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TUNABLE MICROWAVE DEVICES WITH
AUTO-ADJUSTING MATCHING CIRCUIT
CROSS REFERENCE TO RELATED
This application claims the benefit of U.S. Provisional Patent Application No. 60/219,500, filed Jul. 20, 2000.
FIELD OF THE INVENTION
The invention relates to the field of tunable microwave devices. More specifically, the invention relates to impedance matching circuits that utilize a bias voltage to alter the permittivity of a tunable dielectric material.
BACKGROUND OF THE INVENTION
Microwave devices typically include a plurality of components that may have different characteristic impedances. In order to propagate the microwave signal through the device with minimal loss, the impedances of the various components are matched to the characteristic impedance of the input and output signal. By transitioning the impedances so that an input transmission line is matched, most of the available power from the input is delivered to the device. Historically, impedance matching techniques have treated the matching of components with constant characteristic impedances to a constant characteristic impedance of the input line, e.g. to 50 Q. Multi-stage matching circuits have been utilized to obtain minimal reflection loss over a specified frequency range of operation of a device. Numerous techniques, such as the use of radial stubs, quarter wave transformers, and multistage matching circuits with specific distributions, such as Binomial or Tchebychef, etc., have been developed in order to achieve maximum power transfer from the input to the device.
However, the characteristic impedance of the tunable components in tunable microwave devices is not a constant value. The characteristic impedance of the tunable component varies over the operating range of the device from a 4Q minimum to a maximum impedance value. In tunable dielectric devices, a bias voltage applied to tunable dielectric material provides the ability to alter the dielectric constant. The change in the dielectric constant provides a variation in the electrical path length of a microwave signal. As the 45 electrical properties of the tunable dielectric material are varied, the characteristic impedance is also affected.
In practice, a single characteristic impedance within the tunable components minimum/maximum impedance range is selected. This single impedance value is matched using 50 one of the state of the art impedance matching techniques. However, as the tunable microwave device is operated, the impedance of the tunable component varies from the matched impedance and a degradation in the impedance match occurs. 55
Prior tunable dielectric microwave transmission lines have utilized tuning stubs and quarter wave matching transformers to transition the impedance between the input and output. The technique is best for matching a fixed impedance mismatch. U.S. Pat. No. 5,479,139 by Koscica et al. dis- 60 closes quarter wavelength transformers using non-tunable dielectric material for the purpose of impedance matching to a ferroelectric phase shifter device. Similar impedance matching configurations using non-tunable dielectric substrate of background interest are shown in U.S. Pat. No. 65 5,561,407, U.S. Pat. No. 5,334,958, and U.S. Pat. No. 5,212,463. The disadvantage of the above technique is that
the impedance match is optimal at one selective tuning point of the device and degrades as the device is tuned through its range. Hence, the reflection loss due to impedance match increases when the device is tuned away from the matched point.
Another impedance matching approach for tunable devices is presented in U.S. Pat. No. 5,307,033 granted to Koscica et al. That patent discloses the use of spacing of a half wavelength between elements or matching networks for the purpose of impedance matching.
Still another approach utilizes quarter wavelength transformers on tunable dielectric material as disclosed in U.S. Pat. No. 5,032,805, granted to Elmer et al. Other impedance matching configurations are shown in U.S. Pat. Nos. 6,029, 075; 5,679,624; 5,496,795; and 5,451,567. Since it is also desirable to reduce the insertion loss of the matching network, a disadvantage of the above approach is that the quarter wavelength transformer on tunable dielectric material increases the insertion loss.
The disclosures of all of the above-mentioned patents are expressly incorporated by reference.
It would be desirable to minimize the impedance mismatch in tunable microwave device applications. There is a need for a technique for improving impedance matching for tunable microwave components that achieves minimal reflection and insertion losses throughout the range of operation of tunable devices.
SUMMARY OF THE INVENTION
This invention provides an impedance matching circuit comprising a conductive line having an input port and an output port, a ground conductor, a tunable dielectric material positioned between a first section of the conductive line and the ground conductor, a non-tunable dielectric material positioned between a second section of the conductor line and the ground conductor, and means for applying a DC voltage between the conductive line and the ground conductor.
The invention further encompasses an impedance matching circuit comprising a first ground conductor, a second ground conductor, a strip conductor having an input port and an output port. The strip conductor is positioned between the first and second ground conductors and to define first and second gaps, the first gap being positioned between the strip conductor and the first ground conductor and the second gap being positioned between the strip conductor and the second ground conductor. Anon-tunable dielectric material supports the first and second ground conductors and the strip conductor in a plane. A connection point is provided for applying a DC voltage between the strip conductor and the first and second ground conductors. A plurality of tunable dielectric layer sections are positioned between the strip conductor and the first and second ground conductors so as to bridge the gaps between the first and second ground conductors and the strip conductor at a plurality of locations, leaving non-bridged sections in between, defining a plurality of alternating bridged and non-bridged co-planar waveguide sections.
The matching circuits form tunable impedance transformers that are able to match a constant microwave source impedance connected at the input port to a varying load impedance connected at the output port, thereby reducing signal reflections between the microwave source and a variable load impedance.
This invention provides an impedance matching circuit capable of matching a range of impedance values to a
tunable microwave device in order to reduce reflections from impedance mismatch during tuning of the microwave device.
