US3599124A - Crystal filters - Google Patents

Crystal filters Download PDF

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Publication number
US3599124A
US3599124A US723677A US3599124DA US3599124A US 3599124 A US3599124 A US 3599124A US 723677 A US723677 A US 723677A US 3599124D A US3599124D A US 3599124DA US 3599124 A US3599124 A US 3599124A
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electrode means
input
electrodes
output
electrode
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Warren L Smith
Roger A Sykes
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AT&T Corp
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Bell Telephone Laboratories Inc
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
    • H03H9/46—Filters
    • H03H9/54—Filters comprising resonators of piezoelectric or electrostrictive material
    • H03H9/56—Monolithic crystal filters
    • H03H9/566—Electric coupling means therefor

Definitions

  • ABSTRACT Three or more resonator-forming electrode 7/38 pairs and a crystal wafer on which they are mounted, form a Fldd 593mb 333/72- 30; multiresonator crystal filter with respective inductors con- 310/9.5;330/5- nected to two of the pairs.
  • the electrode pairs have masses such as to tune the frequency exhibited by the unconnected [56] References Cmd resonator to a frequency f,
  • the inductors tune the interelec- UNITED STATES PATENTS trode capacitances of the connected resonators to the 3,384,768 5/1968 Shockley 310/95 frequency f,
  • 310/9.5 resonators tune the mechanical resonance of the crystal 3,396,327 8/1968 Nakazawa 333/72 between the electrodes to the frequency f
  • the electrode 3,363,119 1/1968 Koneval 310/95 spacings in view of their masses are such as to achieve 3,437,848 4/1969 Borner et al 310/82 predetermined couplings between resonators.
  • This invention relates to energy transfer devices using the acoustical resonant properties of crystals, and particularly to electric wave filters using an acoustically resonant crystal body having mounted thereon separate electrode pairs to form respective mutually coupled resonators and wherein electrical energy applied to one of the electrode pairs is coupled through the body and removed from another so that the filter transmits a predetermined passband having predetermined characteristics.
  • the passbands be controlled with additional components. Also the before-mentioned copending applications disclose that controlling the masses and spacing of electrode pairs allow the character of the transmission characteristic or passband to be controlled without additional components as long as the passband was limited to a frequency range less than the frequency difference between the socalled resonant and antiresonant frequencies of one electrode pair. It was also discovered that within this frequency range the passband could be controlled by mounting a number of extra electrode pairs on the body between the furtherseparated input electrode pair and output electrode pair. These intermediate electrode pairs served with the body to form resonators that coupled the input resonator, formed by the input electrodes and the crystal body, to the output resonator, formed by the output-electrodes and the crystal body.
  • the other electrode means are given masses and dimensions such that each portion of the body covered by the other two or more electrode means exhibits a predetermined uncoupled mechanical resonant frequency. Also respective inductances tune the capacitances formed by the electrodes of the other electrode means to the predetermined resonant frequency.
  • the first electrode means are made sufficiently massive to form with the portion of the body which it covers a resonator having a tuning frequency in a given relation with the mechanical resonant frequency.
  • this given relation is such that the tuning frequency is equal to the predetermined mechanical resonant frequency.
  • the mechanical resonant frequency of each of the other electrode means is the same.
  • the electrode means which couples the other two electrode means comprises a pair of short-circuited electrodes.
  • the one electrode means which couples the other two electrode means comprises a pair of open-circuited electrodes.
  • the uncoupled mechanical resonant frequency of the body covered by one electrode pair preferably is equal to the predetermined mechanical resonant frequency of the other electrode means.
  • FIG. 1 is a partly schematic, partly plan drawing of a circuit including a filter with a crystal body and embodying features of the invention
  • FIG. 2 is a partly schematic, partly sectional drawing'of the circuit in FIG. 1;
  • FIG. 3 is a schematic drawing of an all-electrical equivalent of the circuit in FIGS. 1 and 2;
  • FIG. 4 is a graph illustrating the transmission characteristic of the filter in FIGS. 1 and 2;
  • FIG. 5 is a partly schematic, partly sectional drawing of a test arrangement for testing couplings between resonators in the circuit of FIGS. 1 and 2;
  • FIG. 6 is a partly schematic, partly sectional drawing of another circuit embodying features of the invention.
  • FIGS. 7, 8 and 9 are drawings illustrating characteristics useful for manufacturing the filters in the circuits of FIGS. 1, 2 and 6.
