WO1999052171A1

WO1999052171A1 - Microstrip cross-coupling control apparatus and method

Info

Publication number: WO1999052171A1
Application number: PCT/US1999/007402
Authority: WO
Inventors: Dawei Zhang
Original assignee: Conductus, Inc.
Priority date: 1998-04-03
Filing date: 1999-04-02
Publication date: 1999-10-14
Also published as: WO1999052171A9; AU3743399A

Abstract

The present invention provides for a method and apparatus to control non-adjacent cross-coupling in a micro-strip filter. In instances of weak cross-coupling, such as a filter circuit on a high dielectric constant substrate material (e.g., LaAIO3 with dielectric constant of 24), a closed loop is used to inductively enhance the cross-coupling. The closed loop increases the transmission zero levels. For strong cross-coupling cases, such as a filter circuit on a lower dielectric constant substrate material (e.g., MgO with dielectric constant of 9.6), a capacitive cross-coupling cancellation mechanism is introduced to reduce the cross-coupling. In the latter instance, the transmission zero levels are moved down.

Description

MICROSTRIP CROSS-COUPLING CONTROL APPARATUS AND METHOD

FIELD OF THE INVENTION

The present invention relates generally to filters for electrical signals, more particularly to control of cross-coupling in narrowband filters, and still more particularly to methods and apparatus to control the placement of transmission zeroes when introducing cross coupling between non-adjacent resonators in a narrowband filter.

BACKGROUND

Narrowband filters are particularly useful in the communications industry and particularly for wireless communications systems which utilize microwave signals. At times, wireless communications have two or more service providers operating on separate bands within the same geographical area. In such instances, it is essential that the signals from one provider do not interfere with the signals of the other provider(s). At the same time, the signal throughput within the allocated frequency range should have a very small loss.

Within a single provider's allocated frequency, it is desirable for the communication system to be able to handle multiple signals. Several such systems are available, including frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and broad-band CDMA (b-CDMA). Providers using the first two methods of multiple access need filters to divide their allocated frequencies in the multiple bands. Alternatively, CDMA operators might also gain an advantage from dividing the frequency range into bands. In such cases, the narrower the bandwidth of the filter, the closer together one may place the channels. Thus, efforts have been previously made to construct very narrow bandpass filters, preferably with a fractional-band width of less than 0.05%. An additional consideration for electrical signal filters is overall size. For example, with the development of wireless communication technology, the cell size (e.g., the area within which a single base station operates) will get much smaller perhaps covering only a block or even a building. As a result, base station providers will need to buy or lease space for the stations. Since each station requires many separate filters, the size of the filter becomes increasingly important in such an environment. It is, therefore, desirable to mmimize filter size while realizing a filter with very narrow fractional-bandwidth and high quality factor Q. In the past, however, several factors have limited attempts to reduce the filter size. For example, in narrowband filter designs, achieving weak coupling is a challenge. Filter designs in a microstrip configuration are easily fabricated. However, very narrow bandwidth microstrip filters have not been realized because coupling between the resonators decays only slowly as a function of element separation. Attempts to reduce fractional-bandwidth in a microstrip configuration using selective coupling techniques have met with only limited success. The narrowest fractional-bandwidth reported to date in a microstrip configuration was 0.6%. Realization of weak coupling by element separation is ultimately limited by the feedthrough level of the microstrip circuit. Two other approaches have been considered for very-narrow- bandwidth filters. First, cavity type filters may be used. However, such filters are usually quite large. Second, filters in stripline configurations may be used, but such devices are usually hard to package. Therefore, by utilizing either of these two types of devices there is an inevitable increase in the final system size, complexity and the engineering cost.

If a quasi-elliptical filter response is desired, it will be appreciated that transmission zeroes on both sides of the passband may be used to enhance the filter skirt rejections. For fewer poles and less Q requirements, a quasi-elliptical filter can achieve similar skirt rejections compared to a Chebyshev filter. Figure 5 illustrates a simulated response of a quasi-elliptical filter compared to a Chebyshev filter.

One method of achieving a quasi-elliptical filter response is to introduce a cross-coupling between two or more specific non-adjacent resonators. In microstrip filter designs, the separation(s) of non-adjacent resonators and the dielectric properties of the substrate determine the strength of the cross coupling. If the layout topology of the filter is constructed such that desired non-adjacent resonators are close together, then the cross-coupling of such non-adjacent resonators can introduce transmission zeroes on both sides of the filter transmission. This results in the layout providing a beneficial parasitic effect in the quasi-elliptical filter response. However, in the past the introduction of such non-adjacent cross- coupling has not been easily controlled. For example, depending upon the required filter size, number of poles and substrate choice, the transmission zeroes may not be provided at the appropriate location. Thus, at times the cross-coupling may not be large enough - such that the transmission zeroes are at very low levels. At other times, the cross-coupling is too large, such that the transmission zeroes are at very high level ~ which interferes with passband performance.

