Abstract:
An RF combiner/divider includes an input switch, an output switch, an impedance-matching transmission network for connecting the input switch to the output switch, and a control circuit connected to the input switch and the impedance-matching transmission network. The RF combiner/divider is used for automatic impedance transformation for impedance-matching. The RF combiner/divider is suitable for use in an RF system with a changeable number of combiner/divider branches.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a radio frequency (“RF”) combiner/divider capable of automatic impedance transformation for impedance-matching and, more particularly, to a combiner/divider for use in an RF system that includes a changeable number of combiner/divider branches. 
         [0003]    2. Related Prior Art 
         [0004]    An RF combiner/divider is used to combine several RF signals into a single output RF signal and divide a single RF signal into several output RF signals. The operation of a divider is opposite to that of a combiner. That is, the structure of a divider can be derived from that of a combiner. The combiner combines several input ports into a single output port while the divider divides a single input port into several output ports. 
         [0005]    Impedance transformation networks are used in the combiners or dividers. When the characteristic impedance at the input port is not matched with the output impedance at the output port, an impedance transformation circuit increases or reduces the impedance stage between the input and output ports to match the output impedance with the characteristic impedance as much as possible. Impedance-matching is important to ensure the maximum power transformation and minimum signal distortion and/or reflection between input and output circuits. 
         [0006]    Korean Patents KR20040069816 and KR20040098857 both describe Wilkinson combiner/dividers based on the Wilkinson Principle. For convenience of description, only the combiners will be discussed for example. Each input branch includes a quarter-wavelength impedance transformer for impedance transformation to match the output impedance with the input impedance. The impedance transformer of each input branch is given limitation. Hence, when the number of the input branches that are combined is changed, the impedance transformer of each input branch must be changed, and this is impractical because such a structure includes a certain number of transformers based on a certain number of channels to be combined, and the impedances of all of the transformers are based on the number of the channels to be combined. Hence, the Wilkinson combiner/dividers based on the Wilkinson Principle are not suitable for systems that include changeable numbers of combined/divided branches. 
         [0007]    U.S. Pat. No. 7,046,101 (“&#39;101”) discloses a combiner/divider that is based on the concept of a series/shunt network instead of the Wilkinson Principle. There is disclosed a divider that includes a single-pole N-way RF switch and a switchable impedance-matching network. The switchable impedance-matching network includes N−1 switch-selectable impedance-matching elements. The impedance-matching elements are arranged along a transmission line that includes an input port at an end and a switching connection point at another end. The switching connection point is for selective contact with several output-port reeds. The impedance-matching elements include different impedance-matching lengths. An impedance-matching distance exists between each impedance-matching element and the switching connection point. In operation, when only one output-port reed is in contact with the transmission line, i.e., only one output port is connected to the input port, the load impedance is matched with the source impedance, without having to activate any impedance-matching element. If the number of output ports connected to the input port is changed, the transmission line is connected to an impedance-matching element in a certain position determined by the number of the output ports that are combined, thus initiating an impedance-modulation mechanism for impedance-matching. In practice, the manufacturing and location of the impedance-matching elements require precision. 
         [0008]    U.S. Pat. No. 6,323,742 discloses an RF combiner that includes N input channels  126   a,    126   b,    126   c  and  126   d  for receiving input signals. These input channels are electrically connected to an electrical connection point  22  or  132 . All of the input signals are combined with one another at the electrical connection point  22  or  132 . Then, a quarter-wavelength impedance transformer  34  or  150  transfers the combination of the input signals to an output port. Each input channel includes a grounding switch  26 ,  28 ,  30  or  32 . There will be high impedance in an input channel if the respective grounding switch is connected to an electrical ground. Hence, the electrical connection point is only connected to an input channel where the grounding switch is open-circuited. An input channel ready for transferring an input signal is defined as an “active input channel.” According to the number of the active input channels, a control circuit  116  controls the connection of a first combiner switch  144  and a second combiner switch  154  to the corresponding impedance transformation line to match the output impedance with the input. The grounding switch provides high impedance to interfere with the ability of the input channels to transfer the signals. That is, the input channels are not actually cut off from the electrical connection point although they cannot smoothly transfer the input signals to the electrical connection point. This practice could easily damage the combiner. Moreover, the structure of the first combiner switch  144  and how it works are not described although it is actually part of an impedance transformer. 
