Patent Publication Number: US-9893409-B2

Title: Branch-line coupler

Description:
FIELD 
     The present disclosure generally relates to couplers, and more particularly to branch-line couplers. 
     BACKGROUND 
     It is well-known that directional couplers are usually used to solve the problems relating to power splitting in many microwave circuits. With the development of mobile communication technology and satellite communication technology, for convenient carrying and moving, the miniaturization of the communication devices becomes more and more important. 
     Branch-line couplers are widely applied to microwave integrated circuits and monolithic integrated circuits. The conventional branch-line coupler, such as the 3 dB branch-line coupler is constituted of four quarter-wavelength lines.  FIG. 1  shows the circuit configuration of a conventional 3 dB branch-line coupler of prior art. In  FIG. 1 , the length and width of the branch-line coupler respectively are 7.4 mm and 9.14 mm. However, the branch-line coupler occupies a large area of the printed circuit board (PCB). Therefore, a minimized and high performance 3 dB branch-line coupler would be preferable. 
     SUMMARY 
     A minimized branch-line coupler to match the demands of communication technology is provided. 
     The present disclosure provides a branch-line coupler, which includes a first port, a second port, a third port, and a fourth port, respectively an input port, a transmitted port, a coupled port, and an isolated port. A first connection part, a second connection part, a third connection part, and a fourth connection part are connected to these ports, and transmission line. A first bent transmission line and a second bent transmission line are electrically connected between the first port and the second port respectively. A third bent transmission line and a fourth bent transmission line are electrically connected between the third port and the fourth port. A first long strip transmission line is electrically connected between the first port and the fourth port. A second long strip transmission line is electrically connected between the second port and the third port. 
     The branch-line coupler of the disclosure occupies a small area and has high performance, which can be suitably applied to the mobile communication product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present technology will now be described, by way of example only, with reference to the attached figures, wherein: 
         FIG. 1  is a circuit configuration schematic diagram of a branch-line coupler according to the prior art. 
         FIG. 2  is a circuit configuration schematic diagram of a branch-line coupler according to an embodiment of the disclosure. 
         FIG. 3  is a s-parameter simulation diagram of a branch-line coupler according to an embodiment of the disclosure, wherein Freq denotes frequency and Mag denotes magnitude. 
         FIG. 4  is a s-parameter simulation diagram of degree of isolation between two output ports of a branch-line coupler, according to an embodiment of the disclosure. 
         FIG. 5  is an output phase difference diagram of two output ports of a branch-line coupler, according to an embodiment of the disclosure. 
         FIG. 6  is an output magnitude difference diagram of two output ports of a branch-line coupler, according to an embodiment of the disclosure. 
         FIG. 7  is a s-parameter simulation diagram of a conventional branch-line coupler. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features of the present disclosure. The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The term “coupled” is defined as connected, whether directly or indirectly through intervening components, and is not necessarily limited to physical connections. The connection can be such that the objects are permanently connected or releasably connected. The term “comprising” when utilized, means “including, but not necessarily limited to”. it specifically indicates open-ended inclusion or membership in the so-described combination, group, series and the like. 
       FIG. 2  shows a branch-line coupler according to an embodiment of the disclosure. The branch-line coupler is symmetrical about X axis and Y axis, in other words, it is symmetrical about the center line of the two sides of the branch-line coupler. The branch-line coupler includes a first port  10 , a second port  11 , a third port  12 , a fourth port  13 , a first bent transmission line  20 , a second bent transmission line  21 , a third bent transmission line  22 , and a fourth bent transmission line  23 . The branch-line coupler also includes a first long strip transmission line  24  and second long strip transmission line  25 , a first gap  30  and a second gap  31 , a first connection part  40 , a second connection part  41 , a third connection part  42 , and a fourth connection part  43 . 
     The first port  10 , the second port  11 , the third port  12 , and the fourth port  13  can be 50 ohms transmission lines. It should be understood that transmission line size is not to be considered as limiting the present disclosure, the transmission line with different impedances can be selected according to meet the demands of port matching. The first port  10  can be an input port, configured to input electromagnetic wave signal. The second port  11  can be a transmitted port, configured to output the electromagnetic wave signal from the input port. The third port  12  can be a coupled port, configured to output coupled electromagnetic wave signal. The fourth port  13  can be an isolated port. The aforesaid port configuration is not to be considered as limiting the present disclosure, the port configuration can be defined freely because the present branch-line coupler is symmetrical about the center lines of the two sides of the branch-line coupler. 
     The size of the first bent transmission line  20 , the second bent transmission line  21 , the third bent transmission line  22 , and the fourth bent transmission line  23  can be 70.7 ohms transmission lines, the first bent transmission line  20  is connected in parallel to the second bent transmission line  21  and they are set in parallel, also they are respectively electrically connected to the first port  10  and the second port  11  through the first connection part  40  and the second connection part  41 . In  FIG. 2 , the first bent transmission line  20  is U shaped, as is the second bent transmission line  21 . Horizontal edge length of the first bent transmission line  20  is shorter than horizontal edge length of the second bent transmission line  21 . In one embodiment, the shape of the first bent transmission line  20  is half-surrounded by the shape of the second bent transmission line  21 , the head and tail of the first bent transmission line  20  and the second bent transmission line  21  are respectively and electrically connected through the first connection part  40  and the second connection part  41 . The first gap  30  width between the first bent transmission line  20  and the second bent transmission line  21  is set longer than 70.7 ohms transmission line width, in other words, the first gap  30  is longer than first bent transmission line  20  width and the second bent transmission line  21  width. The third bent transmission line  22  is connected in parallel to the fourth bent transmission line  23  and they are set in parallel, also they are respectively and electrically connected to the third port  12  and the fourth port  13  through the third connection part  42  and the fourth connection part  43 . In  FIG. 2 , the shape constituted by the third bent transmission line  22  and the fourth bent transmission line  23  is the symmetrical shape of the shape constituted by the first bent transmission line  20  and the second bent transmission line  21 . They are symmetrical about the X axis, also second gap  31  width between the third bent transmission line  22  and the fourth bent transmission line  23  is set longer than the 70.7 ohms transmission line width. It should be understood that the impedances of the transmission lines herein is not to limit the present disclosure, they can be selected freely to meet requirements. Also, the shape constituted by the first bent transmission line  20  and the second bent transmission line  21  and the shape constituted by the third bent transmission line  22  and the fourth bent transmission line  23  are not to be considered as limiting the present disclosure. 
