Abstract:
A hybrid 180° microwave balun device is provided to convert an unbalanced RF signal at the common port into two radio frequency signals with equal amplitude and 180° phase difference at two differential ports. The hybrid device includes a coplanar waveguide connecting to the common port. A power divider separates the coplanar waveguide into two symmetrical slotline waveguides to carry balanced signals. Two broadband multioctave slotline to microstrip transitions constructed in a way that the microstrip lines carry 180° phase separated signals to the differential output ports.

Description:
CROSS-REFERENCE TO PROVISIONAL APPLICATION  
       [0001]     This Patent Application claims the benefit of Provisional Application No. 60/623,287 filed Oct. 29, 2004. 
     
    
     BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to the field of microwave and RF electronics, and more particularly to broadband hybrid structures.  
         [0004]     2. Related Art  
         [0005]     A 180° hybrid is a component that provides a phase-shifted output of unbalanced RF signals. The 180° hybrid is an essential component for a multi-port vector network analyzer (VNA) that offers true differential measurement capability. Differential measurements are becoming more important due to greater use of differential components and circuits in the modern communications industry.  
         [0006]     In order to provide a phase-shifted output, an unbalanced signal must be converted into two balanced signals that are later converted into two unbalanced output signals with equal amplitude and 180° phase shift. To create two balanced signals, a balun is typically employed. A balun is an electronic circuit component that converts an unbalanced Radio Frequency (RF) signal at an input port into a balanced RF signal at an output port. In essence a balun is an unbalanced to balanced transformer.  
         [0007]     A balun-transformer can be implemented using a number of prior art 180° hybrid structures. A low frequency implementation can be achieved with the use of lumped components with constant reactance. The frequency range of application for this type of balun was recently extended into low-gigahertz frequencies.  
         [0008]     Coaxial-line balun transformers have good power handling, but limited bandwidth. These devices are relatively large. As the frequency of application increases, it becomes more difficult to connect the quarter-wave sections in the coaxial-line balun circuit without introducing significant discontinuities that degrade the balun performance. The bandwidth of the best coaxial-line baluns was extended into much lower frequencies by introducing ferrite cores mounted along the outer conductor of a coaxial line. The ferrite cores present a high impedance for the common mode currents along the outer conductors of the balun sections, which corresponds to a good input to output isolation at much lower frequencies.  
         [0009]     Due to the growing demand for ultra-broadband balanced circuits and systems in the optical communications and test and measurement industries, there is a growing demand for very broadband 180° hybrid structures that would cover frequencies from well below 1 GHz up to 40 GHz. It would be desirable to provide a single 180° hybrid structure that could operate over this entire bandwidth.  
       SUMMARY  
       [0010]     According to embodiments of the present invention a hybrid electronic component (planar hybrid transformer, or differential balun) is provided that converts an unbalanced radio frequency signal at the common port into two radio frequency signals with equal amplitude and 180° phase difference at two differential ports.  
         [0011]     The hybrid includes a coplanar waveguide at the common port. A power divider connects the coplanar waveguide to two symmetrical slot lines. In one embodiment, the slotlines are tapered from a wider slot (larger impedance) to a more narrow slot (lower impedance) toward a slotline to microstrip transition to provide a desired impedance matching. The hybrid provides transitions from the two broadband slotlines to microstrip lines in such a matter that the output RF signals have a 180° phase shift with respect to each other. The microstrip lines are formed on the substrate opposite the metalization regions wherein the slotlines are provided.  
         [0012]     Each slotline to microstrip transition includes a loop of the slotline around a ground via connecting the microstrip to the metalization region where the slotline is formed. The slotline to microstrip transitions are done in such a manner that one of the microstrip lines is terminated to the metallization region connected to a central conductor of the input coplanar waveguide and the other microstrip line is terminated to the metallization region connected the coplanar ground plane strips. The grounding in different regions causes the 180° phase difference at two differential ports. The slotlines are terminated after the microstrip to slotline transition in a geometric opening structure formed in the metalization on the substrate to provide an open circuit. In one embodiment, the geometric structure is covered with a magnetic material.  
