US3739304A - Resonator interconnections in monolithic crystal filters - Google Patents

Resonator interconnections in monolithic crystal filters Download PDF

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Publication number
US3739304A
US3739304A US00183863A US3739304DA US3739304A US 3739304 A US3739304 A US 3739304A US 00183863 A US00183863 A US 00183863A US 3739304D A US3739304D A US 3739304DA US 3739304 A US3739304 A US 3739304A
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resonators
resonator
auxiliary
input
output
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A Braun
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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

  • This invention relates to energy translating devices and, more particularly, to monolithic crystal filters.
  • the term monolithic crystal filter as used herein is meant to define the basic filter structure disclosed in US. Pat. No. 3,564,463, issued on Feb. 16,1971 to W. B. Beaver and R. A. Sykes.
  • the Beaver-Sykes apparatus is an energy translating device for translating input oscillatory electrical energy having first characteristics into output oscillatory electrical energy having second characteristics.
  • a filter typically a bandpass filter.
  • such a filter involves the use of a plurality of resonators which share a common piezoelectric body or wafer.
  • the Beaver-Sykes structure is distinguished from other outwardly similar structures by the combination of two features, namely, mass loading and acoustic coupling.
  • mass loading refers to a particular electrode mass which is determined by the nature of the piezoelectric body and its thickness, and by the size and density of the electrodes which make up each of the resonators.
  • Mass loading which conforms to the principles taught by Beaver and Sykes, is evidenced by a number of specific conditions.
  • acoustic energy supplied in or near to one of the resonators is essentially confined or trapped within the boundaries of the resonator so that very little escapes to the surrounding piezoelectric body.
  • the relatively limited amount of acoustic energy that does escape from the energy trapping action of the resonators decreases exponentially in magnitude as the distance from the resonator increases.
  • the contour and dimensions of the outer perimeter of the piezoelectric body have no effect on the nature of the energy transmission accom' plished.
  • Acoustic coupling refers to the existence of an energy channel in the piezoelectric body which effects the transmission of acoustic energy between input and output electrodes. Such coupling is evidenced or manifested by a number of conditions which include, for example, the placing of all resonators within the acoustic field of adjacent resonators.
  • the only physical connecting path between the input and output resonators is in the piezoelectric body and substantially all of the energy transferred from one resonator to another is acoustic energy.
  • the image impedance of the structure or circuit, as a whole conforms to a specifically defined pattern.
  • the structure or circuit, as a whole has an equivalent circuit in the form of a lattice network with resonant and antiresonant frequencies characterized by a specifically defined relation.
  • any effective band filter including a monolithic crystal filter
  • the transfer characteristics of any effective band filter should be marked by steep skirts of attenuation and the passband should be bracketed by distinct attenuating peaks.
  • Monolithic crystal filters have been traditionally designed and used as all pole devices and, in those instances where sharper skirts are desired, it is conventional to increase the order of the transfer function which physically means to increase the number of resonators.
  • 16th order (8 resonator) monolithic crystal filters (MCFs) are known in commercial use, it is evident that increases in the order of the filtering function necessarily increases the complexity and cost of manufacture.
  • An alternate method for realizing sharper skirts in MCFs is to introduce transmission zeros close to the passband.
  • this method has been carried out by both charge cancellation and by phase cancellation.
  • Charge cancellation depends on the interconnection of split electrodes to transfer charge from one resonator to another. At certain frequencies, however, charge cancellation has the effect of inhibiting the electric field across the resonators used to generate the charge transfer, thereby realizing transmission zeros at these frequencies. Attenuation peak location has been shown to depend on and therefore to be controlled by the area ratio of the split electrode. Thus far, however, the effectiveness of this technique has been limited to a tworesonator MCF.
  • Phase cancellation depends on generating two outputs equal in magnitude and opposite in phase at specified frequencies. Upon tying these two outputs together, transmission zeros may be realized at the specified frequencies.
  • phase cancellation technique an example of which is shown in my copending application, Ser. No. 63,204, filed Aug. I2, 1970, it has been found that the peak locations can generally be realized only at discrete frequencies.
  • a general object of the invention is to improve the transmission characteristics of monolithic crystal filters by the establishment of controllable transmission zeros relatively close to the passband while avoiding, however, the shortcomings encountered in the charge and phase cancellation methods.
