Abstract:
A microwave structure having an input section for receiving both a common mode signal and a CPW differential mode signal; an output section; and a CPW transmission line, having a center conductor disposed between a pair of coplanar ground plane conductors, connected between the input section and the output section. The conductors of the CPW transmission line are configured to provide the common mode signal a different attenuation in passing to the output section than the CPW transmission line provides to the differential mode signal passing between the input section and the output section.

Description:
TECHNICAL FIELD 
     This disclosure relates generally to microwave transmission lines and more particular to coplanar waveguide (CPW) microwave transmission lines to provide a different attenuation to an unwanted mode of propagation from that provided to a desired mode of propagation. 
     BACKGROUND OF THE INVENTION 
     As is known in the art, a coplanar waveguides (CPW) structure includes: a center conductor disposed over a surface of a substrate; and a pair of ground plane conductors disposed over the surface of the substrate, the center conductor being disposed between the pair of ground plane conductors. Microwave energy fed to an input of the CPW propagates to an output in a differential transmission mode relative to the pair of ground plane conductor with the electromagnetic field being near the surface substrate. CPW has been and continue to be used in wide variety of integrated circuit and circuit board applications. However, being a three conductor system, CPW structures are vulnerable to propagation of unwanted common mode(s). For example, in many applications the integrated circuit having active elements interconnected on a top, or upper, surface of a common substrate and a conductor is disposed on the bottom surface of the substrate for mounting to a heat sink or to a system ground conductor, for example. In this example, a parallel plate region is formed between the conductors on the upper surface, particularly, when larger ground plane conductors are used for the CPW transmission line, and the conductor on the bottom surface. 
     More particularly, a microwave parallel plate region includes a pair of conductors disposed over opposite surfaces of a substrate. When such parallel plate region is used as a portion of a microwave transmission line, unwanted, parasitic, parallel plate modes may be generated (moding), supported between the pair of conductors, and then transmitted through the parallel plate region. In one application, a substrate may be used to realize a Monolithic Microwave Integrated Circuit (MMIC) chip having an amplifier with a conductor on the bottom of the substrate, for providing a system ground or for soldering to a printed circuit board or heat sink, for example, and conductors on the top of the substrate. In such a chip, transmission lines are used to interconnect elements of the amplifier. As a result of the top and bottom conductors, parallel plate moding may be generated. If the generated moding has frequencies within the bandwidth of the amplifier with magnitudes equal to, or greater than, the forward gain of the amplifier, a portion of the output energy produced by the amplifier may be coupled back to the input of the amplifier providing positive feedback thereby generating unwanted oscillations. 
     Common mode generation may also result from interference from other sources, such as, for example, coupling of external signals generated by other sources, unbalanced excitation or unbalanced ground paths. 
     Thus, while CPW transmission uses a differential mode transmission, these other sources can generate common modes that can propagate through the CPW transmission lines as unwanted signals and become a source of parasitic unwanted common mode signals that propagate through the one or more of the center conductors and pair of ground plane conductors and adversely affect the performance and operation of the MMIC. 
     SUMMARY OF THE INVENTION 
     In accordance with the present disclosure, a transmission line structure is provided having: a substrate; and a coplanar waveguide transmission line disposed over a surface of the substrate. The coplanar waveguide transmission line includes: a center conductor disposed over a surface of the substrate; and a pair of ground plane conductors disposed over the surface of the substrate, the center conductor being disposed between the pair of ground plane conductors. The coplanar waveguide structure is configured to provide a different attenuation to an unwanted mode of propagation from that provided to a desired mode to propagate. 
     In one embodiment, the unwanted mode is a common mode of propagation and the desired mode is a differential mode of propagation. 
     In one embodiment, at least one of the center conductor and the pair of ground plane conductors is configured as an inductor reactive element. 
     In one embodiment, the pair of ground plane conductors and the center conductor is each spiral shaped. 
     In one embodiment, the pair of ground plane conductors and the center conductor is each a meander line. 
     In one embodiment, each one of the center conductor and pair of ground plane conductors provides an inductor to suppress parasitic common mode signal propagation in the center conductor or in either one, or both, of the pair of ground plane conductors. 
