Abstract:
A technique of realizing a termination circuit using coupled resonators in stripline configuration of a circuit presented. The circuit absorbs RF energy incident on its input over a frequency band of interest, and dissipates it in to the dielectric substrate, thereby acting like an effective termination in the frequency band. The resonant elements may be constructed in edge-coupled or broad-side coupled stripline configuration. The technique may be extended to build microstrip line termination with edge-coupled resonators. The technique may further be extended to realize attenuators over a narrow band.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to the effective absorption of radio frequency (RF) energy, and more particularly relates to a termination circuit, such as a stripline termination circuit that can be used to terminate an isolated port in a microwave circuit, over a desired band of frequencies. 
       BACKGROUND OF THE INVENTION 
       [0002]    Antenna feed networks are commonly employed in RF systems that operate in various microwave or millimeter wave frequency bands such as automotive radar, according to one example. Typical antenna feed networks include power splitters, directional couplers, rat-race hybrids, branch-line couplers, etc., that usually require the isolated ports terminated with a resistance about equal to the characteristic impedance of the transmission lines. Generally, an ideal termination circuit is a one-port device that absorbs RF energy incident on the port and reflects none. In general, it is typically sufficient that the termination is effective over the frequency band of operation. 
         [0003]    At higher microwave frequencies, terminations are often achieved by dispensing liquid resistive material over an area on open transmission lines, such as microstrip, that attenuates RF energy. However, this technique is difficult to implement in shielded transmission line structures, such as stripline, where the conducting strip is sandwiched between dielectric substrates, with ground metallization on the outer sides. 
       SUMMARY OF THE INVENTION 
       [0004]    In accordance with one aspect of the present invention, a termination circuit to terminate an isolated port is provided. The termination circuit comprises a first ground plane, a conductive transmission line having a termination end, and a dielectric disposed between the first ground plane and the conductive transmission line to dielectrically isolate the conductive transmission line from the first ground plane. The termination circuit further includes one or more resonating elements electrically coupled to the conductive transmission line for dissipating energy from the transmission line into the dielectric to provide a termination over a desired frequency band, wherein the one or more resonating elements are not in direct contact with the conductive transmission line. 
         [0005]    According to another aspect of the present invention, a stripline termination circuit to terminate an isolated port is provided. The circuit comprises a conductor strip placed between two metalized coplanar ground planes, separated by two dielectric substrates of predesigned thicknesses. RF energy present on the coupled strip is electrically coupled to one or more resonating elements over the desired frequency band and is dissipated in the dielectric substrates. The resonating elements are not in direct contact with the conductor strip, and are constructed in edge-coupled or broad-side coupled stripline configurations, according to various embodiments. 
         [0006]    These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0008]      FIG. 1  is a cross-sectional view of a transceiver, employing a stripline feed network having one or more stripline to waveguide transitions and termination circuitry according to one embodiment; 
           [0009]      FIG. 2  is a perspective cut away view of a portion of the stripline and its feed ports and termination circuitry shown in  FIG. 1 ; 
           [0010]      FIG. 3  is an enlarged top view of a stripline termination circuit shown in  FIG. 2  employing resonator elements according to a first embodiment; 
           [0011]      FIG. 4  is an enlarged top view of a stripline termination circuit employing a resonator element according to a second embodiment; 
           [0012]      FIG. 5  is a top view of a stripline termination circuit employing a resonator element according to a third embodiment; 
           [0013]      FIG. 6  is a top view of a stripline termination circuit employing resonator elements according to a fourth embodiment; 
           [0014]      FIG. 7  is a top view of a stripline termination circuit employing a resonator element according to a fifth embodiment; 
           [0015]      FIG. 8  is a top view of a stripline termination circuit employing resonator elements according to a sixth embodiment; 
           [0016]      FIG. 9  is a top view of a stripline termination circuit employing resonator elements according to a seventh embodiment; 
           [0017]      FIG. 10  is a cross-sectional view of a transceiver, employing a stripline termination circuit having the resonator element(s) provided below the conductive transmission line, in broad-side coupled stripline configuration, according to another embodiment; and 
           [0018]      FIG. 11  is a graph illustrating simulated results achieved with the stripline termination circuit in the embodiment shown in  FIG. 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring to  FIG. 1 , a cross-sectional view of an RF system  10  is generally illustrated comprising a transceiver or module  12 , mounted on an aluminum block  32 , coupled through a waveguide  34  in the block  32 , followed by a transition  30  to a stripline  40  having stripline feed network  42 . The stripline  40  and waveguide  34  are arranged substantially perpendicular (ninety degrees) to each other in this embodiment. The RF system  10  also includes an antenna or radiator  20 . The stripline to waveguide transition  30  transitions RF energy between TEM mode propagation in the stripline  40  and TE 10  mode propagation in the waveguide  34 . The RF system  10  may transmit and receive RF energy for use in various systems, such as an automotive radar system operating in the microwave or millimeter wave frequency band, according to one embodiment. 