  • a filter in accordance with the invention has two transmission paths in parallel one being the conventional path from one acoustically coupled resonator to the next, which path is restricted to the piezoelectric body, and the other being the ungrounded path utilizing the external conductor that is connected between the two nonshorted resonators.
  • the phase differences that arise by virtue of the two transmission paths result in selective cancellation or reinforcement of various frequencies.
  • the passband is shaped by the introduction of transmission zeros relatively close to the passband so that the skirts of the characteristic filter transmission plot are steepened or sharpened, thereby enhancing filter selectivity.
  • the wide number of forms in which the invention may be practiced stems from the fact that either high order filters (many resonators) or low order filters (few resonators) may be employed and further from the fact that a variety of resonator interconnection combinations fall within the principles of the invention.
  • FIG. 1A is a sketch of a conventional n-resonator MCF
  • FIG. 1B is a schematic circuit diagram of an equivalent circuit for the MCF of FIG. 1A;
  • FIGS. 2A, 2B and 2C are schematic circuit diagrams illustrating network transformations
  • FIG. 3A is a schematic circuit diagram of a first embodiment of the invention.
  • FIG. 3B is a plot of the transfer characteristics to be expected from the filter of FIG. 3A;
  • FIG. 4 is a schematic circuit diagram of an equivalent circuit of the MCF filter of FIG. 3A;
  • FIG. 5A is a schematic circuit diagram of a second embodiment of the invention.
  • FIG. 5B is a plot of the transfer characteristics to be expected from the filter of FIG. 5A;
  • FIG. 6 is a schematic circuit diagram of an equivalent circuit for the filter of FIG. 5A;
  • FIG. 7A is a schematic circuit diagram of a third embodiment of the invention.
  • FIG. 7B is a plot of the transfer characteristics to be expected from the filter of FIG. 73;
  • FIG. 8 is a schematic circuit diagram of an equivalent circuit for the filter of FIG. 7A;
  • FIGS. 9A, 9B and 9C illustrate the use of an external capacitor to control the position of transmission zeros in different embodiments of the invention.
  • FIG. 10 is a schematic circuit diagram of an 8- resonator MCF in accordance with the invention.
  • prior art MCFs typically employ a plurality of internal resonators (i.e., excluding the input and output resonators) which are normally shorted to ground since it is known that the short circuit provides the simplest and mostreliable type of resonator termination.
  • filters with other than all pole designs are realized by selectively interconnecting certain ones of the internal resonators without, however, grounding the interconnected portions of those resonators.
  • the underlying theory of the invention may best be explained in terms of the sketch of a conventional monolithic crystal filter shown in FIG. 1A as supplemented by its equivalent circuit shown in FIG. 1B.
  • a conventional MCF employs a piezoelectric body 100 sandwiched between a plurality of electrode pairs l0lA-B, 102A-B, 103A-B, nA-B and 104A-B to form an input resonator 101, an output resonator 104 and intermediate or auxiliary resonators 102, 103 n.
  • a signal source not shown, is normally connected across the input points 11l2 and a load or utilization circuit is connected across the output points 13-14.
  • One side of the filter which includes the input terminal 12, the electrodes 1018, 1028, nB and 1048 and the output terminal 14 is connected to a source of reference potential such as ground.
  • the auxiliary resonators 102, 103 and n are shown open circuited simply by way ofillustration. More generally, these resonators are shorted and connected to ground.
  • the external terminals comprise the terminal set 11-12 and 13-14 located between the outside world and the MCF, i.e., the physical leads.
  • the internal terminals consist of the nodes of the resonator such as nodes l5, l6 and 17 of FIG. 1A.
  • the capacitive inverter CI which, as stated above, represents the piezoelectric coupling.
  • FIGS. 2A, 2B and 2C The effect of these terminations as seen by the internal terminals is illustrated by the transformations shown in FIGS. 2A, 2B and 2C, respectively.
  • FIG. 2A the resonator equivalent consisting of the two series capacitors C, and the shunt capacitors C,,, -C, is transformed into an open circuit by the shorting termination.
  • a source R termination is transformed into an inductance L, and a conductance G, in shunt with a current source I, where:
  • the internal terminals of the input resonator can only be terminated in the shunt combination of a conductance, an inductance and a current source, and the internal terminals of the output resonator can only be terminated in the shunt combination of a conductance and an inductance. Furthermore, the internal terminals of the auxiliary or remaining resonators can only be terminated in an open circuit or coupled to-another resonator through a capacitive 11.
  • interconnecting the external terminals of two resonators is equivalent to capacitively coupling their respective internal terminals and slightly mistuning the resonators being interconnected, the mistuning being the result of the shunt capacitors of the capacitive 11'.