Therefore, there exists a need for a super-narrow-bandwidth filter having the convenient fabrication advantage of microstrip filters while achieving, in a small filter, the appropriate non-adjacent cross-coupling necessary to introduce transmission zeroes which provides an optimized transmission response of the filter.

SUMMARY The present invention provides for a method and apparatus to control non-adjacent cross-coupling in a micro-strip filter. In instances of weak cross- coupling, such as a filter circuit on a high dielectric constant substrate material (e.g., LaAIO₃ with dielectric constant of 24), a closed loop is used to inductively enhance the cross-coupling. The closed loop increases the transmission zero levels. For strong cross-coupling cases, such as a filter circuit on a lower dielectric constant substrate material (e.g., MgO with dielectric constant of 9.6), a capacitive cross- coupling cancellation mechanism is introduced to reduce the cross-coupling. In the latter instance, the transmission zero levels are moved down.

In the preferred embodiment, the present invention is used in connection with a super-narrow band filter using frequency dependent L-C components (such as are described in Zhang, et al. U.S. Ser. No. 08/706,974 which is hereby incorporated herein and made a part hereof by reference). The filter utilizes a frequency dependent L-C circuit with a positive slope k for the inductor values as a function of frequency. The positive k value allows the realization of a very narrow-band filter. Although this filter environment and its topology is used to describe the present invention, such environment is used by way of example, and the invention might be utilized in other environments (for example, other filter devices with non-adjacent resonator devices, such as lumped element quasi-elliptical filters). Further, the environments of communications and wireless technology are used herein by way of example. The principles of the present invention may be employed in other environments as well. Accordingly, the present invention should not be construed as limited by such examples.

As noted above, there have been previous attempts to utilize non-adjacent parasitic coupling to introduce transmission zeroes in filters. However, such efforts have generally been provided purely as a parasitic effect without control. One example of such an attempt is described in S. Ye and R.R. Mansour, DESIGN OF MANIFOLD-COUPLED MULTIPLEXORS USING SUPERCONDUCTIVE LUMPED ELEMENT FILTERS, p. 191, IEEE MTT-S Digest (1994). Still other techniques have been developed to artificially add non-adjacent cross-couplings. Here the efforts have generally introduced transmission zeroes using a properly phased transmission line. Examples of these latter efforts may be found in S.J. Hedges and R.G. Humphreys, EXTRACTED POLE PLANAR ELLIPTICAL FUNCTION FILTERS, p. 97; and U.S. Pat. No. 5,616,539, issued to Hey-Shipton et al. None of these efforts, however, provide the precise control cross-coupling control and flexibility to optimize the filter performance.

Referring more specifically to the device disclosed in the Hey-Shipton patent, conductive elements between non-adjacent capacitor pads in a multi-element lumped element filter are disclosed (see e.g., Figure 13 of that reference). The linear arrangement of the resonators limits the number of elements realizable on a small substrate, while the phase requirements of the connecting line constrain cross- coupling. In addition, the Hey-Shipton patent does not disclose or teach any cancellation approach. Therefore, one feature of the present invention is that it provides a method and apparatus for cancellation techniques to control the location of the transmission zeroes (or decrease the cross-coupling). Another feature is providing the use of a closed loop to enhance the cross coupling. By providing means to increase or decrease cross coupling, control over non-adjacent resonator device cross-coupling is accomplished, and transmission response of the filter is optimized. In a preferred embodiment of the invention, in order to increase cross- coupling of non-adjacent elements, a closed loop coupling element is provided therebetween. In a second preferred embodiment of the invention, in order to decrease cross-coupling of non-adjacent elements, series capacitive elements are provided to cancel (or control) excessive inductive cross-coupling.

Therefore, according to one aspect of the invention, there is provided a filter for an electrical signal, comprising: at least one pair of non-adjacent resonator devices in a micro-strip topology; and a cross-coupling control element between the at least one pair of non-adjacent resonator devices, wherein transmission response of the filter is optimized.

