US2575819A - Crystal unit - Google Patents

Description

Nov. "20, 1951 A. a. LAlRD 2,575,819

CRYSTAL UNIT Filed Feb. 5, 1949 WVEA/TOR 14.6. LA/RD ATTORNEV Patented Nov. 20, 1951 UNI ED STA ES ATE T QFF Arthur G. assignor to Bell Telephone Laboratories,

Incorporated, New

N. Y. a corporation or" New York" Application February 3, 1949, Serial No. 74,380

(Cl. r3 32?) 9 Claims. 1

Th s inve o ates o iezo t ic c s a units, and particularly to a coated crystal having new and desirable characteristics, and a hermetically sealed unit incorporating an improved mounting cage therefor.

Piezoelectric crystals are widely used in the ommunications field be ause of their ty t resonate sharply when subjected to alternating potentials at particular frequencies characteristic of the individual crystal.

This property is made use of in many ways, one use being to control the frequency of vacuum tube oscillators, where it is especially advantaoons because of the high degree of frequency stability exhibited under such control. Another use is in electrical filters, Where the crystal elements provide sharp discrimination between currents of different frequencies.

An object of the invention is a crystal unit construction especially adapted to crystals vibrating in the thickness shear mode of motion, which will give enhanced accuracy and reliability under service conditions.

A related object is a novel construction of coating for crystals which will possess superior electrical properties and at the same time dampen certain undesired vibrations.

A further related object is a mounting of novel construction especially adapted to this type of crystal and crystal coating.

The crystal, in accordance with the present disclosure, is an elongated rectangular quartz blank of AT cut or other suitable cut, vibrating in the thickness shear mode of motion, and arranged to be held by a plurality of spring arms engaging opposite major faces near the crystal ends. In order to prevent spurious modes of vibration and improve the =Q factor of the crystal, it has been found that beveling the ends is beneficial. The end portions of the crystal are also short-,circuited to obtain certain improved electrical characteristics and in order to dampen vibration in the areas engaged by the crystal ase. This ma es t possible t obta n changing contact resistance by the use or" a plurality of specially formed resilient fingers care fully adjusted to a uniform optimum pressure, without being compelled to rely on lead wires permanently connected to the crystal plating.

These fingers are arranged for easy reception and proper positioning of the crystals, which may thus be readily changed or replaced when de;

sired, and also tends .to reduce the transmission of mechanical vibration between the mounting nd c y tal The resilient ringers themselves are designed to provide advantages in case of manufacture and assembly into the crystal cage, and the entire unit'is of a simplified, rugged design.

'lhe invention may be better understood by is en t the d w s i w i h Fig. l is a perspective view of the complete sealed crystal unit;

Rig. 2 is a front elevational View of the crystal 1 n mou tin pa how partially n se with the" cover removed;

Fig. 3 is an exploded view, in perspective, of e elemen s s own n d 2;

Sig. fl is a perspective fragmentary detailed 3 i ual a erred r o the a ed r tal;

Fig. 5 is a perspective view of another preferred form of the crystal during manufacture, illustrating an intermediate stage of production; and

Fig. 6 shows the embodiment of Fig. 5 in coml t d rm- Iteferring now to the drawings, the sealed crystal unit i is shown in Big. 1 as having a metal asi 2. arra ged to be he d n rmal y at round potential through appropriate connection, not shown, to a grounding lug 4. The lower periphery of the casing 2 is sealed to the base 5 by suitable means, such as by sweating it into a moat or trough 5 filled with solder l. A breather hole 9 is provided to permit egress of air expanded by heating during the sweating process. There after the unit may be evacuated, filled with dry air or an inert gas such as nitrogen, and the breather hole sealed with solder. Supporting rods or pins Hi and H afford electrical connection to the crystal within the casing 2.

In Fig. 2, the assembly is shown with the casing 2' removed. Insulating bushings I 2 and I4, through which the parallel supporting rods or pins if and H, respectively, extend, are fixed in the base Soldering lugs 15 and It may be secured by means such as spot welds 11 to rods l6 and II for convenience in connection, as shown in 2, or tubular-shanked, pig-tailed connecting pins l3, as seen in Fig. 3 may be secured to rods 10 and II by conventional techniques.