  • This arrangement has the effect, in accordance with the invention, of generating transmission zeros in a MCF. Three cases will be discussed to illustrate how these transmission zeros are established and, from these illustrative cases, it will be evident that the same principles may be applied in accordance with the invention to any monolithic crystal filter that includes two or more auxiliary (non-input or output) resonators.
  • the first two cases realize one transmission zero below the passband and the third case realizes a pair of transmission zeros, one on either side of the band.
  • Base 1 the case of the filter structure shown in FIG. 3A, which includes an input resonator 101, an output resonator 104, and two auxiliary resonators 1.02 and 103. Each of the latter two resonators has a respective top electrode 102A and 103A which are connected together by a floating conducting path 105, whereas the bottom electrodes 1028 and 1038, along with electrodes 10113 and 1043, are connected to a common reference potential point which may be ground.
  • FIGS. 2A, 2B and 2C Using the transformations of FIGS. 2A, 2B and 2C on the equivalent circuit shown in FIG. 1B, one obtains the equiva lent circuit shown in FIG. 4, i.e., a circuit which is equivalent to the filter of FIG. 3A.
  • Y is the admittance between nodes i and j. From these equations one can obtain the transfer impedance as follows:
  • the zeros of N can be found from:
  • Case 3 The MCF of FIG. 7A is identical to that shown in FIGS. 3A and 5A with the exception of the specific resonator interconnections.
  • resonators 102 and 103 are connected as the input and output elements, respectively, while the end resonators 101 and 104 are connected as the auxiliary elements.
  • the zeros are somewhat closer to the passband and it can be shown that their location can be precisely controlled by the tuning frequency of the auxiliary resonators 101 and 104 and by either one or all three of the coupling coefficients k k and k USE OF AN EXTERNAL CAPACITOR
  • one additional parameter namely the capacitance ratio
  • the capacitance ratio may be employed effectively to control the transmission zero location and hence the location of attenuation peaks in relation to the passband. It is found that by increasing this ratio the'peaks can be moved closer to the band.
  • the capacity ratio may be increased advantageously by the use of an external capacitor which increases the static capacity of the resonators that it interconnects. Additionally, it has been found that this method moves the attenuation peaks closer to the band without in any way disturbing the parameters of the filter.
  • FIGS. 9A, 9B and 9C Examples of the use of an external capacitor in accordance with the invention are shown in FIGS. 9A, 9B and 9C.
  • the MCFs in these figures correspond, respectively, to the MCFs in FIGS. 3A, 5A and 7A, the difference in each instance being the use of an external capacitor C connected in the manner shown.
  • transmission zeros can also be realized in MCFs having more than four resonators. Also, when more than four resonators are used the number of zeros realized and the possible cases to be considered increase. For example, it can readily be verified that for an eight resonator MCF a maximum of four transmission zeros can be realized, with two appearing on either side of the passband.
  • the interconnections required for such a filter are illustrated in FIG. 10 where the auxiliary resonators 101 and 104 are tied together in the manner shown in FIG. 7 and the resonators 102 and 103 are connected as input and output elements. Additionally, however, each of four auxiliary resonators 105, 106, 107 and 108 is shorted and grounded in conventional fashion.
  • a monolithic crystal filter comprising, in combination,
  • a plurality of resonators each including a respective portion of a common piezoelectric body sandwiched between a respective pair of electrodes
  • said resonators including an input resonator, an output resonator and at least two auxiliary resonators,
  • auxiliary resonators a nongrounded, floating, short circuiting, conducting path external to said body connecting one electrode of one of said auxiliary resonators to a corresponding electrode of another of said auxiliary resonators, both of said auxiliary resonators being nonshorted an input point and means independent of said path directly connecting said input point to an electrode of said input resonator, and
  • Apparatus in accordance with claim 2 including means connecting one side of each of said resonators to a source of reference potential.
  • a monolithic crystal filter comprising, in combination,
  • a plurality of resonators each including a respective portion of a common piezoelectric body sandwiched between a respective pair of electrodes
  • said resonators including an input resonator, an output resonator and at least two auxiliary resonators,
  • auxiliary resonators a nongrounded, short circuiting conducting path external to said body connecting one electrode of one of said auxiliary resonators to a corresponding electrode of another of said auxiliary resonators, both of said auxiliary resonators being nonshorted an input point and means independent of said path directly connecting said input point to an electrode of said input resonator, and
  • Apparatus in accordance with claim 7 including a capacitor connected between said path and a source of reference potential.