According to another aspect of the invention, there is provided a bandpass filter, comprising: a plurality of L-C filter elements, each of said L-C filter elements comprising an inductor and a capacitor in parallel with the inductor; a plurality of Pi- capacitive elements interposed between the L-C filter elements, wherein a lumped- element filter is formed with at least two of the L-C filter elements being non-adjacent one another; and means for controlling cross-coupling between the non-adjacent L-C filter elements, wherein quasi-elliptical filter transmission response is achieved.

According to yet another aspect of the invention, there is provided a method of controlling cross-coupling in an electric signal filter, comprising the steps of: connecting a plurality of L-C filter elements, each of the L-C filter elements comprising an inductor and a capacitor in parallel with the inductor; interposing a Pi- capacitive element between each of the L-C filter elements, wherein a lumped-element filter is formed with at least two of the L-C filter elements being non-adjacent one another; and inserting between the non-adjacent L-C filter elements a means for controlling cross-coupling between the non-adjacent L-C filter elements, wherein quasi-elliptical filter transmission response is achieved.

These and other advantages and features which characterize the present invention are pointed out with particularity in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, the advantages and objects attained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described preferred embodiments of the present invention.

BRIEF DESCRD?TION OF THE DRAWINGS

In the Drawings, wherein like reference numerals and letters indicate corresponding elements throughout the several views:

Figure 1 is a circuit model of an nth-order lumped-element bandpass filter showing the tubular structure with all the inductors transformed to the same inductance value.

Figure 2 is a circuit model of an nth-order lumped-element bandpass filter with the L-C filter element apparatus shown as L'(ω).

Figure 3 is an example of a layout of a frequency-dependent inductor realization.

Figure 4 illustrates a realization of lumped-element filters without cross-coupling.

Figure 5 a illustrates the simulation response of a twelve (12) pole filter for both a Chebyshev realization and a Quasi-Elliptical realization. Figure 5b is a graph showing quasi-elliptical performance which enhances filter skirt-rejection.

Figure 6 illustrates a schematic representation of a device which includes cross-coupling cancellation by providing a series capacitive device between non-adjacent resonator devices. Figure 7a is an example topology layout of an HTS quasi-elliptical filter on an MgO substrate utilizing cross-coupling cancellation.

Figure 7b is an illustrative graph showing the transmission response of Fig. 7a with capacitive devices for cancelling (controlling) cross-coupling.

Figures 8a and 8b illustrate filter performance on MgO substrates without cross-coupling cancellation and with cross-coupling cancellation, respectively.

Figure 9 is an example layout utilizing a lumped element filter with cross-coupling cancellation, which layout does not include parallel L-C frequency indicators. Figure 10a illustrates the topology of an HTS filter on an LaAlO₃ substrate utilizing cross-coupling enhancement.

Figure 10b is an enlarged area of Fig. 10a illustrating the closed loop between non-adjacent resonator elements. Figure 10c is a graph based on measurements which illustrates the transmission response of the filter of Fig. 10a with a closed loop enhancement of cross-coupling to -30 dB.

DETAD ED DESCRIPTION The principles of this invention apply to the filtering of electrical signals. The preferred apparatus and method of the present invention provides for control of placement of transmission zeroes to provide greater skirt rejection and optimize the transmission response curve of the filter. Means are provided to increase or decrease the cross-coupling between non-adjacent resonator elements in order to control the zeroes.

As noted above, a preferred use of the present invention is in communication systems and more specifically in wireless communications systems. However, such use is only illustrative of the manners in which filters constructed in accordance with the principles of the present invention may be employed. The preferred environment filter in which the present invention may be employed includes the utilization of frequency-dependent L-C components and a positive slope of inductance relative to frequency. That is, the effective inductance increases with increasing frequency. Figures 1 and 2 illustrate a Pi-capacitor network 10 in which such frequency dependent L-C components may be used. Such networks will be appreciated by those of skill in the art and so will not be discussed in great detail herein. Generally referring to Fig. 1, the schematic Pi-capacitor building block 10 is illustrated. The circuit is comprised of capacitive elements 12 with an inductive element 11 located therebetween. A capacitive element 13 is used at the input and output to match appropriate circuit input and output impedances. Fig. 1 illustrates the case in which each of the inductive elements are established at a similar inductance L. In Fig. 2, an inductor device 30 is utilized which is frequency dependent. Accordingly, the inductance becomes L(ω) and the resulting L-C filter element (shown best in Fig. 2) is L'(ω).

Figure 3 illustrates the L-C filter element 20 which is comprised of an interdigital capacitive element 36 and a half-loop inductive element 34. Figure 4 illustrates a strip-line topology in which Pi-capacitor network 25 is formed of L-C filter elements 20 and capacitor devices 21. In the preferred embodiment of the present invention, this topology may then be modified to locate non-adjacent elements nearer to one another as will be described in more detail below.

