US2157808A - Piezoelectric crystal - Google Patents

Description

May 9, 1939- J. M. woLFsKlLL 2,157,808

PIEZOELECTRIC CRYSTAL Filed Aug. 27, 1955 `faxla YaX/s 1 (eleg-ic] g Patented May 9,1939 2,157,8@8

UNITED STATES earl-:Nr ortica PIEZGELECTRIC CRYSTAL' John M. Wolfskiil, Erie, Pa., assignor to Bliley Electric Company, Erie, Pa., a partnership composed of F. Dawson Bliley and Charles Cullman Application August 27, 1935, Serial No. SiLlEd 3 Claims. (Cl. Til- 327) This invention relates to piezo-electric crystals ultra-high frequency oscillations which it is deand apparatus for employing piezo-electric cryssired to produce and/or stabilize. tais, generally. More particularly this invention My invention greatly simplifies piezo-electric relates to piezo-electric crystals and apparatus quartz crystal controlled oscillation generation 5 for employing piezo-electric crystals adapted for inasmuch as it eliminates the necessity of em- 5 generating ultra-high frequency electric oscilploying frequency doublers and frequency multilemons pliers. This is a material advantage since losses An object oi this invention is to provide a in frequency doublers and multipliers are enorpiezo-electrlc crystal adapted for harmonic opmous when Working with oscillations having 3o eration. frequencies higher than 30.0 megacycles. My inw Another object of this invention is to provide vention readily produces oscillations higher than a piezo-electric crystal adapted to oscillate e- 30.6 megacycles which are frequency stabilized, ciently at frequencies which/are materially highwithout requiring the use of any frequency douer than its fundamental frequency. blers and multipliers.

A further object of this invention is to pro- I accomplish this by cutting or grinding the l5 vide a piezo-electric crystal adapted to sustain crystal in such a manner that its vibrations on oscillations which bear an harmonic relation to the harmonic frequencies are stronger. l prefer its fundamental frequency. to employ a transverse or shear vibration, and in Still a further object of this invention is to so doing greatly increase the strength of the viprovide a piezo-electric crystal of quartz ground brations corresponding to the odd harmonics. 20 to a thickness such that the fundamental fre- Vibrations other than the transverse or shear quency of the crystal is in the neighborhood of type may be employed.

i megacycles and operating the crystal on a By harmonics or harmonic frequencies mean harmonic of the fundamental frequency fOr Sbavarious frequencies called overtones which are bilizing the frequency of electric oscillations o approximately integral multiples of the iunda- 25 said harmonic frequency. mental frequency. Thus, odd harmonics would Still another object of this invention is to probe approximately mid multiples, of the funda- Vide Piezo-electric Crystal OSCillatOr adapted mental frequency. .By selecting odd harmonics to stabilize the frequencies of oscillations of such as the third, nfth, seventh, ninth, eleventh,

39 ultra-high frequencies. and so on, of `for example, a crystal that will 30 Still another object of this invention is to prooscillare on a fundamental frequency of 7,5 megvide a DeZO-el'bl Cri/Stal Cut and gIOUIld in acycles, vibrations of ultra-high frequencies such such a manner that the crystal can be made to as 22.5 megacycles corresponding to the third oscillate at frequencies which hear an harmonic harmonic, 37,5 megacycles Correspnnding to the relation to the fundamental thickness vibration ifm harmonic, 52.5 megacycles corresponding to 35 frequency. the seventh harmonic, 67.5 megacycles corre- Still another object of this invention is to pro- Sponding to the ninth harmonic may be Obtained vide a crystal oscillation generator adapted to directly from the crystal oscillator. The crysoscillate readily and emciently at any one ci a tal will he 3, 5, 7, or 9 times, etc., thicker than o series of progressively increasing high irequenif it were operating on its fundamental at the w cies, same frequency.

Other and further objects of this invention 1D this manner the crystal vibrating on an will be apparent from the following specification 113121110310 @an De used as the pllmaly Source oi and claims. oscillations when employed in a vacuum tube In accordance with this invention l provide CFCF'. and fm1 not necessarily be Simply a 45 means employing an element exhibiting piezo stabilizing device in a self oscillatory circuit. electric properties for generating and/or stabilizcrrgh; 'lerugegnnythb g2? eiuclglgsn cehpezn; erated on its harmonics when used as a resonator roducin some reaction on an associate netthat it readily produces and stabilizes high frevjvork, or git may be used in filter circujts fluency oscillations 0f extremely high frequen' A further advantage of this invention is that cies. This iS aCCOmI-Jlished by Selecting a Crystal, piezo-electric plates or elements may be ground the fundamental frequency 0f which bears an to a thickness sufcient to withstand normal use .55 harmonic relationship to the frequency of the without breakage. Electrical oscillations as high as those produced by piezo-electric elements of myv invention could not be produced with piezo-electric elements of conventional cuts because the piezo-electric elements would have to be ground too thin and fragile, and would be impractical even if the quartz could be ground to the required dimension.

Other features and advantages of this invention will be apparent from the following specification and the accompanying drawing in which, brieiiy, Figure 1 illustrates the manner in which the crystal of my invention is cut from the quartz hexahedron; Figure 2 illustrates the orientation of a crystal with respect to the various crystallographic axes; Figure 3 illustrates an embodiment of a circuit arrangement in which the crystal of my invention may vibrate to produce oscillations and Fig. 4 illustrates enlarged views of the shear mode of vibration in a crystal element.