     In one embodiment, a microwave structure includes: an input section for receiving both a common mode signal and a CPW differential mode signal; an output section; and a CPW transmission line, having a center conductor disposed between a pair of coplanar ground plane conductors, connected between the input section and the output section. The conductors of the CPW transmission line are configured to provide the common mode signal a different attenuation in passing to the output section than the CPW transmission line provides to the differential mode signal passing between the input section and the output section. 
     In one embodiment, the center conductor and the pair of ground plane conductors are each configured as an inductor. 
     In one embodiment, a capacitor is connected in parallel with the inductor. 
     With such an arrangement, the structure presents different impedances to the desired differential mode and the unwanted common mode. The structure provides attenuation of the unwanted common mode while allowing the desired differential mode to propagate. Thus, the structure appears as a spiral inductor to the common mode while appears as a matched transmission line to the differential mode. The structure can be used alone or part of a resonant circuit to block the common mode leaving the differential mode transparent to the resonant circuit. 
     The structure serves as a choke to common mode microwave signals and a CPW transmission line for differential mode microwave signals. 
     In one embodiment, a resistor is connected in parallel with the inductor. 
     The resistor is used for dissipating the energy of the unwanted mode signal. 
     The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a an isometric sketch of a transmission line structure according to the disclosure; 
         FIG. 1A  is an enlarged isometric sketch of a portion of the transmission line structure of  FIG. 1 , such portion being in the area designated by the arrow  1 A- 1 A in  FIG. 1 ; 
         FIG. 1B  is a cross sectional, elevation view of a portion of the transmission line structure of  FIG. 1 , such cross section being taken along line  1 B- 1 B in  FIG. 1 ; 
         FIG. 2  is an isometric sketch of a transmission line structure according to another embodiment, of the disclosure; 
         FIG. 3A  is a schematic diagram of a differential mode equivalent circuit of the transmission line structure of  FIG. 2 ; and 
         FIG. 3B  is a schematic diagram of a common mode equivalent circuit of the transmission line structure of  FIG. 2 . 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 1, 1A and 1B , a transmission line structure  10  ( FIG. 1 ) is shown having: an insulating substrate  12  and a coplanar waveguide transmission line  14  ( FIGS. 1A and 1B ) disposed over an upper surface  16  ( FIGS. 1 and 1A ) of the substrate  12 . The coplanar waveguide transmission line  14  includes: a center conductor  18  disposed over the upper surface  16  of the substrate  12 ; and a pair of ground plane conductors  20 ,  22  ( FIGS. 1 and 1A ) disposed over the upper surface  16  of the substrate  12 , the center conductor, or signal line,  18  being disposed between the pair of ground plane conductors, or strips,  20 ,  22 , as shown. At least one of the center conductor  18  and the pair of ground plane conductors  20 ,  22  is configured as a passive reactive element; here all three conductors  18 ,  20  and  22  are shaped as a spiral inductor, as will be described. It is noted that here a conductor  24  ( FIGS. 1 and 1B ) is disposed on the bottom surface  26  ( FIG. 1 ) of the substrate  12 . Here, the conductor  24  is used for mounting the structure  10  ( FIG. 1 ) to a heat sink, not shown. 
     More particularly, the input to the coplanar waveguide transmission line  14  includes a center conductor input pad  30  ( FIG. 1 ) connected to one end of the center conductor  18  and a center conductor output pad  32  ( FIG. 1 ) connected to the other end of the center conductor  18 . One end of both ground plane conductors  20 ,  22  is connected to a corresponding one of a pair of input ground plane pads  34   a ,  34   b , respectively, as shown, and the other end of each one of the ground plane conductors  20 ,  22  is connected to a corresponding one of a pair of output ground plane pads  36   a ,  36   b , respectively, as shown ( FIGS. 1 and 1A ). It is noted that the ground plane conductors  20 ,  22  are connected by air-bridges  38  that span over the center conductors  18 , as shown ( FIGS. 1, 1A and 1B ). The structure  10  may be formed using conventional photolithographic-etching processes. 
