Coaxial diplexer interface with low passive intermodulation (PIM)

Abstract: A common interface (10) for a PIM sensitive diplexing filter (30) is provided in a non-contacting, or isolated, configuration while providing PIM reliability, ESD conduction and thermal conduction, making it ideal for high power space applications. The common interface (10) is a one-piece construction of a diplexed, or multiplexed, coaxial, or squareax, transmission line that is constructed with a direct non-contacting (34, 36), or connectionless, interface. Terminations (26, 28) connect the inner conductor (20) to the outer conductor (12) of the interface (10) making the device one integral piece yet providing the necessary isolation through non-contacting interface with a PIM sensitive device (30), and terminations (26, 28) that provide thermal and ESD conduction necessary for PIM reliability (Fig. 2).

Description: The present invention relates to an interface for a PIM sensitive device, the interface comprising inner and outer conductors. Such an interface is known from EP 913 878 A1. This document discloses an electric circuit for connection to coaxial lines, the electric circuit comprising several ports as inlet and outlets to coaxial lines with λ/4-lines such that the circuit can be wired as lightning eliminator and as combiner or separator of frequency channels. Lightning currents and overvoltage spikes entering one of the ports are eliminated towards Earth via a λ/4 short-circuiting line. A representation of a mechanical realization shows an inner conductor that is connected to the ports and to an open λ/2-line and to the λ/4-short-circuiting line by means of metal-to-metal contacts. The article "Elimination of fine tuning in high power, low-PIM diplexers for combined transmit/receive antennas", IEE colloquium on microwave filters and multiplexers, (digest number 158), London, UK, 19 th November 1990, describes the design and performance of a low-PIM diplexer. The described diplexer is based on the utilization of rectangular waveguides. A rectangular waveguide, in general, comprises a conducting tube of rectangular cross section. Electromagnetic waves propagate inside the tube in interaction with a varying distribution of electrical charge on the outer conducting walls. Such a waveguide, in general, does not comprise an inner conductor. The article "Principles of low PIM hardware design", 13 th National Radio Science Conference, NRSC 96 (CAT. No. 96 th 8-28), Cairo, Egypt, 19-21 March 1996, addresses the problem of passive intermodulation (PIM) interference due to passive nonlinearities in a high-power transmission path in a space RF communication payloads comprising high power multi-channel transmitters and broadband receivers having shared RF feeds. Non-linear metallic junctions are described as causing such PIM interference. According to this article, PIM interference can be minimized by using contactless technique in designing low PIM components. The present invention relates, in general, to a PIM sensitive diplexing or multiplexing filter and more particularly to an interface for the coaxial common port of a diplexing or multiplexing filter. A common coaxial transmission line must be connected to the resonating elements of a filter section in such a manner as to reliably avoid the production of passive intermodulation (PIM). The highest reliability in the avoidance of PIM is accomplished by coupling the transmission line and the filter in a non-contacting, or "isolated" configuration, i.e. a capacitive joint and/or an inductive joint. However, while this electrical isolation avoids PIM, it introduces other problems. For example, there is no bleed path for electrostatic charge build-up. Another potential problem is a build up of heat from poor heat dissipation of the inner conductor because there are no conduction paths that are inherent with "directly" contacting conductors. In an attempt to overcome these problems, a thermal shunt, or other thermally conductive path consisting of a direct electrical and thermally conductive path between the inner conductor and the outer conductor has been added to the interface. This requires that the inner conductor of the PIM sensitive hardware must be intimately attached to the outer conductor. At least one fastening attachment, such as a screw, is normally used. Unfortunately, this assembly is not very reliable in terms of PIM avoidance. It is an objective of the present invention to provide for an interface for a PIM sensitive device comprising inner and outer conductors that allows for a transfer of high power RF energy from a resonating filter to another component while avoiding the risk of passive intermodulation generation and providing for a dissipation of heat and static electric charges. In particular, there is a need for a PIM