Patent Publication Number: US-8125302-B2

Title: Signal selecting device

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a signal selecting device used in transmission, reception or transmission/reception of information. In the field of radio communication using radio waves, necessary signals and unnecessary signal are separated by extracting signals at a particular frequency from a large number of signals. Filters that perform this function comprise a resonator and an impedance transforming circuit and are incorporated in many radio devices. Such filters cannot change design parameters, such as the center frequency and the bandwidth. Therefore, a radio communication device using a plurality of combinations of center frequencies and bandwidths has to have a number of filters equal to the number of combinations of center frequencies and bandwidths and select a filter for use by means of a switch or the like. For example, a non-patent literature 1 (DoCoMo Technical Journal Vol. 14, No. 2, pp. 31-37) discloses a related art in which a filter for use is selected from among a plurality of filters by means of a switch. 
     Related arts, such as that disclosed in the non-patent literature 1, have a problem that, as the number of combinations of center frequencies and bandwidths increases, the circuit area and the number of components also increase. An object of the present invention is to provide a filter capable of appropriately changing a center frequency and a bandwidth by controlling characteristics of a resonator and an impedance transforming circuit and to reduce the number of filters used even when a plurality of combinations of center frequencies and bandwidths is used. 
     SUMMARY OF THE INVENTION 
     A signal selecting device according to the present invention has two input/output ports, a plurality of resonating parts, a plurality of impedance transforming parts, and a controlling part. The resonating parts have a ring conductor having a length equal to one wavelength at a resonant frequency or an integral multiple thereof and a plurality of switches each of which is connected to a different part of the ring conductor at one end and to a ground conductor at the other end. The controlling part controls the state of the switches. The resonating parts are disposed in series between the two input/output ports. The impedance transforming parts are disposed between the input/output ports in such a manner that the impedance transforming parts at the both ends are disposed between the input/output port and the resonating part and the remaining impedance transforming parts are disposed between the resonating parts. That is, the number of the impedance transforming parts is greater than the number of resonating parts by one. The impedance transforming parts adjust the impedance between the outside and the resonating parts or between the resonating parts. The term “ring conductor” means a conductor (a transmission line) having the opposite ends thereof connected to each other and is not limited to a particular shape. That is, the shape of the ring conductor is not limited to a circular shape, but the ring conductor can have any other shape, such as a polygonal shape. 
     The impedance transforming parts may be capable of changing the characteristics. In that case, the controlling part controls the characteristics of the impedance transforming parts. In particular, in a case where the signal selecting device has an odd number of resonating parts, all the impedance transforming parts can be configured to have the same characteristics at the operational frequency of the signal selecting device. Alternatively, in a case where the signal selecting device has an even number of resonating parts (it means that the number of the impedance transforming parts is an odd number), the impedance transforming part disposed at the center alone can be controlled to have characteristics different from those of the remaining impedance transforming parts. 
     Three or more variable reactance means can be connected to the ring conductor at regular intervals. In that case, the controlling part controls the characteristics of the variable reactance means. 
     One or more branch parts can be disposed between the impedance transforming parts and the resonating parts, and a switch part can be disposed between one of the input/output port and the impedance transforming parts. In that case, switching can be performed so that one of the branch parts is selected and is connected to the switch part. 
     EFFECT OF THE INVENTION 
     According to the present invention, the resonating parts having the ring conductor and the switches can arbitrarily change the susceptance slope parameter highly independently of the resonant frequency. Therefore, the signal selecting device can be easily designed to have desired characteristics. In addition, the bandwidth and the in-band and out-band characteristics can also be changed by changing the susceptance slope parameter of the resonating parts. 
     Furthermore, in a case where the resonating parts have variable reactance means connected to the ring conductor at appropriate intervals, the signal selecting device can change the center frequency highly independently of the bandwidth and the in-band and out-band characteristics. In addition, in a case where the characteristics of the impedance transforming parts can be changed, the signal selecting device can more appropriately adjust the bandwidth and the in-band and out-band characteristics. 
     Furthermore, in a case where the signal selecting device has the branch parts and the switch part, the number of resonators can be changed. That is, the bandwidth and the in-band and out-band characteristics can be more flexibly adjusted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 1; 
         FIG. 2A  is a diagram showing a configuration of a resonating part; 
         FIG. 2B  is a diagram showing an equivalent circuit using a lossless transmission line model; 
         FIG. 3  is a graph showing a relationship between the susceptance slope parameter and θ in a single resonator; 
         FIG. 4  is a diagram showing a section of the signal selecting device that includes resonating parts and impedance transforming parts; 
         FIG. 5  is a diagram for explaining characteristics of a typical J-inverter; 
         FIG. 6  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 2; 
         FIG. 7  is a graph showing frequency characteristics of the signal selecting device grounded at determined positions; 
         FIG. 8  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 3; 
         FIG. 9  is a diagram showing a section of the signal selecting device having four resonating parts and five impedance transforming parts that includes the resonating parts and the impedance transforming parts; 
         FIG. 10  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 4; 
         FIG. 11  is a diagram showing an exemplary configuration in which arrangement of variable reactance means is modified; 
         FIG. 12  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 5; 
         FIG. 13  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 6; 
         FIG. 14  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 7; 
         FIG. 15  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 8; 
         FIG. 16A  is a diagram showing an example of the impedance transforming part that is formed by a transmission line having a characteristic impedance of Z and a length equal to a quarter wavelength at a resonant frequency; 
         FIG. 16B  is a diagram showing an example of the impedance transforming part that is formed by a capacitor; 
         FIG. 16C  is a diagram showing an example of the impedance transforming part that is formed by a coil; 
         FIG. 16D  is a diagram showing an example of the impedance transforming part that is formed by lines coupled by electromagnetic induction; 
         FIG. 16E  is a diagram showing an example of the impedance transforming part that is formed by a combination of the examples shown in  FIGS. 16A to 16D ; 