         [0020]    The transceiver device  12  may include a monolithic millimeter wave integrated circuit (MMIC)  14  mounted onto a low temperature co-fired ceramic (LTCC) substrate  16 . MMIC  14  may include one or more amplifiers, mixers, and other electrical circuitry. The substrate  16  is shown mounted on the conductive block  32  which has the waveguide  34  formed therein. The waveguide  34  may be realized in aluminum/copper/FR4 or any other rigid support, according to various embodiments. The waveguide  34  is perpendicular to the stripline  40  and its transmission line  42  in the embodiment shown. 
         [0021]    The stripline  40  includes a conductive strip or transmission line  42  separated from first (upper) and second (lower) ground planes  44  and  46  by a dielectric  48  such that line  42  is sandwiched by the dielectric  48 . The dielectric is an electrically nonconductive substrate that may be made of two dielectric sheets, according to one embodiment. RF energy is coupled to the antenna or radiator strip  20  on the antenna dielectric substrate  18  through an aperture  45  in the bottom ground plane  46 , according to one embodiment. According to other embodiments, a slot radiator or other radiator may be employed. 
         [0022]    The stripline  40  is a shielded transmission line with conductive strip or line  42  sandwiched between two dielectric substrates  48 , with ground metallization  44  and  46  on either sides of the structure. The stripline  40  offers a cost-effective implementation of the feed network. To effect a signal transmission, stripline  40  is connected by its transmission line  42  to a conductive stripline patch  60 . 
         [0023]    The stripline  40  is shown in  FIG. 2  employing one or more stripline termination circuits  100 . The stripline termination circuits  100  may be used to terminate isolated ports of the splitters, such as rat-race hybrid and dummy antenna ports in the feed layer. Input impedance of the stripline termination circuit  100  is approximately equal to 50Ω, according to one embodiment. The stripline termination circuits  100  are essentially formed as printed circuits fabricated on top of the bottom sheet of the dielectric and are shown formed coplanar with other portions of the conductive transmission line  42 , according to one embodiment. The stripline termination circuits  100  are provided with resonator elements  110  that serve as resonators to absorb electrical energy at the termination port over a desired bandwidth. The resonators  110  are designed to achieve the desired level of absorption (or input return loss S11 dB) over the desired bandwidth. In the example, simulated response of the termination showed over 10 dB return loss in 75.5 GHz to 77.3 GHz frequency range. However, it should be appreciated that the stripline termination circuit  100  may be designed to cover different frequency bands. 
         [0024]    As shown in  FIG. 2 , the stripline  40  is formed on top of the bottom dielectric layer such that the conductive transmission line  42  is separated from and sandwiched between the first and second ground planes  44  and  46  by the intermediate dielectric  48 . As such, the conductive transmission line  42  is electrically isolated from the upper and lower ground planes  44  and  46  which electrically shield the transmission line  42 . In the embodiment shown, the conductive transmission line  42  forming the termination circuit  100  is formed coplanar with the remaining conductive transmission line  42  that is coupled to feed transitions one or more waveguides. However, it should be appreciated that the termination circuit  100  may be fabricated above or below other portions of the conductive transmission line, according to other embodiments. 