BRIEF DESCRIPTION OF THE DRAWINGS 5
FIG. 1 is a plan view of a first embodiment of the auto adjusting matching network of this invention in the form of the microstrip;
FIG. 2 is a cross-sectional view of the embodiment of 10 FIG. 1 taken along line 2—2, showing a microstrip line geometry;
FIG. 3 is a cross-sectional view of the embodiment of FIG. 1 taken along line 3—3, showing a microstrip line geometry; 15
FIG. 4 is a plan view of a second embodiment of the auto adjusting matching network of this invention in the form of the stripline;
FIG. 5 is a cross-sectional view of the embodiment of FIG. 4 taken along line 5—5, showing a stripline geometry; 20
FIG. 6 is a cross-sectional view of the embodiment of FIG. 4 taken along line 6—6, showing a stripline geometry;
FIG. 7 is a cross-sectional view of a third embodiment for the auto adjusting matching network of this invention based 25 on a coaxial geometry;
FIG. 8 is a cross-sectional view of the embodiment of FIG. 7 taken along line 8—8, showing the coaxial transmission line geometry;
FIG. 9 is a plan view of another embodiment for the auto 30 adjusting matching network of this invention including multiple partial stages on tunable material;
FIG. 10 is a plan view of another embodiment for the auto adjusting matching network of this invention based on a slotline or finline geometry, and including multiple partial 35 stages;
FIG. 11 is a cross-sectional view of the embodiment of FIG. 10 taken along line 11—11, showing a slotline geometry;
FIG. 12 is a plan view of another embodiment for the auto adjusting matching network of this invention based on a co-planar waveguide geometry, and including multiple partial stages;
FIG. 13 is a cross-sectional view of the embodiment of 45 FIG. 12 taken along line 13—13, showing a coplanar waveguide geometry; and
FIG. 14 is a block diagram showing a matching network of this invention coupled to a tunable dielectric device.
DESCRIPTION OF THE PREFERRED
The preferred embodiments described herein are each designed for use within a certain arbitrary frequency range. 5J For this reason all references to a "wavelength" will refer to the center frequency of the design.
Referring to the drawings, FIG. 1 is a plan view of a first embodiment of an auto adjusting matching network of this invention in the form of the microstrip circuit 10. FIG. 2 is 60 a cross-sectional view of FIG. 1 taken along line 2—2, showing the microstrip line geometry. FIG. 3 is a crosssectional view of FIG. 1 taken along line 3—3.
The device has two ports 12 and 14 for input and output of a guided electromagnetic wave. It includes a multi-stage 65 microstrip line 16, having sections 18 and 20 of various widths and lengths, deposited on a non-tunable dielectric
substrate 22, which in turn is supported by ground plane 24; and a microstrip line section 26 deposited on a voltage tunable dielectric substrate 28 which in turn is supported by ground plane 30. Abiasing electrode 32 in the form of a high impedance microstrip line is connected to microstrip section 20.
The biasing electrode 32 serves as a means for connecting an external variable DC bias voltage supply 34 to the auto-adjusting impedance matching circuit. The connection of the biasing electrode 32 to the circuit is not limited to microstrip section 20, but may be made to any other part of the circuit that is electrically connected to microstrip line section 26. Ground planes 24 and 30 are electrically connected to each other. While ground planes 24 and 30 are shown as separate elements, it should be understood that they may alternatively be constructed as a single ground plane.
The microstrip line section 26, which comprises a conducting strip, is directly supported by a dielectric layer 28, which is the voltage tunable layer. A ground plane 30 supports the dielectric layer 28. The microstrip line 26 is less than a quarter wavelength long and forms an approximately quarter wavelength long transformer when joined to section 20 of the matching network on the non-tunable dielectric substrate 22.
The non-tunable stages 36 and 38 of the matching network 10 form a multistage matching circuit 40 directly supported by the non-tunable dielectric layer 22. The multistage matching circuit 40 can be any number of stages of varying widths and lengths, not limited to quarter wavelength sections. If the non-tunable and tunable substrates 22 and 28 respectively are not of the same height, the last stage 38 of the matching network, which abuts the tunable dielectric 28, would be electrically connected to microstrip line section 26 via a step 42.
FIG. 4 is a plan view of a second embodiment of the auto adjusting matching network 44 of the invention in the form of a stripline. FIG. 5 is a cross-sectional view of FIG. 4 taken along line 5—5, showing a stripline geometry. FIG. 6 is a cross-sectional view of FIG. 4 taken along line 6—6.
The device 44 has two ports 46 and 48 for input and output of the guided electromagnetic wave. It comprises a stripline 50 having sections 52 and 54 of various widths and lengths embedded in a non-tunable dielectric substrate 56 supported by top and bottom ground planes 58 and 60, an additional section 62 of stripline 50 embedded in a tunable dielectric substrate 64 supported by top and bottom ground planes 66 and 68, and a biasing electrode 70 in the form of a high impedance stripline is connected to stripline section 54.
The connection of biasing electrode 70 to the circuit is not limited to stripline section 54, but may be made to any other part of the circuit that is electrically connected to stripline section 62. The biasing electrode 70 serves as means for connecting the auto adjusting impedance matching circuit to an external adjustable DC voltage bias source 72. Ground planes 66 and 68 may or may not be the same ground planes as for the tunable microwave device to which the matching circuit is connected. Ground planes 66 and 68 are electrically connected to the ground planes of the tunable microwave device. Ground planes 66 and 68 are electrically connected to ground planes 58 and 60.
The stripline section 62, which is a conducting strip, is directly embedded in the tunable dielectric layer 64, which is the voltage tunable layer. Ground planes 66 on the top and 68 on the bottom support the dielectric layer 64. The