  • a source S applies a high frequency potential across a tuning inductor L and across the first of eight pairs of electrodes l2, l4; 16, 18; 20, 22; 24, 26; 28, 30; 32, 34; 36, 38; and 40, 42, that are vapor-deposited or plated in alignment along a chosen axis, such as the Z crystallographic axis on a rectangular AT-cut quartz crystal wafer or body 44.
  • a chosen axis such as the Z crystallographic axis on a rectangular AT-cut quartz crystal wafer or body 44.
  • the thicknesses of the electrodes and wafer in FIG. 1 are exaggerated. The individual electrodes of each of the pairs oppose each other across the wafer.
  • the source S by applying the high frequency potential across the input electrodes 12 and 14 piezoelectrically generates thickness shear vibrations in the crystal wafer 44.
  • the vibrations excite vibrations of equal frequency in the crystal wafer portions between successive pairs of electrodes 12 to 42 and generate electrical energy in the electrodes 40 and 42 across which a tuning inductor L appears.
  • Each electrode pair with the wafer forms a resonator coupled to the adjacent resonators.
  • a load resistor R receives electrical energy appearing across the output electrodes 40 and 42 over a predetermined bandwidth B,,..
  • the intermediate pairs of electrodes [6 through 38 are all short circuited to each other and grounded.
  • the masses of the electrodes 12 through 42 affect the resonant frequency of each of the resonators when considered alone by lowering their respective frequencies from the fundamental thickness shear mode frequency of the crystal wafer 44.
  • Each of the masses is such as to tune the individual resonator in the absence of the others to the same frequency f when measured in the short circuit condition.
  • the inductor L tunes the total effective electrical interelectrode capacitance C,,, of the first electrode pair 12 and 14, including the stray capacitances, to the resonant frequency f,, so that f,, equals lgg'flLic l
  • the inductor L tunes the interelectrode capacitance of the last pair of electrodes 40, 42 to the same frequency f,, so that fi, equals l/(2 1r ⁇ L cl, In FIG. 1 the electrodes 12 through 42 are substantially equal so that C,,, C C and L, L,,.
  • FIG. 1 The operation of the filter in FIG. 1 may best be understood by considering it in connection with the equivalent circuit shown in FIG. 3 wherein the portions representing the structure composed of the crystal body 44 and the electrodes 12 through 42 are designated F.
  • the source S applies high frequency potentials across a capacitor C,, representing the electrical interelectrode capacitance of the electrodes 12 and 14.
  • the source S has an internal resistance R
  • a capacitance transformer composed of a capacitance Tee having two capacitor C, in the series arm and a capacitor C, in the shunt leg represent the piezoelectric coupling between the electrodes 12 and 14 and the wafer 44 and serve to apply the input energy from the source S to a shunt resonant circuit composed of an inductor L, and capacitor C, representing the resonant structure of the wafer 40 between the electrodes 12 and 14.
  • the resonant circuit composed of inductor L, and C is tuned to the frequency f,, on the basis of the thickness of wafer 44, the dimensions and masses of electrodes 12 and 14.
  • the inductor L connected across the capacitor C,,, and the electrodes 12 and 14 tunes the capacitor C,,, .to the frequency f so that the latter equals ll(2rr ⁇ /L,-C,,,).
  • the energy appearing in the resonator L,C excites aplufiility of resonators L C L,,, C,,; L C L C L C,,; L,, C,; and L,,, C,,.
  • the respective resonators L C,, etc. each represent the vibration frequency of the individual resonating portion of the wafer when the other resonating portions are detuned from the passband region.
  • the inductive sections 8, S etc. represent the coupling between successive resonators and result in frequency shifts of the vibrating portions between the wafers.
  • the electrical energy appearing there passes to a load resistor R
  • the inductor L tunes the capacitance C,,, to the frequency f,.
  • the portion W represents the equivalent circuit of the wafer material.
  • the portion T represents the piezoelectric coupling between the wafer material and the electrodes and the portion E represents the total electrical capacitances of the electrodes.
  • the electrodes 16 through 38 are short circuited. Therefore, the capacitances C,,; to C are also short circuited.
  • the effect of these short circuits is actually to place an infinite impedance or opencircuit across each one of the resonators C,, L, to C,, L,. This can be seen from computing the values of the total capacitances C, across one of the capacitances such as C,.
  • the capacitor C, in the leg of the capacitance transformer circuit to C is shunted across the capacitor C,.
  • the capacitor C on the other hand is then in series with the capacitor C, in the other arm of the capacitance Tee.
  • the capacitance thus equals zero and the corresponding reactance XgFl/ZflfC is infinite. Therefore the effect of the short circuits across each of resonators L C,,, to L C is effectively to place infinite reactances across the particular resonators and have substantially no effect upon their tuning.
  • the inductors L, and L and the impedances R, and R which form shunt resonant circuits with the capacitors C and C represent respective impedances composed of infinite reactances and respective resistances R, and R at the frequency fl