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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)
US00183863A 1971-09-27 1971-09-27 Resonator interconnections in monolithic crystal filters Expired - Lifetime US3739304A (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3944951A (en) * 1974-11-21 1976-03-16 Bell Telephone Laboratories, Incorporated Monolithic crystal filter
US4484158A (en) * 1982-07-07 1984-11-20 General Electric Company Monolithic crystal filter and method of manufacturing same
US4604543A (en) * 1984-11-29 1986-08-05 Hitachi, Ltd. Multi-element ultrasonic transducer
US5231327A (en) * 1990-12-14 1993-07-27 Tfr Technologies, Inc. Optimized piezoelectric resonator-based networks

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5787228A (en) * 1980-11-18 1982-05-31 Oki Electric Ind Co Ltd Monolithic filter

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2198684A (en) * 1938-09-20 1940-04-30 Bell Telephone Labor Inc Wave filter
US2373431A (en) * 1943-03-30 1945-04-10 Bell Telephone Labor Inc Electric wave filter
US3334307A (en) * 1966-11-14 1967-08-01 Zenith Radio Corp Multi-electrode acoustic amplifier with unitary transducing and translating medium
US3396327A (en) * 1961-12-27 1968-08-06 Toyotsushinki Kabushiki Kaisha Thickness shear vibration type, crystal electromechanical filter
US3609601A (en) * 1970-01-12 1971-09-28 Collins Radio Co Monolithic filter having "m" derived characteristics
US3656180A (en) * 1970-08-12 1972-04-11 Bell Telephone Labor Inc Crystal filter

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2198684A (en) * 1938-09-20 1940-04-30 Bell Telephone Labor Inc Wave filter
US2373431A (en) * 1943-03-30 1945-04-10 Bell Telephone Labor Inc Electric wave filter
US3396327A (en) * 1961-12-27 1968-08-06 Toyotsushinki Kabushiki Kaisha Thickness shear vibration type, crystal electromechanical filter
US3334307A (en) * 1966-11-14 1967-08-01 Zenith Radio Corp Multi-electrode acoustic amplifier with unitary transducing and translating medium
US3609601A (en) * 1970-01-12 1971-09-28 Collins Radio Co Monolithic filter having "m" derived characteristics
US3656180A (en) * 1970-08-12 1972-04-11 Bell Telephone Labor Inc Crystal filter

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3944951A (en) * 1974-11-21 1976-03-16 Bell Telephone Laboratories, Incorporated Monolithic crystal filter
US4484158A (en) * 1982-07-07 1984-11-20 General Electric Company Monolithic crystal filter and method of manufacturing same
US4604543A (en) * 1984-11-29 1986-08-05 Hitachi, Ltd. Multi-element ultrasonic transducer
US5231327A (en) * 1990-12-14 1993-07-27 Tfr Technologies, Inc. Optimized piezoelectric resonator-based networks
US5404628A (en) * 1990-12-14 1995-04-11 Tfr Technologies, Inc. Method for optimizing piezoelectric resonator-based networks

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