Referring to Fig. 1 of the drawing in detail, reference numeral I designates a section of a quartz hexahedron from which a plate or slab 2,

adapted to be used as a piezo-electric crystal, is

cut. The position of the plate 2 in the quartz hexahedronal crystal is shown in Fig. 1a, with respect to the :c or electric axis and the y or mechanical axis. Fig 1b shows the angle at which the crystal plate is cut with respect to the z or optic axis. In Fig. la the quartz hexahedron is shown with the optic or 2 axis at right angles to the :I: and y axes which are illustrated in the same plane. Fig. 1b, however, shows the y and z axes in the same plane and the a axis at right angles to that plane. The plate 2 is cut from the crystal I in the manner shown, with the principal surfaces 3 and 4 of the plate, cut at an angle il with respect to the optic or z axis. The principal surfaces 3 and 4 are cut substantially parallel to the electric or a: axis.

'Ihe values of the angle e may vary over rather wide limits, however, for best operation on harmonic frequencies, I have found that the plate should be cut at angles of plus 30 degrees or minus 19 degrees. For example, a crystal which would oscillate at 15 megacycles would be approximately .013" thick for the 30 out and approximately .018" thick for the 19 cut. These thicknesses give strong, practical crystal plates satisfactory for use in electrical circuits.

The minus 19 degree angle produces a plate having a high frequency thickness coeilicient, and the plus 30 degree angle produces a crystal having a low frequency-thickness coeiiicient.

There are two kinds of quartz, namely, righthanded and left-handed. In the case of righthanded quartz, the convention is adopted that a positive angle is a clockwise rotation of the principal axis (optic axis) when the electrically positive face as determined by a squeeze, is up. For left-handed quartz, a positive angle is a counterclockwise rotation of the principal axis when the electrically positive face is up.

Angles other than those given in the foregoing paragraph may be used, however, if the angle o departs from the angles given by more than plus or minus 10 degrees, the operation of the crystal plate as an harmonic oscillator becomes unsatisfactory, apparently due to the lower activity of the crystal plate as an oscillator and also because of the higher coupling coeilicient in the crystal to other vibrations.

The orientation of the crystal plate 2 with respect to the crystallographic axes x, y and z is illustrated in Fig. 2, wherein the substantially parallel relation of the sides 3 and l to the a: axis, and an angle o between these sides and the z axis is shown.

One of the possible circuit arrangements illustrated in Fig. 3 lshows the crystal plate 2 connected to control the frequency of an electric discharge device oscillation generator. The contact members 6 associated with the crystal plate 2 are of brass, bronze, Icopper, and like materials and are connected across the grid resistor 'I which is connected to the grid electrode 8 and cathode 9 of the tube 5. The anode I is connected to the oscillatory circuit I I which includes an inductance unit I2 and a variable condenser I3. A source of anode current supply I4, shunted by the by-pass condenser I5, is connected to the voscillatory circuit II and the cathode 9.

The operation of the circuit illustrated in Fig. 3 is as follows: The condenser I3 is adjusted to tune the oscillatory circuit I I to an harmonic frequency of the crystal plate 2, corresponding to the frequency of electrical oscillations that it is desired to produce. The crystal plate 2 impresses electric charges in the form of oscillations on the grid 8 of the tube 5 corresponding to the frequency to which the circuit II is tuned. These oscillations may be of a frequency much higher than the fundamental frequency of the crystal, as brought out in the preceding paragraphs herein, and yet electrical oscillations of ultra-high frequencies, derived from the oscillation generator and impressed upon the inductance I6, are of sufficient power and potential to be themselves suitable for driving an amplifier.

Fig. 4 illustrates the crystal plate of my invention vibrating at its fundamental frequency in shear or transverse vibration. The distribution of the positive and negative electrical charges on the crystal plate, is shown for a particular instant. 'Ihis same crystal plate, vibrating at the third harmonic, is illustrated in Fig. 4b. The frequency of the oscillations derived from the crystal, vibrating as illustrated in Fig. 4b is three times the frequency` derived from the crystal plate, vibrating as illustrated in Fig. 4a. The crystal in Fig. 4b may be considered as divided into three imaginary crystal plates a, b and c as defined by the dotted lines. The -electrical charges on these imaginary crystal plates are designated, for a given instant by the plus and minus signs and it is seen that the polarity of the imaginary plate b is the reverse of that of the plates a and c, making the polarity on the surfaces of the crystal at a given instant, of opposite sign; a condition necessary for sustaining oscillations and a condition which always exists when operating the crystal at the odd harmonics.

By defining the angle 0 of the electrode faces of my crystal element or plate as substantially -19 degrees or substantially +30 degrees with respect to the a axis of the crystalline'body, in the claims forming a part hereof, it is intended that this angle 9 may vary within limits set forth in preceding parts of this specification.

It willbe observed that; have described an embodiment of my invention in detail however, I do not desire to limit this invention to the exact details set forth except insofar as those details are defined by the following claims.

What I claim and desire to secure by Letters Patent of the United States is as follows:

l. A piezo-electric crystal cut from a crystalline body and`adapted for operation in a shear mode at an odd harmonic, said odd harmonic being a high or ultra high frequency, said crystal being so cut that its electrode faces are substantially parallel to an :c axis of the crystalline body and make an angle of substantially -19 degrees with the z axis of the crystalline body.

2. A method of operating a crystal at an odd harmonic in a shear mode which comprises so cutting the crystal that its electrode faces are parallel to the :v axis and make an angle of substantially -19 degrees with the z axis and applying a eld of a frequency corresponding to the desired shear mode harmonic.

3. A piezo-electric crystal cut from a crystal-v line body adapted for operation in a shear mode at an odd harmonic said odd harmonic being a high or ultra high frequency, said crystal being so cut that its electrode faces are substantially parallel to an x axis of the crystalline body and make an angle of substantially -19 degrees with the z axis of the crystalline body, said angle being such as will produce a 10W coupling coefficient in the crystal JOHN M. WOLFSKILL.

to other mode vibrations. 10

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