VERTICAL INTERCONNECT BETWEEN COAXIAL AND RECTANGULAR COAXIAL TRANSMISSION LINE VIA COMPRESSIBLE CENTER CONDUCTORS

Abstract: An RF interconnect between a rectangular coaxial transmission line including a coaxial center conductor (122) and a dielectric structure (124) with a rectilinear cross-sectional configuration fitted around the coaxial center conductor and an RF circuit (130) separated from the airline circuit by a separation distance. The RF interconnect includes a compressible conductor structure (152) having an uncompressed length exceeding the separation distance, and a dielectric sleeve structure (154) surrounding at least a portion of the uncompressed length of the compressible conductor structure. The RF interconnect structure is disposed between the rectangular coaxial transmission line and the RF circuit such that the compressible conductor is placed under compression between th substrate and the RF circuit. Examples of the RF circuit include a vertical coaxial transmission line or a grounded coplanar waveguide circuit disposed in parallel with the center conductor of the rectangular coaxial transmission line.

Description: VERTICAL INTERCONNECT BETWEEN COAXIAL AND RECTANGULAR COAXIAL TRANSMISSION LINE VIA COMPRESSIBLE CENTER CONDUCTORS TECHNICAL FIELD OF THE INVENTION This invention relates to microwave devices, and more particularly to structures for interconnecting between coaxial or coplanar waveguide transmission line and rectangular coaxial transmission line. BACKGROUND OF THE INVENTION A typical technique for providing a vertical RF interconnect with a coaxial line uses hard pins. Hard pin interconnects do not allow for much variation in machine tolerance. Because hard pins rely on solder or epoxies to maintain electrical continuity, visual installation is required, resulting in more variability and less S- Parameter uniformity. Some interconnect structures employ pin/socket structures. These pin/ socket interconnects usually employ sockets which are much larger than the pin they are capturing. This size mismatch may induce reflected RF power in some packaging arrangements. For interconnects to rectangular coaxial transmission line, stripline or similar transmission lines, a pin would have to be soldered onto the surface of the circuit, causing more assembly and repair time. SUMMARY OF THE INVENTION The transition from coaxial line or coplanar waveguide transmission line to rectangular coaxial transmission line is made with a compressible center conductor. The compressible center conductor is captured within a dielectric, such as REXO- LITE (TM), TEFLON (TM), TPX (TM), and allows for a robust, solderless, vertical interconnect. The center conductor in an exemplary embodiment is a thin, gold plated, metal wire (usually tungsten or beryllium copper), which is wound up into a knitted, wire mesh cylinder. The compressible center conductor is captured within the dielectric in such a way as to form a coaxial transmission line. The compressibility of the center conductor allows for blindmate, vertical interconnects onto rectangular coaxial transmission lines while maintaining a good, wideband RF connection. The compressible center conductor also maintains a good physical contact without the use of solder or conductive epoxies. The RF interconnect can be applied to either side of the circuit board. BRIEF DESCRIPTION OF THE DRAWING These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which: FIG. 1 is an unsealed side cross-sectional diagram of an embodiment of the invention for an interconnect between an rectangular coaxial transmission line and a grounded coplanar waveguide (GCPW) circuit. FIG. 2 is an isometric view of the rectangular transmission line and RF interconnect of FIG. 1, without the outer conductive housing. FIG. 3 is an isometric view of the rectangular transmission line of FIG. 1, without the outer conductive housing. FIG. 4A is an unsealed top view of the GCPW substrate of FIG. 3. FIG. 4B is an unsealed bottom view of the GCPW substrate; FIG. 4C is an unsealed cross- sectional view taken along line 4C-4C of FIG. 4A. FIG. 5 is a side cross-sectional view illustrating an alternate embodiment, providing an interconnect between a rectangular coaxial line and a transverse coaxial line. FIGS. 6A-6C illustrate three embodiments of the compressible conductor structure of an RF interconnect in accordance with the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with aspects of the invention, a vertical interconnect between a rectangular coaxial or "squarax" transmission line and a coaxial or a coplanar waveguide transmission line is made with a compressible center conductor. An exemplary embodiment of the vertical interconnect in an RF circuit 100 for interconnecting to a grounded coplanar waveguide (GCPW) transmission line is illustrated in FIGS. 1-3. A rectangular or squarax transmission line is essentially a coaxial transmission line, but with a rectangular or square shaped dielectric instead of a round cross-sectional configuration. Thus, the rectangular transmission line 120 includes a center conductor 122 having a circular cross-section, and an outer dielectric sleeve 124 fabricated with a square or rectilinear cross-section. In this exemplary embodiment, the center conductor has a diameter of .040 inch, and the dielectric sleeve has a width dimension of .120 inch and a height dimension of .060 inch. The circuit 100 includes a conductive housing structure comprising an upper metal plate 102 and a lower metal plate 104. The upper and lower plates sandwich the rectangular coaxial line 