US3697903A - Equal-resonator piezoelectric ladder filters - Google Patents

Equal-resonator piezoelectric ladder filters Download PDF

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Publication number
US3697903A
US3697903A US730002A US3697903DA US3697903A US 3697903 A US3697903 A US 3697903A US 730002 A US730002 A US 730002A US 3697903D A US3697903D A US 3697903DA US 3697903 A US3697903 A US 3697903A
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resonator
resonators
branch
series
shunt
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US730002A
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Franz L Sauerland
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Clevite Corp
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Clevite Corp
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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 devices; Electromechanical resonators
    • H03H9/46—Filters
    • H03H9/54—Filters comprising resonators of piezoelectric or electrostrictive material
    • H03H9/542—Filters comprising resonators of piezoelectric or electrostrictive material including passive elements

Definitions

  • the filter selectivity may be changed without [52] US. Cl ..333/72, 333/70 S changing the resonators and Without requiring the [51] Int. Cl. ..H03h 7/08 frequencies of P insertion 1088 to be ehanged-
  • This [58] Field of Search ..333/70, 71, 72, 74; 317/234; yp of filter will be referred to as equal-resonator 3lO/8.5, 8.3 r
  • a piezoelectric ladder filter section is made up with two identical piezoelectric resonators, one in a series branch and one in a shunt branch and capacitors are included in the circuit.
  • the capacitors include what amounts to a T section or an equivalent pi section between a shunt piezoelectric branch and a series piezoelectric branch. These and other capacitors in the circuit are varied in capacitance in order to obtain the desired bandwidth and skirt ratio (sometimes referred to as selectivity).
  • a ladder filter is designed by computing the values of capacitors required to produce such characteristics.
  • FIG. I is a diagram illustrating the cascading of networks
  • FIGS. 2a and 2b are diagrams of identical resonator cascade networks in accordance with the invention.
  • FIGS. 3a and 3b are diagrams of ladder filter sections in accordance with the invention particularly suited to cascading where more than two identical sections are to be cascaded,
  • FIG. 4 is a graph showing a family of response curves with different bandwidths for ladder filter sections of the type in FIG. 3a, with identical resonators (Z 2
  • FIG. 5 is a diagram of the responses of a cascade of two sections of type 5 of FIG. 4 and five sections of type 9 of FIG. 4, respectively,
  • FIG. 6 is a graph of a nominal response curve of a cascade of two basic sections of the type shown in FIG. 3a,
  • FIG. 7 is a perspective diagram of the construction which may be employed for the T network of capacitors.
  • I-2 Piezoelectric Ladders The insertion loss peaks in the lower stopband are caused by series resonance (i.e., short circuit) of the shunt resonators, while the peaks in the upper stopband are due to antiresonance (i.e., open circuit) of the series resonators.
  • One way of describing the selectivity of a bandpass filter is by the minimum stopband rejection (the difference between the minimum loss in the band pass and the minimum loss in the band reject regions). and the skirt ratio S.
  • S P/B the skirt ratio
  • P and B are defined respectively as the difference in frequency for the two infinite rejection points which bound the pass band and the difference in frequency of the 3 db loss points in the pass band.
  • this ratio is usually less than 5; in ceramic filters it is frequently increased by choosing extreme geometries (for example, extremely thin and thick disks) for the shunt and series resonators. As will be discussed in Section IV, this presents problems with regard to spurious resonator responses.
  • the stopband rejection is limited.
  • the maximally obtainable stopband rejection for a 2-resonator ladder without additional capacitors is presently about 26 db.
  • Piezoelectric ladder filters whose selectivity may be changed without changing the resonators and without requiring the frequencies of peak insertion loss to be changed are called equal resonator filters.
  • Equal-resonator filters all of whose resonators are identical are called identical resonator filters.
  • Image parameter theory teaches one that one can cascade two filters to obtain a more complex filter as shown in FIG. 1.
  • Image parameter theory is also well known. An example of this explanation is given on pages 176-177 of Transmission Networks and Wave Filters by T. E. Shea published by D. Van Nostrand,
  • FIG. 1 a series combination of the two resonators denoted Z acts and contributes as only one resonator of impedance 22.
  • the filter shown in FIG. 1 can provide at most the filtering provided by conventional three-resonator circuits. This drawback can be avoided by using filters which, when cascaded, result in filters shown in FIGS. 2a and 2b.
  • the filters shown in FIGS. 2a and 2b can provide the filtering provided by conventional four-resonator ladder filters.
  • FIGS. 3a and 3b typically represent capacitive T sections and II sections, respectively. However, either may be regarded as a combination of two L sections.