The filter devices of the invention are preferably constructed of materials capable of yielding a high circuit Q filter, preferably a circuit Q of at least 10,000 and more preferably a circuit Q of at least 40,000. Superconducting materials are suitable for high Q circuits. Superconductors include certain metals and metal alloys, such a niobium as well as certain Perovskite oxides, such as

Methods of deposition of superconductors on substrates and of fabricating devices are well known in the art, and are similar to the methods used in the semiconductor industry.

In the case of high temperature oxide superconductors of the Perovskite-type, deposition may be by any known method, including sputtering, laser ablation, chemical deposition or co-evaporation. The substrate is preferably a single crystal material that is lattice-matched to the superconductor. Intermediate buffer layers between the oxide superconductor and the substrate may be used to improve the quality of the film. Such buffer layers are known in the art, and are described, for example, in U.S. Patent No. 5,132,282 issued to Newman et al., which is hereby incorporated herein by reference. Suitable dielectric substrates for oxide superconductors include sapphire (single crystal Al₂O₃), lanthanum aluminate (LaAlO₃), magnesium oxide (MgO) and yttrium stabilized zirconium (YSZ). Turning now to Fig. 5b, a graphical representation of the quasi- elliptical performance enhancement showing improved filter skirt-rejection is illustrated. Fig. 5b illustrates that the transmission zeroes (or notches) provide sharper skirt rejection with fewer poles needed. Additionally, such performance requires lower loss or less Q.

Utilizing these principles in a micro-strip design, the cross coupling of the non-adjacent resonator devices may beneficially provide zeroes which introduce the quasi-elliptical performance. However, by controlling the placement of zeroes, transmission response is improved to further optimize the filter performance. Fig. 6 illustrates that in the event there is too much cross-coupling, then a capacitive cross-coupling technique may be employed between non-adjacent resonator devices. In Fig. 6, there are schematically illustrated series capacitors 73 located between non-adjacent resonator devices 71 and 72. Those of skill in the art will appreciate that there are five pairs of non-adjacent resonators in Fig. 6. However, only one pair of non-adjacent resonator devices 71 and 72, as well as one series capacitance 73, is specifically marked with numerical designations.

Fig. 7a illustrates more specifically a topology of an HTS quasi- elliptical filter on an MgO substrate in which cross-coupling cancellation may be employed. This MgO substrate may have a dielectric constant of 9.6. Depending on distance between the devices, additional capacitance between the non-adjacent devices to cancel cross-coupling may improve the filter performance.

Resonator elements 71 and 72 normally include cross-coupling due to their proximity to one another. In order to cancel (or control) cross-coupling, series capacitor 73 is inserted into that area located between the elements 71 and 72. Fig. 7b illustrates the output of a PCS D-Block (5MHz) filter with cross coupling. Representative specifications for such a filter include a filter passband frequency of 1865-1870 MHZ, with a 60dB rejection at 1 MHZ from the band edge. As an example circuit, all inductors are identical within the filter with

100 micron linewidth. All interdigital capacitor fingers are 50 micro wide. Equivalent inductance of this capacitively-loaded circuit is about 12 nanoHenries at 1.6 GHz. The whole filter structure may be fabricated on a MgO substrate with a dielectric constant of about 10. The substrate is about 0.5 millimeter thick. Other substrates also used in this type of filters could be lanthanum aluminate and sapphire.

The YBCO is typically deposited on the substrate using reactive co- evaporation, but sputtering and laser albation could also be used. A buffer layer may be used between the substrate and the YBCO layer, especially if sapphire is the substrate. Photolithography is used to pattern the filter structure.

Figs. 8a and 8b illustrate (for comparison) filter performance on MgO substrates without cross-coupling cancellation (Fig. 8a) and with cross-coupling cancellation (Fig. 8b).

As will be apparent to those of skill in the art, the principles of cross coupling may be used in environments in which frequency transformation inductive elements are not employed. For example, Fig. 9 illustrates a representative arrangement of a lumped element filter utilizing cross-coupling cancellation (without frequency dependent inductors).

Turning now to Figures 10a and 10b, an HTS filter laid out on an LaAlO₃ substrate is illustrated. Since this substrate exhibits a high dielective constant, cross-coupling is generally low (based in part on distance between the devices). Therefore, in this type of arrangement, cross-coupling enhancement may be necessary to optimize the filter performance.

