The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore.
1. Field of the Invention
This application relates to electromagnetic filters, and in particular, to compact planar microwave blocking filters.
2. Background
Cryogenic electronics systems may contain low-noise devices such as Josephson tunnel junctions, coulomb blockade devices, and bolometric detectors. Microwave thermal blocking filters may be utilized in the cryogenic electronics systems to realize isolation between cooled elements of the low-noise devices and room temperature readout and bias electronics. The use of thermal blocking filters may prevent degradation of the detector performance from Johnson noise emitted at warmer elements of electronics required by the sensor system.
Providing a low-noise DC bias line to the detectors is possible using a large value shunt capacitor. However, realizing this function with a broadband readout capability is challenging. A dissipative conventional approach includes utilizing a resistor loaded filter. In this approach, microwave power is absorbed along the filter structure to provide broadband attenuation. However, to provide sufficient attenuation at high frequency, the resister loaded filter requires a long line, which in turn creates a large capacitance and limits the operating bandwidth of the signal. Additionally, this approach requires the use of lossy, or loaded, dielectric materials, which are not compatible with thin film fabrication processes. A non-dissipative approach may be effective at blocking thermal noise power, however, the approach must adequately address spurious transmission resonances and sensitivity to impedance matching.
Thus, it may be beneficial to provide microwave thermal blocking filters which overcome these problems.
A compact planar microwave blocking filter includes a dielectric substrate and a plurality of filter unit elements disposed on the substrate. The filter unit elements are interconnected in a symmetrical series cascade with filter unit elements being organized in the series based on physical size. In the filter, a first filter unit element of the plurality of filter unit elements includes a low impedance open-ended line configured to reduce the shunt capacitance of the filter.
A compact planar microwave blocking filter includes a thin dielectric substrate and a plurality of filter unit elements disposed on the substrate. The filter unit elements are interconnected in a symmetrical series cascade with filter unit elements being organized in the series based on increasing physical size. In the filter, each filter unit element of the plurality of filter unit elements includes a low impedance open-ended line configured to reduce the total shunt capacitance of the filter and reduce radiation loss of the filter.
A compact planar microwave blocking filter includes a thin dielectric substrate, a plurality of filter unit elements disposed on the substrate, and a housing disposed on the thin dielectric substrate and covering the plurality of filter unit elements. The filter unit elements are interconnected in a symmetrical series cascade with filter unit elements being organized in the series based on increasing physical size. The housing includes two input pockets on either side of a largest filter unit element of the plurality of filter unit elements. In the filter, each filter unit element of the plurality of filter unit elements includes a low impedance open-ended line configured to reduce the total shunt capacitance of the filter and reduce radiation loss of the filter.
Detailed illustrative embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein.
Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments. Like numbers refer to like elements throughout the description of the figures.
It will he understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Hereinafter, example embodiments will be described with reference to the attached drawings.
Example embodiments of the present invention include a compact filter design technique utilizing a cascaded band-stop filter to produce large broadband microwave blocking capability. Example embodiments further include techniques to reduce a filter's equivalent DC shunt capacitance and to control radiation loss.
Example embodiments of the present invention are thus directed to compact planar broadband microwave blocking filters. The filters may be constructed from multiple sections of band-stop filters with means to control radiation loss. The filters are scalable thereby enabling blocking of a plurality of frequencies depending upon any particular scale used.
According to example embodiments, a microwave blocking filter includes a plurality of compact band-stop filters or unit elements. The unit elements are scaled to suppress various frequency bands such that the microwave blocking filter provides relatively low radiation leakage and achieves a targeted total filter shunt capacitance. The microwave blocking filter includes stepped impedance coupled line filters arranged as the unit elements. Hereinafter, a plurality of unit elements are described with reference to
Each unit element 100, 200, 300, and 400 is scaled to an electrical length ideally generating three transmission zeros frequencies f0, f1, and f2 in the stop-band, thus increasing and/or maximizing bandwidth, as shown in
Increased and/or maximized bandwidth may occur if the electrical lengths θ of unit elements 200, 300, and 400 are approximately equal to a quarter-wavelength at the center frequency f0. Thereafter, f1 and f2 may be analytically determined through Equations (1) and (2) below:
Zc,e and Zc,0 are even-mode and odd-mode characteristic impedances of the coupled line Zc. The coupling coefficient ‘c’ of Zc, is defined in Equation (3) below:
The overall filter length (e.g., including all unit elements) may be reduced through use of a relatively small f1/f0 ratio. Referring to Equation (1), filter length may be reduced by adjusting c, and Z1. Referring to Equation (1) and (2), increasing Z0/Zc ratio does not affect f1 and f2, however the increase in Z0/Zc increases the filter's stop-band attenuation and pass-band return loss level as shown in
To minimize the total capacitance of the microwave blocking filter, Zc and c are set to the highest allowable value. Z0 is also set to a high value to reduce and/or minimize the line capacitance that is used to interconnect unit elements. For example, the highest allowable value may correspond to the maximum or near maximum value attainable through a particular manufacturing process. It is noted that in practice the maximum value may not necessarily be fixed for any manufacturing process. Thus example embodiments of the present invention should not be limited to a single fixed value, as materials and processing methods change.