         [0025]    One stripline termination circuit  100  shown in  FIG. 2  is illustrated further in  FIG. 3  having a plurality of resonator elements  110  separate from and in close proximity to the conductive transmission line  42  such that they are electrically coupled to the conductive transmission line  42 , according to a first embodiment. In this embodiment, the resonator elements  110  each have a portion that is parallel to the conductive transmission line  42  and is physically separated therefrom such that there is no direct contact therebetween. The resonator elements  110  further have a curved portion that forms a closed path or ring for each resonator element  110 . 
         [0026]    In the embodiment shown in  FIG. 3 , there are six resonator elements  110  with three elements on one lateral side of the conductive transmission line  42  and three resonator elements  110  on the opposite lateral side of the conductive transmission line  42 . Each resonating element  110  is physically separated from the conductive transmission line  42 , such that there is no direct contact between line  42  and resonating element  110 . Instead, each resonating element  110  is electrically coupled via electromagnetic radiation such that RF energy is coupled to the resonators  110  that are placed in close proximity to the transmission line  42 . By providing a series of resonator elements  110  (e.g., three resonators on each side), the RF energy is progressively coupled to the resonator elements  110  from the main transmission line  42  at the termination portion. The electrical energy that is coupled to the resonator elements  110  is circulated in the closed path of each resonator  110 , according to the embodiment shown, and the energy is thereby dissipated into the dielectric substrate  48  due to substrate losses. The resonator elements  110  may be suitably designed to make use of a material loss (Tan σ) property to dissipate the energy, effectively acting like a termination over the desired frequency band. 
         [0027]    The stripline termination circuit  100  is further shown having a plurality of plated via holes  52  extending between the top and bottom ground planes  44  and  46  and generally located around the outside of the conductive transmission line  42  and resonator elements  110 . The plated via holes  52  form a fence along the stripline that minimizes interference with adjacent circuitry, and minimizes undesirable parallel plate modes. The plurality of via holes  52  may be formed in a single row, or may be formed in multiple rows in various shapes and sizes. It should be appreciated that the plurality of vias  52  may be provided in various numbers, orientations and shapes and may further be provided with a conductive plating to form the conductive vias. The dielectric substrate  48  may have a thickness and the via hole fence may have a width (edge-to-edge) distance between via hole rows on either side of the conductive transmission line  42  and resonator elements  52  as desired to provide desired functioning of the stripline termination circuit  100 . 
         [0028]    While the first embodiment of the resonator elements shows a closed loop that is generally ring-shaped, it should be appreciated that other shaped resonating elements may be employed to attenuate RF energy at the termination  100 . Referring to  FIG. 4 , a stripline termination circuit  100  employing a resonating element  110 A according to a second embodiment. In this embodiment, the resonating element  110 A generally is configured as a U-shape or horseshoe-shape ring extending on both lateral sides of the conductive transmission line  42  and wrapping around the termination end. The U-shaped ring  110 A couples electrical energy from the conductive termination line  42  into the dielectric substrate. 
         [0029]    Referring to  FIG. 5 , a stripline termination circuit  100  is illustrated employing a pair of resonator elements  110 B, according to a third embodiment. In this embodiment, each of the resonator elements  110 B is fabricated as a U-shape element having parallel linear portions that are parallel to the transmission line  42  and a curved connecting portion at one end. A first U-shaped element  110 B is provided on one lateral side of the conductive transmission line  42  and a second U-shape resonating element  110 B is provided on the opposite second lateral side of the conductive transmission line  42 . 
         [0030]    Referring to  FIG. 6 , a stripline termination circuit  100  is illustrated employing a pair of resonator elements  110 C, each in a generally U-shape, according to a fourth embodiment. The resonator elements  110 C have a shape similar to that shown in  FIG. 5  in the third embodiment, except one of the resonator elements  110 C is oriented one hundred eighty degrees (180°) relative to the other resonating element  110 C. 
         [0031]    Referring to  FIG. 7 , a stripline termination circuit  100  is illustrated employing a resonator element  110 D, according to a fifth embodiment. In this embodiment, the resonator ring includes portions that are similar to the resonator element  110 B shown in the embodiment of  FIG. 5 , the exception that one end of each of the resonator elements  110 D are connected together by a conductive element that wraps around the outer terminating end of the transmission line  42 . 
