EP0068345A1 - Symmetrical coupled line coplanar waveguide filter - Google Patents

Symmetrical coupled line coplanar waveguide filter Download PDF

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Publication number
EP0068345A1
EP0068345A1 EP82105340A EP82105340A EP0068345A1 EP 0068345 A1 EP0068345 A1 EP 0068345A1 EP 82105340 A EP82105340 A EP 82105340A EP 82105340 A EP82105340 A EP 82105340A EP 0068345 A1 EP0068345 A1 EP 0068345A1
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conductors
filter
coplanar waveguide
conductor
waveguide filter
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Ronald E. Stegens
Gary G. Hawisher
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Comsat Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
    • H01P1/00—Auxiliary devices
    • H01P1/20—Frequency-selective devices, e.g. filters
    • H01P1/201—Filters for transverse electromagnetic waves
    • H01P1/2013—Coplanar line filters

Definitions

  • This invention is directed to microwave filters, and more particularly to such filters having a co- planar waveguide (CPW) construction.
  • CPW co- planar waveguide
  • Edge-coupled bandpass filters of the stripline or microstrip variety are. well-known. These may typically comprise a configuration as shown in figures 1 and 2 in which first and second stripline conductors 10 and 12 are deposited on the upper surface of a dielectric substrate 14 having a ground plane 16 on the lower surface thereof. In such a filter configuration, a signal on one of the conductors, for example conductor 10, will be coupled across a gap 18 to the other conductor 12, and these filters perform substantially as desired even when the transmission lines are asymmetrical.
  • the design techniques for microstrip filters are well-known and are described, for example, in Design of Microwave Filters, Impedance-Matching Networks, and Coupling Structures, by Matthaei et al, Section 8.09. The synthesis procedure generally begins from a low pass prototype, and yields required values for even and odd mode impedances Z oe and Z oo' respectively, for each coupled section.
  • the odd mode impedance Z can be considered the impedance of either conductor when both conductors have opposite potentials (i.e., +1 and -1 volt), while the even mode impedance Z oe can be considered the impedance of either conductor when both have the same potential.
  • the input and output conductors and the ground plane are all coplanar.
  • conductors 20 and 22 are disposed on the upper surface of the dielectric 24 in a configuration similar to that of Figure 1, and the ground plane 16 in Figure 2 is replaced by a pair of grounded conductors 26 on the upper surface of the dielectric and on either side of the conductors 20 and 22.
  • CPW edge-coupled filters as shown in Figure 3 using two edge-coupled lines exhibiting the desired Zoe and Z 00 values perform poorly due to their asymmetrical construction.
  • the conductor 20 will be closer than the conductor 22 to the upper ground conductor 26, and conversely the conductor 22 will be closer than the conductor 20 to the lower ground conductor 26.
  • This asymetrical coupling of the conductors 20 and 22 to the different ground conductors can cause field asymmetry about the center line of transmission 28, and this may result in the excitation of an odd mode of propagation between the ground planes.
  • This odd mode of propagation tends to cause unwanted transmission responses which are difficult to predict, and some type of suppression, e.g. the use of bond wires placed over the line from one ground plane to the other, is usually required to obtain even partially satisfactory performance.
  • CPW coplanar waveguide
  • the CPW filter according to the present invention comprises first and second conductors and first and second ground planes all disposed on the same surface of a dielectric substrate, the first conductor having an enlarged coupling portion and the second conductor having a bifurcated coupling portion interposed between said first conductor and each of said ground planes.
  • the first and second conductors are symmetrical with respect to the transmission center line at all points.
  • the filter according to the present invention is based on the recognition that the odd mode propagation resulting in spurious filter response characteristics in known CPW filters could be substantially eliminated by utilizing a filter structure which is symmetrical about the center line of transmission.