The pass-band for the microwave blocking filter is defined by f1 of the unit element. The filter size may be reduced significantly by designing the filter at f0 that is much greater than f1. Referring to Equation (1), adjusting Z1 to a minimal value would reduce filter size. However, this requires a wide transmission line in the Z1 section (as shown in
Despite its strong out-of-band spurious responses, the shunt capacitance in the Z1 section is reduced about 2-11% in
Turning to
The filter 800 further includes two ports 809 and 819, configured to interface with a communications line. Once connected the filter 800 blocks microwaves within the stop-band of the filter 800's design. For example, the filter 800 may be scaled to block any number of frequencies as discussed above, and different manufacturing materials and methods may yield different material properties, thereby affecting the stop-band of the filter 800.
Hereinafter a more detailed explanation of an actual example microwave blocking filter is described with reference to
It is noted that although
Microstrip transmission is used for the filter unit elements #1-#7. An electrically thin dielectric substrate is used in the design to avoid surface wave propagation at the highest frequencies of interest. Metal walls are used to enclose the structure to prevent unimpeded radiation propagation through the filter housing. The physical limits due to these factors are computed in terms of maximum operating frequency and summarized in Table 1 given below:
From Table 1, the maximum frequency at which the filter may operate is limited by the microstrip physical dimensions. Therefore, the small metal enclosure is designed to suppress waveguide propagation mode above 45 GHz and filter is constructed using seven different element sizes connected in series as shown in
Each element is placed in series increasing in size towards the center of the structure. This gradual increase in size allows high frequency signals to be blocked by smaller elements with low radiation loss before a signal reaches section #1 and #2 which have high radiation around the center of the filter. Short transmission line length with Z0=53 Ohm is used to connect between elements to minimize the total filter length. In addition, a small input pocket is implemented around the filter terminal as shown in
It is noted again that although particular measurements, dimensions and other implementation specific details have been discussed above with reference to
As discussed above, example embodiments of the present invention are directed to microwave blocking filters. The filters may provide isolation between the cooled elements of low-noise sensors and room temperature readout and bias electronics in cryogenics electronics systems without the drawbacks of the conventional art. Further, given the high blocking capabilities of the filters described, example embodiments also provide low-pass filters for microwave communication systems (e.g., to suppress out-of-band interferences).
Example embodiments include transmission line elements that produce band-stop frequency responses. The filters disclosed use signal reflection, due to transmission line impedance contrast and transmission zeros generated by coupled lines, to block microwave transmission. The level of reflection is dependent on the number of filter unit elements combined in series and the proper enclosure size to cover these filter unit elements. In addition, the enclosed package may prevent additional filter radiation from reaching the input/output terminals. This results in an ultra-high microwave signal blocking capability.
Example embodiments include filters consisting of two sections. The first section is a metal/conductive pattern (e.g., metal, copper, aluminum, niobium, superconductive material, etc) printed on a substrate (e.g., dielectric, semiconductor, degeneratively-doped semiconductor, etc). The second section is an enclosure/housing of conductive material (e.g., metal, metalized polymer, conductive layer, etc). The enclosure is configured to be attached to/cover the first section. Finally, the input/output ports of the filter may be configured according to various different types of external interface.
Example blocking filters include unit elements combined in series. Each unit element has three transmission zero frequencies that provide limited suppression bandwidth. By combining many unit elements of various sizes in series, many transmission zero frequencies are generated that are spread across a wide frequency band and provide significant signal blocking capability. High impedance contrast is used to ensure a compact filter design. In addition, the overall equivalent shunt capacitance may be reduced/minimized using split-end transmission line elements. The reflection capability is enhanced by using electrical wall around the filter. The wall may inhibit low frequency signal from radiation and this frequency is set by the dimension of the enclosure walls.
The filters disclosed may be scaled to different sizes to reflect microwave signals at various frequency bands. For example, according to at least one example embodiment, a microwave blocking filter produces a band-stop/frequency response in the THz range.
While the invention is described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalence may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to the teachings of the invention to adapt to a particular situation without departing from the scope thereof. Therefore, it is intended that the invention not be limited the embodiments disclosed for carrying out this invention, but that the invention includes all embodiments falling with the scope of the appended claims. Moreover, the use of the terms first, second, etc. does not denote any order of importance, but rather the terms first, second, etc. are used to distinguish one element from another.
This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application Ser. No. 61/086,318 entitled “COMPACT PLANAR MICROWAVE BLOCKING FILTER” filed on Aug. 5, 2008, the entire contents of which are hereby incorporated by reference.
Number | Date | Country | |
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61086318 | Aug 2008 | US |