US2139918A

US2139918A - Piezoelectric crystal mounting

Info

Publication number: US2139918A
Application number: US103382A
Authority: US
Inventors: Franklin W Smalts
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1936-09-30
Filing date: 1936-09-30
Publication date: 1938-12-13
Anticipated expiration: 1955-12-13
Also published as: GB499225A; GB499295A

Description

Dec. 13, 1938. F. w, SMALTS PIEZOELECTRIC CRYSTAL MOUNTING Patented Dec. 13, 1938 UNITED STATES PATENT OFFICE PIEZOELECTRIC CRYSTAL MOUNTING Delaware Application September 30, 1936, Serial No. 103,382

Claims.

'I'his invention relates to the art of mounting piezo-electric crystals and has for its principal object the provision of a low-capacity mounting for piezo-electric resonators and oscillators.

Other objects and advantages will be apparent and the invention itself will be best understood by reference to the following specification and to the accompanying drawing, wherein:

Figure l is a sectional View of a cabinet containing a crystal mounting of a known type marked with phantom capacitors in order to visually indicate the problem with which the instant invention is primarily concerned,

Fig. 2 is a sectional view of a cabinet similar to that of Fig. 1 but enclosing a crystal mounting constructed in accordance with the principle of the invention,

Fig. 3 is a sectional plan view taken on the line 3--3 of Fig. 2, and

Fig. 4 is a chart of the operating characteristics of a piezo-electric crystal when mounted in the holder of Fig. 2 as compared with the operating characteristics of the same crystal mounted in the prior art holder of Fig. l.

Numerous mounting arrangements for piezoelectric crystal elements are known in the art and various advantages are claimed for each type. Perhaps the most common form of mounting is the simple "air-gap type of holder illustrated in Fig. l. Here the crystal, which is designated I0, is simply laid without clamping pressure upon the surface of the lower of two electrode plates. Both the bottom plate I2 and the upper plate I4 are made entirely of metal and are spaced one from the other by means of adjustable screws I6 which occupy space on the plates somewhat removed from the crystal element. The surface area of each of these electrode plates I2 and I4 is preferably considerably greater than that of the crystal element. Further, these electrode plates are preferably of considerable mass. A large electrode surface area has been found to facilitate accurate alignment of the plates and the extra mass not only provides a more rigid, tremor-proof, support but also tends to maintain the temperature of the crystal constant during slight Variations in the temperature of the ambient medium. The principal disadvantages of mounting a piezo-electric crystal element in the manner described are (a) the band width and (b) the current increment (explained in connection with Fig. 4) of the crystal element are considerably less than theoretically possible of achievement.

55 The present invention is predicated upon a proper appreciation of the fact that excess capacitance is the factor inhibiting optimum performance of crystals mounted in the manner dictated by the prior art.

Excess capacitance has been found to exist not only in the area between the electrode plates remote from the crystal, as indicated by the phantom capacitors C but also exists between the side surfaces of the plates and the inner wall v of the metal container or casing I8 in which the -l0 assembly is housed, as indicated at C. The excess capacitance originating at C is especially great where, as is frequently the case, the inner wall of the enclosure I8 is close to the side surfaces ofthe plates I2 and I4. v15

The above described and other factors inhibiting optimum performance of crystals mounted in accordance with the prior art are obviated, in accordance with the present invention, by the provision of a pair of electrodes having a surface v20 area corresponding substantially to that of the principal faces of the crystal element with which they are associated. As shown in Figs. 2 and 3 the metal electrodes, here designated 20 and 3D are mounted, respectively, upon platesv 2| and 3l 25 which are formed of insulating material. The electrodes are preferably contained in recesses 22 and 32 provided for that purpose in the surfaces of the insulating plates. The electrode surface of each electrode is preferably fiush with 30 the surface of the insulating plate in which it is embedded. Preferably each electrode is formed with an integral, central, rod-

like extension

23, 33, respectively, which passes through the body of the insulating material and terminates in a 35 screw threaded portion which accommodates a clamping nut 24--34 and a second screw nut 25-35. Each outerV screw retains one of the necessary current carrying leads 26-36 for actuating the piezo-electric element 40. The entire 40 electrode assembly is supported upon insulating risers 42 upon the base 44 of a metal heat box 46.