  • Figure 5 illustrates a "threefinger" CPW filter construction utilizing symmetrical interleaved transmission lines.
  • a first conductor 30 and second conductor 32 are disposed on a dielectric substrate 34 between ground planes 36.
  • the conductors 30 and 32 are each provided with coupling portions 38 and 40, respectively, and the coupling portion 40 is bifurcated and extends around either side of the portion 38.
  • the conductors 30 and 32 and their respective coupling portions 38 and 40 are symmetrically disposed with respect to the center tine of transmission 42 so that all electric and magnetic fields will have even field symmetry about the center line 42. Due to the symmetrical construction, the filter shown in Figure 5 will not tend to excite unwanted transmission modes, and no mode suppression is required.
  • the dimensions of the interleaved portions 38 and 40 of the transmission lines are chosen so that the total capacitances from each line to the CPW grounds planes 36 will be equal. Since the bifurcated portion 40 is much closer than the inner portion 38 to each of the ground planes 36, the dimensions of the bifurcated portions will generally be much narrower than the enlarged portion 38, i.e., the dimension (C-B) will be much less than the dimension A in Figure 5.
  • the line structure shown in Figure 5 can be designed using conformal mapping techniques for a zero conductor thickness, infinite dielectric and equal line capacitance for the pair of coupled lines.
  • the design can be implemented according to the following procedure which can be used to map the three-finger structure of Figure 5 from a coupled stripline model.
  • the symmetric coupled line problem revolves around the conformal mapping of a cross sectional capacitance problem, using elliptic integrals.
  • the cell in Figure 6 represents the capacitance problem as a part of a stripline model of the coupled transmission lines in cross section.
  • the cell, shown on the complex plane, is in reality part of a structure periodic along both real and imaginary axes. This is mapped into the coupled CPW line shown in Figure 7.
  • the steps used to solve the symmetric coupled CPW line problem are summarized below.
  • the dimensions of the coupled line are determined from the desired even and odd mode impedances.
  • (K(k)/K'(k)) Ce/ ⁇ can be solved for k by simultaneously solving Equations (6) and (7) and k can be plugged back into each of equations (6) and (7) to obtain K(k) and K'(k) corresponding to K 2e and K' 2e , respectively.
  • a two-pole filter utilizing two of the filter sections of Figure 5 coupled in cascade and designed for a 1 GHz bandwidth and centered at 5.0 GHz was built by first utilizing prior art filter synthesis techniques to determine the required even and odd-mode impedances for each filter section and then utilizing the conformal mapping techniques of the above- described program to calculate the pattern dimensions. After the proper dimensions were calculated, the filter was fabricated using gold conductors on a 50 mil thick alumina substrate. The filter layout and dimensions are shown in Fig. 8. A 100 mil thickness of resistive material was added to the bottom of the substrate for further odd mode suppression, and the measured filter response agrees very closely with theoretical calculations as shown in Figure 9.
  • the filter according to the present invention may have either semi-infinite or finite ground planes on either side of the center conductors, and it should be appreciated that patterns other than that shown in Figure 5 may be used as long as symmetry is preserved. Other modifications could also be made to the disclosed filter structure without departing from the spirit and scope of the invention as defined in the following claims.

Abstract

A coplanar waveguide filter is disclosed including a pair of grounded conductors (36) disposed on a surface (34) of a dielectric substrate equidistant from a transmission center line (42), and first (30) and second (32) conductors on said substrate surface (34) between said grounded conductors (36) and symmetrically disposed with respect to the transmission center line (42) by virtue of one conductor (32) being bifurcated, and the other not. The structure produces coupled sections of coplanar waveguide transmission lines which are both physically and electrically symmetrical. Solutions for the even and odd-mode impedance are given, allowing synthesis of a wide range of filter functions.

