US2470609A - Piezoelectric crystal - Google Patents

Description

y 1949- 'w. c. ELMORE ETAL 2,470,609 A I PIEZQELECTRIC CRYSTAL Filed Dec. 10, 1945 INVENTOR. WILLIAM G. ELMORE JOHN F. MARSHALL ATTORNEY Patented May 17, 1949 ,=PIEZOELECTRIC .CRYSTAL r -William (irwillmore, Swarthmore; and-John F. ,Marshall- Primos, Pa., assignors to-the;United @States of. America asrepresented by the-Secretary of .War

i. Application December 10, 1945,-Serial No.- 634,083

8 Claims. (01. file-327) .fIhis-v invention relates to piezoelectric crystals and-,more particularly. to. means for. .mounting piezoelectric. crystals.

Piezoelectric plates, usually cut from. natural crystals of quartz or turmaline,.find extensive use, for example in. frequency controlof oscillating. circuits as is well known in the radio art. There are many'idifierent ways invwhich such crystal plates, generally termed crystals, may be mounted for the purpose of utilizing the. piezoelectric. .effect. Among the"difficulties nor-- mally encountered are. theoscillation damping efiec'ts inherent inthe usual means of mounting the-crystal in aJholderJ; These damping effects may be used, therchoice depending primarily upon the desired frequencyof-operation and upon the .rdesired frequency-temperature: characteristics. Similarly, 'the crystal: may bemutilized in anydesi-redwmode of:osci1lation. Under any such circumstances,--however, ra noda-l plane can be found, so that the crystal may be supported in the manner hereinafter described. In accordance with the invention it istproposed to use strings orthreadsengaging the crystal at or near the nodal .plane to provide; a support for the. crystal. Theterm thread as a used herein. includes any filamentous or thread-.like-Jnaterial which'may or may not be resilient.

areievidenced by a reducedcrystal Q, and gen- Referring. .iagainr tor-the .,-drawing,vtwoloops erally poorer, frequency stability characteristics. l3r-.and-- M of filamentous: material,;each loop Therefore'amongtheobjects ofthis invention slightly greater in length than. the perimeter are to provide a. means for mounting a. piezoof the 'crystal in a .plane perpendicular to the electric crystal so as to improve the operating Q Y axis,--areused. 1 .The loopsimay be engaged with of the crystal, and to "reduce damping effects zo'thabar-crystal byzslippingthemover the crystal, to which, the crystal maybesubjected durin for example fromoppositeiends thereof,- loop oscillations. l31beingadvanced r-romtheleft-andloop l4 being Other objects, advantages and novel features advanced from-the right, .as-shown in the drawof the inventionwillrbecome apparent from-the ing, so that .they meet :andcontact the crystal description contained-hereinzwherein reference at-substantially thenodalplane...Loops l3 and is made-to the accompanying drawing.

A- crystal is *oftendesignated in accordance with its orientationwith-respect to the crystallographicaxes of the natural crystalline structure. Thus, an X-cut quartz crystal used at relatively low frequencies, for-example, may be in the form of arectangular bar so-cut-iromthe natural crystal that its longitudinal axis-is oriented perpendicularly to an X (electric) axis. Forfuller description-of crystallographic axes of quartz-and various crystal'cuts-,=reiereneemay be had to Radio Engineers' Handbook by Frederick E. Terman, page 488 et-seqr, published by McGraw Hill Book Company, 1943.

Referring now to the drawing, there is shown, by way of example; an -X-cut bar crystal it, and at-orie sidethereof is drawna set of mutually perpendicular--crystal1ographic axes (X, Y. and Z), for convenience in illustrating its orientation. A fundamental mode ot'oscillation of such a crystal is that in which mechanicat vibrations occur substantially along the Y axispcrpendicw larly to-the X and Z axes as designated in the drawing. Such a-mode of oscillation establishes a nodal=planexsubstantially at the center of thelong-itudinal axis=of crystal iii.

Although an X-cut bar crystal of-- rectangular cross-section is rhere shown and referred to in detail,-it will be understood that a crystal of any othershape or relative dimensions or type of cut I 4 are engaged by suitable clasping. means, for example. byhooks .-(not-..shown) on by other threadsl3, I3", I and. I4", .respectively knotted thereto. .Thusloop. I3 is engaged by 'knots l5 and is lying in the Y-Z plane to threads I3 and; l3,.respective1y, and loop M by knots l1 and I8 lying in the XY plane to threads'M', and M", respectively. Spring or yieldable meansmay be utilized between the -mounting (not shown) and any or all of the threads 13-, 13,- M and I4 to apply tension to the threads inthe-respective directions illustrated'. Alternatively, thethreads may be elastic or include elastic portions to supply the desired tension.

The tensions applied at'knots i Sand It include components of -iorce in the Y- direction; likewise, -the tensions applied at knots l'I-and i8'have components ofii'orce along the Y axis. However,

the components of force applied to knots l5 and I6 along the Y axis are opposite in direction to the components offoroe'resolved along the sameaxis. applied to -knots l1 and I8. Thereby, threads l3 and! are held in engagement-with each other, and against the lateral edges and faces of crystal Ill. The loops and-supporting threads: areengaged and tensioned in -such manner. that threads. I3'-= and" I3" extend substantially along a Y?rZ planeand have components of forces .resolvedmainlyzzalong the Y and Z axes; threads M and 14" extend substantially along an X-Y plane and have components of forces resolved mainly along the X and Y axes. The effect of applying tension at the seveknots of the loop and thread structure is clear if one assumes threads [3, I3, I l and i i to be tensioned, for example by Springs, in a suitable framework or holder. It is apparent that bar crystal ill will assume a position of relative to the holder.