  • the capacitors C, and C alone constitute an L section or if the L sections were shown separately, C, and a capacitor'of half the capacitance of capacitor C would constitute one L section; and capacitor C and a capacitor of half the capacitance of C shunting the other half capacitance would constitute the second L section. It will-be observed, for example, in FIG.
  • an L section would result in the case of the degenerate capacitive circuit in which one of the series capacitors of the T network was short circuited or one of the shunt capacitors of a II network was open circuited or became very large or very small, respectively, so that it could be approximated by a short circuit or an open circuit.
  • the input and output image impedances of section FIG. 3a are of the form i.e., they are independent of the frequencies of peak insertion loss.
  • An insertion-loss peak frequency fp in the upper stopband may be reduced by shunting a capacitor to the series resonator that produces f
  • An insertion-loss peak [P in the lower stopband may be increased by connecting a capacitor in series with the shunt resonator that producesf
  • Piezoelectric resonators have spurious responses, i.e., resonances at frequencies other than and in addition to the desired frequency. These spurii may be harmonic, inharmonic or subharmonic responses of the desired mode of vibration. In addition, they may be due to undesired modes of vibration. As a result of spurii, piezoelectric filters exhibit undersired passbands in addition to the desired one.
  • a large stopband rejection R requires a large ratio C ,,,/C,, m of the static capacitances of shunt and series resonators, respectively.
  • this can for example be achieved by using a thin shunt resonator and a thick and possibly partially electroded series resonator, i.e., by resorting to extreme resonator geometries and without regard for the geometry that would enhance the suppression of spurii.
  • the resonators may be predetermined, and the static capacitances of the shunt and series resonator may be chosen to conform to the resonator geometry that is optimum with regard to suppression of spurii.
  • the equal-resonator ladder therefore, allows use of resonators designed for minimum spurious response.
  • Table II shows analogous results for another identical resonator filter.
  • FIG. 5' in curve a, shows the response of a cascade of two sections of the type No. 5 of FIG. 4 and in curve b, shows the response of a cascade of five sections of the type No. 9 of FIG. 4.
  • capacitors C C and'C of FIG. 3a could be made in one piece as suggested in FIG. 7, with one common electrode on the opposite face of the disk.
  • electrodes and interconnections for all capacitors of one or more basic section(s) could be combined on a ceramic substrate that could at the same time serve as circuit board for the resonators.
  • the capacitors C C C C,,, C may be made either adjustable in capacitance or continuously variable in order that a selected bandwidth and skirt ratio may be obtained while the filter is in circuit.
  • a bandpass electric wave filter having ladder structure with shunt and series branches, a piezoelectric resonator in a shunt branch, a piezoelectric resonator in a series branch, and a capacitive circuit separating the resonators, the capacitive circuits being selected from a group consisting of ll, T and L circuits, in the case of an L circuit a capacitor in a series branch being connected to the shunt branch resonator and a capacitor in a shunt branch being connected to a series branch capacitor, the capacitive circuit providing high stop band rejection corresponding to stop band rejection of resonators with larger ratios of static capacitance of shunt resonators to static capacitance of series resonators and no separating capacitive circuit, the values of capacitances in the capacitive circuit being such as result from a transformation to a capacitive, II, T or L network, of a capacitor-transformer coupling between resonators to give the equivalent of large ratios of
  • a bandpass electric wave filter having ladder structure with shunt and series branches comprising capacitors and at least three piezoelectric resonators in combination, each resonator being connected in either a series branch or a shunt branch with at least one resonator in a shunt branch, and at least one resonator in a series branch, with at least one capacitive circuit separating a resonator in a series branch from a resonator in a shunt branch, the capacitive circuits being selected from a group consisting of II, T and L circuits, in the case of an L circuit the separation being such that a resonator in a shunt branch is connected to a capacitor in a series branch, which is connected to a capacitor in a shunt branch, which in turn, is connected to a resonator in a series branch, the capacitive circuit rovidin hi h sto band re'ection corres ondin to gtop ban d rectioii
  • a ladder filter as described in claim 2 having at least three resonators with all of the resonators in series branches being substantially identical and all of the resonators in shunt branches being substantially identical.
  • a ladder filter as described in claim 3 with a resonator in a shunt branch and a resonator in a series branch separated by capacitive ll circuit.
  • a ladder filter as described in claim 7 with a resonator in a shunt branch and a resonator in a series branch separated by capacitive L section.
  • a ladder filter as described in claim 7 with a resonator in a shunt branch and a resonator in a series branch separated by a capacitive ll section.
  • a ladder filter as described in claim 7 with a resonator in a shunt branch and a resonator in a series branch separated by a capacitive T section.