     The first long strip transmission line  24  and the second long strip transmission line  25  can be 50 ohms transmission line, the first long strip transmission line  24  is electrically connected to the first connection part  40  and the fourth connection part  43 . The second long strip transmission line  25  is electrically connected to the second connection part  41  and the third connection part  42 , and the second long strip transmission line  25  and the first long strip transmission line  24  are respectively connected to each port and other transmission line through a connection part. 
     The first connection part  40 , the second connection part  41 , the third connection part  42 , and the fourth connection part  43  can be transmission lines. The first connection part  40  is electrically connected to the first port  10 , the first bent transmission line  20 , the second bent transmission line  21 , and the first long strip transmission line  24 . The second connection part  41  is electrically connected to the second port  11 , the first bent transmission line  20 , the second bent transmission line  21 , and the second long strip transmission line  25 . The third connection part  42  is electrically connected to the third port  12 , the third bent transmission line  22 , the fourth bent transmission line  23 , and the second long strip transmission line  25 . The fourth connection part  43  is electrically connected to the fourth port  13 , the third bent transmission line  22 , the fourth bent transmission line  23 , and the first long strip transmission line  24 . The aforesaid transmission lines can be microstrip lines or other transmission lines. 
     As shown in  FIG. 2 , the length and width of the disclosed branch-line coupler respectively are 3.24 mm and 9.13 mm. It should be understood that the size of the branch-line coupler is decided by the required frequency of branch-line coupler, it is not to limit the present disclosure, and any size of the branch-line coupler can be chosen to adapt the requirement of the frequency. 
       FIG. 3  shows an s-parameter simulation diagram of a branch-line coupler according to an embodiment of the disclosure. In  FIG. 3 , the frequency band of the branch-line coupler corresponding to the parameter of S 11  below −10 dB is between 4.6 Ghz and 6.6 Ghz, the center frequency is 5.6 Ghz. The S 12  and S 13  parameters have 3 dB power loss at that frequency band. The parameters S 22 , S 33 , and S 44  of the second port  11 , the third port  12 , and the fourth port  13  are approximate to parameter S 11  of the first port  10 . For simplicity, diagrams for S 22 , S 33 , and S 44  are not given. 
       FIG. 4  shows an s-parameter simulation diagram of isolation degree of two output ports of a branch-line coupler according to an embodiment of the disclosure. As  FIG. 4  shows, the two outputs of the branch-line coupler have a high degree of isolation at the frequency band of 4.6 Ghz-6.8 Ghz. 
       FIG. 5  shows an output phase difference diagram of two output ports of a branch-line coupler according to an embodiment of the disclosure. In  FIG. 5 , the second port  11  and the third port  12  have a small phase difference at the frequency band of 4.6 Ghz to 6.8 Ghz. Specifically, the output phase difference of the second port  11  and the third port  12  is less than 20°. 
       FIG. 6  shows a magnitude difference in output of two output ports of a branch-line coupler according to an embodiment of the disclosure. In  FIG. 6 , the second port  11  and the third port  12  of the branch-line coupler have a small magnitude output difference at the frequency band 4.8 Ghz-6.8 Ghz. Specifically, the magnitude output difference between the second port  11  and the third port  12  is less than 2 dB. 
       FIG. 7  shows an s-parameter simulation diagram of a conventional branch-line coupler. As  FIG. 7  shows, the frequency band corresponding to the parameter S 11  of the conventional branch-line coupler below −10 dB is 4.6 Ghz-6.6 Ghz. The center frequency is 5.6 Ghz, and the S 12 , S 13  parameters have 3 dB power loss at the frequency band of 4.6 Ghz to 6.6 Ghz. 
     Comparing  FIG. 3  with  FIG. 7 , the branch line coupler has a performance as good as that of a conventional branch-line coupler. 
     The branch-line coupler formed by bent transmission lines decreases the size by 56% as compared with the conventional branch-line coupler. In addition, the coupler has good performance at the frequency band 4.6 Ghz to 6.6 Ghz, and the S 11  parameter is below −10 dB at the aforesaid frequency band. The magnitude of output and output phase of the two output ports have little difference and the two ports of the branch-line coupler have a high degree of isolation. The present coupler not only overcomes the disadvantage of occupying a large PCB area, but also has good performance, and is very suitable in mobile communication products. 
     The foregoing description, for purposes of explanation, is with reference to specific embodiments. However, the illustrated embodiments are not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The various modifications which are possible within the principles of the disclosure will therefore be protected within the scope of the claims.