The nodal plane of a crystal may not, in some instances, be symmetrically located in the crystal plate. For example, the nodal plane may lie substantially centrally in the bar crystal I here shown, but inclined at a small angle to the Z axis. The window formed by the supporting loops does not then coincide with the nodal plane, for the loops tend to form the window in a plane at a right angle to the longitudinal axis. The operation of the crystal under such circumstances is not greatly affected, however, for the restraint upon the crystal by the thread support is slight. If it is desired to position the loops accurately at a nodal plane which is inclined with respect to a physical axis of the crystal, the loops may be held in place by applying some varnish such as Glyptal, which has the further advantage of improving the breaking strength of many types of thread which may be used. Furthermore, if the threads are coated with Glyptal, engaged on the crystal, and then baked at about 100 centigrade for a few hours, the threads will be more firmly cemented to the sides of the crystal. Another manner in which the loops may be caused to retain engagement with the crystal at a true nodal plane is by forming small nicks or depressions in the edges of the crystal bar at nodal positions, so that the string supporting structure may be engaged in these depressions.

The engagement of the crystal at substantially a nodal plane, the lightness of the crystal thread supporting structure, and the small tension forces required result in very low damping of the crystal and correspondingly improved Q, improved stability and improved frequency-temperature characteristics.

Any suitable framework or holder (not shown) may be utilized in which the supporting threads may be anchored to secure the crystal and string arrangement here shown. The holder may be provided with conventional electrode elements supported near, but not touching, suitable faces of the crystal, and with external leads or connection terminals as is usual. Such crystal faces, for example faces l9 and which are perpendicular to the X axis in the illustrated embodiment, may, if desired, be plated with conductive material. The need for separate electrode elements may be eliminated :by providing contact means between the plated faces and the external connection terminals, as by lightly positioning, against the plated surfaces, connecting wires or foil fine enough not to impose excessive restraint upon the crystal.

It will be apparent to those skilled in the art that many variations of the invention are possible. It is not desired to restrict the spirit or scope of the invention by the particular embodiment herein described.

What is claimed is:

1. In a holder for a piezoelectric crystal having a longitudinal axis and opposed pairs of rectangular faces substantially parallel thereto, a supporting structure including two loops extending about said crystal substantially in a plane perpendicular to said axis, four supporting threads, means engaging one pair of said threads to one said loop at portions adjacent an opposed pair of crystal faces, means engaging the other pair of said threads to the other said loop at portions adjacent the other opposed pair of crystal faces, and yieldable means supplying tension forces in the supporting structure whereby said loops are drawn into engagement with each other and with said faces to maintain said crystal in substantially fixed position within said holder.

2. A structure for supporting within a holder an object having a longitudinal axis and faces substantially parallel thereto, including two loops having resilient portions and extending about said solid substantially in a plane perpendicular to said axis, a first pair of attaching means engaging one said loop substantially on opposed ends of a first line in said plane, a second pair of attaching means engaging the other said loop substantially on opposed ends of a second line in said plane at right angles to said first line, said attaching means connecting said loops to said holder, whereby said loops are drawn by said resilient portions into engagement with each other and with said object to maintain said object in a substantially fixed position.

3. A structure for supporting an object, including two loops extending about said object substantially in a plane, a holder, a first pair of tensioning means mounted on said holder and secured to one said loop substantially on opposed ends of a first line in said plane, a second pair of tensioning means mounted on said holder and secured to the other said loop substantially on opposed ends of a second line in said plane at right angles to said first line, whereby said loops are drawn into engagement by said first and second pairs of tensioning means to maintain said object in a substantially fixed position within said holder.

i. A method of suspending a crystal in a holder to allow substantially undamped oscillations thereof which comprises placing two loops around said crystal perpendicular to its longitudinal axis and substantially at its nodal plane, connecting a pair of strands to one of said loops at points adjacent to opposite faces of said crystal, connecting another pair of strands at points on opposite faces adjacent to said previously mentioned faces and applying tension to said strands.

5. A method of suspending a piezoelectric crystal to allow substantially undamped oscillation which comprises placing two flexible loops about said crystal perpendicular to its longitudinal axis, engaging one loop at points adjacent to opposing faces of said crystal by resilient strands, engaging the other loop at points adjacent to a second pair of opposing faces by resilent strands, and applyin tension forces to all said strands.

6. The method of forming a holder for a crystal which will allow substantially undamped oscillations which comprises placing two loops of non-rigid material about said crystal perpendicular to its longitudinal axis and substantially at its nodal plane, attaching a pair of strands to one of said loops at points adjacent to opposin faces of said crystal, attaching a second pair of strands to the second of said loops at points adjacent to a second pair of opposing faces and applying tension forces to all of said strands to cause said loops to engage and retain said crystal in the desired position.

'7. The method of forming a holder for a crystal having a longitudinal axis and pairs of parallel opposed faces perpendicular thereto, which comprises placing about said crystal and perpendicular to said longitudinal axis, two loops having a pair of strands attached to each at diagonally opposite points and applying tension forces to said strands to caus said loop to engage and support said crystal.

8. A method for suspending a vibrating object to allow substantially undamped oscillations thereof which comprise placin pliable threadlike loops of material about said object perpendicular to its axis of vibration and substantially at its nodal plane, applying tension to said loops to cause said loops to engage whereby said object is tightly encircled at said nodal plane, applying Glyptal-like substance to said supporting REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,228,813 Osley June 5, 1917 1,567,225 Barbin Dec. 29, 1925 1,692,063 Trogner Nov. 20, 1928 1,875,324 Bower Sept. 6, 1932 1,907,986 Little May 9, 1933 2,255,495 Scholes et al. Sept. 9, 1941

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