  • the capacitance Tees reflect these low resistances and parallelresonant circuits L,, C,,, and L C,,,across resonators C,, L, and C,,, L; as high resistances and 'zero reactances if the values of R, and R are low.
  • each of the resonators C,, L, to C,,, L,,, representing the resonators formed by the electrodes 12 and 42 and the wafer 44 is tuned to substantially the same frequeny fo-
  • the passband formed by such tuning depends upon the coupling between each successive pair of resonators.
  • FIG. 4 illustrates a passband available from a circuit such as shown in FIGS. 1 and 2. The desired degree of coupling necessary to produce particular passbands is available from ordinary circuit theory. The actual coupling between adjacent resonators can be measured by determining the frequency shifts imparted by one ofa pair of resonators upon the other.
  • the circuit of FIG. 5 illustrates the method for determining the coupling between successive pairs of electrodes.
  • a variable frequency source 60 applies a high frequency signal across one of the two pairs of electrodes between which the coupling is to be measured.
  • a meter 62 measures the input voltage.
  • the resonator to which the coupling is to be measured such as that formed by the electrodes 24 to 26 is short circuited.
  • the remaining electrodes are maintained open circuited to detune the resonators formed by them.
  • the applied frequency from the source 60 is noted at the two lowest voltages measured by the meter 62 as the frequency output of the source 60 varies.
  • the crystal body is composed of an AT-cut quartz crystal 1 inch long, 0.400 inches wide and approximately 0.0061 inches thick.
  • the dimensions of the electrode pairs 12 through 42 are 0.0734 inches along the long direction of the crystal body, that is along the Z axis, by 0.0916 inches along the X-axis.
  • the electrode separations d, through d, between the edges having the long dimensions are:
  • the masses of the electrodes are such as to achieve respective platebacks of 2 percent.
  • the term plateback is defined in the before-mentioned copending applications and represents a measure of the masses or the effects of the masses of the electrodes.
  • plateback constitutes the fractional drop (ff,,)/f in the resonant frequency f,, of a crystal body electroded with a single pair of electrodes, from the fundamental thickness shear frequency f of the unelectroded crystal body due to increasing masses of the electrodes. This takes into account the fact that as the masses of the electrodes are increased, the resonantfrequency of the individual resonator as measured with other resonators detuned is lowered.
  • the resulting coupling coefficients k between successive pairs from left to right in FIGS. 1 and 2 are 1.54X1O 1245x10 1.200X10, 1.192X10, 1200x10 l.245 l and 1.54X
  • the structure of FIGS. I and 2 passes the midband frequency of 10.7 megahertz.
  • the width of the passband is kilohertz.
  • the resonator inductance L, through L is 34 millihenries.
  • the filter is intended to' operate between a source having an impedance of 3000 ohms and a load of 3000 ohms.
  • the detuning capacitance C in each case equals C,[C,C o/Cia) 1l/[ fiCiQKCfiCQTQi1; E l C1 is q l C, /C,,. Therefore, the open-circuit 12,, frequency which can be tested as shown in FIG. 5 except by open circuiting the electrodes 24 and 26 and short circuiting the electrodes 12, l4, l6, 18, 28, 20, 32, 34, 46, 38, 40 and 42, includes the effects of the interelectrode capacitances C and the effect of the piezoelectric coupling. This constitutes a raising of the frequency from f, because the resultant capacitance is negative.
  • the resonators formed by the electrodes 12 and 14 as well as the electrodes 40 and 42 as represented by L, C, and L,,,, C are alone, without the effect of C,,, tuned to the frequency fl
  • the effects of piezoelectric coupling of capacitors C and C are obviated by tuning the capacitances C and C with the inductors L, and L, to the frequency of the resonators L,, C, and L,,, C,,. I-Iere that frequency is f
  • I-Iere that frequency is f
  • the tuning of the uncoupled resonators, as considered alone, formed by electrodes 16 to 38, and represented by L C to L C-,, capacitors C,, C, and C,,, to the frequency f is accomplished by making the masses of the electrodes 16 to 38 to be greater than the electrodes 12, 14 and 40, 42. This reduces the uncoupled resonant frequency 1, of each until the total resulting frequency of each open-circuited resonator is f
  • the increased masses achieve the previously mentioned plateback that reduces the resonant frequency of each resonator.
  • the relative masses of each electrode pair, that is, the relative platebacks are determined not only to achieve proper tuning but to achieve the desired coupling'The greater the plateback on successive resonators, the smaller the coupling between them.
  • the crystal body is composed of an AT-cut quartz crystal, 1.4 inches long, 0.400 inches wide and approximately 0.0087 inches thick.
  • the dimensions of the electrode pairs 12 through 42 are 0.1050 inches along the long direction of the body, that is along the Z axis, by 0.1304 inches across the Z axis.
  • the electrode separation d, to d, between the edges having the long dimensions are:
  • the spacing dimensions have tolerances of 10.0001 inches, respectively.
