Abstract:
A balancing/unbalancing (balun) structure includes a microstrip line printed circuit board (PCB). Two input ports are coupled to a differential signal. An isolated port is connected to ground through a resistance. An output port is coupled to a single-ended signal corresponding to the differential signal. A plurality of traces on the PCB connect the two input ports, the load connection port and the output port, wherein distance between adjacent traces is approximately twice PCB thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application No. 10/232,617, filed on Sep. 3, 2002, entitled COMPACT BALUN WITH REJECTION FILTER FOR 802.1 1a AND 802.1 1b SIMULTANEOUS OPERATION, Inventor: Franco De Flaviis, which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to balancing/unbalancing structures, or “baluns,” for use in gigahertz wireless applications. 
     2. Related Art 
     A balun (short for BALanced to Unbalanced) is a transformer connected between a balanced source or load (signal line) and an unbalanced source or load (signal line). A balanced line has two signal line conductors, with equal currents in opposite directions. The unbalanced signal line has just one conductor; the current in it returns via a common ground or earth path. Typically, an RF balun function is implemented as an off-chip transformer or as a quarter wave hybrid (lumped or microstrip) integrated into an RF circuit board. 
     RF wireless circuits utilize balanced outputs of signals to minimize the effect of ground inductance and to improve common mode rejection. Circuits that benefit from balanced operation include mixers, modulators, IF strips, differential amplifiers and voltage controlled oscillators. These balanced outputs, moreover, consist of differential signals which must be combined to provide a single ended output signal. Thus, a balun is a RF balancing network or electric circuit for coupling an unbalanced line or device and a balanced line or device for the purpose of transforming from balanced to unbalanced or from unbalanced to balanced operation, with minimum transmission losses. A balun can be used with an unbalanced input and a pair of balanced outputs or, in the reverse situation, a pair of balanced sources and an unbalanced load. Baluns can be used to interface an unbalanced input with a balanced circuit by dividing the signal received at its unbalanced terminal equally to two balanced terminals, and by providing the signal at one balanced terminal with a reference phase and the signal at the other balanced terminal with a phase that is 180° out-of-phase relative to the reference phase. Plus or minus 180° baluns can be used to interface a balanced or differential input from a balanced port of a balanced circuit providing output signals which are equal in magnitude but 180° out-of-phase and an unbalanced load driven by a single-ended input signal. The balun combines the signals of the balanced input and provides the combined signal at an another port. 
     The balanced structure can improve performance in devices such as mixers, modulators, attenuators, switches and differential amplifiers, since balanced circuits can provide better circuit-to-circuit isolation, dynamic range, and noise and spurious signal cancellation. A balanced load is defined as a circuit whose behavior is unaffected by reversing the polarity of the power delivered thereto. A balanced load presents the same impedance with respect to ground, at both ends or terminals. A balanced load is required at the end of a balanced structure to ensure that the signals at the balanced port will be equal and opposite in phase. 
     Depending on the implementation, baluns can be divided into two groups: active and passive. Active baluns are constructed by using several transistors (so-called active devices). Although active baluns are very small, they are not generally preferred for the following reasons. First, due to the employment of active devices, noise will be introduced into the system. Also, active devices tend inherently to waste power. Additionally, the low-cost fabrication of active baluns is limited to semiconductor manufacture. Conversely, passive baluns are quite popular. Passive baluns include lumped-type baluns and distributed-type baluns. 
     Lumped-element-type baluns employ discrete components that are electrically connected, such as lumped element capacitors and lumped element inductors. Advantages of lumped-element-type baluns include small size and suitability for low frequency range usage. On the other hand, the performance of lumped-element-type baluns is not good in high frequency ranges (several GHz), because the lumped elements are very lossy and difficult to control. Also, the operational bandwidth of lumped-element-type baluns is small (&lt;10%, typically). 
     A 180° hybrid device is constructed from several sections of quarter-wavelength transmission lines and a section of half-wavelength transmission line. The drawbacks of the 180° hybrid device are larger size, difficulty in achieving a high impedance transformation ratio, and limitation to a balanced pair of unbalanced outputs. 
     In general, low return loss, low insertion loss, and good balanced characteristics are required for balun applications. In addition, bandwidth is another figure of merit. 