120, contacting the dielectric sleeve 124. A coaxial connector 106 is attached to the coaxial conductor 124 and to the housing structure. The GCPW circuit 130 includes a dielectric substrate 132 having conductive patterns formed on both the top surface 132A and the bottom surface 132B. In this exemplary embodiment, the substrate is fabricated of aluminum nitride. The top conductor pattern is shown in FIG. 4A, and includes a conductor center trace 134 and top conductor groundplane 136, the center trace being separated by an open or clearout region 138 free of the conductive layer. The bottom conductor pattern is illustrated in FIG. 4B, and includes the bottom conductor groundplane 140 and circular pad 142, separated by clearout region 144. The top and bottom conductor groundplanes 136 and 140 are electrically connected together by plated through holes or vias 146. The vertical RF interconnect 150 between the rectangular coaxial line 120 and the GCPW line 130 comprises a compressible center conductor 152. In this exemplary embodiment, the compressible center conductor is fabricated from a thin, gold plated, metal wire (usually tungsten or beryllium copper), which is wound up into a knitted, wire mesh cylinder. The wire mesh cylinder is captured within a dielectric body 154 in such a way as to form a 50 ohm, coaxial transmission line. In this exemplary embodiment, the compressible center conductor 152 has an outer diameter of .040 inch. The dielectric 154 is made of TEFLON (TM), a moldable material with a dielectric constant of 2.1. The dielectric 152 has an inner diameter of .040 inch and an outer diameter of .120 inch. The compressible center conductor is inserted into the dielectric sleeve 154, forming a 50 ohm, coaxial transmission line. The dielectric sleeve 154 is captured within the housing metal structure, which also supplies the outer ground for the rectangular coaxial transmission line and the vertical interconnect coaxial transmission line. When the dielectric sleeve 154 is inserted into the housing structure, it makes physical contact with the surface of the rectangular transmission line. The lower end of the compressible center conductor 152 makes electrical contact with the center conductor 122 of the rectangular coaxial line. In order to maximize the amount of contact between the compressible center conductor 152 and the pin 122, the center conductor pin 122 and dielectric sleeve 122 have been milled flat at the interface location with the vertical interconnect as shown in FIG. 3. The upper end of the compressible center conductor 152 makes contact with a conductive sphere 148 attached to pad 142 of the GCPW line 130, where the RF signal is transitioned from a coaxial structure to a co-planar waveguide circuit. The sphere 148 ensures good compression of the conductor 152. The co-planar waveguide circuit can be terminated in a connector or connected to other circuitry. FIG. 5 illustrates an alternate embodiment of the invention, wherein an RF circuit 180 provides an interconnect 150 between a rectangular coaxial line and a transverse coaxial line. The rectangular transmission line 120 as in the embodiment of FIGS. 1-4 includes a center conductor 122 having a circular cross-section, and an outer dielectric sleeve 124 fabricated with a square or rectilinear cross-section. The circuit 180 includes a conductive housing structure comprising upper metal plates 184, 186 and a lower metal plate 182. The upper and lower plates sandwich the rectangular coaxial line 120, contacting the dielectric sleeve 124. A coaxial connector 106 is attached to the coaxial conductor 124 and to the housing structure. A vertical coaxial connector 190 with center conductor 192 is positioned for entry of the vertical coaxial center conductor 192 through the opening formed in the upper plates 184, 186. The vertical RF interconnect 150 between the rectangular coaxial line 120 and the coaxial connector 190 comprises the compressible center conductor 152. In this exemplary embodiment, the compressible center conductor is fabricated from a thin, gold plated, metal wire (usually tungsten or beryllium copper), which is wound up into a knitted, wire mesh cylinder. The wire mesh cylinder is captured within the dielectric body 154 in such a way as to form a 50 ohm, coaxial transmission line. The pin 192 of the vertical coaxial connector has the same diameter as the diameter of the compressible center conductor 152 to maintain 50 ohm impedance when engaging the vertical interconnect. When the pin 192 is inserted into the dielectric sleeve 154 of the vertical interconnect, the pin 192 makes electrical contact with the top of the compressible center conductor 152 while the bottom end of the conductor 152 is pushed down to make electrical connection with the center conductor 122 of the rectangular coaxial line. The conductor 152 is compressed to take up physical variation in center conductor lengths. Three alternate types of compressible center conductors suitable for use in interconnect circuits embodying the invention are shown in FIGS. 6A-6C. FIG. 6A shows a compressible wire bundle 200 in a dielectric sleeve 202, and is the embodiment of compressible center conductor illustrated in the embodiments of FIGS. 1-5. FIG. 6B shows an electroformed bellow structure 210 in a dielectric sleeve 212; the bellows is compressible. FIG. 6C shows a "pogo pin" spring loaded structure 220 in a dielectric sleeve 222; the tip 220A is spring-biased to the extended position shown, but will retract under compressive force. It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.