Rods It and H carry the lower mounting plate assembly L8 and upper mounting plate assembly IS, constituting therewith the cage in which the crystal 20 is supported. The assemblies l8 and 19 are of substantially identical construction. Each consists of a plate 21, formed of insulating material such as mica, bored .to receive elongated bular m al eyeletszi arranged to fit siidably over the parallel supporting rods I and H. Each eyelet head 24 engages one side of the plate 2|. Longitudinal strips 25, cut from the elongated tubular portion 26 of the eyelet 22, are rolled back against the opposite side of the plate, as shown at 21, to fix the eyelet therein. The rolled-back portion 21 of one of the eyelets in each of the mounting plate assemblies l8 and L also secures aconnecting lug 29, extending from the spring clip 50, to plate 2| and makes electrical contact between that clip and one of the rods I!) or II. Clip 39 is held securely to plate 2| by a tubular rivet 3| disposed centrally through the base 32 thereof. Rivet 3| also secures to base 2| an auxiliary spring clip 34, which provides lateral and longitudinal positioning for the crystal 20.

Spring clip 30 is formed from a flat sheet of metal such as a beryllium-copper alloy which has high electrical conductivity and is mechanically resilient. From opposite sides of a base 32 and normal thereto extend a plurality of spaced parallel fingers 35 and 36. The fingers 35 and 35,- extending from opposite sides of the base 32, are rolled terminally inward, forming opposed contact surfaces 3'! for the reception of the crystal 2!]. Between the contact surfaces 31 and the base 32 of spring clip 36 is disposed the auxiliary spring clip 34. This auxiliary clip is shown in its normal position in the lower portion of the exploded view of Fig. 3, and by itself in the upper portion of that figure in order to delineate the details of construction more plainly. From the base portion 39, which is bored to permit entry of the rivet3l, resilient arms 40, extend at a 45 degree angle. Arms 40 are then bent in a double curve to offer longitudinally-positioning portions 4| and laterally-positioning portions 42 to crystal 29. Spring clips 30 and auxiliary spring clips 34 are goldplated to insure good conductivity and prevent corrosion. As illustrated in Figs. 2 and 3, the coated crystal 20, or 48, may be inserted into the cage assembly by pressing the crystal downwardly to a position where the lower end thereof is inserted and held between the spring fingers 31 of the lower spring clip 36 and also between the positioning portions- 4i. and 42 of the lower auxiliary spring clip 34, and then sliding the upper spring clip assembly l9 downwardly on its support rods [0 and H to. a position where the upper end of the crystal 20 is inserted and engaged between the spring fingers 31 of the upper spring clip 30 and also between the positioning portions 4| and 42 of the upper auxiliary spring clip 34. The crystal 20 may be removed, and also replaced in the cage assembly, by a similar procedure. of the cage with the crystal therein, the parts may be secured to the support pins H) and H by means suchas spot-soldering or spot-welding thereto the tubular portions 26 of .the eyelets 22, as indicated at 43, forming a rugged, shockresistant mounting for the crystal.

The crystal 20 itself is, as shown in the fig- 'It is anticipated that, after assembly awash) ures, of elongated rectangular form. Plating is applied in a particular manner which has been found to give improved performance. The crystal is in this case an AT cut quartz plate,

with the width along the X or electrical axis,-

tion introduces difiiculties in mounting, since there are no nodal points, at which the vibration would be negligible, available for connection or contact. This is overcome by the means described below. The AT cut quartz plates designed for operation below about 2.5 megacycles per second are beveled at the ends in order to obtain a better Q and modified boundary conditions. Above that frequency, it has not been found necessary to use the beveled construction.

If the beveled construction is used, it is preferred that the chamfer angle be substantially 30 degrees to the adjacent major face, as shown at in Fig. 4. The chamfering may terminate in a blunted edge 46, as shown in Fig. 4, or in a sharp edge 47, as shown in Fig. 3. If the former, it is preferable that the blunted edge be about 15/5 in width, where t is the thickness of the plate.

The crystal blanks 48 are given terminal undercoatings of silver on each major face adjacent the ends as shown in Fig. 5, the terminal undercoatings 49 are applied to the reverse side, and similar terminal undercoatings 50 to the obverse side of the blank 48. This may be a thin layer of the commercially known Hanovia paste, or other mixture of finely divided silver in a suitable binder. These undercoatings are fired to create a firm bond with the crystal surface. The bonded layer affords a hard contact surface which will not be readily abraded away as the crystal is engaged with the mounting cage.