Description

    BACKGROUND OF THE INVENTION
  • This invention is directed to microwave filters, and more particularly to such filters having a co- planar waveguide (CPW) construction.
  • Edge-coupled bandpass filters of the stripline or microstrip variety are. well-known. These may typically comprise a configuration as shown in figures 1 and 2 in which first and second stripline conductors 10 and 12 are deposited on the upper surface of a dielectric substrate 14 having a ground plane 16 on the lower surface thereof. In such a filter configuration, a signal on one of the conductors, for example conductor 10, will be coupled across a gap 18 to the other conductor 12, and these filters perform substantially as desired even when the transmission lines are asymmetrical. The design techniques for microstrip filters are well-known and are described, for example, in Design of Microwave Filters, Impedance-Matching Networks, and Coupling Structures, by Matthaei et al, Section 8.09. The synthesis procedure generally begins from a low pass prototype, and yields required values for even and odd mode impedances Zoe and Zoo' respectively, for each coupled section.
  • For purposes of discussion, the odd mode impedance Z can be considered the impedance of either conductor when both conductors have opposite potentials (i.e., +1 and -1 volt), while the even mode impedance Zoe can be considered the impedance of either conductor when both have the same potential. In obtaining a desired filter response, it is important that each of the conductors 10 and 12 have substantially the same even mode impedance Zoe, and odd mode impedance Zoo ; this is generally not difficult in a microstrip configuration since each of the conductors 10 and 12 is separated from the same ground plane by the same thickness of dielectric 14.
  • In a CPW configuration, the input and output conductors and the ground plane are all coplanar. As shown in figures 3 and 4, conductors 20 and 22 are disposed on the upper surface of the dielectric 24 in a configuration similar to that of Figure 1, and the ground plane 16 in Figure 2 is replaced by a pair of grounded conductors 26 on the upper surface of the dielectric and on either side of the conductors 20 and 22. CPW edge-coupled filters as shown in Figure 3 using two edge-coupled lines exhibiting the desired Zoe and Z 00 values perform poorly due to their asymmetrical construction. Note that in the center-coupling section of the filter of Figure 3, the conductor 20 will be closer than the conductor 22 to the upper ground conductor 26, and conversely the conductor 22 will be closer than the conductor 20 to the lower ground conductor 26. This asymetrical coupling of the conductors 20 and 22 to the different ground conductors can cause field asymmetry about the center line of transmission 28, and this may result in the excitation of an odd mode of propagation between the ground planes. This odd mode of propagation tends to cause unwanted transmission responses which are difficult to predict, and some type of suppression, e.g. the use of bond wires placed over the line from one ground plane to the other, is usually required to obtain even partially satisfactory performance.
  • As a consequence of the above difficulties, the use of CPW filters has been limited to applications where high quality filters are not required, and improvements in CPW filter technology are needed.
    • - SUMMARY OF THE INVENTION
  • It is an object of this invention to provide a coplanar waveguide (CPW) filter which is substantially free of the above difficulties and performs satisfactorily even where high quality filters are required.
  • Briefly, the CPW filter according to the present invention comprises first and second conductors and first and second ground planes all disposed on the same surface of a dielectric substrate, the first conductor having an enlarged coupling portion and the second conductor having a bifurcated coupling portion interposed between said first conductor and each of said ground planes. The first and second conductors are symmetrical with respect to the transmission center line at all points.
    • BRIEF DESCRIPTION OF THE DRAWINGS
    • Figure 1 is a plan view of a conventional stripline microwave filter;
    • Figure 2 is a sectional view along lines II-II in Figure 1;
    • Figure 3 is a plan view of a prior art coplanar waveguide microwave filter;
    • Figure 4 is a sectional view along lines II-II in Figure 3;
    • Figure 5 is a plan view of a coplanar waveguide filter according to the present invention;
    • Figures 6 and 7 are diagrams for explaining the design of a filter according to the present invention;
    • Figure 8 is a plan view of a three section filter according to the present invention; and
    • Figure 9 is a graphical illustration of the measured and theoretical response of the filter of Figure 8.
    DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
  • The filter according to the present invention is based on the recognition that the odd mode propagation resulting in spurious filter response characteristics in known CPW filters could be substantially eliminated by utilizing a filter structure which is symmetrical about the center line of transmission. Figure 5 illustrates a "threefinger" CPW filter construction utilizing symmetrical interleaved transmission lines. In this filter structure, a first conductor 30 and second conductor 32 are disposed on a dielectric substrate 34 between ground planes 36. The conductors 30 and 32 are each provided with coupling portions 38 and 40, respectively, and the coupling portion 40 is bifurcated and extends around either side of the portion 38. The conductors 30 and 32 and their respective coupling portions 38 and 40 are symmetrically disposed with respect to the center tine of transmission 42 so that all electric and magnetic fields will have even field symmetry about the center line 42. Due to the symmetrical construction, the filter shown in Figure 5 will not tend to excite unwanted transmission modes, and no mode suppression is required.