         [0032]    Referring to  FIG. 8 , a stripline termination circuit  100  is illustrated employing a plurality of resonator elements  110 E, according to a sixth embodiment. In this embodiment, the resonator elements  110 E are fabricated as circular rings provided with three rings each on opposite lateral sides of the conductive transmission line  42 . The middle pair of conductive circular rings  110 E is shown having a slightly smaller size and diameter as compared to the outer pairs of resonator elements  110 E. 
         [0033]    Referring to  FIG. 9 , a stripline termination circuit  100  is illustrated employing a plurality of resonator elements  110 F, according to a seventh embodiment. In this embodiment, each of the resonator elements  110 F are shown formed as substantially rectangular closed loops or rings. While various shapes, sizes, numbers of resonator elements  110 - 110 F are shown and described herein, it should be appreciated that other resonator elements may be employed to couple RF energy such that it is attenuated from the stripline termination circuit  100  into the dielectric substrate  48 . 
         [0034]    The stripline termination circuit  100  has been shown and described herein in connection with one or more resonator elements that are located edge side coupled on the lateral sides of the conductive transmission line  110 . However, it should be appreciated that the resonator elements may be formed at other locations other than the side lateral locations. Referring to  FIG. 10 , the resonator elements  110  may be located below the conductive transmission line  42  such that it is broad side coupled. Additionally, it should be appreciated that the resonator elements  110  may be located above the conductive transmission line  42 , according to another embodiment. Accordingly, the resonator elements are located near the conductive transmission line  42  and its isolated termination port sufficiently close to provide an electromagnetic coupling, however, are not physically in direct contact with the conductive transmission line  42 . 
         [0035]    The graph shown in  FIG. 11  generally illustrates simulated results in decibels (dB) versus frequency in gigahertz (GHz) for RF signal dissipation in the stripline termination circuit  100 . As can be seen, the stripline termination circuit  100  provides an efficient dissipation of RF energy centered about a frequency of about seventy-six and one-half gigahertz (76.5 GHz) and provides good termination and attenuation of electrical signals in the frequency range of seventy-six to seventy-seven gigahertz (76-77 GHz), according to one embodiment. 
         [0036]    The stripline termination circuit  100  comprises a conductor strip placed between two metalized coplanar ground planes, separated by two dielectric substrates of predesigned thicknesses. RF energy present on the coupled strip is electrically coupled to one or more resonating elements over the desired frequency band and is dissipated in the dielectric substrates. The resonating elements are not in direct contact with the conductor strip, and are constructed in edge-coupled or broad-side coupled stripline configurations, according to various embodiments. 
         [0037]    Accordingly, the stripline termination circuit  100  advantageously provides for the dissipation of RF energy from a conductive transmission line to one or more resonators via electrical coupling to dissipate energy into the dielectric substrate  48 . The stripline termination circuit  100  advantageously employs the use of a stripline and eliminates the need for dispensable liquid absorbers, resistors and other hard to accommodate or expensive components and processes that are typically required to terminate an isolated port in the feed layer. 
         [0038]    While a termination circuit is shown and described herein in connection with a stripline determination circuit  100 , it should be appreciated that the termination circuit may be used to provide a termination circuit for other circuits. According to another embodiment, the termination circuit  100  may terminate an isolated port of a microstrip circuit which employs a single ground plane dielectrically isolated from a conductive transmission line, in contrast to a pair of ground planes. In a microstrip application, the termination circuit includes a first ground plane, a conductive transmission line having a termination end, a dielectric disposed between the first ground plane and the conductive transmission line to dielectrically isolate the conductive transmission line from the first ground plane, and one or more resonating elements electrically coupled to the conductive transmission line for dissipating energy from the transmission line into the dielectric to provide a termination over a desired frequency band, wherein the one or more resonating elements are not in direct contact with the conductive transmission line. The microstrip circuit may employ an edge-coupled configuration in which the resonators are coupled to an edge of the conductive transmission line within the same plane. It should further be appreciated that the termination circuit may be employed in other devices including, but not limited to, two-port devices such as an attenuator circuit according to further embodiments. 
         [0039]    It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.