sensitive diplexing-filter common interface that provides PIM reliability in conjunction with ESD conduction and thermal dissipation, making it ideal for high power space applications. This objective is achieved by an interface as mentioned at the outset, comprising an one-piece integrated configuration for the inner and outer conductors, the one-piece configuration having predefined paths for providing direct electrical and thermal conduction therebetween. The present invention is a one-piece interface connector for a PIM sensitive diplexing filter. In the present invention there is an absence of contacting connections of the inner-conductor. The inner-conductor and outer-conductor are one piece, thereby eliminating any direct metal-to-metal connections to the high current carrying inner-conductor. The only direct connection is to the outer-conductor that can be connected by any means proven to have high reliability in the avoidance of PIM generation, such as a high-pressure connection. The present invention allows the transfer of high power RF energy from the resonating element of a cavity resonating filter to another component, such as an antenna feed element. The transfer is such that it avoids the risk of PIM generation while providing a thermally conductive path and an electrostatic conductive path to dissipate heat and dissipate static electric charges from the transmission line inner conductor. The inner-conductor of the interface is integral with the outer-conductor of the interface, thereby eliminating any need to connect the inner-conductors of the interface to the outer conductor. The outer-conductor of the interface has flange, or other structure, which allows for a connection to the outer-conductor of a transmission line or filter housing. The result is a "one-piece" construction of a diplexed, (or multiplexed), coaxial, (or squareax), transmission line so as to provide a direct path for thermal dissipation and ESD ground and having a non-contacting, integral inner-conductor interface. Other features and advantages of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. In order that the invention may be well understood, there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which: FIGURE 1 is a perspective view of a coaxial diplexer interface of the present invention; and FIGURE 2 is a cross-sectional view of the coaxial diplexer interface of the present invention in communication with a filter housing. Figure 1 is a perspective view of the low passive intermodulation (PIM) coaxial diplexer interface 10 of the present invention. It should be noted that while the present invention is being described herein in conjunction with a diplexed coaxial transmission line, it is possible to incorporate the present invention with a multiplexed coaxial or squareax transmission line as well. One of ordinary skill in the art will have knowledge sufficient, in conjunction with the information in the present disclosure, to apply the present invention to the multiplexed coaxial or squareax transmission line application. An outer conductor 12 of the interface 10 has a common port 14 and a flange member 16 having structure 18 for receiving a high-pressure interface (not shown). Integral to the outer conductor 12 is an inner conductor 20. The inner conductor 20 has several branches 21, 23, 25 and 27, leading to terminations of the inner conductor 20. The first branch 21 leads to a non-contacting coupling with a resonating element of a section of a PIM sensitive device (not shown) by way of a quarter wavelength coupling probe 22. Branch 23 leads to a non-contacting coupling with a resonating element of another section of the PIM sensitive device (not shown) by way of probe 24. Branches 25 and 27 lead to terminations 26 and 28 at the flange 16 making the inner conductor 20 and the outer conductor 12 an integral piece. The terminations 26 and 28 provide the necessary isolation and at the same time provide the electrical and thermal conduction required for PIM reliability. Because the terminations 26 and 28 are integral to the flange member 16, it is possible to manufacture the inner and outer conductors as one integral part, as for example, by a machining process. The terminations 26 and 28 provide a direct thermal dissipative path and ESD ground. The branches 21 and 23 provide a connectionless interface with a PIM sensitive device (not shown). Figure 2 is a cross-sectional view of the interface 10 of the present invention in communication with a PIM sensitive filter 30. Only non-contacting connections are present at the inner conductor branches 21 and 23 by way of probes 22, 24. The only direct connection is provided at the outer conductor 12 of the interface 10 where it is connected at the flange 16 with a high pressure fitting 31. It is known that a high-pressure