         [0009]    The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art. 
       SUMMARY OF INVENTION 
       [0010]    It is an objective of the present invention to provide an inexpensive and efficient RF combiner/divider. 
         [0011]    It is another objective of the present invention to provide an RF combiner/divider for use in an RF system that includes a changeable number of combiner/divider branches, wherein the RF combiner/divider is used for automatic impedance transformation for impedance-matching. 
         [0012]    To achieve the foregoing objective, the RF combiner includes an input switch, an output switch, an impedance matching transmission network and a control circuit. The input switch includes several input channels for receiving input signals. The output switch includes the same number of input channels as the input channels of the input switch. All of the input channels of the output switch are electrically connected to an output port. The impedance matching transmission network includes switching elements and impedance transmission lines. The number of the switching elements is identical to that of the input channels. The impedance transmission lines are arranged between the switching elements and the input channels of the output switch. The control circuit controls the number of the input channels of the input switch that are electrically connected to a center connection point, and selectively connects an impedance-matched one of the switching elements to the center connection point based on the number. 
         [0013]    The control circuit controls the on/off of the input channels via digital inputs at the input channels electrically connected to the input switch. 
         [0014]    Other objectives, advantages and features of the present invention will be apparent from the following description referring to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]    The present invention will be described via detailed illustration of four embodiments referring to the drawings wherein: 
           [0016]      FIG. 1  is a block diagram of an RF combiner/divider according to the first embodiment of the present invention, showing that a quarter of a wavelength of an impedance matching transmission network is longer than the electrical length of the switching elements; 
           [0017]      FIG. 2  is a block diagram of a control circuit connected to the RF combiner/divider shown in  FIG. 1 ; 
           [0018]      FIG. 3  is a block diagram of an RF combiner/divider according to the second embodiment of the present invention, showing that an impedance transformer includes a multiple-stage quarter-wavelength transmission line; 
           [0019]      FIG. 4  is a block diagram of an RF combiner/divider according to the third embodiment of the present invention, showing that a quarter of a wavelength of an impedance matching transmission network is shorter than the electrical length of the switching elements; and 
           [0020]      FIG. 5  is a block diagram of an RF combiner/divider according to the fourth embodiment of the present invention, showing that an electrical length measured from a center connection point of an input switch to an output port of an output switch is identical to a quarter of a wavelength. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0021]    The present invention is related to an RF combiner/divider. Only a combiner is however described referring to the drawings since a divider and a combiner are identical to each other regarding the structure but opposite to each other regarding the operation. 
         [0022]    Referring to  FIGS. 1 and 2 , an RF combiner includes an input switch  10 , an output switch  60 , an impedance-matching transmission network  30  and a control circuit  80  according to an embodiment of the present invention. The impedance-matching transmission network  30  connects the input switch  10  to the output switch  60  electrically. The control circuit  80  is electrically connected to the input switch  10  and the impedance matching transmission network  30 . 
         [0023]    The input switch  10  is preferably a single-poled 2N-throw RF switch such as a single-poled 8-throw (“SP8T”) switch. Half of the stationary contacts of the single-poled 2N-throw RF switch are used as input channels  11 ,  12 ,  13  and  14  of the input switch  10 . The other stationary contacts of the single-poled 2N-throw RF switch are used as switching elements  21 ,  22 ,  23  and  24  of the impedance-matching transmission network  30 . The input channels  11 ,  12 ,  13  and  14  and the switching elements  21 ,  22 ,  23  and  24  are connected to a center connection point  20  under the control of the control circuit  80 . 
