Frequency Control with Quartz Crystals
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Engineering Bulletin E-6: Frequency Control with Quartz Crystals |
Foreign matter on a crystal can cause erratic performance or prohibit oscillation entirely. A crystal will not oscillate if there is any grease, oil, wax, or similar substance on its faces. Such substances are removed during manufacture by a special degreasing process but can be deposited by handling of the crystal after manufacture.
Dust is probably the greatest offender. It can cause erratic performance or prevent oscillation entirely. Corona can develop when particles of dust separate the crystal and its electrodes since points of high potential naturally appear at each particle. If the crystal is subjected to rather high excitation, a radio-frequency arc can result. The arc will modulate the oscillator output giving it a rough note, and, if allowed to continue, the concentrated heat of the arc may fracture the crystal.
To protect the crystals from dust, modern crystal holders are designed to have closefitting assemblies. In addition, each holder is thoroughly washed before actual use. Sometimes, however, due to handling in shipment, minute particles of dust may be deposited on the crystal causing non-oscillation. This is more common with very high frequency crystals for, naturally, they will be more sensitive to foreign matter than crystals at lower frequencies. A simple cleaning of the crystal and electrodes is usually all that is necessary to restore correct oscillation; further cleanings generally will be unnecessary for long periods of service.
The best cleansing agent is carbon tetrachloride but other solvents such as Carbona Cleaning Fluid may be used providing they have no dissolved or suspended impurities. Clean soap and water is effective but requires greater care as a more vigorous scrubbing action is necessary. The crystal should be carefully washed and then dried with a clean lint-free cloth. In drying, care should be exercised to prevent the crystal from becoming entangled in the cloth and subsequently broken. After cleaning, the fingers should not be allowed to come into contact with the major faces as the oil from the fingers will offset the cleaning operation; the crystal can be handled by grasping it by its edges, or, by employing a pair of tweezers. The same procedure should be followed with the electrodes but, as they are not fragile and have only one active face, the operation is considerably simplified.
Care must be exercised, when replacing the crystal in its holder, so as not to chip the corners or to break the crystal by placing it in such a position that it will bind. Where both of the crystal electrodes are separate from the holder assembly, the crystal is merely placed between its two electrodes and inserted into the holder cavity; the edge of the crystal should not protrude beyond the edge of the electrodes as chipping might result. It should be noticed that one face of each electrode is very finely finished while the other face is rough, in comparison it is imperative that the finely finished faces be in contact with the crystal.
In some types of holders, one electrode is part of the assembly and cannot be removed. This electrode may be slightly larger than the crystal or it may be a small circular "button". It generally fits into a recess in the holder body and has a spiral spring beneath it. The button-type holders such as the Bliley BC3 and HF2, necessitate the exercise of care in reassembly to prevent binding the crystal when the cover electrode is placed in position. If the spiral spring prevents the electrode from seating in its recess, the electrode can be held in position, for reassembly, by the tip of a screw driver.
In other types of holders, such as Bliley BC6 and CM2, the bottom electrode is fixed and the removable top electrode is held by a flat spring in the top of the assembly. The spring pressure is adjustable by bending the spring until the desired tension is obtained. If the second electrode is a small disc, for use with high-frequency crystals, the position of the disc electrode, and its pressure, should be determined by experiment for optimum crystal performance.