         FIG. 17A  is a diagram showing an example of an impedance transforming part capable of changing the characteristics that is formed by a transmission line having a characteristic impedance of Z and a length equal to a quarter wavelength at a resonant frequency and variable capacitors connected in parallel to the transmission line; 
         FIG. 17B  is a diagram showing an example of the impedance transforming part capable of changing the characteristics that is formed by a variable capacitor; 
         FIG. 17C  is a diagram showing an example of the impedance transforming part capable of changing the characteristics that is formed by a variable coil; 
         FIG. 17D  is a diagram showing an example of the impedance transforming part capable of changing the characteristics that is formed by lines variably electromagnetically coupled to each other; 
         FIG. 17E  is a diagram showing an example of the impedance transforming part capable of changing the characteristics that is formed by two kinds of transmission lines that have a length equal to a quarter wavelength at a resonant frequency and different characteristic impedances and are switched from one to another; 
         FIG. 17F  is a diagram showing an example of the impedance transforming part capable of changing the characteristics that is formed by two kinds of transmission lines that have a length equal to a quarter wavelength at different resonant frequencies and the same characteristic impedance and are switched from one to another; 
         FIG. 18A  is an example in which a switch that makes a short circuit is used as a switch when ring conductors are connected in series to a signal line; 
         FIG. 18B  is an example in which a switch that makes a short circuit via a transmission line is used as a switch when ring conductors are connected in series to a signal line; 
         FIG. 18C  is an example in which a switch that establishes a connection of a transmission line having an open end is used as a switch when ring conductors are connected in series to a signal line; 
         FIG. 19A  is a diagram showing an exemplary functional configuration of a controlling part according to the embodiments 1, 2 and 8; 
         FIG. 19B  is a diagram showing an exemplary functional configuration of a controlling part according to the embodiment 3; 
         FIG. 19C  is a diagram showing an exemplary functional configuration of a controlling part according to the embodiment 4; 
         FIG. 20A  is a diagram showing another exemplary functional configuration of the controlling part according to the embodiments 1 and 2; 
         FIG. 20B  is a diagram showing another exemplary functional configuration of the controlling part according to the embodiment 3; 
         FIG. 20C  is a diagram showing another exemplary functional configuration of the controlling part according to the embodiment 4; 
         FIG. 21A  is an example of processing means that is composed of a calculation unit, a storage unit and a control unit; and 
         FIG. 21B  is an example of the processing means that is composed of a retrieval unit, a storage unit and a control unit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIG. 1  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 1. A signal selecting device  100  has two input/output ports  111  and  112 , N resonating parts  120   1  to  120   N , N+1 impedance transforming parts  130   0,1  to  130   N,N+1 , and a controlling part  140 . The resonating part  120   n  (n represents any integer in a possible range and is an integer from 1 to N in this case) has a ring conductor  121   n  having a length equal to one wavelength at a resonant frequency or an integral multiple thereof, and M switches  122   n - 1  to  122   n -M each of which is connected to a different part of the ring conductor  121   n  at one end thereof and to a ground conductor at the other end thereof. The controlling part  140  controls the state of the N*M switches  122   1 - 1  to  122   N -M. (“N*M” shows multiplying N by M.) The resonating parts  120   1  to  120   N  are disposed in series between the two input/output ports. The impedance transforming parts  130   0,1  to  130   N,N+1  are disposed between the input/output ports in such a manner that the impedance transforming parts  130   0,1  and  130   N,N+1  at the both ends are disposed between the input/output port and the resonating part and the remaining impedance transforming parts  130   1,2  to  130   N−1,N  are disposed between the resonating parts. Specifically, the impedance transforming part  130   n,n+1  (n represents any integer in a possible range as described above and is an integer from 1 to N−1 in this case) is disposed between the resonating part  120   n  and the resonating part  120   n+1  and adjusts the impedance between the resonating parts  120   n  and the resonating part  120   n+1 . The impedance transforming part  130   0,1  changes the impedance between the outside on the input/output port  111  and the resonating part  120   1 . The impedance transforming part  130   N,N+1  changes the impedance between the resonating part  120   N  and the outside on the input/output port  112 . The ring conductor  121   n  means a conductor (a transmission line) having the opposite ends thereof connected to each other and is not limited to a particular shape. That is, while the ring conductor has a circular shape in  FIG. 1 , the ring conductor can have a polygonal or other shape instead of the circular shape. 
       FIG. 2A  shows a configuration of the resonating part  120   n .  FIG. 2B  shows an equivalent circuit using a lossless transmission line model. Z in  denotes the input impedance of the resonating part  120   n  from the point P. An operation of the resonating part  120   n  will be described by determining the input impedance Z in  of the model shown in  FIG. 2B  in a case where the switch  122   n - 3  shown in  FIG. 2A  is in the on state. At a resonant frequency f r , a transmission line  121   n - 1  has an electrical length of π and a characteristic impedance of Z 1 , a transmission line  121   n - 2  has an electrical length of θ and a characteristic impedance of Z 2 , and a transmission line  121   n - 3  has an electrical length of (π-θ) and a characteristic impedance of Z 3 . That is, the total sum of the electrical lengths of the transmission lines  121   n - 1 ,  121   n - 2  and  121   n - 3  is 2π (360 degrees). A path P A  composed of the transmission line  121   n - 1  and the transmission line  121   n - 2  is a path extending clockwise from the point P to the switch  122   n - 3  in  FIG. 2A . A path P B  composed of the transmission line  121   n - 3  is a path extending counterclockwise from the point P to the switch  122   n - 3  in  FIG. 2A . Z L  denotes the impedance between the switch  122   n - 3  to the ground. 
     In this case, the input impedance Z in  is expressed by the following formula (1). In this formula, j denotes an imaginary unit. 
                     Z   in     =         y   22     +     Y   L             y   11     ⁡     (       y   22     +     Y   L       )       -       y   12     ⁢     y   21                   (   1   )               
In this formula,
 
 y   11   =−jY   2  cot θ+ jY   3  cot θ
 
 y   12   =−jY   2  csc θ+ jY   3  csc θ
 
 y   21   =−jY   2  csc θ+ jY   3  csc θ
 
 y   22   =−jY   2  cot θ+ jY   3  cot θ
 
 Y   2 =1 /Z   2   ,Y   3 =1 /Z   3   ,Y   L =1/Z L ,
 
where L denotes the length of the ring conductor, and θ=x/2πL (rad). As can be seen from the formula (1), when Y 2 =Y 3 , the impedance Z in  is infinity except when θ is 0 or an integral multiple of π. When θ is 0 or an integral multiple of π, Z in =Z L . That is, when the line length (physical length)×changes, the resonant frequency is constant except when the line length reduced to an electrical length at the resonant frequency is 0 or an integral multiple of π. Next,  FIG. 3  shows a relationship between θ and the susceptance slope parameter in a single resonator in a case where the impedances Z 1 , Z 2  and Z 3  are 50Ω. The susceptance slope parameter b is determined by the following formula.
