant frequency by varying the electrode area. The motional resistance (R) represents the molecular friction losses of the quartz crystal plus the mechanical damping through the system. The parallel capacity (Co) is the capacity between the vacuum deposited metal electrodes and the quartz material as a dielectric and we have:

where

f = resonant frequency

a = electrode area

n = order no of overtone (1, 3, 5, 7)

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CHARACTERISTICS OF FREQUENCY VS. LOAD CAPACITANCE

For many applications, there are requirements to pull the crystal frequency by using a load reactive element.

This may be necessary in order to trim out the manufacturing tolerance or in phase locked loop and frequency modulation applications.

Fig. 5a, 5b and 5c show respectively.

- a crystal unit alone

- a crystal unit with a series connected load capacitance CL

- a crystal unit with a parallel connected load capacitance CL

Fig. 5 Influence of the Load Capacitance CL

For example in oscillator circuit combination of a crystal unit and a capacitor (Fig 5b) acts as a crystal unit with the load resonance frequency fL in a similar low impedance to Fig 5a.

In Fig 5c, the load capacitance CL is in parallel with the crystal unit, and the combination will act as a crystal unit with a load resonance frequency fS.

Frequency vs. Holders