  • In order to maintain equal even and odd mode impedances Zoe and Zoo for each of the conductors 30 and 32, the dimensions of the interleaved portions 38 and 40 of the transmission lines are chosen so that the total capacitances from each line to the CPW grounds planes 36 will be equal. Since the bifurcated portion 40 is much closer than the inner portion 38 to each of the ground planes 36, the dimensions of the bifurcated portions will generally be much narrower than the enlarged portion 38, i.e., the dimension (C-B) will be much less than the dimension A in Figure 5.
  • The line structure shown in Figure 5 can be designed using conformal mapping techniques for a zero conductor thickness, infinite dielectric and equal line capacitance for the pair of coupled lines. The design can be implemented according to the following procedure which can be used to map the three-finger structure of Figure 5 from a coupled stripline model.
  • The symmetric coupled line problem revolves around the conformal mapping of a cross sectional capacitance problem, using elliptic integrals. The cell in Figure 6 represents the capacitance problem as a part of a stripline model of the coupled transmission lines in cross section. The cell, shown on the complex plane, is in reality part of a structure periodic along both real and imaginary axes. This is mapped into the coupled CPW line shown in Figure 7.
  • The steps used to solve the symmetric coupled CPW line problem are summarized below. The dimensions of the coupled line are determined from the desired even and odd mode impedances.
    • 1) Express the even and odd mode impedances in terms of the normalized line capacitances Ce/e and Co/ε using equations (1), (2) and (3).
      Figure imgb0001
      Figure imgb0002
      Figure imgb0003
      where Zoe, and Zoo are the even and odd mode impedances, µo/εo is the free space impedance, εr is the relative dielectric constant of the CPW substrate.
    • 2) Define K2e/K'2e and K2o/K'2o from Ce/ε and Co/ε using equations (4) and (5).
      Figure imgb0004
      Figure imgb0005
    • 3) Find K2e and K2o from K2e/K'2e and K2o/K'2o using the general definitions of the complete elliptic integral given in equations (6) and (7). This will require numeric techniques of the type described in Jacobi and Elliptic Functions, L.M. Milne-Thomas, Dover Publ., 1950.
      Figure imgb0006
      Figure imgb0007
  • Basically, for example, (K(k)/K'(k)) = Ce/ε can be solved for k by simultaneously solving Equations (6) and (7) and k can be plugged back into each of equations (6) and (7) to obtain K(k) and K'(k) corresponding to K2e and K'2e, respectively.
    • 4) Define k1 from k2e and k2o using equation (8).
      Figure imgb0008
    • 5) Find K1 and K'1 from k1 using the complete elliptic integral definitions in equations (6) and (7).
    • 6) Define Ko/K'o from K1 and K'1 using equation (9).
      Figure imgb0009
    • 7) Find Ko and ko from Ko/K'o using the complete elliptic integral definitions given in equations (6) and (7).
    • 8) Define the quantity
      Figure imgb0010
      (shown in Figure 1) from K1, k1 and k2o using equation (10), and the definition of the incomplete elliptic integral given in equation (11).
      Figure imgb0011
      Figure imgb0012
    • 9) The final dimensions D, A, B and C, shown in Figures 5 and 7, are obtained from Ko, k and c/a using equations (12), (13), (14) and (15). The Jacobi elliptic function sn(x,k) is the inverse of the incomplete elliptic integral function in equation (11). This relationship is shown in equation (16).
      Figure imgb0013
      Figure imgb0014
      Figure imgb0015
      Figure imgb0016
    Figure imgb0017
  • A computer program using this procedure will generate tabulated output data as shown in the following TABLES I-V where ER is the dielectric constant of the substrate, ZE is the even mode impedance Zoe, ZO is the odd mode impedance Zoo, and A through D represent the dimensions illustrated in Figure 5.
  • Results are shown for a popular dielectric material (Al2O3) for which ER = 10.
    Figure imgb0018
    Figure imgb0019
    Figure imgb0020
    Figure imgb0021
    Figure imgb0022
  • A two-pole filter utilizing two of the filter sections of Figure 5 coupled in cascade and designed for a 1 GHz bandwidth and centered at 5.0 GHz was built by first utilizing prior art filter synthesis techniques to determine the required even and odd-mode impedances for each filter section and then utilizing the conformal mapping techniques of the above- described program to calculate the pattern dimensions. After the proper dimensions were calculated, the filter was fabricated using gold conductors on a 50 mil thick alumina substrate. The filter layout and dimensions are shown in Fig. 8. A 100 mil thickness of resistive material was added to the bottom of the substrate for further odd mode suppression, and the measured filter response agrees very closely with theoretical calculations as shown in Figure 9.
  • The filter according to the present invention may have either semi-infinite or finite ground planes on either side of the center conductors, and it should be appreciated that patterns other than that shown in Figure 5 may be used as long as symmetry is preserved. Other modifications could also be made to the disclosed filter structure without departing from the spirit and scope of the invention as defined in the following claims.