 
                     b   =           ω   0     2     ⁢       ⅆ   B       ⅆ   ω         ⁢     ❘     ω   0           ,           (   2   )               
where B=Im (Y in ), and Y in =1/Z in .
 
From this drawing, it can be seen that the susceptance slope parameter b can be changed without changing the resonant frequency by changing the value θ, or in other words, changing the switch to be turned on. In addition, as can be seen from the formula (2), the susceptance slope parameter b indicates the variation of the imaginary part of the admittance with respect to the frequency. As the susceptance slope parameter b becomes greater, the admittance changes more greatly with respect to the difference frequency with respect to the resonant frequency. Therefore, in a band-pass filter using parallel resonance, for example, the bandwidth becomes narrower. As described later, the in-band and out-band characteristics are determined by the susceptance slope parameter b. That is, the bandwidth and the in-band and out-band characteristics can be changed by the resonating part, and the bandwidth can be changed by changing the susceptance slope parameter b while keeping the center frequency constant.
 
     A principle of changing the bandwidth and the in-band and out-band characteristics of the filter has been described above. Actually, in order to change the bandwidth and the in-band and out-band characteristics of the filter, an appropriate switch  122   n -m (m represents any integer in a possible range and is an integer from 1 to M in this case) to be turned on has to be selected from among the large number of switches. In the signal selecting device  100  shown in  FIG. 1 , the controlling part  140  selects the switch  122   n -m to be turned on. In order for the controlling part  140  to select the appropriate switch  122   n -m, the controlling part  140  has to consider the relationship between the position of the switch  122   n -m to be turned on and the susceptance slope parameter b of the resonating part  120   n  and the relationship between the susceptance slope parameter b and the characteristics of the signal selecting device  100 . The relationship between the position of the switch  122   n -m and the susceptance slope parameter b has already been described with reference to  FIG. 3 . In the following, the relationship between the susceptance slope parameter b and the characteristics of the signal selecting device  100  will be described. 
       FIG. 4  is a diagram showing a section of the signal selecting device shown in  FIG. 1  that includes the resonating parts and the impedance transforming parts. There are N resonating parts  120   1  to  120   N  and N+1 impedance transforming parts  130   0,1  to  130   N,N+1 . The impedance transforming parts  130   0,1  to  130   N,N+1  are disposed between the input/output ports  111  and  112  in such a manner that the impedance transforming part  130   0,1  is disposed between the input/output port  111  and the resonating part  120   1 , the impedance transforming part  130   N,N+1  is disposed between the input/output port  112  and the resonating part  120   N , and the remaining impedance transforming parts  130   1,2  to  130   N−1,N  are disposed between the remaining resonating parts. Admittance  911  and  912  are port admittances of the input/output ports  111  and  112 , respectively. The impedance transforming parts  130   0,1  to  130   N,N+1  transform the impedance of a component connected thereto (a circuit or an element, for example) to an impedance that is proportional to the inverse thereof. The ring conductor  121   n  of the resonating part  120   n  used in the signal selecting device  100  is connected in parallel with a transmission line that connects the impedance transforming part  130   n−1, n  and the impedance transforming part  130   n,n+1 . The impedance transforming parts  130   0,1  to  130   N,N+1  in this case are referred to as admittance inverter or J-inverter.  FIG. 5  is a diagram for explaining the characteristics of a typical J-inverter. The characteristics of the J-inverter shown in this drawing is expressed by the following formula. 
                   Y   =       J   2       Y   ′               (   3   )               
That is, the admittance parameter J of the J-inverter is a coefficient that determines the number by which the admittance inverted by the J-inverter is multiplied.
 
     The admittance parameter J n−1,n  of the impedance transforming part  130   n−1, n  are expressed by the following formulas using the bandwidth (fractional bandwidth), the in-band and the out-band characteristics. 
                     J     0   ,   1       =           Gb   1     ⁢   w         g   0     ⁢     g   1                   (   4   )                 J       n   -   1     ,   n       =     w   ⁢           b     n   -   1       ⁢     b   n           g     n   -   1       ⁢     g   n                     (   5   )                 J     N   ,     N   +   1         =           Gb   N     ⁢   w         g   N     ⁢     g     N   +   1                     (   6   )               
In these formulas, G denotes the port admittance, and b n  denotes the susceptance slope parameter of the n-th resonating part  120   n . w denotes the fractional bandwidth of the signal selecting device  100 , g n  denotes an element value of an original low pass filter, and these values determine the bandwidth and the in-band and out-band characteristics of the signal selecting device  100 . When these parameters satisfy the relationships expressed by the formulas (4) to (6), the signal selecting device  100  has desired characteristics. Of these parameters, the fractional bandwidth w and the element value g n  of the original low pass filter are determined from the characteristics of the signal selecting device  100  to be achieved. The port admittance G depends on the circuits preceding and following the signal selecting device  100 . Therefore, the admittance parameter J n−1, n  or the susceptance slope parameter b n  can be adjusted to satisfy the relationship expressed by the formulas (4) to (6).
 
     Conventional signal selecting devices (filters) cannot arbitrarily change the susceptance slope parameter b n . Therefore, after the fractional bandwidth w and the element value g n  of the original low pass filter are determined, the admittance parameter J n−1,n  that satisfies the formulas (4) to (6) has to be designed with the susceptance slope parameter b n  being fixed. In addition, conventionally, a capacitor is often used as the J-inverter. However, if the bandwidth is changed by changing the capacitance of the capacitor, the operational frequency of the J-inverter also changes. That is, the center frequency also changes. Therefore, it is difficult to design the J-inverter that satisfies the formulas (4) to (6). 