  • the masses of the electrodes are such as to achieve respective platebacks of 2 percent.
  • the resulting respective coupling coefficients k between successive pairs from left to right in FIG. 6 are 154x10, 1.245 10, 1.200X 10", 1.192X10, 1.200X10, l.245 1() and 1.54X10'
  • the structure of FIG. 6 passes a midband frequency of 7.5 megahertz and a passband width of about 17.5 kilohertz.
  • the resonator inductance in each resonator is 48.5 millihenries.
  • the filter is intended to be driven by a source impedance of 3000 ohms and when the output of the electrodes 40 and 42 is applied across a load R of 3000 ohms.
  • the invention thus eliminates the limitations previously placed upon such filters by the piezoelectric coupling of the electrodes.
  • the coupling is controlled between adjacent resonators, a wider passband is available than would be obtained otherwise with the particular crystal material.
  • the crystal structure of FIGS. 1, 2 and 6 is manufactured by first selecting the total bandwidth and calculating on the basis of ordinary circuit theory the needed coupling coefficients between each pair of electrodes. Electrode sizes and suitable platebacks are chosen from curves such as in FIGS. 7, 8 and 9 which have been developed for structures wherein two pairs of electrodes are coupled to each other. Where I is the thickness of the wafer and r is the width of the electrodes, r/t is generally made equal to 12 although in practice any value between 6 and 20 is usable. A value of 152 is frequently chosen as the length of the electrodes normal to the coupling axis for good suppression of other modes.
  • the fundamental thickness shear mode frequency f is chosen from the formula Ff /(b-P where P,, is the fractional shift in frequency due to mass loading of the electrodes and equal to (ff,,)/ f.
  • the manufacture of filters such as shown in FIGS. 1, 2 and 6 starts by first cutting a wafer from a quartz crystal having the desired crystallographic orientation such as an AT cut. The wafer is then lapped and etched to a thickness 1 corresponding to the fundamental shear mode index frequency f. Masks with cutouts placed on each face of the crystal wafer serve for depositing the electrodes. The geometry of the electrodes is determined by considering the desired bandwidths and convenient platebacks.
  • the proper separation d between the electrodes of adjacent pairs may be determined from graphs such as those of FIGS. 7, 8 and 9 which show variation in coupling (f f,,)/'Vf f for various ratios of electrode separation d to wafer thickness and for various platebacks as well as various values of r/t at one center frequency.
  • the invention furnishes a reliable energy-translating system and filter which can be constructed not only simply but to cover wide-bandwidth passbands.
  • FIGS. 7, 8 and 9 are examples of empirically derived graphs for a filter having two coupled resonators operating unaffected by other resonators within a frequency range and about a center frequency.
  • the graphs are useful for determining suitable parameters.
  • the value (f,,f,)/f A is an approximation where f is close tofl.
  • r is the electrode dimension in the direction along which the electrodes are aligned.
  • An electromechanical filter comprising, in combination,
  • intermediate electrode means sandwiching said body therebetween located between said input and output electrode means
  • each of said electrode means having sufficient mass to decrease exponentially the amplitude of acoustic energy in said body as the distance from said electrode means increases,
  • each of said electrode means being spaced at a preselected distance within the acoustic field of each adjacent one of said electrode means
  • inductor means connected to said input and output electrode means, thereby to facilitate accurate shaping of the passband characteristics of said filter, said intermediate electrode means comprising a single pair of short-circuited electrodes, said inductor means form ing a respective tuning circuit with each of said input and output electrode means, and
  • each of said tuning circuits being tuned to the mechanical resonant frequency of a respective one of the resonators formed by said input and output means.
  • An electromechanical filter comprising, in combination,
  • intermediate electrode means sandwiching said body therebetween located between said input and output electrode means
  • each of said electrode means having sufficient mass to decrease exponentially the amplitude of acoustic energy in said body as the distance from said electrode means increases, thereby to confine said acoustic energy substantially to a limited acoustic field in said body close to said electrode means and away from the edges of said body,
  • each of said electrode means being spaced at a preselected distance within the acoustic field of each adjacent one of said electrode means
  • inductor means connected to said input and output electrode means, thereby to facilitate accurate shaping of the passband characteristics of said filter
  • said intermediate electrode means comprising a plurality of pairs of electrodes
  • said inductor means forming a respective tuning circuit with each of said input and output electrode means
  • each of said tuning circuits being tuned to the mechanical resonant frequency of a respective one of the resonators formed by said input and output means.