Claims (7)

1. A coplanar waveguide filter having at least one filter section formed of a pair of grounded conductors (36)disposed on a substrate surface (34) and first (30) and second (32) conductors disposed on said substrate surface (34) between said grounded conductors (36), characterized in that
a) said pair of grounded conductors (36) is equidistant from a transmission center line (42), and that
b) each of said first (30) and second (32) conductors is symmetrically disposed with respect to said transmission center line (42).
2. A coplanar waveguide filter as defined in claim 1, characterized in that said first (30) and second (32) conductors include longitudinally overlapping end portions (38, 40).
3. A coplanar waveguide fileter as defined in claim 1, characterized in that said first (30) and second (32) conductors include interleaved end portions (38, 40).
4. A coplanar waveguide filter as defined in claim 1, characterized in that said first conductor (30) includes an end portion (38) and said second conductor (32) includes at least first and second end portions (40) disposed on either side of said first conductor end portion (38) between said first conductor (30) and each of said grounded conductors (36).
5. A coplanar waveguide filter as defined in any one of claims 2, 3 or 4, characterized in that each of said first (30) and second (32) conductors has the same total capacitance to ground.
6. A coplanar waveguide filter as defined in claim 5, characterized in that the total width (2[C-B]) of said second conductor (32) is less than the total width (2A) of said first conductor (30), said widths being measured in a direction along said substrate (34) perpendicular to said transmission center line.
7. A coplanar waveguide filter as defined in any one of claims 1, 2 or 3, characterized in that said least one filter section comprises a plurality of filter sections (Fig. 8) connected in cascade.
EP82105340A 1981-06-25 1982-06-18 Symmetrical coupled line coplanar waveguide filter Withdrawn EP0068345A1 (en)

