US2249933A - Piezoelectric plate - Google Patents

Description

Patented July 22, 1941 PIEZOELECTRIC PLATE Rudolf Bcchmann, Berlin, Germany, assignor t Telefunken Gesellschaft fiir Drahtlose Telegraphie m. b. H., Bcrlin, Germany, a corporation of Germany Application September 26, 1939, Serial No. 296,573 In Germany September 24, 1938 Claims.

This invention relates to an electrode arrangement for a piezo-electric quartz crystal plate.

In the use of quartz crystal resonators, more especially as selection means, the single wave performance of the quartz plates plays a decisive part. In the case of shorter waves, the manufacture of single wave plates involves much greater difiiculties than in the case of longer waves. In accordance with an earlier proposition, the single wave performance of quartz plates was obtained by a tapering and a rounding-off of the edges of the quartz plates.

The multiple wave performance of quartz plates is due primarily to two different conditions, one condition being the multiple wave performance and has to do with coupling phenomena between the various degrees of freedom of plate oscillations, such as is the case, for instance, in coupling phenomena between the thickness oscillations and the harmonics of any natural oscillations determined by the transverse dimensions.

These coupling phenomena can be eliminated with the previously described means for working the edges and with the damping by covering the edge regions with an elastic layer. Beyond this, especially in the case of plates whose diameter is large compared with the thickness (larger than 20:1), 2. further multiple wave performance occurs. This invention will be more clearly understood by referring to the accompanying drawing, in which:

Figure 1 shows a round crystal plate with electrodes having a slightly smaller area than the crystal plate;

Fig. 1a is a resonance curve obtained by using electrodes of Fig. 1;

Fig. 2 shows a crystal plate with electrodes still smaller in area than those of Fig. 1;

Fig. 2a is a resonance curve obtained by using electrodes of Fig. 2;

Fig. 3 shows a crystal with two electrodes, each one having different areas;

Fig. 3a. is a resonance curve obtained by using electrodes of Fig. 3; 1

Fig. 4 shows a crystal with two electrodes, each of different areas but still different from that shown in Fig. 3;

Fig. 4a. is a resonance curve obtained by using electrodes of Fig. 4.

A resonance curve whose diameter is large campared with the thickness is shown in Fig. 1a in which in place of a single resonance spot determined by the thickness, several adjacent resonance places appear having almost the same values. The physical performance is seen in that inside the plate whose diameter is assumed to be large against the thickness, multiple reflection phenomena appear which entail such multiple resonators. The plate diameter in the very thin plates under consideration is chosen large in view of the fact that the mounting of small plates entails considerable difficulties. In the case of large plates which are slightly faceted, the mounting can be carried out in the known manner at the circumference without the involvement of substantial damping.

In the invention, such multiple wave phenomena are eliminated more especially in the case of plates in which the diameter is large against the thickness, by dimensioning in a suitable manner the electrical field which excites the oscillations, and when utilizing an excitation surface which is smaller, or small as compared with the surface of the quartz crystal. In this manner, the oscillation performances at the border regions are substantially weakened and single wave resonators are obtained. The use of electrodes having varying sizes was found to be especially effective. The physical explanation for this lies in the fact that aside from the electrical field extending at right angle to the surface of the plate, a. small stray component of the electrical field lies parallel to the surface of the plate when using electrodes of different sizes. The field parallel to the surface of the plate has a compensating effect upon coupling phenomena still existing between the thickness oscillations and the transversal oscillations.

The idea of the present invention will be elucidated in reference to the Figs. 1 to 4a, inclusive. In these figures, there is shown by the upper picture of each figure a cross-section through the crystal and the electrodes and by the lower picture of each figure a curve in which the abscissa represents the frequency and the ordinate the apparent conduction value of the crystal. The curves have been recorded with an apparatus of the type described in Telefunkenzeitung No. '76, July, 1937, page 10. Fig. 1 is concerned with a round disk whose frequency is 7000 kilocycles and whereby the diameter of the quartz crystal is 25 mm. and its thickness is 0.5 mm. The energizing electrode likewise had a round cross-section and both of them had a diameter of 23 mm. In Fig. 2, the electrodes for the same plate were reduced to 20 mm. In Fig. 3, the one electrode diameter was 20, the other one was 12 mm. and in Fig. 4, which finally represents a single wave plate, the one diameter was 13 mm. and the other one 17 mm. The present invention relates to thickness oscillations of quartz plates which carry out shearing oscillations. In addition to the proper choice of the dimensions of the electrodes also the aforementioned rounding-ofi or tapering may obviously be resorted to. Furthermore, plates are employed preferably whose temperature coefficient is less than five times per Celsius degree, such as can be realized by proper cutting.

What is claimed is:

1. A piezo-electric crystal arrangement which will function in the thickness mode of the crystal to give a single frequency response when subjected to high frequencies, comprising a piezoelectric crystal, two electrodes each having an area which is smaller than one-half the area of said crystal and arranged each side thereof, one of said electrodes having a smaller area than the other electrode.

2. A piezo-electric crystal arrangement which will function in the thickness mode of the crystal to give a single frequency response when subjected to high frequencies, comprising a circular piezo-electric crystal, two electrodes each having an area which is smaller than one-half the area of said crystal and arranged each side thereof, one of said electrodes having a smaller area than the electrode.

3. A piezo-electric crystal arrangement which will function in the thickness mode of the crystal to give a single frequency response when subjected to high frequencies in the order of 7,000 kilocycles, comprising a piezo-electric crystal, two electrodes each having a smaller area than said crystal and arranged each side thereof, one of said electrodes having a diameter of seventeen millimeters and the other electrode having a diameter of thirteen millimeters.

4. A piezo-electric crystal arrangement which will function in the thickness mode of the crystal to give a single frequency response when subjected to high frequencies in the order of 7,000 kilocycles, comprising a piezo-electric crystal, two electrodes each having a smaller area than said crystal and arranged each side thereof, one of said electrodes having a diameter of twenty millimeters and the other electrode having a diameter of twelve millimeters.

5. A piezo-electric crystal arrangement which will function in the thickness mode of the crystal to give a single frequency response when subjected to high frequency comprising a piezoelectric crystal having a high ratio of dimensional difference between its diameter and its thickness, two electrodes arranged each side of said crystal, the area of said electrodes being smaller than one-half the area of said crystal whereby multiple wave phenomena are eliminated.

RUDOLF BECHMANN.

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