     To the contrary, the signal selecting device  100  according to the present invention has the resonating part  120   n  incorporating the ring conductor  121   n  and therefore can arbitrarily change the susceptance slope parameter b n . That is, the characteristics of the signal selecting device  100  can be changed by changing the susceptance slope parameter b n  of the resonating part  120   n . Therefore, in case of designing of the signal selecting device  100 , the fractional bandwidth w and the element value g n  of the original low pass filter are determined and the admittance parameter J n−1,n  is calculated from the characteristics of the circuit of the impedance transforming part  130   n−1,n  (J-inverter). Then, the switch to be turned on can be selected among the switches  122   n - 1  to  122   n -M so that the susceptance slope parameter b n  satisfies the formulas (4) to (6). That is, the condition that the formulas (4) to (6) have to be satisfied does not have to be considered in design of the J-inverter, so that the J-inverter can be easily designed. 
     Furthermore, when the bandwidth and the in-band and out-band characteristics are to be changed, the switch  122   1 - 1  to  122   N -M to be turned on can be changed to meet the desired characteristics. In this case, the resonant frequency of the resonating part  120   n  does not change, and the admittance parameter J n−1,n  also does not change, so that the center frequency can be kept constant. In actual, the number of switches is finite, so that the possible susceptance slope parameters b n  are discrete. Therefore, a switch  122   1 - 1  to  122   N -M that provides a value closest to the required susceptance slope parameter b n  is selected. 
     As described above, in a signal selecting device according to the embodiment 1, the resonating part having the ring conductor and the switches can arbitrarily change the susceptance slope parameter highly independently of the resonant frequency. Therefore, the signal selecting device can be easily designed to have desired characteristics. In addition, the bandwidth and the characteristics can be changed by changing the susceptance slope parameter of the resonating part. 
     Embodiment 2 
     In the embodiment 1, a signal selecting device according to the present invention has been generally described. In an embodiment 2, a signal selecting device according to the present invention will be specifically described.  FIG. 6  is a diagram showing an exemplary functional configuration of a signal selecting device according to the embodiment 2. A signal selecting device  200  has input/output ports  211  and  212 , three resonating parts  220   1  to  220   3 , four impedance transforming parts  230   0,1  to  230   3,4 , and a controlling part  240 . The resonating part  220   n  has a ring conductor  221   n . Although not shown in  FIG. 6 , the resonating part  220   n  has switches as in the embodiment 1. The input/output ports  211  and  212  have a port impedance of 50Ω. The resonating part  220   n  has a resonant frequency of 5 GHz, and the ring conductor  221   n  has a characteristic impedance of 50Ω. For the convenience of explanation, it is assumed that the position of grounding of the resonator is changed instead of selecting the switch to be turned on. The positions of the switches are shown by θ 1  to θ 3  in the drawing. The impedance transforming parts  230   0,1  to  230   3,4  are transmission lines, which have a characteristic impedance of 50Ω and a length equal to a quarter of the wavelength at 5 GHz. At this time, the admittance parameter of the impedance transforming parts  230   0,1  to  230   3,4  is 0.02 S. In addition, since the port impedance is 50Ω, the port admittance is 0.02 S. 
     Next, there will be specifically described a way of changing the positions θ 1  to θ 3  of the switches when the characteristics to be achieved of the signal selecting device  200  is changed. For example, there will be considered three cases where the characteristics to be achieved of the signal selecting device  200  are Butterworth characteristics with a fractional bandwidth of 3%, Butterworth characteristics with a fractional bandwidth of 5%, and Chebyshev characteristics (with a ripple of 0.1 dB) with a fractional bandwidth of 3%. In any of the cases, the center frequency is supposed to be 5 GHz. 
     First, two cases where the signal selecting device has Butterworth characteristics will be considered. In the case of the Butterworth characteristics, the element values g 0  to g 4  of the original low pass filters of the three resonating part  220   1  to  220   3  are 1, 1, 2, 1 and 1, respectively. For the cases where the fractional bandwidth is 0.03 (3%) and 0.05 (5%), the susceptance slope parameters b 1  to b 3  are determined using the formulas (4) to (6). Then, in the case where the fractional bandwidth is 3%, b 1 =0.67, b 2 =1.33, and b 3 =0.67. In the case where the fractional bandwidth is 5%, b 1 =0.4, b 2 =0.8, and b 3 =0.4. Then, the grounding positions θ 1  to θ 3  that provide these values are determined. The susceptance slope parameters b 1  to b 3  and the grounding positions θ 1  to θ 3  are shown by the formula (2) and in  FIG. 3 . The grounding positions θ 1  to θ 3  determined using  FIG. 3  are about 18 degrees, 13 degrees and 18 degrees, respectively, in the case where the fractional bandwidth is 3%, and about 23 degrees, 16 degrees and 23 degrees, respectively, in the case where the fractional bandwidth is 5%. 
     Next, the case where the signal selecting device has Chebyshev characteristics, and the fractional bandwidth to be achieved is 3% will be considered. In the case of the Chebyshev characteristics with a ripple of 0.1 dB, the element values g 0  to g 4  of the original low pass filters of the three resonating part  220   1  to  220   3  are 1, 1.0315, 1.1474, 1.0315 and 1, respectively. Based on the fractional bandwidth of 0.03 (3%), the susceptance slope parameters b 1  to b 3  are determined using the formulas (4) to (6). Then, b 1 =0.69, b 2 =0.76, and b 3 =0.69. From  FIG. 3 , the grounding positions θ 1  to θ 3  that provide these susceptance slope parameters determined from  FIG. 3  are about 17 degrees, 17 degrees and 17 degrees, respectively. 
       FIG. 7  shows frequency characteristics of the signal selecting device  200  grounded at the positions determined as described above. In this way, switching among the Butterworth characteristics with the fractional bandwidth of 3%, the Butterworth characteristics with the fractional bandwidth of 5% and the Chebyshev characteristics (with a ripple of 0.1 dB) with the fractional bandwidth of 3% can be achieved by changing the grounding positions. That is, it can be seen that the in-band and out-band characteristics can be changed by selecting the switch to be turned on. The grounding positions can also be determined in an analytical manner instead of using a graph as in this embodiment. 