         [0024]    Each of the input channels  11 ,  12 ,  13  and  14  of the input switch  10  receives an input signal. Thus, there is characteristic impedance Z o  at each of the input channels  11 ,  12 ,  13  and  14 . The input signals include but not limited to RF signals, microwave frequency signals and signals at higher frequencies. 
         [0025]    There is respective transformation impedance at each of the switching elements  21 ,  22 ,  23  and  24  as part of the impedance-matching transmission network  30 . To this end, each of the switching elements  21 ,  22 ,  23  and  24  is sized according to the respective transformation impedance. The size includes length and/or cross-sectional width. 
         [0026]    The output switch  60  is preferably a high-power single-pole N-throw switch such as single-pole 4-throw (“SP4T”) switch. The single-pole N-throw switch includes four input channels  61 ,  62 ,  63  and  64  which are all connected to an output port  65  electrically. The input channels  61 ,  62 ,  63  and  64  are connected to the switching elements  21 ,  22 ,  23  and  24  via impedance transmission lines  31 ,  32 ,  33  and  34 , respectively. Hence, impedance at each of the input channels  61 ,  62 ,  63  and  64  is identical to the impedance at a corresponding one of the input channels  11 ,  12 ,  13  and  14  of the input switch  10 . 
         [0027]    The impedance-matching transmission network  30  includes the switching elements  21 ,  22 ,  23  and  24  and the impedance transmission lines  31 ,  32 ,  33  and  34  for connecting the switching elements  21 ,  22 ,  23  and  24  to the input channels  61 ,  62 ,  63  and  64 . The impedance transmission lines  31 ,  32 ,  33  and  34  are impedance-controlled RF transmission lines including but not limited to coaxial cables, coaxial structures built therein, circuit board transmission lines and microstriplines. 
         [0028]    The on/off of the input channels  11 ,  12 ,  13  and  14  of the input switch  10  are under the control of the control circuit  80  based on digital inputs  81 ,  82 ,  83  and  84  thereat. The digital input at each of the digital inputs  81 ,  82 ,  83  and  84  may be “1” to represent the turning on of a corresponding one of the input channels  11 ,  12 ,  13  and  14 . The digital input at each of the digital inputs  81 ,  82 ,  83  and  84  may alternatively be “0” to represent the turning off of a corresponding one of the input channels  11 ,  12 ,  13  and  14 . 
         [0029]    A selector  85  is connected to the control circuit  80  and operable to select a number of the input channels  11 ,  12 ,  13  and  14  to be turned on. Based on the selected number, the control circuit  80  turns on at least some of the input channels  11 ,  12 ,  13 ,  14  and connects the input switch  10  to the output switch  60  via a selected one of the impedance transformers  35 ,  36 ,  37  and  38  of the impedance-matching transmission network  30  for impedance transformation in an impedance-matched manner. 
         [0030]    For example, three of the input channels of the input switch  10  may be turned on. The characteristic impedance Z 0  at each turned-on input channel is 50 Ω (Z 0 =50 Ω). The total impedance at the center connection point  20  is Z 0 /N (50 Ω/3=16.66 Ω). By using the impedance-matching transmission network  30  for impedance transformation, the output impedance at the output switch  60  is matched with the characteristic impedance Z 0 , i.e., Z 0 /N is transformed to Z 0  for output. 
         [0031]    For example, only one of the input channels of the input switch  10  is turned on. The control circuit  80  connects the input switch  10  to the output switch  60  via the impedance transformer  35  where the impedance is Z 0 /√{square root over (1)}. 
         [0032]    For example, two of the input channels of the input switch  10  are turned on. The control circuit  80  connects the input switch  10  to the output switch  60  via the impedance transformer  36  where the impedance is Z 0 /√{square root over (2)}. 
         [0033]    For example, three of the input channels of the input switch  10  are turned on. The control circuit  80  connects the input switch  10  to the output switch  60  via the impedance transformer  37  where the impedance is Z 0 /√{square root over (3)}. 