     An example of a conventional 180° hybrid is shown in  FIG. 1 , which shows four hybrids, all of which have a rat-race arrangement. The hybrids are suitable for 5.3 GHz operation, and a single hybrid is shown in  FIG. 2 , along with representative dimensions. As may be seen from  FIG. 2 , the footprint of each hybrid is approximately 473 mm 2  (18.2×26 mm), including the feeding arms, and the overall size of the board in  FIG. 1 , which includes the four hybrids, is about 2916 mm 2  (54×54 mm). As shown in  FIG. 3 , the hybrid of  FIGS. 1 and 2  may be thought of as a 3-port microwave device, with an input port, and two output ports, one of which outputs the signal with a phase of 0° at −3 dB, and the other one outputs the signal at 180°, at −3 dB. It will be appreciated that for a passive device such as illustrated in  FIGS. 1 and 2 , the designation of “input” or “output” is purely arbitrary. In practical applications, the single-ended input (or output) may, for example, be connected to an antenna, while the differential output (or input) may be connected to a differential amplifier, or differential driver. 
     However, many of the known passive balun structures are relatively large, which is often unacceptable in modem wireless applications. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a compact balun for 802.11a applications that substantially obviates one or more of the problems and disadvantages of the related art. 
     There is provided a balancing/unbalancing (balun) structure including a microstrip line printed circuit board (PCB). Two input ports are coupled to a differential signal. An isolated port is connected to ground through a resistance. An output port for coupling to a single-ended signal corresponding to the differential signal. A plurality of traces on the PCB connects the two input ports, the load connection port and the output port, wherein distance between adjacent traces is approximately twice PCB thickness. 
     In another aspect there is provided a 180° hybrid balun including a microstrip line printed circuit board (PCB). Two input ports on one side of the balun are coupled to a differential signal. An isolated port is connected to ground through a resistance. An output port on an opposite side of the balun is coupled to a single-ended signal corresponding to the differential signal, a direction from the one side to the opposite side defining a horizontal axis. A plurality of traces on the PCB connect the two input ports, the load connection port and the output port. The plurality of traces includes a plurality of folded λ/4 elements oriented along the horizontal axis. 
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure and without particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIG. 1  shows an arrangement of four conventional 180° hybrid baluns of a rat race type for 5.3 GHz application. 
         FIG. 2  shows a single rat race 180° hybrid balun. 
         FIG. 3  shows a 180° hybrid balun as a 3-port microwave device. 
         FIGS. 4 and 17  show a 180° hybrid balun of one embodiment of the present invention. 
         FIG. 5  shows scattering parameters S 14  and S 12  of the embodiment of FIG.  4 . 
         FIG. 6  shows the scattering parameter S 11  of the embodiment of FIG.  4 . 
         FIG. 7  shows the phase difference for signals passing through the balun of FIG.  4 . 
         FIG. 8  shows another embodiment of a 180° of the present invention. 
         FIG. 9  shows scattering parameters S 14  and S 12  of the embodiment of FIG.  4 . 
         FIG. 10  shows the scattering parameter S 11  for the embodiment of  FIG. 4   
         FIG. 11  shows the phase difference for signals passing through the balun of FIG.  8 . 
         FIG. 12  shows a photograph of an implementation of the 180° hybrid balun of FIG.  4 . 
         FIG. 13  shows a photograph of an implementation of the balun shown in FIG.  8 . 
         FIG. 14  shows measured scattering parameters S 14 , S 12  for the balun of FIG.  4 . 
         FIG. 15  shows a measured scattering parameter S 11  for the balun of FIG.  4 . 
         FIG. 16  shows a measured phase response of the balun of FIG.  4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 4  illustrates one embodiment of a 180° hybrid balun  400  of the present invention. As shown in  FIG. 4 , the hybrid balun  400  has 4 ports, P 1 , P 2 , P 3  and P 4 , and a number of traces (transmission lines) connecting the ports to each other. The differential signal is viewed at input ports P 2  and P 3 , and is outputted at port P 1 . It will be appreciated that the designation of the ports as either “input” or “output” is arbitrary, and the balun  400 , being a passive device, works identically in both directions. 
     For ease of reference, the hybrid in  FIG. 4  may be said to have a horizontal direction from left to right, and vertical direction from top to bottom in the figure. It will, of course, be understood by one of ordinary skill in the art that such designations are purely nominal, and only serve to explain the illustrations of the embodiment (while having no particular significance in actual implementation on a circuit board). 
     Port P 4  is connected to a ground through a matched resistance (for example, 50 ohms). The balun  400  shown in  FIG. 4  has a “folded in on itself” topology, in other words, the amount of open space inside the balun  400  is kept to a minimum by folding the transmission lines inward. Specifically, all the adjacent traces are arranged such that the spacing between the traces is approximately double the thickness of the printed circuit board (PCB). Note that trying to bring the traces closer than that will likely result in unwanted cross-coupling, and degraded performance. 
     The transmission line distance between the port P 2  and the port P 1  is λ/4. The transmission line distance between the port P 2  and the port P 3  is λ/4. The transmission line distance between the port P 3  and the port P 4  is λ/4. The transmission line distance between the port P 4  and the port P 1  is 3λ/4. 
     The embodiment illustrated in  FIG. 4  has the following characteristics: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Center frequency of operation 
                 f 0  = 5.3 GHz 
               