Finally, with the aid of conventional masking and evaporating equipment, a gold plating 5| isapplied to each major face, and continued over one pair of the end undercoatings 49 and 59 to form the terminal plating 52. It will of course be understood that the usual heat aging and baking operations are used, in conjunction with the gold plating, which is preferably carried out in vacuo, at a pressure of 1 to 2 microns.

The preferred arrangement is seen most clearly in Fig. 4, where the terminal coating 52 is partly broken away to show the undercoating 50, for the blanks having beveled ends, and in Fig. 6 for those having square ends. Another preferred construction for beveled ends is seen inFig. 3.

With particular reference 'to the embodiment of Fig. 4, the face coating 5| covering the obverse major face is shown as continuous with terminal coating 52 extending over the opposed terminal undercoating 49 and 50 of one end, and terminally around the sides at 54 to join these opposite undercoatings. It is to be understood that the coating covering the reverse major face is similarly joined to both terminal undercoatings" at the opposite end, not shown, of the crystal. The same sort of an arrangement is shown in Fig. 6. In both of these cases, it should be observed that none of the plating is permitted to extend onto the end proper, whether it be the beveled portion 45 or the square end 55.

In the embodiment of Fig. 3, however, the connection between opposite end platings is obtained solely by extending the plating over the sharp end 41, and no plating is placed in the side position corresponding to 54 in Fig. 4. Either of these arrangements effectively short-circuits each of the end portions of the crystal 2!] or 48, and hence reduces the motion to a negligible amount at the ends. Hence a pressure clamping arrangement may be used without difficulty in maintaining good connection and constant conlagers-1s re {and variable-contactresistancefrequently; found inmountings which do not use an electricalcon- 'neetion fixedv tothe platings.

,What is claimed is: l.--I-n apiezoelectric crystal devicathecombination iof an elongated rectangular crystal clam -'f-=having length {and breadth large compared 'th the thickness thereof, beveled portions defining *the opposite endsof said crystal blank, m'etallic end, coating continuous about said crystal blank adjacent each of the: end portions thereof but 1 not extending onto said beveled. portions, ametallic coating covering a substantial portion of a first"'-maj'orface''of said crystal blank and joined to one of said end coatings, and a metallic coating covering -a substantial portion of a second major face opposite said first major face and joined to" the other of'said end coatings.

2. A coated crystal comprising an elongated rectangular'wvafer of piezoelectric material of length and breadth large in comparison with the,

thickness thereof, opposed beveled end'portions rerminan defining said water, end und'er'c'oatgs appliedtooppositemajor'faces of saidw'afer 'a'uijacent',ibut'notextending"o "0; each 'cfsaid 'o" posed jb'eveleden'd p'ortionsfanr'st face i co ting extending over a substantial portion of onerna'jor face and one end portion, including opposite ones of said undercoatings, but not extending onto said beveled portions of said wafer, and a second face coating extending over a substantial portion of the major face of said wafer opposite to that covered by said first face coating and over the opposite end portion, including opposite ones of said nndercoatings, but not extending onto said beveled portions of said wafer.

3. In combination, a coated piezoelectric crystal comprising an elongated substantially rectangular crystal blank adapted for vibration in the thickness shear mode of motion and having opposed major faces and opposite end faces, separated end undercoatings applied to each of said opposed major faces of said blank adjacent, but not extending onto the said opposite end faces thereof, a first face coating covering a substantial portion of one of said opposed major faces, a secondface coating covering a substantial portion of t e other of said o e m r aces a first terminal coating joined to said first face coatin and e t n n a ou said b an adiacent, but not extend n on n f sai en iac s, an a sec nd termin l coat n j ine to said second face coating and extending around said blank adjac nt, u not extend n o t th other of said end faces and electrical connec! tions to said terminal coatings comprising resilient metallic members bearing against the metal terminal portions on said opposed major faces adjacent said Opposite end faces.

4. In a coated piezoelectric crystal, the combination of an elongated substantially rectangular crystal 'blan'k' adapted forvibrationin the "thickness shear'mode of motion, electrodes fixed on "each of two opposed major-faces of said blank and covering a. substantial, portion of the-active area central thereof," and means for reducing the piezoelectric I response at each end of said blank, :comprising an end electrode disposed on eachvof thesaidopposed major faces, connecting :electrodes disposed on the sides of saidcrystal blank extending betweenopposed' ones of said-end electrodes; means for joining two, opposed and connected-end electrodes to one-of. saidmajor faces near one end' of said'blank, and means forjoining-the other of-said major faces tc two other opposed and connectediend electrodes-nearthe other end of-said blank, and mounting means for said blank comprising a pair of clamps one at-each end :of said blank and each having sets of resilient fingers bearing against the opposed maj or faces of said blank.