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2153155A (en) * 1984-01-24 1985-08-14 Secr Defence Improvements on or relating to microwave filters
EP0589704A1 (en) * 1992-09-24 1994-03-30 Matsushita Electric Industrial Co., Ltd. Microwave filter
GB2295277A (en) * 1994-11-16 1996-05-22 Philips Electronics Uk Ltd RF circuits with microstrip coupler
EP1562255A1 (en) * 2004-02-03 2005-08-10 NTT DoCoMo, Inc. Coplanar filter
EP1691443A1 (en) * 2005-02-09 2006-08-16 NTT DoCoMo INC. Coupling structure, resonator excitation structure and filter for coplanar-waveguide circuit
DE102007041125B3 (en) * 2007-08-30 2009-02-26 Qimonda Ag Sensor e.g. position sensor, for detecting measured variable of coplanar waveguide, has structures with dielectric characteristics, respectively, where measured variable influences characteristics or relationship between structures
US7782066B2 (en) 2007-08-30 2010-08-24 Qimonda Ag Sensor, method for sensing, measuring device, method for measuring, filter component, method for adapting a transfer behavior of a filter component, actuator system and method for controlling an actuator using a sensor

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2859417A (en) * 1952-05-08 1958-11-04 Itt Microwave filters
US3820041A (en) * 1972-08-28 1974-06-25 J Gewartowski Resonance control in interdigital capacitors useful as dc breaks in diode oscillator circuits

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2859417A (en) * 1952-05-08 1958-11-04 Itt Microwave filters
US3820041A (en) * 1972-08-28 1974-06-25 J Gewartowski Resonance control in interdigital capacitors useful as dc breaks in diode oscillator circuits

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
CONFERENCE PROCEEDINGS OF THE 6th EUROPEAN MICROWAVE CONFERENCE, 14th/17th September 1976, pages 49-53, Sevenoaks (GB); *

Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2153155A (en) * 1984-01-24 1985-08-14 Secr Defence Improvements on or relating to microwave filters
EP0589704A1 (en) * 1992-09-24 1994-03-30 Matsushita Electric Industrial Co., Ltd. Microwave filter
US5461352A (en) * 1992-09-24 1995-10-24 Matsushita Electric Industrial Co., Ltd. Co-planar and microstrip waveguide bandpass filter
CN1050703C (en) * 1992-09-24 2000-03-22 松下电器产业株式会社 Electric filter
GB2295277A (en) * 1994-11-16 1996-05-22 Philips Electronics Uk Ltd RF circuits with microstrip coupler
US7378924B2 (en) 2004-02-03 2008-05-27 Ntt Docomo, Inc. Filter with improved capacitive coupling portion
CN100385729C (en) * 2004-02-03 2008-04-30 株式会社Ntt都科摩 Coplanar filter
EP1562255A1 (en) * 2004-02-03 2005-08-10 NTT DoCoMo, Inc. Coplanar filter
EP1691443A1 (en) * 2005-02-09 2006-08-16 NTT DoCoMo INC. Coupling structure, resonator excitation structure and filter for coplanar-waveguide circuit
KR100820285B1 (en) * 2005-02-09 2008-04-07 가부시키가이샤 엔티티 도코모 Coupling structure, resonator excitation structure and filter for coplanar-waveguide circuit
US7397331B2 (en) 2005-02-09 2008-07-08 Ntt Docomo, Inc. Coupling structure, resonator excitation structure and filter for coplanar-waveguide circuit
CN100466374C (en) * 2005-02-09 2009-03-04 株式会社Ntt都科摩 Coupling structure, resonator excitation structure and filter for coplanar-waveguide circuit
EP2065964A1 (en) * 2005-02-09 2009-06-03 NTT DoCoMo, Inc. Coupling structure, resonator excitation structure and filter for coplanar-waveguide circuit
DE102007041125B3 (en) * 2007-08-30 2009-02-26 Qimonda Ag Sensor e.g. position sensor, for detecting measured variable of coplanar waveguide, has structures with dielectric characteristics, respectively, where measured variable influences characteristics or relationship between structures
US7782066B2 (en) 2007-08-30 2010-08-24 Qimonda Ag Sensor, method for sensing, measuring device, method for measuring, filter component, method for adapting a transfer behavior of a filter component, actuator system and method for controlling an actuator using a sensor

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