Claims: CLAIMS What is claimed is: 1. An RF interconnect between a rectangular coaxial transmission line (120) including a coaxial center conductor (122) and a dielectric structure (124) with a rectilinear cross-sectional configuration fitted around the coaxial center conductor and an RF circuit vertically separated from the rectangular coaxial transmission line by a separation distance, the RF interconnect comprising: a compressible conductor structure (152) having an uncompressed length exceeding the separation distance; a dielectric sleeve structure (154) surrounding at least a portion of the uncompressed length of the compressible conductor structure; and wherein said RF interconnect structure is disposed between said rectangular coaxial transmission line and said RF circuit such that said compressible conductor is placed under compression between said coaxial center conductor and said RF circuit. 2. An RF interconnect according to Claim 1 wherein said RF circuit is a coaxial transmission line (190) including a circuit center conductor (192), said circuit center conductor extending transverse to said coaxial center conductor of the rectangular coaxial transmission line, said compressible conductor under compression between said circuit center conductor and said coaxial center conductor. 3. An RF interconnect according to Claim 1 wherein said RF circuit is a grounded coplanar waveguide (GCPW) circuit (130) including a GCPW dielectric substrate (130) with a first surface having a conductor center trace and a ground conductor pattern formed thereon, said compressible conductor under compression between said GCPW substrate and said airline substrate. 4. An RF interconnect according to Claim 3 wherein said GCPW substrate is parallel to the coaxial center conductor. 5. An RF interconnect according to any preceding claim wherein a first end of the compressible conductor structure (152) is in contact with said RF circuit at a first contact area, a second end of the compressible conductor structure is in contact with the rectangular coaxial transmission line at a second contact area, and wherein the first and second contact areas are free of any permanent solder or epoxy material. 6. An RF interconnect according to any preceding claim, wherein the dielectric sleeve structure of the RF interconnect has a circular cross-sectional configuration, and wherein the dielectric structure of the rectangular coaxial line is relieved to form a region into which the dielectric sleeve structure is fitted. 7. An RF interconnect according to Claim 6 wherein the coaxial center conductor (122) has a flat area formed therein at a contact point with the compressible conductor. 8. An RF interconnect according to any preceding claim wherein the compressible conductor is transverse to the rectangular coaxial center conductor. 9. An RF interconnect according to any preceding claim wherein the compressible conductor structure includes a densely packed bundle of thin conductive wire. 10. An RF interconnect according to any of Claims 1-8 wherein the compressible conductor structure includes a compressible bellows structure. 11. An RF interconnect according to any of Claims 1-8 wherein the compressible conductor structure includes a spring-loaded retractable probe structure. 12. A method for forming an RF interconnect between a rectangular coaxial transmission line including a coaxial center conductor and a dielectric structure with a rectilinear cross-sectional configuration fitted around the coaxial center conductor and an RF circuit vertically separated from the rectangular coaxial transmission line by a separation distance, the method comprising: providing a compressible conductor structure (152) having an uncompressed length exceeding the separation distance, the compressible conductor structure in a dielectric sleeve structure surrounding at least a portion of the uncompressed length of the compressible conductor structure; placing the RF interconnect structure between said rectangular coaxial transmission line and said RF circuit such that the compressible conductor is placed under compression between the rectangular coaxial transmission line and the RF circuit. 13. A method according to Claim 12 wherein a first end of the compressible conductor structure is in contact with said RF circuit at a first contact area after said placing, a second end of the compressible conductor structure is in contact with the rectangular coaxial transmission line at a second contact area after said placing, and wherein the first and second contact areas are free of any permanent solder or epoxy material.

Inventors: COX GERALD A, HUBBARD DOUGLAS A, KEESEY TIMOTHY D, KERNER STEPHEN R, QUAN CLIFTON, ROBERTS DAVID E, SCHUTZENBERGER CHRIS E, TUGWELL RAYMOND C, COX, GERALD, A., HUBBARD, DOUGLAS, A., KEESEY, TIMOTHY, D., KERNER, STEPHEN, R., QUAN, CLIFTON, ROBERTS, DAVID, E., SCHUTZENBERGER, CHRIS, E., TUGWELL, RAYMOND, C.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 2001
Main classification: H01P5
IPC codes: H01P1, H01P5