Frequency Control with Quartz Crystals
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Engineering Bulletin E-6: Frequency Control with Quartz Crystals |
As previously pointed out, the resonant and anti-resonant properties of a quartz crystal are manifested when the crystal is placed in a radio frequency field. This is true whether the field is produced by an external source of energy or by feedback action in an oscillator circuit. The direct, and obvious, method of producing the necessary field is to place the crystal between two metal electrodes connected to the source of radio-frequency potential. The complete assembly consisting of the two electrodes and a dust-proof insulating body is known as a crystal holder or crystal mounting. The crystal holder, when supplied complete with a calibrated crystal, is termed a crystal unit.
There are four types of crystal holders in general use today: (1) pressure mountings, (2) air-gap mountings, (3) knife-edge mountings, and (4) temperature-controlled mountings. An additional type is the pressure-air-gap which combines 1 and 2.
The pressure-type holder is best suited for installations where the crystal is to develop comparatively high potentials or where the mounting will be subject to external vibration or shock as would be encountered in mobile or portable applications. In the pressure holder, the electrodes are maintained in intimate contact with the crystal faces under pressure exerted by a spring. Holders used with a wide range of crystal frequencies sometimes are provided with a variable spring pressure feature such that optimum pressure can be obtained for each particular crystal. Crystal units manufactured in production for a given frequency, or a given band of frequencies, often incorporate fixed electrode pressure since the optimum pressure can be predetermined and does not vary widely from crystal to crystal.
Pressure holders are suitable for frequencies from 400kc. to 30,000kc. In the frequency range from 400kc. to about 7000kc., both electrodes have essentially the same face area as the crystals. At higher frequencies, however, one electrode is made in the form of a disc, generally with a diameter in the neighborhood of 1/2 inch, so as to reduce the holder capacity (C1, figure 2). This reduction of capacity promotes better crystal oscillation.
To offset the obvious difficulty of manufacturing a disc electrode holder having high mechanical stability (which is a requisite for stable crystal performance) a new type of disc electrode (Footnote 6) was developed by Bliley Engineers. This electrode is formed by recessing a portion of the active face of a full sized electrode such that the remaining center portion has the shape of a disc while a small raised section remains at each extreme corner. The relieved area reduces holder capacity, the center section acts as a disc electrode in the usual manner, and the corners serve to clamp the crystal for mechanical stability. This type of electrode is employed for frequencies up to 11,000kc. At higher frequencies, the simple disc electrode presents the only practical arrangement.
In the air-gap crystal holder, there is an air gap between the crystal and either one, or both, of the electrodes. Holders of this type, which are manufactured for oscillator frequency control crystals, are generally provided with a means for varying the spacing of the air gap. This is usually accomplished by attaching one electrode to a micrometer screw such that the electrode can be moved in a direction parallel to the plane of the crystal faces. A variation of this arrangement, employed in 80-meter and 40-meter amateur frequency crystal units (Bliley types VF1 and VF2), makes use of an adjustable angular air gap. (Footnote 7) The angular air gap, by discouraging arcing and greatly reducing the detrimental effects of air-gap air resonance, extends the usefulness of the crystal for variable frequency purposes. This arrangement is superior to the parallel air gap where the crystal is expected to develop comparatively high potentials and where a relatively wide adjustable frequency range is wanted. The holder design does not, however, readily lend itself to the precise mechanical assembly possible with the parallel air-gap mounting. For this reason, the use of the angular air-gap holder is largely confined to amateur applications for which it is admirably suited.
The specific advantage of the variable air-gap holder lies in the fact that the oscillating frequency can be varied over an appreciable range. This is a most convenient feature in applications where the oscillating frequency must be accurately maintained within very close limits of a specified value. It is not always conveniently possible to finish a crystal directly for each particular transmitter but, through the use of a variable air-gap holder, the crystal can be calibrated in a standard test oscillator. The station engineer can then make any necessary readjustments of frequency simply by changing the air-gap setting.
The variable air-gap holder is particularly useful in amateur transmitting equipment for the purpose of shifting frequency to avoid severe interference. It is equally advantageous for operating near the edge of any band of frequencies because the operator can set his frequency much closer to the edge than would be possible by working with a fixed-frequency crystal.
