Hairpin microstrip line electrically tunable filters

Details Hairpin microstrip line electrically tunable filters Hairpin microstrip line electrically tunable filters

2002-04-16

2004-04-06

Summons, Barbara (Department: 2817)

Wave transmission lines and networks

Coupling networks

Wave filters including long line elements

C333S235000

Reexamination Certificate

active

06717491

ABSTRACT:

FIELD OF INVENTION
This invention generally relates to electronic filters, and more particularly, to tunable microstrip line resonator filters.
BACKGROUND OF INVENTION
The number of wireless communication systems has increased in the last decade, crowding the available radio frequency spectrum. Filter products used in radios have been required to provide improved performance with smaller size. Efforts have been made to develop new types of resonators, new coupling structures and new filter configurations. One of the techniques for reducing the number of resonators is to add cross couplings between non-adjacent resonators to provide transmission zeros. As a result of these transmission zeros, the filter selectivity is improved. However, in order to achieve these transmission zeros, certain coupling patterns have to be followed. This impedes the size reduction effort.
Electrically tunable microwave filters are highly desirable for communications applications. Magnetically and mechanically tunable filters are large and heavy. Electrically tunable filters use electrically tunable varactors in combination with the filter resonators. When the varactor capacitance is electrically tuned, the resonator resonant frequency is adjusted, which results in a change in the filter frequency response. Electrically tunable filters have the important advantages of small size, light weight, low power consumption, simple control circuits, and fast tuning capability. Traditional electronically tunable filters use semiconductor diode varactors. Compared with the semiconductor diode varactors, tunable dielectric varactors have the merits of lower loss, higher power-handling, higher IP3, and faster tuning speed. For most tunable filter applications, it is desirable to keep the filter configuration simple, otherwise it will be hard to tune the filter from one frequency to the other and still to maintain reasonable filter performance.
Tunable filters for wireless mobile and portable communication applications must be small in size and must have a relatively uncomplicated coupling structure. These design requirements mean that adding cross coupling to achieve transmission zeros, especially of the elliptic function type, is not a good option.
For miniaturization, a hairpin resonator structure has been widely used in microstrip line filters, especially for filters employing high temperature superconductor (HTS) materials. See for example, U.S. Pat. No. 3,745,489 by Cristal et al. for “Microwave And UHF Filters Using Discrete Hairpin Resonators”. It has been noticed that such filters have a transmission zero near the low end of the operating frequency, which results in an improvement in the filter selectivity at the low frequency side, but a degradation in the filter selectivity at the high frequency side, even though, theoretical analysis shows that the transmission zero should be at the high frequency side. See, George L. Matthaei, Neal O. Fenzi, Roger J. Forse, and Stephan M. Rohlfing, “Hairpin-Comb Filters for HTS and Other Narrow-Band Applications,” IEEE Trans. On MTT-45, August 1997, pp 1226-1231.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called “ferroelectric” because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BSTO) or BSTO composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled “Ceramic Ferroelectric Material”; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material-BSTO-MgO”; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material—BSTO-ZrO
2
”; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled “Ceramic Ferroelectric Composite Material-BSTO-Magnesium Based Compound”; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled “Multilayered Ferroelectric Composite Waveguides”; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled “Thin Film Ferroelectric Composites and Method of Making”; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled “Method of Making Thin Film Composites”; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled “Electronically Graded Multilayer Ferroelectric Composites”; U.S. Pat. No. 5,635,433 to Sengupta, entitled “Ceramic Ferroelectric Composite Material-BSTO-ZnO”; and U.S. Pat. No. 6,074,971 by Chiu et al. entitled “Ceramic Ferroelectric Composite Materials with Enhanced Electronic Properties BSTO-Mg Based Compound-Rare Earth Oxide”. These patents are hereby incorporated by reference. The materials shown in these patents, especially BSTO-MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
In addition, the following U.S. patent applications, assigned to the assignee of this application, disclose additional examples of tunable dielectric materials: U.S. application Ser. No. 09/594,837 filed Jun. 15, 2000, entitled “Electronically Tunable Ceramic Materials Including Tunable Dielectric and Metal Silicate Phases” (International Publication No. WO 01/96258 A1); U.S. application Ser. No. 09/768,690 filed Jan. 24, 2001, entitled “Electronically Tunable, Low-Loss Ceramic Materials Including a Tunable Dielectric Phase and Multiple Metal Oxide Phases”; U.S. application Ser. No. 09/882,605 filed Jun. 15, 2001, entitled “Electronically Tunable Dielectric Composite Thick Films And Methods Of Making Same” (International Publication No. WO 01/99224 A1); U.S. application Ser. No. 09/834,327 filed Apr. 13, 2001, entitled “Strain-Relieved Tunable Dielectric Thin Films”; and U.S. Provisional Application Serial No. 60/295,046 filed Jun. 1, 2001 entitled “Tunable Dielectric Compositions Including Low Loss Glass Frits”. These patent applications are incorporated herein by reference.
Examples of filters including tunable dielectric materials are shown in U.S. patent application Ser. No. 09/734,969 (International Publication No. WO 00/35042 A1), the disclosure of which is hereby incorporated by reference.
There is a need for tunable electronic filters that maintain structural simplicity, are relatively small, and provide transmission zeros.
SUMMARY OF THE INVENTION
An electronic filter constructed in accordance with this invention includes a first microstrip line hairpin resonator including first and second arms, a first varactor connected between a first end of the first arm and a first end of the second arm of the first microstrip line hairpin resonator, a first capacitor connected between a second end of the first arm and a second end of the second arm of the first microstrip line hairpin resonator, the first and second arms being coupled to provide a first transmission zero, an input coupled to the first microstrip line hairpin resonator, a second microstrip line hairpin resonator including third and fourth arms, a second varactor connected between a first end of the third arm and a first end of the fourth arm of the second microstrip line hairpin resonator, a second capacitor connected between a second end of the third arm and a second end of the fourth arm of the second microstrip line hairpin resonator, the third and fourth arms being coupled to provide a second transmission zero, and an output coupled to the second microstrip line hairpin resonator. The first and second arms and the third and fourth arms are substantially parallel to each other.
The capacitance of the varactors, and thus the frequency response of the filter, can be controlled by applying a control voltage to each of the first and second varactors. The first and second microstrip line hairpin resonators can be coupled to form a Chebyshev type of filter

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