Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
2001-06-19
2003-10-14
Tokar, Michael (Department: 2819)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S09900R, C333S185000, C505S700000, C505S866000
Reexamination Certificate
active
06633208
ABSTRACT:
TECHNICAL FIELD
The present invention is directed to electric filters, and more particularly to multi-resonator electric filters.
BACKGROUND OF THE INVENTION
Electrical filters are generally known and often include electrical components, such as inductors, capacitors, and resistors. Filters are often used to select desired electric signal frequencies that will be passed through the filter while blocking or attenuating other undesirable electric signal frequencies. Filters may be classified in some general categories that include low-pass filters, high-pass filters, band-pass filters, and band-stop filters, indicative of the type of frequencies which are selectively passed by the filter. Further, filters can be classified by type, such as Butterworth, Chebyshev, Inverse Chebyshev, and Elliptic, indicative of the type of bandshape response (frequency cutoff characteristics) the filter provides relative to the ideal.
Further, the filters often include capacitors and inductors in series or parallel and may include multiple stages or poles that may be resonators. For example, a capacitor and inductor set may make up a resonator, and a four-pole filter may include four resonators each having a capacitor (C) and inductor (L) set. For example, a circuit schematic for an eight-pole band-pass filter is provided in FIG.
1
. In this case, each L and C pair are resonators and each of the resonators are capacitively coupled to one another in series. The first resonator
101
includes two capacitors, C
1
and C
2
, and an inductor L
1
. There are eight such resonators
101
-
108
making up the eight-pole band-pass filter.
Filters are often used in communication systems. For example, one particular application is for cellular communications and includes the formation of filters useful in the microwave range, such as frequencies above 500 MHz, for base-station transceivers.
Considering the case of conventional microwave filters, there have been basically four types. First, lumped-element filters have used separately fabricated air wound inductors and parallel-plate capacitors, wired together into a filter circuit. These conventional components are relatively small compared to the wave length, and accordingly, make for a fairly compact filter. However, the use of separate elements has proved to be difficult in manufacture, and resulting in large circuit to circuit differences. The second conventional filter structure utilizes mechanical distributed element components. Coupled bars or rods are used to form transmission line networks that are arranged as a filter circuit. Ordinarily, the length of the bars or rods is ¼ or ½ of the wave length at the center frequency of the filter. Accordingly, the bars or rods can become quite sizeable, often being several inches long, resulting in filters over a foot in length. Third, printed distributed element filters have been used. Generally they comprise a single layer of metal traces printed on an insulating substrate, with a ground plane on the back of the substrate. The traces are arranged as transmission line networks to make a filter. Again, the size of these filters can become quite large. The structures also suffer from various responses at multiples of the center frequency. Fourth, cavity filters have been used. They also suffer from various responses at multiples of the center frequency and can be quite large.
Various thin-film lumped-element structures have been proposed. Swanson U.S. Pat. No. 4,881,050, issued Nov. 14, 1989, discloses a thin-film microwave filter utilizing lumped elements. In particular, a capacitor &pgr; network utilizing spiral inductors and capacitors is disclosed. Generally, a multi-layer structure is utilized, a dielectric substrate having a ground plane on one side of the substrate and multiple thin-film metal layers and insulators on the other side. Filters are formed by configuring the metal and insulation layers to form capacitive &pgr;-networks and spiral inductors. Swanson U.S. Pat. No. 5,175,518 entitled “Wide Percentage Band With Microwave Filter Network and Method of Manufacturing Same” discloses a lumped-element thin-film based structure. Specifically, an alumina substrate has a ground plane on one side and multiple layer plate-like structures on the other side. A silicon nitride dielectric layer is deposited over the first plate on the substrate, and a second and third capacitor plates are deposited on the dielectric over the first plate.
Historically, such lumped element circuits were fabricated using normal, that is, non-superconducting materials. These materials have an inherent loss and, as a result, the circuits have various degree of lossiness. For resonant circuits, the loss is particularly critical. The Q of a device (assumed to be “unloaded” throughout this document) is a measure of its ability to store energy and thus inversely related to its power dissipation or lossiness. Resonant circuits fabricated from printed normal metals have Q's at best on the order of a few hundred.
With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from these materials. The microwave properties of the high temperature superconductors have improved substantially since their discovery. Epitaxial superconductive thin films are now routinely formed and commercially available. See, e.g., R. B. Hammond, et al., “Epitaxial Tl
2
Ca
1
,Ba
2
Cu
2
O
8
Thin Films With Low 9.6 GHz Surface Resistance at High Power and Above 77 K”, Appl. Phys. Lett., Vol. 57, pp. 825-27, 1990. Various filter structures and resonators have been formed. Other discrete circuits for filters in the microwave region have been described. See, e.g., S. H. Talisa, et al., “Low-and High-Temperature Superconducting Microwave Filters,” IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 9, September 1991, pp. 1448-1554.
The need for compact, reliable narrow-band filters has never been stronger. Applications in the telecommunication fields are of particular importance. As more users desire to use the microwave band, the use of more narrow-band filters helps to increase the number of users in the spectrum. The area from 700 to 2,000 MHz is of particular interest. In the United States, the 800 to 900 MHz range is used for analog and digital cellular communications. The personal communications services (PCS) are in the 1,800 to 2,000 MHz range.
Many passive microwave devices, for example, resonators, filters, antennas, delay lines, and inductors, have been fabricated in planar form utilizing high temperature superconducting thin films. As described, such structures are often smaller than conventional technologies in terms of physical size. However, these devices are also limited in their size given the constraints of fabricating high quality, epitaxial films. As a result, devices fabricated in HTS films are often of a quasi-lumped element nature, that is, where the nominal size the device is smaller than the wavelength of operation. This often results in folding of devices, which leads to significant coupling between lines.
Despite the clear desirability of improved electrical circuits, including the known desirability of converting circuitry to include superconducting elements, efforts to date have not always been satisfactory. It has proved to be difficult in substituting high temperature superconducting materials to form circuits without degrading the intrinsic Q of the superconducting film. These problems include circuit structure, radiative loss and tuning, and have remained in spite of the clear desirability of an improved circuit. Some of these problems have been overcome by the inventions discloses in U.S. patent application Ser. Nos. 5,888,942 and 6,026,311. However, there is still room for further improvements of relatively high Q and reduced intermodulation distortion (IMD) of electric filters in general. This need is particularly applicable to superconducting electric filters used in, for example, wireless telecommunication systems such as cellular communic
Fenzi Neal
Hammond Robert B.
Salkola Markku I.
O'Melveny & Myers LLP
Superconductor Technologies Inc.
Tan Vibol
Tokar Michael
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