Fig. 10b shows an enlarged area with non-adjacent resonator devices 61 and 62 illustrated. It will be appreciated that such devices 61 and 62 may be comprised of a lumped capacitive inductive element such as the element designated 20 in Fig. 3. The resonator elements 61 and 62 include an area therebetween in which a weak cross-coupling occurs due to the layout of the elements on the substrate. In order to enhance the cross-coupling, a loop device 63 is located therebetween (e.g., in the area in which no element previously resided). This closed loop enhances the cross- coupling between the devices 61 and 62. Further, because no device was previously located within that area, the additional element does not require real estate on the layout, nor does it interfere with the other devices. It will be appreciated that multiple choices of the loop could be made, including circular, rectangular, an arc, triangular and combinations thereof.

Fig. 10c illustrates that closed loop device 63 enhances transmission zero level to -30 dB. Such a filter, before using the transmission zero enhancement has a transmission level of -70dB .

As will be apparent to those of skill in the art, the principles of this style of cross coupling may also be used in environments in which frequency transformation elements are not employed (e.g., a lumped element filter).

It will be appreciated, that the principles of this invention apply to control cross-coupling between non-adjacent resonant devices in order to improve filter performance. In the examples provided herein, this is accomplished in two ways, namely either adding inductive or capacitive elements. The examples also illustrate that the control may be based on the substrates utilized

It is to be understood that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only and changes may be made in detail. Other modifications and alterations are well within the knowledge of those skilled in the art and are to be included vvithin the broad scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A filter for an electrical signal, comprising: a. at least one pair of non-adjacent resonator devices in a micro-strip topology; and b. a cross-coupling control element between the at least one pair of non-adjacent resonator devices, wherein transmission response of the filter is optimized.

2. The filter of claim 1 , wherein the cross-coupling control element includes a capacitive element located between the pair of non-adjacent resonator devices.

3. The filter of claim 2, wherein the capacitive element is a series capacitor element connected to each of the pair of non-adjacent resonator devices.

4. The filter of claim 1, wherein the means for controlling cross-coupling includes a loop element located between the pair of non-adjacent resonator devices.

5. The filter of claim 4, wherein the closed loop element is an inductive loop which passes proximate each of the pair of non-adjacent resonator devices.

6. The filter of claim 1, wherein the means for controlling cross-coupling includes either a capacitive element or an inductive element.

7. The filter of claim 1, wherein the micro-strip topology includes a dielectric substrate of either MgO, LaAlO₃, Al₂O₃, or YSZ

8. The filter of claim 6, wherein each of the pair of non-adjacent resonator devices includes a capacitive device and an inductive device so as to form a lumped element device.

9. The filter of claim 8, wherein the capacitive device, the inductive device, and the capacitive or inductive element are formed from a conductive material on the dielectric substrate.

10. The filter of claim 9, wherein the capacitive device is formed from interdigitized fingers of the inductive device.

11. The filter of claim 1, wherein the resonator devices comprise a superconductive material.

12. A bandpass filter, comprising: a. a plurality of L-C filter elements, each of said L-C filter elements comprising an inductor and a capacitor in parallel with the inductor; b. a plurality of Pi-capacitive elements interposed between the L-C filter elements, wherein a lumped-element filter is formed with at least two of the L-C filter elements being non-adjacent one another; and c. means for controlling cross-coupling between the non-adjacent L-C filter elements, wherein quasi-elliptical filter transmission response is achieved.

13. The filter of claim 12, wherein the means for controlling cross-coupling includes a capacitive element located between the non-adjacent L-C filter elements.

14. The filter of claim 13, wherein the capacitive element is a series capacitor element connected to the L-C filter elements.

15. The filter of claim 12, wherein the means for controlling cross-coupling includes a closed loop element located between the L-C filter elements.

16. The filter of claim 15, wherein the closed loop element is an inductive loop which passes proximate each of the L-C filter elements.

17. The filter of claim 12, wherein the means for controlling cross-coupling includes either a capacitive element or an inductive element.

18. A method of controlling cross-coupling in an electric signal filter, comprising the steps of: a. connecting a plurality of L-C filter elements, each of the L-C filter elements comprising an inductor and a capacitor in parallel with the inductor; b. interposing a Pi-capacitive element between each of the L-C filter elements, wherein a lumped-element filter is formed with at least two of the L-C filter elements being non-adjacent one another; and c. inserting between the non-adjacent L-C filter elements a means for controlling cross-coupling between the non-adjacent L-C filter elements, wherein quasi-elliptical filter transmission response is achieved.