US1967839A - Piezo-electric crystal - Google Patents

Description

Patented July 24, 1934 UNITED STATES 1,967,839 PIEZO-ELECTRIC CRYSTAL Mendel Osnos, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. 11., Berlin, Germany, a corporation of Germany Application June 30,

In Germany 1932, Serial No. 620,278

July 11, 1931 1 Claim. (Cl. 1'71327) It is known that the natural period of a piezoelectric crystal experiences changes with its temperature. In the case of a quartz crystal which is so cut out of a mother crystal that its maximum surface is parallel to an electrical axis, the frequency generally grows with increasing temperature. Of course, this constitutes a serious disadvantage whenever constant or stable frequency is desired. Now, experiments have shown that if in a crystal cut as above pointed out the circumference or contour of the largest surface or facet is formed by an irregular polygon the change in the crystal frequency with the change of temperature is not uniform, indeed, that in some temperature ranges it is more marked and in others less so.

Hence, if the temperature of such a crystal is kept inside the limits where the change in frequency is but small the variations in wave length will be but small. As a matter of fact, a perfectly stable or nearly stable frequency can be insured inside wide temperature ranges if two sides of the crystal are made parallel to two of its electrical axes or nearly so.

A better understanding of the present invention may be had by referring to the following description accompanied by drawing wherein Figures l and 2 illustrate the manner of obtaining crystals for use in accordance with the present invention, and Figure 3 illustrates such a crystal in a particular circuit embodiment for obtaining substantially constant frequencies within definite limits.

For instance, in the case of a crystal of \=123.5 m as in Figure 1 where the sides a and b are out along the electrical axes, a constant frequency has been maintainable between 32 and 45 degrees C. approximately.

For another crystal of \==l08 m (see Figure 2), a stable wave length has been obtained between 25 and 36 degrees C.

The arabic numbers in Figures 1 and 2 designate millimeters and in general indicate relatively the dimensions of the sides of the crystals.

As can be seen both crystals are of widely different dimensions. The only common factor between them is that the angle between the sides or faces a and b was 120 degrees, that is to say, the angle which in a quartz crystal is known to be formed between the electrical axes thereof.

In order that a crystal according to this invention may be made wholly independent of temperature influences all that is necessary is to maintain its temperature by artificial ways and means approximately at a mean temperature. For instance, the temperature of the crystal Figure l should be maintained around 40 degrees C., and the temperature of the crystal Figure 2 at about 30 degrees C. But it is by no means necessary to stabilize the temperature in an absolute sense. The mean temperature can be kept in a simple way, for instance, by electric heating so that the heating current is only roughly stabilized.

One embodiment of the idea is shown in Figure 3, where 1 the lower electrode of the crystal holding means, 2 the top electrode, 3 a piezoelectric crystal, 4 an electrical heater, 5 a regulating resistance (rheostat), and 6 an ammeter.

If the maximum room temperature (ambient temperature) is above the maximum limiting permissible temperature, a mean admissible temperature of the crystal could be'secured by cooling, say, by the aid of a ventilating fan.

I claim:

A quartz crystal element having an irregular shaped electrode surface, said surface being bound by two lines parallel to each other, a third line perpendicular to the parallel lines and a fourth line at an angle of 120 degrees to one of said parallel lines.

MENDEL OSNOS.

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