The circumferential dimension of the insulating plates 2| and 3I may be the same or similar to that of the large metal electrodes (IIJ-I2 of 45 Fig. 1) whereby sufficient space is provided to accommodate screws 48 for adjusting the airgap between the crystal 40 and the surface of the upper electrode 30. These screws extend through insulating washers 50 positioned be- 50 tween the electrode supporting plates and are provided adjacent their upper ends with spring 52 for equalizing the clamping force applied to the top plate.

Referring now to the chart of Fig. 4. The 55 curves A and B show current plotted against frequency for a quartz piezo-electric resonator when mounted in a standardl air-gap holder (curve A) similar to that shown in Fig. 1 and when mounted in the low capacity holder (curve B) of Figs. 2 and 3. The crystal was cut to respond to a band of frequencies slightly above 570 kilocycles. The applied radio frequency voltage was the same in each case, i. e. approximately 150 volts. The air-gap, i. e. the space between the crystal and its upper electrode, was the same for both tests. The holder in each case was installed in a heat box of a standard type.

Analyzing these curves it will be noticed that the band of frequencies passed Vvby the crystal when mounted in the holder of Fig. l was 78 cycles. This is shown on curve A, and is the frequency difference between points of maximum current (resonance) and approximately zero current (anti-resonance). The band of frequencies `passed by the crystal when mounted in the low-capacity holder -of the present invention, as shown by curve B, `was 226 cycles, which represents an increase of approximately 290% in usable band width. The current passed by lthe crystal when mounted in the prior art holder of Fig. l, as shown by curve A was 390 microarnperes (410 max., minus 30 min.) and the current passed by the crystal when mounted in the holder of Fig. 3, as shown by curve B, was 845 microamperes (890 max., minus 45 min.). This represents an increase of approximately 220% in amount of current passed by the crystal under test.

From the foregoing it will be seen that the mounting of the present invention possesses all of the mechanical advantages, and none -of the undesired electrical characteristics of lprior art mountings. While the invention has been described as embodied in an air-gap holder it is to be understood that such embodiment is merely illustrative and is not to be interpreted in a limiting sense vexcept as required by the prior art and the spirit of the appended claims.

What is claimed is:

l. In combination, `a pair of metal electrodes, a piezo-electric crystal interposed between said electrodes and contacting at least one of said electrodes, the area of the electrode surface of each electrode corresponding substantially to the area of the crystal surface adjacent thereto, a

pair of plates composed of insulating material for supporting said metal electrodes, each of said insulating plates having a surface area considerably greater than that of the electrode which it supports, and means connected between said insulating plates for varying the spacing between said metal electrodes.

2. The invention as set forth in claim 1 wherein said electrodes are embedded in said insulating plates with their electrode surfaces ush with the surface of the plates in which they are embedded.

3. A mounting for piezo-electric crystals comprising a pair of metal electrodes, the area of the electrode surface of each electrode corresponding substantially to the area of the surface of `the crystal -to be used therewith, a pair of plates composed -of insulating material for supporting said metal electrodes, each of said insulating plates having `a surface area and mass considerably greater than that of the electrode which it supports, and means connected between said insulating plates for varying the spacing between said metal electrodes.

4. The invention as set forth in claim 3 Wherein said Velectrodes are embedded in said insulating plates with their electrode surfaces flush with the surface of the plates in which they are embedded.

5. In a piezo-electric crystal mounting, a pair of flat plates composed of insulating material, each having a metal electrode embedded in its central portion and with its electrode surface flush with the surface thereof, means for maintaining said plates and electrodes in spaced relation, a .piezo-electric element interposed between said electrodes, -a metal cabinet, and means for supporting said crystal mounting in said cabinet with `the Yelectrodes substantially in the center thereof so that the capacity between said velectrodes and rmetal cabinet is minimized.

FRANKLIN W. SMALTS.