     Embodiment 3 
     In the embodiment 2, all the impedance transforming parts have the same, fixed characteristics. If such identical impedance transforming parts are used in this way, the signal selecting device can be easily designed and fabricated. However, the impedance transforming parts do not always have to have the same characteristics but can have different characteristics or variable characteristics.  FIG. 8  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 3. A signal selecting device  300  has two input/output ports  311  and  312 , N resonating parts  320   1  to  320   N , N+1 impedance transforming parts  330   0,1  to  230   N,N+1  capable of changing the characteristics, and a controlling part  340 . While all the impedance transforming parts  330   0,1  to  330   N,N+1  are shown as being capable of changing the characteristics in  FIG. 8 , only one particular impedance transforming part may be capable of changing the characteristics. The resonating part  320   n  has a ring conductor  321   n  having a length equal to one wavelength at a resonant frequency or an integral multiple thereof, and M switches  322   n - 1  to  322   n -M each of which is connected to a different part of the ring conductor  321   n  at one end thereof and to a ground conductor at the other end thereof. The controlling part  340  controls the state of the N*M switches  322   1 - 1  to  322   N -M and the characteristics of the impedance transforming parts  330   0,1  to  330   N,N+1 . The resonating parts  320   1  to  320   N  are disposed in series between the two input/output ports. The impedance transforming parts  330   0,1  to  330   N,N+1  are disposed between the input/output ports in such a manner that the impedance transforming parts  130   0,1  and  130   N,N+1  at the both ends are disposed between the input/output port and the resonating part and the remaining impedance transforming parts  130   1,2  to  130   N−1,N  are disposed between the resonating parts. The configuration shown in  FIG. 8  has a high design flexibility and facilitate achievement of desired filter characteristics. In the two examples described below, the impedance transforming parts  330   0,1  to  330   N,N+1  (J-inverters) need to have variable characteristics. 
     One example is a case where an even number of resonating parts are used. In this specification, a signal selecting device using four resonating parts and five impedance transforming parts will be described.  FIG. 9  shows a section of the signal selecting device  300  shown in  FIG. 8  having four resonating parts and five impedance transforming parts that includes the resonating parts and the impedance transforming parts. For example, the signal selecting device  300  having four resonating parts  320   1  to  320   4  is designed to have Chebyshev characteristics with a center frequency of 5 GHz, a fractional bandwidth of 5% and a ripple of 0.1 dB. The element value g 0  to g 5  of the original low pass filters are 1, 1.1088, 1.3061, 1.77.3, 0.8180 and 1.3554, respectively. The fractional bandwidth is 0.05. In the embodiment 2, each susceptance slope parameter b n  is determined on the assumption that the admittance parameter is 0.02 S because all the impedance transforming parts (J-inverters) are quarter-wave transmission lines having a characteristic impedance of 50Ω. However, in the case of the signal selecting device having four resonating parts, the solutions that satisfy the formulas (4) to (6) cannot be found if the same admittance parameter is substituted in the formulas. This is because the element values g n  of the original low pass filters are not symmetrical if there are an even number of stages of components having Chebyshev characteristics. In other words, the sequence of the element values g n  of the original low pass filters viewed from the leading end differs from the sequence of the same element values g n  viewed from the trailing end. Thus, in order to satisfy all the relationships expressed by the formulas (4) to (6), the admittance parameter of at least one impedance transforming part has to be different from that of the other impedance transforming parts. In the case of the Butterworth characteristics, the sequence of the element values of the original low pass filters is always symmetrical, and therefore, all the impedance transforming parts can have the same admittance parameter. 
     That is, in order for the signal selecting device having an even number of resonating parts to switch between the Chebyshev characteristics and Butterworth characteristics, at least one impedance transforming part has to be variable. Any of the impedance transforming parts can be variable. However, the central impedance transforming part is preferably variable because the central impedance transforming part can change the filter characteristics widely. The reason for this will be described in detail with reference to  FIG. 9 . First, in the case where the impedance transforming part  330   4,5  closest to the input/output port is variable, to achieve Chebyshev characteristics with a fractional bandwidth of 5% and a ripple of 0.1 dB, the admittance parameter is 0.017, and the susceptance slope parameters b 1  to b 4  are 0.444, 0.522, 0.708 and 0.327, respectively. Next, in the case where the impedance transforming part  330   3,4  next closest to the input/output port is variable, the admittance parameter is 0.023, and the susceptance slope parameters b 1  to b 4  are 0.444, 0.522, 0.708 and 0.443, respectively. In the case where the central impedance transforming part  330   2,3  is variable, the admittance parameter is 0.017, and the susceptance slope parameters b 1  to b 4  are 0.444, 0.522, 0.522 and 0.443, respectively. As can be seen, the susceptance slope parameters b 1  to b 4  in the case where the central impedance transforming part  330   2,3  is variable are less variable than the susceptance slope parameters b 1  to b 4  in the cases where the impedance transforming part  330   4,5  is variable and where the impedance transforming part  330   3,4  is variable. The susceptance slope parameters b 1  to b 4  of the resonating parts  320   1  to  320   4  vary with the grounding position and reach a maximum value when θ is 90 degrees. However, the value depends on the characteristic impedances of the ring-shaped lines forming the respective resonating part, and therefore, if the resonating part is formed by a line having a fixed characteristic impedance, the maximum value is set during design and cannot be changed. As the variation of the susceptance slope parameters b 1  to b 4  becomes smaller, the range to which the resonating parts can be applied becomes wider. Thus, when the central impedance transforming part  330   2,3  is variable, the range of the filter characteristics variation is widest. 
     As described above, in addition to achieving the same effect as a signal selecting device according to the embodiment 1, the signal selecting device according to the embodiment 3 can increase the design flexibility and enable switching between Chebyshev characteristics and Butterworth characteristics in case of the signal selecting device having an even number of resonating parts. 