         [0013]     In one alternative, one of the differential slotlines provided from the power divider is terminated in a large geometric structure without transition to a microstrip line. The large geometric structure is filled in with a thin film resistive material to form a termination. The second slotline is then provided directly as an output to a balanced port of the hybrid device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Further details of the present invention are explained with the help of the attached drawings in which:  
         [0015]      FIG. 1  is a block diagram illustrating a 180° hybrid component according to the present invention;  
         [0016]      FIG. 2  shows a top view of an embodiment of the 180° hybrid component in accordance with the present invention;  
         [0017]      FIG. 3  illustrates the instantaneous electric (E) field polarities of signals carried at terminals of the power divider shown in  FIG. 2 ;  
         [0018]      FIG. 4  illustrates details of the microstrip to slotline transition of  FIG. 2 ;  
         [0019]      FIG. 5  illustrates details of the open circuit termination region connected to the slotline in  FIG. 2 ;  
         [0020]      FIG. 6  illustrates an alternate substrate for the 180° hybrid component wherein magnetic material is applied over the slotline terminations;  
         [0021]      FIG. 7  illustrates the substrate of  FIG. 2  as provided in a support fixture with connectors;  
         [0022]      FIG. 8  shows simulation results for S 12  and S 13  measurements through a 180° hybrid device having components as shown in  FIG. 2 ;  
         [0023]      FIG. 9  shows simulation results for S 11 , S 22  and S 33  measurements through a 180° hybrid device having components as shown in  FIG. 2 ;  
         [0024]      FIG. 10  shows simulation results for phase difference measurements between S 12  and S 13  measurements made through a 180° hybrid device having components as shown in  FIG. 2 ; and  
         [0025]      FIG. 11  shows an alternative embodiment for a 180° hybrid device according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]      FIG. 1  is a block diagram illustrating an embodiment of a 180° hybrid component according to the present invention. The hybrid includes a common port  1  and differential ports  2  and  3 . The common port  1  connects to coplanar waveguide  10 . The coplanar waveguide  10  leads to a power divider  12 . In one embodiment, the impedance of the coplanar waveguide  10  is set at 50 Ohms. The power divider  12  in one embodiment transitions the 50 Ohms from the coplanar waveguide  10  to an impedance of 100 Ohms. The outputs of the power divider  12  are connected to two slotlines  14  and  16 . In one embodiment, the slotlines  14  and  16  are tapered to transition the 100 Ohm impedance from the power divider  12  down to 50 Ohms. The slotlines  14  and  16  pass the signal to slotline to microstrip transitions  18  and  20 . The slotline to microstrip transition passes the signal to two microstrip lines  22  and  24 .  
         [0027]     The two microstrip lines  22  and  24  feed into the differential ports  2  and  4 . The two microstrip lines  22  and  24  are each terminated (by a connection as illustrated in  FIG. 4 ) into oppositely polarized metalized areas (metalization where the slotlines  14  and  16  are formed) by the slotline to microstrip transitions  18  and  20 , thus producing a phase difference of 180° . The microstrip line  22  which feeds port  2  is terminated in the metalized area that has the same polarity as the ground plane conductor of the input coplanar waveguide  10 . The microstrip line  24  which feeds port  3  is terminated into the metalized area that has the same polarity as the central conductor of the coplanar waveguide  10 .  
         [0028]      FIG. 2  shows a top view of an embodiment of the 180° hybrid component in accordance with the present invention. Etching on both sides of a substrate  6  making up the 180° hybrid device is shown. The solid lines represent the microstrip transmission lines connected to the differential ports  2  and  3  formed on a first side of the substrate  6 . The dashed lines represent components formed on an opposing side of the substrate  6  including: coplanar waveguide  10  (made up of center conductor  10 A and outer conductor  10 B) connected to port  1 , power divider  12 , slotlines  14  and  16  extending from power divider  12 , two slotline open circuit terminations  28  and  29  and two slotline to microstrip transitions  18  and  20 .  