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  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Piezo-Electric Or Mechanical Vibrators, Or Delay Or Filter Circuits (AREA)
US730002A 1968-05-17 1968-05-17 Equal-resonator piezoelectric ladder filters Expired - Lifetime US3697903A (en)

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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5047726A (en) * 1990-02-16 1991-09-10 Ericsson Ge Mobile Communications Inc. Method and apparatus for implementing four-frequency measuring process for coupled-dual resonator crystals using both resonator ports
US5051711A (en) * 1989-04-27 1991-09-24 Ten-Tec, Inc. Variable bandwidth crystal filter with varactor diodes
US20050174198A1 (en) * 2003-12-29 2005-08-11 Stmicroelectronics S.A. Electronic circuit comprising a resonator to be integrated into a semiconductor product
EP1739831A1 (de) * 2005-07-01 2007-01-03 STMicroelectronics S.A. Bandpassfilter mit akustischen Resonatoren
US20070035365A1 (en) * 2005-03-18 2007-02-15 Stmicroelectronics S.A. Integrable tunable filter circuit comprising a set of baw resonators
US20080024244A1 (en) * 2006-07-28 2008-01-31 Stmicroelectronics Sa Filtering circuit fitted with acoustic resonators
US20090072925A1 (en) * 2007-09-03 2009-03-19 Stmicroelectronics Sa Frequency tuning circuit for lattice filter
US20190058456A1 (en) * 2016-02-24 2019-02-21 Telefonaktiebolaget Lm Ericsson (Publ) Band reject filters

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1795204A (en) * 1927-01-03 1931-03-03 American Telephone & Telegraph Electrical wave filter
US2045991A (en) * 1931-09-19 1936-06-30 Bell Telephone Labor Inc Wave filter
US2165509A (en) * 1938-04-22 1939-07-11 Bell Telephone Labor Inc Oscillation generator
US2248776A (en) * 1938-07-28 1941-07-08 Bell Telephone Labor Inc Wave filter
US2547133A (en) * 1947-12-04 1951-04-03 Bell Telephone Labor Inc Wave filter
US3344368A (en) * 1967-09-26 Fettweis bandpass filter

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3344368A (en) * 1967-09-26 Fettweis bandpass filter
US1795204A (en) * 1927-01-03 1931-03-03 American Telephone & Telegraph Electrical wave filter
US2045991A (en) * 1931-09-19 1936-06-30 Bell Telephone Labor Inc Wave filter
US2165509A (en) * 1938-04-22 1939-07-11 Bell Telephone Labor Inc Oscillation generator
US2248776A (en) * 1938-07-28 1941-07-08 Bell Telephone Labor Inc Wave filter
US2547133A (en) * 1947-12-04 1951-04-03 Bell Telephone Labor Inc Wave filter

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
Mason Bell System Tech. Journal Vol. 13 July 1934 p. 410 414. *

Cited By (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5051711A (en) * 1989-04-27 1991-09-24 Ten-Tec, Inc. Variable bandwidth crystal filter with varactor diodes
US5047726A (en) * 1990-02-16 1991-09-10 Ericsson Ge Mobile Communications Inc. Method and apparatus for implementing four-frequency measuring process for coupled-dual resonator crystals using both resonator ports
US20050174198A1 (en) * 2003-12-29 2005-08-11 Stmicroelectronics S.A. Electronic circuit comprising a resonator to be integrated into a semiconductor product
US7492242B2 (en) 2005-03-18 2009-02-17 Stmicroelectronics S.A. Integrable tunable filter circuit comprising a set of BAW resonators
US7825748B2 (en) 2005-03-18 2010-11-02 Stmicroelectronics Sa Integrable tunable filter circuit comprising a set of BAW resonators
US20090174503A1 (en) * 2005-03-18 2009-07-09 Stmicroelectronics Sa Integrable tunable filter circuit comprising a set of baw resonators
US20070035365A1 (en) * 2005-03-18 2007-02-15 Stmicroelectronics S.A. Integrable tunable filter circuit comprising a set of baw resonators
FR2888060A1 (fr) * 2005-07-01 2007-01-05 St Microelectronics Sa Circuit de filtrage passe-bande dote de resonateurs acoustiques
US7525400B2 (en) 2005-07-01 2009-04-28 Stmicroelectronics S.A. Band pass filtering circuit fitted with acoustic resonators
US20070030099A1 (en) * 2005-07-01 2007-02-08 Stmicroelectronics S.A. Band pass filtering circuit fitted with acoustic resonators
EP1739831A1 (de) * 2005-07-01 2007-01-03 STMicroelectronics S.A. Bandpassfilter mit akustischen Resonatoren
US20080024244A1 (en) * 2006-07-28 2008-01-31 Stmicroelectronics Sa Filtering circuit fitted with acoustic resonators
US7696844B2 (en) 2006-07-28 2010-04-13 Stmicroelectronics Sa Filtering circuit fitted with acoustic resonators
US20090072925A1 (en) * 2007-09-03 2009-03-19 Stmicroelectronics Sa Frequency tuning circuit for lattice filter
US7920036B2 (en) 2007-09-03 2011-04-05 Stmicroelectronics S.A. Frequency tuning circuit for lattice filter
US20190058456A1 (en) * 2016-02-24 2019-02-21 Telefonaktiebolaget Lm Ericsson (Publ) Band reject filters
US11063575B2 (en) * 2016-02-24 2021-07-13 Telefonaktiebolaget Lm Ericsson (Publ) Band reject filters

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GB1272104A (en) 1972-04-26
DE1925154A1 (de) 1970-05-21
FR2008772A1 (de) 1970-01-23

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