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  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Piezo-Electric Or Mechanical Vibrators, Or Delay Or Filter Circuits (AREA)
US723677A 1968-04-24 1968-04-24 Crystal filters Expired - Lifetime US3599124A (en)

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US (1) US3599124A (enrdf_load_stackoverflow)
BE (1) BE731938A (enrdf_load_stackoverflow)
CH (1) CH493166A (enrdf_load_stackoverflow)
ES (1) ES366488A1 (enrdf_load_stackoverflow)
FR (1) FR2006841A1 (enrdf_load_stackoverflow)
GB (1) GB1266565A (enrdf_load_stackoverflow)
NL (1) NL6906268A (enrdf_load_stackoverflow)
SE (1) SE362759B (enrdf_load_stackoverflow)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4329666A (en) * 1980-08-11 1982-05-11 Motorola, Inc. Two-pole monolithic crystal filter
US4825467A (en) * 1986-11-25 1989-04-25 International Telesystems, Inc. Restricted access television transmission system
US4980598A (en) * 1989-12-21 1990-12-25 Lucas Schaevitz Inc. Monolithic resonator for a vibrating beam accelerometer
EP0437304A3 (en) * 1990-01-12 1992-05-06 Ngk Spark Plug Co. Ltd. Method of adjusting a frequency response in a stripline filter device
US6205315B1 (en) * 1999-11-24 2001-03-20 Xerox Corporation Tuned transducer, and methods and systems for tuning a transducer
US20030127944A1 (en) * 2001-12-06 2003-07-10 Clark William W. Tunable piezoelectric micro-mechanical resonator