         [0029]      FIG. 3  illustrates the instantaneous electric (E) field polarities of signals carried at the coplanar waveguide and at the terminals of the power divider  12 . The slotline  16  that feeds port  3  is terminated by the open circuit  28  in a metalized area of substrate  6 . The microstrip line  24  is terminated into the metal strip connected to the center conductor  10 A. The ground plane for microstrip line  24  is electrically isolated from the metalized area connected to conductor  10 A with a large size open termination  28  which establishes the phase component for the RF signal at Port  3  The microstrip line that feeds port  2  is terminated in the metalized area that has the same polarity as the ground plane conductor of the coplanar line  10  ( 10 B). The ground plane for microstrip line  22  is electrically isolated from the metalized area connected to the conductor  10 B of coplanar waveguide  10  by a large size slotline open termination  29  which establishes the phase component for the RF signal at port  2 . This way the phase shift between the signals at port  2  and port  3  is maintained at 180° over an extremely wide frequency range.  
         [0030]      FIG. 4  illustrates details of the microstrip to slotline transition  20 . The physical connection from the microstrip  24  to the metallization area at one side of slot  16  is made using a via  26  through the hybrid substrate  6 . The energy carried in each of slotlines  14  and  16  is coupled to the metalized pad in the microstrip lines  22  and  24  through the substrate  6  by the vias, such as  26 . To improve the transition, in one embodiment, the slotlines  14  and  16  make a 270° turn, or “spiral” under the microstrip pad around the corresponding via holes and then is abruptly terminated with an open circuit regions  28  and  29 .  
         [0031]      FIG. 5  shows details of the slotline  16  as connected to an open circuit region  28 . The frequency bandwidth of the 180° hybrid structure is greatly expanded at frequencies below 3 GHz by optimizing the shape, size and position of the slotline open circuits  28  and  29 . In some embodiments, the optimization of the slotline-to-microstrip transitions  18  and  20 , including the slotline open circuits  28  and  29  and the remainder of the 180° hybrid structure can be performed through the use of commercially available high accuracy 3-D high frequency structure simulator software. In one embodiment a 180° hybrid can be fabricated on a 0.01 inch thick substrate.  
         [0032]     For the substrate of  FIG. 2 , the metalization layer in which the slotlines  14  and  16  are formed can be gold, copper, silver or a other desired conductive material. The metallization is etched away to form the coplanar waveguide structure  10 , power divider  12 , slotline structures  14  and  16  and slotline open terminations  28  and  29 .  
         [0033]     In some embodiments, impedance transformation is used in one or both of the coplanar waveguide  10  and the slotlines  14  and  16 . The characteristic impedance of the balanced slotlines  14  and  16  are gradually transformed from 100 Ohms at the power divider  12  to 50 Ohms at slotline to microstrip transitions  18  and  20  by gradually reducing the width of both slots along the length of the slotlines  14  and  16 . The coplanar waveguide  10  is likewise shown gradually transitioned from the unbalanced port  1  gradually toward the power divider  12 . The impedance transformation in the power divider  12  with an unbalanced-to-balanced transformer can be accomplished by using a gradual taper in the width of metal conductors and in the width of the slots. In some embodiments a 50 Ohm coplanar structure is transformed into two 100 Ohm slotline structures. In other embodiments, the characteristic impedance of the balanced slotline structures is gradually transformed from 100 Ohms to 50 Ohms.  
         [0034]      FIG. 6  illustrates an alternate substrate for the 180° hybrid component wherein magnetic material  30  and  32  is applied over the slotline terminations  28  and  29 . The particular type of magnetic material used to form regions  30  and  32  depends on the application requirements. Polyiron mix or a variety of ferrite materials may be applied according to the bandwidth requirements. The shape of the regions  30  and  32  and the extent of coverage over the termination regions  28  and  29  can be selected according to design requirements.  
