Abstract:
A terminal circuit is applied to a bi-directional coupler. The terminal circuit includes a transmission line having a first end and a second end, a first resistor connecting the first end and a first ground and a second resistor connecting the second end and a second ground. A resistance value of the first resistor is substantially identical to that of the second resistor.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a bi-directional coupler, and more particularly to a bi-directional coupler using an improved terminal circuit. 
     2. Description of Related Art 
     A directional coupler is a radio frequency (RF) component/device which provides three communication ports, an input port, an output port and a coupled port. RF signals enter the directional coupler via the input port, where only a small portion thereof is output via the coupled port while the remaining is output via the output port. 
       FIG. 5  shows a schematic view of one such bi-directional coupler  100 . The bi-directional  100  provides 4 communication ports: an input port  110 , an output port  120 , a coupled port  130  and an isolated port  140 . The bi-directional coupler  100  further provides two terminal circuits  150  and  160  which respectively comprise terminal resistors  155  and  165 , each connecting to a ground. The resistance value of each of the resistors  155  and  165  is 50 ohm (Ω), in one example. 
       FIG. 6  shows a schematic view of an equivalent circuit of the bi-directional coupler  100  shown in  FIG. 1 . When a transmitter (TX)  170  delivers RF signals to the bi-directional coupler  100  via the input port  110 , a large portion of the RF signals are forwarded to an antenna  180  via the output port  120 . Meanwhile, a lesser portion of the RF signals are transmitted to the coupled port  130  while no RF signal is output via the isolated port  140  (in an ideal manner). On the contrary, when RF signals are delivered to the bi-directional coupler  100  via the output port  120 , a large portion of the RF signals are forwarded to the transmitter  170  via the input port  110 . Meanwhile, a lesser portion of the RF signals is transmitted to the isolated port  140  while no RF signal is output via the coupled port  130  (in an ideal manner). 
     However, accuracy of termination values of a terminal resistor may be affected due to manufacturing processes and temperature variations and parasitical effects of parasitical capacitors, thereby increasing return loss and diminishing isolation of a coupler. In other words, referring to  FIG. 2 , the terminal resistor  165  of the terminal circuit  160  involves an allowed tolerance range so that signal transmission may be impeded due to variations of a resistance value of the terminal circuit  160 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, all the views are schematic, and like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  shows a schematic view of an embodiment of a bi-directional coupler in accordance with the present disclosure. 
         FIG. 2  shows a schematic view of an equivalent circuit of the bi-directional coupler shown in  FIG. 2  in accordance with the present disclosure. 
         FIG. 3  shows a schematic view of an improved terminal circuit provides two terminal resistors which are connected with a transmission line, compared with a traditional terminal circuit providing a single terminal resistor. 
         FIG. 4  shows a schematic view of a verification result, using the Monte Carlo Simulation method, for return loss and isolation of an embodiment of a bi-directional coupler in accordance with the present disclosure. 
         FIG. 5  shows a schematic view of one example of a bi-directional coupler of the prior art. 
         FIG. 6  shows a schematic view of an equivalent circuit of the bi-directional coupler shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     An embodiment of a bi-directional coupler of the present disclosure uses improved terminal circuits to reduce return loss and enhance isolation for resistors and achieve high directivity of the bi-directional coupler. In other words, two terminal resistors, which are separated by a transmission line replace a terminal resistor residing in a conventional terminal circuit. Therefore, resistance tolerance influence caused by manufacturing processes and temperature variations and parasitical effect influence from parasitical capacitors is minimized, thereby enhancing accuracy of terminal circuits of the bi-directional coupler. 