5.-A mounting V for rectangular piezoelectric crystals having end platings, comprising a base,

insulating bushings disposed through-said; base and adapted to secure support rodsyapairof support rods fixed through said bushings normal to said base, and upper and lower crystal-engaging members disposed on said support rods where- }pair of support rods, and means for connecting "those'of said c'onductingfingerssecuredto the lower of said crystal engaging members to the otherof said pair of support rods. n x

6. A holder for-piezoelectric crystals having a coating at one end extending over afirst'maj'o'r face, anda coating at the other end extending over'a secondm'aj'or'face' opposite to the coating on said first major face, comprising a base, a pair of parallel support rods insulated from said base and extending therethrough, electrically separate upper and lower crystal-engaging members arranged to receive a crystal therebetween disposed on and extending between said rods, means for connecting one of said crystal-engaging mem bers to one of said pair of support rods, and means for connecting the other of said crystal-engaging members to the other of said pair of support rods.

7. In a holder for coated piezoelectric crystals having connected terminal coatings on opposite major faces, the combination of a base, support pins insulated from and held by said base and adapted to act as external terminals, upper and lower crystal-engaging members disposed on said support pins, each of said members having an insulating plate adapted to fit over said supp rt pins, resilient conducting fingers secured to said p at and arr ed t provide a p al ty 01.? cm posed surfaces for support and electrical connection to a crystal, means for connecting said fine em. n he l r an upper rystab neasine members to difierent ones of said support pins, and a cover scaled to said base and having a scal able aperture therein adapted to permit evacua= tion of air therefrom.

8. The combination in a piezoelectric crystal unit of a hermetically sealed cover and base adapted to enclose a crystal and mounting. wherein said mounting comprises a pair of support pins fixed parallelly in, but insulated from, said base, upper and lower crystal-engaging members disposed thereon, each of said members comprising an insulating plate apertured to fit over said pins, eyelets secured through said plate and arranged to fit slidably over and be secured to said pins, a

spring clip comprising a plurality of flat, oppo sitely curved resilient fingers fixed to said plate and arranged to receive a crystal therebetween,

shear-F mode, a first face coating covering a sub-- stantial portion of one major face of said blank,

a second face coating covering a substantial portion of the major face of said blank opposite to that covered by said first face coating, end under coatings applied to opposite major faces of said waferadjacent, but not extending onto, each of the opposite end portions of said blank, a first terminal coating formed integrally with said first face coating and extending over opposite undercoatings and completely around said blank adjacent, but not extending onto, one end thereof, a second terminal coating formed integrally with said second face coating and extending over opposite ones of said undercoatings and completely around said blank adjacent, but not extending onto, the other end thereof.

9. In a piezoelectric crystal mounting, the combination of a base, insulating bushings fixed in said base, a pair of parallel support pins fixed through said bushings and arranged to serve as external connection members, upper and lower crystal-engaging members disposed on said pins, each of said members comprising an insulating plate, elongated tubular headed eyelets disposed through, and having the headed portion engag- 8 ing one side of said plate, arranged to fit slidably over said support pins, longitudinal strips rolled back from the elongated tubular portion of said eyelets against the other side of said plate, means for fixing each of said eyelets to the one of said pins over which it fits, a spring clip secured to said plate and having a plurality of resilient fingers extending in two spaced rows normally to said plate and rolled terminally inward of said rows to provide opposed crystal-engaging surfaces; an'auxiliary spring clip secured to said clip and arranged to maintain the lateral and longitudinal position of a crystal received in said spring clip, means for connecting one of said spring clips to one of said support pins, means for connecting the spring clip associated with one ARTHUR G. LAIRD.

REFERENCES orrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,095,376 Bechmann Oct. 12, 1937 2258,96? Bokovoy Oct. 14, 1941 2,321,358 Bokovoy June 8, 1943 2,443,700 Sylvester June 22, 1948 2,445,719

Sylvester July 20, 1948

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