     As noted above, at least one of the center conductor  18  and the pair of ground plane conductors  20 ,  22 ; here all three conductors  18 ,  20  and  22  are shaped as a spiral inductor. The spiral inductors are to provide an impedance to the common mode signals to suppress such common mode signals in attempting to pass between the input pad  30  and the output pad  32 ; however, the three conductors  18 ,  20  and  22  forming a CPW transmission line, allow differential mode signals at the input pad  30  to pass to the output pad  32  substantially unattenuated. Thus, the structure resembles a spiral inductor, however unlike the common spiral inductor where the signal line only wraps around, in structure two ground conductor strips  20 ,  22  also follow the signal line  18  and wraps around as well. 
     It is noted that the ground plane conductors  34   a ,  34   b  are separated from ground plane conductors  36   a ,  36   b  by a portion of the surface of the substrate  12 . The ground plane conductors  34   a ,  34   b  is electrically connected to ground plane conductors  36   a ,  36   b  through a resistor R and a capacitor C ( FIGS. 1 and 1A ) and, the resistor R and the capacitor C being in parallel with the spiral shaped inductors (the spiral shaped conductors  18 ,  20  and  22 ). The capacitor C and the spiral shaped inductors (the spiral shaped conductors  18 ,  20  and  22 ) form L-C tank circuits tuned to the undesired common mode signals; however, because the CPW transmission line formed by three conductors  18 ,  20  and  22  provide a differential line (the signal line  18  has its own ground plane lines  20 ,  22  on either side and on the same surface, differential mode signals pass through the CPW line without being effected by the tank circuits. The resistor R dissipates common mode energy in the tank circuits.  FIG. 3A  is a schematic diagram of a differential mode equivalent circuit of the transmission line structure of  FIG. 2 ; and  FIG. 3B  is a schematic diagram of a common mode equivalent circuit of the transmission line structure of  FIG. 2 . 
     Referring now to  FIG. 2 , a coplanar waveguide transmission line  14 ′ includes: a center conductor  18 ′ disposed over the upper surface  16  of the substrate  12 ; and a pair of ground plane conductors  20 ′,  22 ′ disposed over the upper surface  16  of the substrate  12 , the center conductor, or signal line,  18 ′ being disposed between the pair of ground plane conductors, or strips,  20 ′,  22 ′, as shown. At least one of the center conductor  18 ′ and the pair of ground plane conductors  20 ′,  22 ′ is configured as a passive reactive element; here all three conductors  18 ′,  20 ′ and  22 ′ are shaped as a meander line inductor, as will be described. It is noted that the ground plane conductors  20 ′,  22 ′ are connected by air-bridges  38 ′ that span over the center conductors  18 ′, as shown. The structure  10 ′ may be formed using conventional photolithographic-etching processes. Thus, here there are two, serially connected inductors L 1  and L 2  formed by each one of the three conductors  18 ′,  20 ′ and  22 ′. Capacitors  60 ,  62  are connected in parallel with each corresponding one of the inductors L 1 , L 2  forming a pair of serially connected L-C resonant tank, circuits  50 ,  52 , respectively as shown. These tank circuits  50 ,  52  are tuned to the undesired common mode signals; however, because the CPW transmission line formed by three conductors  18 ′,  20 ′ and  22 ′ provide a differential line (the signal line  18 ′ has its own ground plane lines  20 ;  22 ′ on either side and on the same surface, differential mode signals pass through the CPW line without being effected by the tank circuits  50 ,  52 . Resistors R 1  and R 2  are connected in parallel with L-C tank circuits  50 ,  52 , respectively, to dissipate common mode energy in the tank circuits  50 ,  51 . 
       FIG. 3A  is a schematic diagram of a differential mode equivalent circuit of the transmission line structure of  FIG. 2  having a pair of ground plane conductors  20 ′,  22 ′ and a center conductor  18 ′ configured to connect to loads.  FIG. 3B  is a schematic diagram of a common mode equivalent circuit of the transmission line structure of  FIG. 2  having a pair of ground plane conductors  20 ′,  22 ′ and the center conductor  18 ′. 
     A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.