       FIG. 1  shows a schematic view of an embodiment of a bi-directional coupler  200  in accordance with the present disclosure. The bi-directional  200  provides four communication ports, comprising an input port  210 , an output port  220 , a coupled port  230  and an isolated port  240 . In an embodiment, the bi-directional coupler  200  further provides two improved terminal circuits  250  and  260 . The improved terminal circuit  250  comprises terminal resistors  251  and  253  and a transmission line  310  connecting the terminal resistors  251  and  253 , wherein the terminal resistors  251  and  253  have substantially the same resistance value and each of them connects to a ground. The improved terminal circuit  260  comprises terminal resistors  261  and  263  and a transmission line  320  connecting the terminal resistors  261  and  263 , where the terminal resistors  261  and  263  have substantially the same resistance value and each of them connects to a ground. 
       FIG. 2  shows a schematic view of an equivalent circuit of the bi-directional coupler  200  shown in  FIG. 1  in accordance with the present disclosure. As described, an embodiment of the bi-directional coupler uses improved terminal circuits to reduce return loss and enhance isolation as well as achieve high directivity. Referring to  FIGS. 4 and 5 , the improved terminal circuit  250  includes the terminal resistors  251  and  253 , which are connected with the transmission line  310 . Similarly, the improved terminal circuit  260  includes the terminal resistors  261  and  263 , which are connected with the transmission line  320 . 
     Referring to  FIG. 3 , compared with a traditional terminal circuit providing a single terminal resistor, an improved terminal circuit provides two terminal resistors, which are connected with a transmission line, where resistance values of both are substantially the same. An impedance Zin (i.e. an input resistance value) of the terminal circuit is calculated using the following equation: 
               Zin   =     Zc   ⁢       (     β   +     j   ⁢           ⁢   tan   ⁢           ⁢   θ       )         (     α   +   β     )     +       j   ⁡     (     αβ   +   1     )       ⁢   tan   ⁢           ⁢   θ             ,         
where Zc represents a characteristic impedance of the transmission line (e.g. 100Ω), α represents a ratio of the resistance vale Z 1  of one terminal resistor and the resistance value Zc, β represents a ratio of the resistance vale Z 2  of the other terminal resistor and the resistance value Zc, θ represents a length of the transmission line (e.g. λ/2), and j represents √{square root over (−1)}.
 
     The efficiency of an embodiment of the bi-directional coupler using the improved terminal circuits is verified using a Monte Carlo Simulation method. Resistor-related parameters and transmission-line-related parameters are preset. Regarding resistor-related parameters, the “Tolerance of Resistor” is set as 2% and the “Line Width Variation” is set as 0.5 pico-farad (pF). Regarding the transmission-line-related parameters, the “Substrate Thickness Variation” is set as 2%, the “Line Width Variation” is set as 2%, the “Metal Thickness Variation” is set as 2%, and the “Dielectric Constant Variation” is set as 2%. 
       FIG. 4  shows a schematic view of a verification result, using the Monte Carlo Simulation method, for return loss and isolation of an embodiment of a bi-directional coupler in accordance with the present disclosure. As illustrated, B 1 , B 2  and B 3  respectively represent return loss, coupled energy and isolation for a bi-directional coupler using a traditional terminal circuit, while A 1 , A 2  and A 3  respectively represent return loss, coupled energy and isolation for a bi-directional coupler using an improved terminal circuit of the present disclosure. It can be seen that the coupled energy for the improved terminal circuit is equivalent to that of the traditional terminal circuit. By contrast, the return loss has improvement in 5˜10 dB and the isolation has improvement in 10˜15 dB. 
     In conclusion, the return loss and isolation of an embodiment of the bi-directional coupler are maximized by using improved terminal circuits and high directivity can be achieved. In other words, an embodiment of the bi-directional coupler uses an improved terminal circuit providing two terminal resistors which are separated by a transmission for replacing a traditional terminal circuit providing a single terminal resistor. Thus, resistance tolerance influence caused by manufacturing processes and temperature variations and parasitic influence from parasitic capacitors is minimized, thereby enhancing accuracy of the terminal circuits and achieving high directivity of the bi-directional coupler. 
     Although the features and elements of the present disclosure are described as embodiments in particular combinations, each feature or element can be used alone or in other various combinations within the principles of the present disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.