               
                   
                 Bandwidth 
                 BW = 0.15 GHz (0.3 dB roll off) 
               
               
                   
                 Substrate thickness 
                 H = 0.2286 mm (top layer) 
               
               
                   
                 Relative dielectric constant 
                 ε r  = 3.783 
               
               
                   
                 Dielectric loss at 5.3 GHz 
                 tanδ = 0.01 
               
               
                   
                 Minimum line width 
                 Δs = 0.127 mm 
               
               
                   
                 Substrate material: 
                 FR-4 
               
               
                   
                   
               
             
          
         
       
     
     The hybrid shown in  FIG. 4  has an approximately square shape. It will be appreciated that the shape is not necessarily a perfectly geometric square. However, generally, the closer that such a shape is to a square, the less overall area the hybrid takes up. It will also be appreciated that the hybrid of  FIG. 4  may be “stretched” in either the vertical or horizontal dimension, to result in a more “rectangular” layout. Additionally, one of the advantages of the present invention is that a hybrid such as that shown in  FIG. 4  may be modified to fit into “odd” shapes, to the extent such oddly shaped free areas are available on the printed circuit board. 
     Further with reference to  FIG. 4 , the hybrid balun  400  has a plurality of folded λ/4 elements formed by the traces. For example, one such element is formed by the traces designated  401 ,  402  and  403  in FIG.  4 . This folded λ/4 element connects the port P 2  and the port P 3 , and is oriented in a horizontal direction, or along a horizontal axis. Another such “folded element” is formed by the traces designated  404 ,  405  and  406 , which is also oriented along the horizontal axis, but is shorter than the folded λ/4 element formed by the traces  401 ,  402  and  403 . Thus, due to the use of the folded λ/4 elements, the entire balun  400  has a topology that is “folded in on itself,” as discussed above, and the overall area occupied by the balun  400  is minimized. The corners of the folded λ/4 elements are shown as being at 45° in FIG.  4 . In actual applications, the use of rounded corners may provide slightly better performance, but from a manufacturing standpoint, the use of 45° corners may be preferred. 
     Compared to the conventional balun illustrated in  FIG. 2 , the overall size of the balun  400  of this embodiment is reduced by a factor of 12 in the footprint area (from 473 mm 2  to 37 mm 2 ). Also, the input ports P 2 , P 3  are located on the same side. This avoids the need of long connecting arms thus reducing the insertion loss. The design is almost insensitive to the line width with nearly 60% tolerance, and is almost insensitive to the parasitics associated with the load on the isolated port P 4 . 
     It will also be appreciated that the hybrid balun  400  shown in  FIG. 4  may be manufactured on a single layer PCB, where the “bottom” of the PCB is grounded, and the traces shown in  FIG. 4  are on the “top.” If additional area reduction is required, the balun  400  of  FIG. 4  may be folded further using a third layer of tracing (i.e., using a two-substrate PCB), where the middle tracing layer is ground, and the two halves of the “folded in” balun  400  of  FIG. 4  are formed on opposite sides of the PCB (see  FIG. 17  for an isometric view, with the around layer and the PCB material not shown). Such an arrangement, while reducing the area occupied by the balun  400 , requires the addition of vias, which tends to increase parasitics, and reduce the bandwidth. Also, to the extent the space on the bottom of the two substrate PCB was available for use in placing other components, it would obviously not be available if it is used for the folded balun  400  as described above. 