Variable air-gap holders can be used with crystals from 100kc. to 11,000kc. At frequencies much above 5000kc., however, the holder must be very carefully manufactured and special efforts taken in the finishing of the crystal. In view of the limited frequency swing which can be realized at high frequencies, the major advantage in the use of a variable air-gap holder is in enabling the manufacturer to work to a closer finishing frequency tolerance.
The total frequency range over which a crystal can be adjusted by means of can air gap varies with frequency and is somewhat dependent on the amount of circuit capacity appearing in parallel with the crystal. At 4000kc., with a type VF1 unit, the range is about 6kc. while with a type VF2 unit, the range is extended to 12kc. The frequency swing obtainable with the parallel-gap holder, which is used primarily for fixed frequency operation, is considerably less than with the angular gap mounting. With either arrangement, as the air gap is increased, the effective activity of the crystal is decreased (refer to section entitled CRYSTAL ACTIVITY). If the air gap is made too large, the crystal will not oscillate.
Fixed air-gap holders, in the exact sense of the term, are not widely used. Their application is confined mostly to crystals at relatively low frequencies where the cost of a variable air-gap or knife-edge holder is not warranted for the particular application (low frequency crystals are quite sensitive to electrode pressure and, accordingly, are best mounted in an air-gap or knife-edge holder). Mechanically, the fixed air-gap holder consists of two electrodes spaced apart by insulating washers or by an insulating ring. The distance between the electrodes is made a few thousandths of an inch greater than the crystal thickness creating, thereby, the fixed air gap.
In modified form, the fixed air-gap mounting is rather extensively applied in pressure-type holders. An oscillating quartz plate seldom vibrates uniformly over its entire facial surfaces; maximum motion usually occurs at the central area and minimum motion exists at the corners. This means that it is possible to apply greater pressure at the corners than at the center before vibration will be impeded. An obvious method for taking advantage of this fact to increase mechanical stability and to improve general performance is to cut away the central portion of the holder electrodes. This results in the development of a fixed air gap (no electrode pressure) over the major portion of the crystal faces while permitting high pressure to be applied at the extreme corners of the crystal. A practical further modification consists of distorting the electrodes such that the faces toward the crystal are concave surfaces. Either method is applicable to crystals having frequencies from 400kc. to about 7000kc. At higher frequencies, the modified disc electrode previously described, is applied.
The principle of corner clamping can also be followed with variable air-gap mountings in which it is usual practice to locate the crystal with respect to the electrodes by means of a loosely fitting retainer ring. In this case, a ring or frame is used to apply pressure only to the corners of the crystal while leaving the center free for the variable upper electrode. By clamping the crystal in such a manner, small frequency changes, which could occur by shock or vibration causing displacement of the crystal relative to its electrodes, can be eliminated. Corner clamping for air-gap mounted crystals is practical for frequencies above approximately 1500kc.
The knife-edge holder is applicable to bar type crystals in the frequency range from 16kc. to 275kc. Briefly, the crystal electrodes are formed directly on the crystal faces with a pure metal, generally silver, and the crystal is rigidly supported between knife-edges placed at a nodal point (position on the crystal where zero motion exists as a result of standing waves). knife-edge mounting is advantageous because fairly heavy shocks cannot harm the crystal or change its frequency and because the crystal activity is less affected by the holder than by other types. Furthermore, the crystal never requires cleaning.
Temperature control is employed where the crystal frequency must be held essentially constant under widely varying temperature conditions. Temperature-controlled mountings combine an automatic temperature control feature with a crystal holder. The holder generally consists of a large metal block, whose temperature is regulated by a heater and thermostat, a second electrode and an enclosing protective casing. The crystal holder proper can be variable air-gap, variable or fixed pressure, or knife-edge mounting. Temperature control can also be accomplished by placing any type of crystal holder in a box-type constant temperature oven. The box-type oven possesses the closest degree of temperature regulation since better heat distribution and insulation is possible. The self-contained temperature controlled mounting is, however, more regularly employed because of its compactness and lower cost. When used with low-drift crystals, it is adequate for all applications but those requiring the utmost in frequency stability.
Stainless steel, Monel metal and Duralumin are the metals most commonly used for the electrodes in crystal holders. The choice of material is based on corrosion resistance, machinability, uniformity, hardness, freedom from warpage and lack of foreign substance, such as oil, which might work out from the metal and interfere with normal crystal performance. Stainless steel is most widely used and is usually heat treated to discourage warping. In temperature-controlled mountings, where thermal conductivity is an important factor, Duralumin is employed.