     Embodiment 4 
     In the embodiment 3, one of the cases where the impedance transforming parts need to have variable characteristics has been described. In this embodiment 4, the other of the cases will be described.  FIG. 10  is a diagram showing an exemplary functional configuration of a signal selecting device according to the embodiment 4. A signal selecting device  400  has two input/output ports  411  and  412 , N resonating parts  420   1  to  420   N , N+1 impedance transforming parts  430   0,1  to  430   N,N+1  capable of changing the characteristics, and a controlling part  440 . The resonating part  420   n  has a ring conductor  421   n  having a length equal to one wavelength at a resonant frequency or an integral multiple thereof, M switches  422   n - 1  to  422   n -M each of which is connected to a different part of the ring conductor  421   n  at one end thereof and to a ground conductor at the other end thereof, and three variable reactance means  423   n - 1  to  423   n - 3  connected to the ring conductor  421   n  at regular intervals. The controlling part  440  controls the state of the N*M switches  422   1 - 1  to  422   N -M, the characteristics of the impedance transforming parts  430   0,1  to  430   N,N+1  and the characteristics of the variable reactance means  423   1 - 1  to  423   N - 3 . The resonating parts  420   1  to  420   N  are disposed in series between the two input/output ports. The impedance transforming parts  430   0,1  to  430   N,N+1  are disposed between the input/output ports in such a manner that the impedance transforming parts  430   0,1  and  430   N,N+1  at the both ends are disposed between the input/output port and the resonating part and the remaining impedance transforming parts  430   1,2  to  430   N−1,N  are disposed between the resonating parts. In this embodiment, if the ring conductors  421   n  have the same characteristic impedance, the signal selecting device can be easily designed. 
     The resonating part  420   n  of the signal selecting device  400  has three variable reactance means  423   n - 1  to  423   n - 3  connected to the ring conductor  421   n  at regular intervals. Therefore, the signal selecting device  400  can change the resonant frequency and the zero point highly independently. To change the resonant frequency, the impedance has to be appropriately changed at the respective resonant frequencies, so that the impedance transforming parts  430   0,1  to  430   N,N+1  also have to be variable. 
     As described above, since each resonating part has the variable reactance means connected to the ring conductor at appropriate intervals, the center frequency can be changed highly independently of the bandwidth and the in-band and out-band characteristics. Furthermore, the variable impedance transforming circuits allows appropriate adjustment of the bandwidth and the in-band and out-band characteristics. 
     While the signal selecting device has been described as having three variable reactance means in this embodiment, the same effect can be achieved if the signal selecting device has four or more variable reactance means. 
       FIG. 11  shows a modified configuration of the signal selecting device shown in  FIG. 10  in which the variable reactance means are not disposed at regular intervals. With the configuration shown in  FIG. 11 , the center frequency, the bandwidth and the in-band and out-band frequency characteristics can be changed by appropriately designing the positions of the variable reactance means and the reactances thereof. For example, in the case of a signal selecting device  400 ′, the reactance of the variable reactance means  423   n - 2  can be set at a half the value of the variable reactance means  423   n - 1  and  423   n - 3 . In this way, even if the arrangement of the variable reactance means changes, the same effect as that of the signal selecting device  400  can be achieved. In addition, the number of the variable reactance means of the signal selecting device  400 ′ is not limited to three, and the same effect can be achieved if the signal selecting device  400 ′ have four or more variable reactance means. 
     Embodiment 5 
       FIG. 12  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 5. A signal selecting device  500  has the configuration of the signal selecting device  100  according to the embodiment 1 additionally provided with N−1 branch parts and a switch part. Specifically, a signal selecting device  500  has two input/output ports  511  and  512 , N resonating parts  120   1  to  120   N , N+1 impedance transforming parts  130   0,1  to  130   N,N+1 , a controlling part  540 , N−1 branch parts  530   1,2  to  530   N−1, N , and a switch part  550 . The branch part  530   n,n+1  has three terminals and switches the state of connection between a predetermined terminal (one terminal) and the remaining terminals (two terminals). The switch part  550  has N+1 terminals and switches the state of connection between a predetermined terminal (one terminal) and the remaining terminals (N terminals). The predetermined terminal of the switch part  550  is connected to the input/output port  512 , and one of the remaining terminals is connected to the impedance transforming part  130   N,N+1  (or, in other words, disposed between the input/output port  512  and the impedance transforming part  130   N,N+1 ). The predetermined terminal of the branch part  530   n,n+1  is connected to the impedance transforming part  130   n,n+1  (on the side of the input/output port  511 ), and one of the remaining terminals is connected to the resonating part  120   n+1  (or, in other words, disposed between the impedance transforming part  130   n,n+1  and the resonating part  120   n+1 ). The other of the remaining terminals of the branch part  530   n,n+1  is connected to one of the remaining terminals of the switch part  550 . The controlling part  540  controls the state of the N*M switches  122   1 - 1  to  122   N -M, the state of connection of the branch parts  530   1,2  to  530   N−1,N  and the state of connection of the switch part  550 . 
     For example, in the case where all the branch parts  530   n,n+1  connect the impedance transforming parts  130   n,n+1  to the resonating parts  120   n+1 , and the switch part  550  connects the impedance transforming part  130   N,N+1  to the input/output port  512 , the signal selecting device  500  functions as a signal selecting device having N resonators. In the case where one branch part  530   n,n+1  connects the impedance transforming part  130   n,n+1  to the switch part  550 , and the switch part  550  connects the impedance transforming part  130   n,n+1  to the input/output port  512 , the signal selecting device  500  functions as a signal selecting device having n resonators. That is, the number of resonators can be changed by controlling which branch part  530   n,n+1  is connected to the switch part  550 . Therefore, the bandwidth and the in-band and out-band frequency characteristics can be more flexibly adjusted. 
     Embodiment 6 
       FIG. 13  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 6. A signal selecting device  600  has the configuration of the signal selecting device  300  according to the embodiment 3 additionally provided with N−1 branch parts  630   1,2  to  630   N−1,N  and a switch part  650 . The way of connection between the branch parts  630   1,2  to  630   N−1,N  and the switch part  650 , the way of control, and the effects are the same as those in the embodiment 5. 
     Embodiment 7 
       FIG. 14  is a diagram showing an exemplary functional configuration of a signal selecting device according to an embodiment 7. A signal selecting device  700  has the configuration of the signal selecting device  400  according to the embodiment 4 additionally provided with N−1 branch parts  730   1,2  to  730   N−1,N  and a switch part  750 . The way of connection between the branch parts  730   1,2  to  730   N−1,N  and the switch part  750 , the way of control, and the effects are the same as those in the embodiment 5. 