         [0034]    For example, four of the input channels of the input switch  10  are turned on. The control circuit  80  connects the input switch  10  to the output switch  60  via the impedance transformer  38  where the impedance is Z 0 /√{square root over (4)}. 
         [0035]    Referring to  FIG. 3 , to satisfy the need for a larger bandwidth, the impedance transformers  35 ,  36 ,  37  and  38  may include multiple-stage quarter-wavelength transformation lines  31 ,  32 ,  33  and  34  according to another embodiment of the present invention. 
         [0036]    Each of the switching elements  21 ,  22 ,  23  and  24  is connected to a corresponding one of the impedance transmission lines  31 ,  32 ,  33  and  34  to form a corresponding one of the quarter-wavelength impedance transformers  35 ,  36 ,  37  and  38  as in the embodiment shown in  FIG. 1 . The electrical length of the impedance-matching transmission network  30 , a quarter of the wavelength, is longer than the electrical length of each of the switching elements  21 ,  22 ,  23  and  24 , and terminates prior to the input channels  61 ,  62 ,  63  and  64  of the output switch  60 . Hence, the impedance at initial ends of the impedance transmission lines  31 ,  32 ,  33  and  34  are Z 0 /√{square root over (1)}, Z 0 /√{square root over (2)}, Z 0 /√{square root over (3)} and Z 0 /√{square root over (4)}, respectively. The impedance is increased to Z 0  at a certain point where the electrical length of each of the switching elements  21 ,  22 ,  23  and  24  is subtracted from the electrical length of a quarter of the wavelength. Hence, the impedance at each of the input channels  61 ,  62 ,  63  and  64  of the output switch  60  is Z 0 . 
         [0037]    Each of the impedance-switching elements  21 ,  22 ,  23  and  24  forms a corresponding one of the quarter-wavelength impedance transformers  35 ,  36 ,  37  and  38  according to another embodiment of the present invention referring to  FIG. 4 . The electrical length of the impedance-matching transmission network  30 , a quarter of the wavelength, is shorter than the electrical length of each of the switching elements  21 ,  22 ,  23  and  24 . The impedance-matching transmission network  30  is connected to the switching elements  21 ,  22 ,  23  and  24  via changing the size. Hence, the impedance at the entire impedance-matching transmission network  30  and the impedance at the output switch  60  are Z 0 . 
         [0038]    Each of the switching elements  21 ,  22 ,  23  and  24 , a corresponding one of the impedance transmission lines  31 ,  32 ,  33  and  34  and a corresponding one of the input channels  61 ,  62 ,  63  and  64  are interconnected serially to form a corresponding one of the quarter-wavelength impedance transformers  35 ,  36 ,  37  and  38  according to another embodiment of the present invention referring to  FIG. 5 . The total electrical length measured from the center connection point  20  of the input switch  10  to the output port  65  is identical to a quarter of the wavelength. That is, the total electrical length that is formed by interconnecting the switching elements  21 ,  22 ,  23  and  24 , the impedance transmission lines  31 ,  32 ,  33  and  34  and the input channels  61 ,  62 ,  63  and  64  is identical to a quarter of the wavelength. Hence, the impedance at the channel that consists of the switching element  21 , the impedance transmission line  31  and the input channel  61  is Z 0 /. The impedance at the channel that consists of the switching element  22 , the impedance transmission line  32  and the input channel  62  is Z 0 /√{square root over (2)}. The impedance at the channel that consists of the switching element  23 , the impedance transmission line  33  and the input channel  63  is Z 0 /√{square root over (3)}. The impedance at the channel that consists of the switching element  24 , the impedance transmission line  34  and the input channel  64  is Z 0 /√{square root over (4)}. 
         [0039]    The present invention has been described via the detailed illustration of the embodiments. Those skilled in the art can derive variations from the embodiments without departing from the scope of the present invention. Therefore, the embodiments shall not limit the scope of the present invention defined in the claims.