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3222622A (en) * 1962-08-14 1965-12-07 Clevite Corp Wave filter comprising piezoelectric wafer electroded to define a plurality of resonant regions independently operable without significant electro-mechanical interaction
US3334307A (en) * 1966-11-14 1967-08-01 Zenith Radio Corp Multi-electrode acoustic amplifier with unitary transducing and translating medium
US3344368A (en) * 1967-09-26 Fettweis bandpass filter
US3363119A (en) * 1965-04-19 1968-01-09 Clevite Corp Piezoelectric resonator and method of making same
US3384768A (en) * 1967-09-29 1968-05-21 Clevite Corp Piezoelectric resonator
US3396327A (en) * 1961-12-27 1968-08-06 Toyotsushinki Kabushiki Kaisha Thickness shear vibration type, crystal electromechanical filter
US3401283A (en) * 1965-04-19 1968-09-10 Clevite Corp Piezoelectric resonator
US3437848A (en) * 1964-09-24 1969-04-08 Telefunken Patent Piezoelectric plate filter

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3344368A (en) * 1967-09-26 Fettweis bandpass filter
US3396327A (en) * 1961-12-27 1968-08-06 Toyotsushinki Kabushiki Kaisha Thickness shear vibration type, crystal electromechanical filter
US3222622A (en) * 1962-08-14 1965-12-07 Clevite Corp Wave filter comprising piezoelectric wafer electroded to define a plurality of resonant regions independently operable without significant electro-mechanical interaction
US3437848A (en) * 1964-09-24 1969-04-08 Telefunken Patent Piezoelectric plate filter
US3363119A (en) * 1965-04-19 1968-01-09 Clevite Corp Piezoelectric resonator and method of making same
US3401283A (en) * 1965-04-19 1968-09-10 Clevite Corp Piezoelectric resonator
US3334307A (en) * 1966-11-14 1967-08-01 Zenith Radio Corp Multi-electrode acoustic amplifier with unitary transducing and translating medium
US3384768A (en) * 1967-09-29 1968-05-21 Clevite Corp Piezoelectric resonator

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
Electronic Comm. Eng. of Japan...M. Onoe Piezo-Elec. Resonators Vibrating In Trapped Modes Sept. 1965 p.84 93 *

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4329666A (en) * 1980-08-11 1982-05-11 Motorola, Inc. Two-pole monolithic crystal filter
US4825467A (en) * 1986-11-25 1989-04-25 International Telesystems, Inc. Restricted access television transmission system
US4980598A (en) * 1989-12-21 1990-12-25 Lucas Schaevitz Inc. Monolithic resonator for a vibrating beam accelerometer
EP0437304A3 (en) * 1990-01-12 1992-05-06 Ngk Spark Plug Co. Ltd. Method of adjusting a frequency response in a stripline filter device
US6205315B1 (en) * 1999-11-24 2001-03-20 Xerox Corporation Tuned transducer, and methods and systems for tuning a transducer
US20030127944A1 (en) * 2001-12-06 2003-07-10 Clark William W. Tunable piezoelectric micro-mechanical resonator
US6943484B2 (en) * 2001-12-06 2005-09-13 University Of Pittsburgh Tunable piezoelectric micro-mechanical resonator

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GB1266565A (enrdf_load_stackoverflow) 1972-03-15
CH493166A (de) 1970-06-30
BE731938A (enrdf_load_stackoverflow) 1969-10-01
NL6906268A (enrdf_load_stackoverflow) 1969-10-28
DE1919024B2 (de) 1973-02-01
FR2006841A1 (enrdf_load_stackoverflow) 1970-01-02
ES366488A1 (es) 1971-03-16
SE362759B (enrdf_load_stackoverflow) 1973-12-17
DE1919024A1 (de) 1969-12-04

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