       FIG. 5  is an illustration of predicted S scattering parameters for transmission between the ports P 1  and P 4 , and the ports P 1  and P 2  of the balun  400 . As may be seen from  FIG. 5 , at the frequency of interest (here, 5.3 GHz), the scattering parameter is approximately −3 dB (or about −3.6 dB, as seen from FIG.  5 ). In other words, the predicted performance of the 180° hybrid balun  400  is what is required of a device such as this. 
       FIG. 6  shows a predicted scattering parameter S 11  that represents whether the balun  400  is properly matched to the transmission characteristics of the printed circuit board. As may be seen from  FIG. 6 , the scattering parameter S 11  at all frequencies of interest is substantially less than −20 dB. 
       FIG. 7  shows a predicted phase response of the balun  400 , specifically the phase of the transmitted signal from port P 2  to port P 1  minus the transmitted signal from port P 4  to port P 1 . As may be seen from  FIG. 7 , the phase response at 5.3 GHz, the frequency of interest, is −180°, or exactly what is required. 
     One of the advantages of the topology of  FIG. 4  is that the input ports P 2  and P 3  are on one side of the device, while the output port P 1  is on the opposite side of the device. This often has an advantage in the layout of the PCB, particularly where a connection to an antenna at the port P 1  is involved. 
       FIG. 8  illustrates another embodiment of a balun  800  of the present invention. As may be seen from  FIG. 8 , the balun  800  includes four ports P 1 , P 2 , P 3 , P 4 , similar to the embodiment of  FIG. 4 , as well as a plurality of folded λ/4 elements. The major difference is that the use of an extension  801 , which connects to the balun  800  itself at point  802 . This is done in order to have the differential inputs P 2  and P 3  on one side, and the single ended output P 1  on the other. If having the inputs and the output of the balun  800  on the same side is acceptable, then there would obviously be no need for the extension  801 . 
     It will also be appreciated that the structure shown in  FIG. 8  includes the folded λ/4 elements, as described above with reference to FIG.  4 . 
       FIG. 9  is an illustration of the predicted S scattering parameters for transmission between ports P 1  and P 4 , and ports P 1  and P 2  of the balun  800 . As may be seen from  FIG. 5 , at the frequency of interest, 5.3 GHz, the scattering parameter is approximately −3 dB, which is required. 
       FIG. 10  shows a predicted scattering parameter S 11  that represents whether the balun  800  is properly matched to the transmission characteristics of the printed circuit board. As may be seen from  FIG. 10 , the scattering parameter S 11  at all frequencies of interest is substantially less than −20 dB. 
       FIG. 11  shows a predicted phase response of the balun  400 , specifically the phase of the transmitted signal from port P 2  to port P 1  minus the transmitted signal from port P 4  to port P 1 . As may be seen from  FIG. 11 , the phase response at 5.3 GHz, the frequency of interest, is −180°. 
       FIG. 13  shows a photograph of an implemented embodiment of  FIG. 8 , together with exemplary dimensions. 
       FIG. 12  shows a photograph of an implemented embodiment of  FIG. 4 , together with exemplary dimensions, and  FIGS. 14 ,  15  and  16  show measured scattering parameters and phase response for the balun  400  of FIG.  4 . As may be seen from these figures, the measured response and the predicted response shown in  FIGS. 5 ,  6  and  7  closely match. Also, although both the embodiments of FIG.  4  and the embodiment of  FIG. 8  provide good performance, the performance of the  FIG. 4  embodiment is slightly better. 
     It will be appreciated that the balun of the present invention is applicable to any number of applications that require conversion from single ended to differential signal, and not just to 802.11a applications. 
     It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.