     Embodiment 8 
     In the embodiments 1 to 7, the ring conductors are connected in parallel to the signal line. In an embodiment 8, the ring conductors are connected in series to the signal line.  FIG. 15  is a diagram showing an exemplary functional configuration of a signal selecting device according to this embodiment. A signal selecting device  800  has the same configuration as the signal selecting device  100  according to the embodiment 1 except that the resonating parts  120   1  to  120   N  are replaced with resonating parts  820   1  to  820   N . The resonating part  820   n  has a ring conductor  821   n  having a length equal to one wavelength at a resonant frequency or an integral multiple thereof and M switches  822   n - 1  to  822   n -M each of which is connected to a different part of the ring conductor  821   n  at one end thereof and to a ground conductor at the other end thereof. Two signal lines in the resonating part  820   n  are connected to the ring conductor  821   n  at positions spaced apart by a distance equal to an integral multiple of a half of the wavelength at the resonant frequency. That is, the two signal lines are connected to the ring conductor  821   n  at positions spaced apart by an integral multiple of π in terms of electrical length. A switch  822   n -m is not limited to a switch capable of simply making a short circuit but can be a switch capable of making a short circuit via a transmission line having a certain line length or a switch capable of establishing a connection of a transmission line having an open end. 
     If θ is set at 0, and the part having the impedance Z L  is a signal line in  FIG. 2 , the resulting resonating part is equivalent to the resonating part  820   n . With reference to  FIG. 2 , it has been described that, when θ=0, the impedance Z L  at the resonant frequency of the resonator  120   n  is equal to the input impedance Z in . This means that if the part having the impedance Z L  is not a short circuit but a signal line, a signal is transmitted at the resonant frequency, and a filter function (a signal selecting function) is provided. In the case where the ring conductors  821   n  are connected in series to each other, the paths in which all the switches  822   n -m are in the OFF state have a length equal to an integral multiple of a half of the wavelength at the resonant frequency and, therefore, do not affect the frequency characteristics of the respective resonating parts  820   n . Therefore, only the paths that include a switch  822   n -m in the ON state affect the frequency characteristics of the respective resonating parts  820   n . The frequency characteristics of the resonating part  820   n  differs from the frequency characteristics of the resonating part  120   n  in this regard. 
     As described above, in the signal selecting device  800 , the resonating parts having a ring conductor and switches can arbitrarily change the susceptance slope parameter highly independently of the resonant frequency, as with the signal selecting device  100  according to the embodiment 1. Therefore, the signal selecting device can be easily designed to have desired characteristics. In addition, the bandwidth and the in-band and out-band characteristics can also changed by changing the susceptance slope parameter of the resonating parts. In practice, in the case where the ring conductors are connected in series, the resonating parts are typically designed using a reactance slope parameter (a parameter in a one-to-one relationship with the susceptance slope parameter). 
     The signal selecting device  800  shown in  FIG. 15  has the configuration of the signal selecting device  100  according to the embodiment 1 in which the resonating parts  120   1  to  120   N  are replaced with the resonating parts  820   1  to  820   N . However, the resonating parts of the signal selecting devices  200 ,  300 ,  400 ,  400 ′,  500 ,  600  and  700  according to the embodiments 2 to 7 can also be replaced with the resonating parts  820   1  to  820   N . In those cases, the same effect can be achieved. 
     Specific Examples of Components 
     Finally, circuits or elements that can be used to form the components shown in the embodiments 1 to 8 will be described. 
     As shown in  FIGS. 16A to 16E , the impedance transforming part used in the signal selecting devices according to the present invention can be: 
     a transmission line having a characteristic impedance of Z and a length equal to a quarter wavelength at the resonant frequency ( FIG. 16A ); 
     a capacitor ( FIG. 16B ); 
     a coil ( FIG. 16C ); 
     lines coupled by electromagnetic induction ( FIG. 16D ); or 
     combinations thereof ( FIG. 16E ). As shown in  FIGS. 17A to 17F , the variable impedance transforming circuit can be: 
     a transmission line having a characteristic impedance of Z and a length equal to a quarter wavelength at the resonant frequency to which variable capacitors are connected in parallel with each other ( FIG. 17A ); 
     a variable capacitor ( FIG. 17B ); 
     a variable coil ( FIG. 17C ); 
     lines variably electromagnetically coupled to each other ( FIG. 17D ); 
     two kinds of transmission lines that have a length equal to a quarter wavelength at the resonant frequency and different characteristic impedances and are switched from one to another ( FIG. 17E ); or 
     two kinds of transmission lines that have a length equal to a quarter wavelength at different resonant frequencies and the same characteristic impedance and are switched from one to another ( FIG. 17F ). However, the present invention is not limited to the circuit examples listed above. Furthermore, the resonating part used in the signal selecting device according to the present invention has been described as a circular-ring-shaped line, the resonating part is not limited to the circular-ring-shaped line but can have any ring shape other than a circular ring. 
       FIGS. 18A to 18C  show exemplary configurations of the switch connected to the ring conductor. For example, the switch can be: 
     a switch that makes a short circuit ( FIG. 18A ); 
     a switch that makes a short circuit via a transmission line ( FIG. 18B ); or 
     a switch establishes a connection of a transmission line having an open end ( FIG. 18C ). Different types of switches can be used, or switches having transmission lines of different lengths can be used. Alternatively, a switch having a transmission line whose length can be changed can be used. Furthermore, a switch that establishes a connection to a capacitor or a coil can also be used. 