interface of 10 kPSI provides a reliable PIM avoidance connection, a good thermal conduction path, and a good ESD conduction path. It is possible, however, to substitute the high pressure interface shown with another suitable connection method. The various connections of the interface 10 are connected to the filter 30 in the following manner. A first resonating element 32 of the transmit filter section is coupled to probe 22 by way of a non-contacting choke joint. In a choke joint, the surface of the connection is covered with a dielectric material 34 to isolate the connection, making it non-contacting. The first resonating element 36 of the receive filter section is also coupled to the inner conductor 20 by the second probe 24, also by way of a choke joint isolated by dielectric material 38. The first and second probes 24, 22 maintain a length, or phase, relationship such that the transmit and receive filter sections are multiplexed at a termination 40 of the inner conductor 20. The termination 40 is coupled to an antenna element, (not shown), also by a choke joint. The terminations 26 and 28 of the inner conductor 20 are directly integrated to the outer conductor 12 in a one-piece construction as described above. The terminations 26 and 28 maintain a length, or phase, relationship such that an "open" circuit appears respectively at transmit and receive bands, yet maintains a short circuit for thermal conduction from the inner conductor 20 and ESD conduction to the outer conductor 12. The flange 16 is a plate-shaped member. The two terminations 26 and 28 are located at one side of the flange 16. Further, while termination 28 is located above the inner conductor 20 which penetrates the flange 16, termination 26 is located at an opposite side thereof. The flange 16 has approximately a constant thickness as is indicated at 42, where termination 26 is integrally connected with the flange 16. On the other hand, the flange 16 is, at the side facing the common port 14, provided with an indentation 44. Thus, the flange 16 is thinner in the area where termination 28 is connected integrally with the flange 16. Using a high-pressure interface 31, the outer conductor of the filter housing 30 is directly connected to the outer conductor of the coaxial diplexer interface 12, yet the inner conductor 20 is connectionless, thereby avoiding the generation of any PIM through direct connections.

Claims: An interface (10) for a PIM sensitive device (30), said interface (10) comprising inner and outer conductors, characterized by : an one-piece integrated configuration (10) for said inner (20) and outer (12) conductors, said one-piece configuration having predefined paths for providing direct electrical and thermal conduction therebetween. The interface of claim 1, characterized by a flange member (16) for connection to said PIM sensitive device (30). The interface of claim 2, characterized in that said flange member (16) further comprises fastening members (18) for a high-pressure interface (31). The interface of claim 2, characterized in that said inner conductor (20) branches into a plurality of terminations (26, 28), at least one of which connects to said outer conductor (12) at said flange member (16). The interface of any of claims 1-4, characterized in that said inner conductor (20) and said PIM sensitive device (30) are connected by at least one non-contacting choke joint (34, 38). The interface of claim 4, characterized in that said inner conductor (20) further comprises at least one branch (25, 27) from said inner conductor (20) to said flange member (16) such that a short circuit is provided for thermal and ESD conduction. The interface of any of claims 4-6, characterized in that said inner conductor (20) further comprises at least one branch (23, 25) from said inner conductor (20) away from said flange member (16) to provide an open circuit to said PIM sensitive device (30). The interface of claim 7, characterized in that said at least one branch (23, 25) further comprises a branch (22) for a transmit band and a branch (24) for a receive band. The interface of claim 7 or 8, characterized in that said at least one branch (23, 25) extending away from said inner conductor (20) is coupled to said PIM sensitive device (30) by way of a choke joint (34, 38). The interface of any of claims 7-9, characterized by at least one branch (22) for coupling to a transmit filter section and at least one branch (24) for coupling to a receive filter section. The interface of claim 10, characterized in that said couplings (22, 24) are non-contacting choke joints (34, 38).

Inventors: HENDRICK LOUIS W, KICH ROLF, REYNOLDS ROBERT L, HENDRICK, LOUIS W., KICH, ROLF, REYNOLDS, ROBERT L.
Applicants: BOEING CO, THE BOEING COMPANY
Application year: 2000
Main classification: H01P1
IPC codes: H01R24, H01P1