         [0035]      FIG. 7  illustrates the substrate  6  for the 180° hybrid component of  FIG. 2  as provided in a support fixture  36  with connectors  41 - 43 . The displayed hybrid component  6  of  FIG. 2  is presented only for illustrative purposes. It should be clear to those of ordinary skill in the art that any number of physical designs could be used. Components carried over from  FIG. 2  are similarly labeled in  FIG. 7 , as are components carried over in other figures.  
         [0036]      FIGS. 8-10  illustrates simulation results for measurements from a 180° hybrid component in accordance with one embodiment of the present invention using components as illustrated in  FIG. 2 . In  FIGS. 8-9 , the magnitude vs. frequency plots are shown, while  FIG. 10  provides a phase difference between port  2  and port  3  signals vs. frequency plot.  
         [0037]      FIG. 8  provides S-parameters with transmission coefficient measurements S 12  ( 51 ) and S 13  ( 52 ) superimposed. The measurement for S 12  ( 51 ) is shown with a solid line, while S 13  ( 52 ) is shown with a dashed line. For the measurement S 12 , a signal is applied at port  1  and then measured at port  2 , while for S 13  the signal is applied at port  1  and results measured at port  3 . The magnitude is plotted in 1 dB per division from −3 dB to −10 dB, while frequency ranges from 0-40 GHz. As shown, both the plots S 12  and S 13  remain between −3.5 dB and −5.5 dB from 3 to 40 GHz. In some embodiments the frequency range of application was extended down to 900 MHz  
         [0038]      FIG. 9  provides S-parameters with reflection coefficient measurements S 11  ( 61 ) and S 22  ( 62 ) and S 33  ( 63 ) superimposed. The reflection measurements are made by applying a signal to a port and measuring results from the same port. The measurement for S 11  ( 61 ) is shown with a solid line, while S 22  ( 62 ) is a dashed line with long dashes and S 33  ( 63 ) is a dashed line with short dashes. The magnitude is plotted in 5 dB per division from 0 dB to −25 dB, while frequency ranges from 0-40 GHz. As shown, reflection coefficients remain below −5 dB from 0-40 GHz for all of the reflection measurements S 11 , S 22  and S 33 .  
         [0039]     The phase plot demonstrates the phase difference between port  2  and port  3  differential output signals.  
         [0040]      FIG. 10  illustrates the simulation result plot  65  showing the phase difference for the signals S 21  and S 31 . For the frequency range of 0-25 GHz, the phase difference remains within two degrees of 180 degrees. From 0-40 GHz, the phase difference remains within four degrees of 180 degrees.  
         [0041]     Referring to  FIGS. 8-10 , it has been determined that as the frequency of the signal decreases there is a certain ratio of the diameter of open circuit circular termination  28  and  29  to the length of the slotline at which the slotline mode becomes the non-dominant mode for the propagation of electromagnetic energy along the structure. Thus, in the present embodiments, the frequency bandwidth of the 180° hybrid structure is greatly expanded at frequencies below 3 GHz by optimizing the shape, size and position of the slotline open circuits  28  and  29  in the slotline-to-microstrip transition.  
         [0042]      FIG. 11  shows an alternate embodiment of a planar balun in accordance with the present invention. The illustrated embodiment is based on a unbalanced to balanced transformer utilized in a 180° hybrid similar to  FIG. 2  with components similarly labeled. In  FIG. 11  one of the slotlines  14  is terminated into tapered thin film resistive media. The resistive material is simply applied over the etched out metalization region. The resistive material value in ohms-per-square can be selected to meet design requirements. By doing so approximately 50% of the energy of input signal is absorbed by a resistor and another 50% of the input signal energy is coupled to balanced slotline  16 . The characteristic impedance of slotline  16  can be adjusted to meet any particular design requirements. The gradual tapered impedance transformer shown can be used to meet the desired bandwidth requirements. The gradual taper introduced to the resistive termination in the illustrated coplanar to slotline greatly improves the bandwidth of this structure.  
         [0043]     Although the present invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many additional modifications will fall within the scope of the invention, as that scope is defined by the following claims.