       FIGS. 19A to 19C  show exemplary functional configurations of the controlling part.  FIG. 19A  shows an exemplary functional configuration of the controlling parts  140 ,  240  and  840  according to the embodiments 1, 2 and 8, respectively. A decoder  141 ,  241 ,  841  serves to perform switching among a plurality of preset states. When a signal indicating a state is input to the decoder  141 ,  241 ,  841 , the decoder instructs switch controlling means  142 ,  242 ,  842  to select and turn on a switch corresponding to the state. The switch controlling means  142 ,  242 ,  842  controls the state of the switches of the resonating parts  120   1  to  120   N ,  220   1  to  220   3 ,  820   1  to  820   N  according to the instruction.  FIG. 19B  shows an exemplary functional configuration of the controlling part  340  according to the embodiment 3. A decoder  341  controls the characteristics of the impedance transforming parts in addition to serving the same function as the decoder  141 ,  241 ,  841 . The decoder  341  issues an instruction to impedance transforming part controlling means  343  according to an input signal. The impedance transforming part controlling means  343  changes the characteristics of the impedance transforming parts  330   0,1  to  330   N,N+1  according to the instruction.  FIG. 19C  shows an exemplary functional configuration of the controlling part  440  according to the embodiment 4. A decoder  441  controls the characteristics of the variable reactance means in addition to serving the same function as the decoder  341 . The decoder  441  issues an instruction to variable reactance means controlling means  444  according to an input signal. The variable reactance means controlling means  444  changes the characteristics of the reactance variable means according to the instruction. The dotted lines in  FIGS. 19A to 19C  represent branch part controlling means  548 ,  648 ,  748  and switch part controlling means  549 ,  649 ,  749 , which are added to the controlling part in the case where the signal selecting device has the branch parts and the switch part as shown in the embodiments 5 to 7. In this case, the controlling part also controls the branch parts and the switch part. Therefore, the decoder  141 ,  241 ,  341 ,  441 ,  841  also issues an instruction to the branch part controlling means  548 ,  648 ,  748  and the switch part controlling means  549 ,  649 ,  749  according to the input signal. The branch part controlling means  548 ,  648 ,  748  and the switch part controlling means  549 ,  649 ,  749  change the state of connection between the branch parts and the switch part according to the instruction. 
       FIGS. 20A to 20C  show other exemplary functional configurations of the controlling part.  FIG. 20A  shows an exemplary functional configuration of the controlling parts  140  and  240  according to the embodiments 1 and 2, respectively. Processing means  145 ,  245  receives the bandwidth w and the in-band and out-band characteristics (whether the characteristics is Butterworth characteristics or not, whether the characteristics is Chebyshev characteristics or not, what decibel the ripple is in the case of Chebyshev characteristics, or the like) as an input signal. The processing means  145 ,  245  determines which switch is to be turned on based on the input signal and issues an instruction to switch controlling means  146 ,  246 . The switch controlling means  146 ,  246  controls the state of the switches of the resonating parts  120   1  to  120   N ,  220   1  to  220   3  according to the instruction.  FIG. 20B  shows an exemplary functional configuration of the controlling part  340  according to the embodiment 3. Processing means  345  controls the characteristics of the impedance transforming parts in addition to serving the same function as the processing means  145 ,  245 . The processing means  345  determines the way of changing the characteristics of the impedance transforming parts based on the input signal and issues an instruction to impedance transforming part controlling means  347 . The impedance transforming part controlling means  347  changes the characteristics of the impedance transforming parts  330   0,1  to  330   N,N+1  according to the instruction.  FIG. 20C  shows an exemplary functional configuration of the controlling part  440  according to the embodiment 4. Processing means  445  controls the characteristics of the variable reactance means in addition to serving the same function as the processing means  345 . An input signal to the processing means  445  includes information about the center frequency. The processing means  445  determines the way of changing the characteristics of the variable reactance means based on the input signal and issues an instruction to variable reactance means controlling means  448 . The variable reactance means controlling means  448  changes the characteristics of the reactance variable means according to the instruction. The dotted lines in  FIGS. 20A to 20C  represent the branch part controlling means  548 ,  648 ,  748  and the switch part controlling means  549 ,  649 ,  749 , which are added to the controlling part in the case where the signal selecting device has the branch parts and the switch part as shown in the embodiments 5 to 7. The processing means  145 ,  245 ,  345 ,  445 ,  845  also issues an instruction to the branch part controlling means  548 ,  648 ,  748  and the switch part controlling means  549 ,  649 ,  749  according to the input signal. The branch part controlling means  548 ,  648 ,  748  and the switch part controlling means  549 ,  649 ,  749  change the state of connection between the branch parts and the switch part according to the instruction. 
       FIGS. 21A and 21B  show exemplary functional configurations of the processing means.  FIG. 21A  shows an example of the processing means composed of a calculation unit, a storage unit and a control unit. A calculation unit  1451  determines the susceptance slope parameter according to the formulas (4) to (6) using information, such as the bandwidth and the in-band and out-band characteristics. Then, the calculation unit  1451  determines θ from the susceptance slope parameter. Furthermore, the calculation unit  1451  selects a switch closest to the determined θ based on switch position information or the like stored in a storage unit  1452  and instructs a control unit  1453  to turn on the selected switch. According to the instruction, the control unit  1453  controls the switch controlling means, the impedance transforming part controlling means, the variable reactance means controlling means, the branch part controlling means and the switch part controlling means.  FIG. 21B  shows an example of the processing means composed of a retrieval unit, a storage unit and a control unit. In this case, a storage unit  1455  stores a lookup table, for example. A retrieval unit  1454  retrieves a condition closest to the condition indicated by an input signal from the lookup table and obtains information about the current state of the switch, the impedance transforming part, the variable reactance means, the branch part controlling means and the switch part controlling means. Then, the retrieval unit  1454  issues an instruction to a control unit  1456 . Alternatively, the examples shown in  FIGS. 21A and 21B  can be combined to each other. For example, if the condition indicated by the input signal is found in the lookup table, the condition can be used, and if the condition indicated by the input signal is not found in the lookup table, calculation can be performed. 
     As the impedance transforming part controlling means that controls the impedance transforming parts capable of changing the characteristics, circuits described below can be used. In the case where the impedance transforming parts change the characteristic impedance in a discrete manner (a case where a plurality of switches are used to control the characteristics, for example), a digital variable impedance transforming circuit controlling circuit can be used as the impedance transforming part controlling means. In the case where the impedance transforming part change the characteristic impedance in a continuous manner (a case where a varactor using a diode is used, for example), a variable impedance transforming circuit controlling circuit, such as a D/A converter, can be used as the impedance transforming part controlling means. The same holds true for the variable reactance means controlling means.