Abstract:
A millimeter-band switching circuit easily corrects a varied off capacitance of a switching element without shifting position of the switching element. The millimeter-band switching circuit includes: a coupling line having a line length that can be changed; a first input and output terminal; a second input and output terminal; a first transmission line connected between the first input and output terminal and a first end of the coupling line; a second transmission line connected between the input and output terminal and a second end of the coupling line; a first field effect transistor (FET) connected in parallel with the first transmission line; and a second FET connected in parallel with the second transmission line and turned ON/OFF simultaneously with turning ON/OFF of the first FET.

Description:
BACKGROUND OF-THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a millimeter-band switching circuit which operates in a millimeter-band. 
         [0003]    2. Description of the Related Art 
         [0004]    In a general millimeter-band switching circuit, switching elements are connected in parallel with a signal line to reduce an insertion loss. In order to ensure isolation, two or more switching elements are connected with one another through a transmission line. 
         [0005]    A known millimeter-band single-pole single-throw (SPST) switching circuit will be described with reference to  FIGS. 7 and 8 .  FIG. 7  is an equivalent circuit diagram showing a structure of the known millimeter-band SPST switching circuit (see, for example, JP 09-093001 A).  FIG. 8  shows an insertion loss and an isolation characteristic of the known millimeter-band SPST switching circuit. The characteristics shown in  FIG. 8  are obtained by simulation. 
         [0006]    The known millimeter-band SPST switching circuit shown in  FIG. 7  includes a transmission line L 2  connected with a signal input and output terminal P 1 , a transmission line L 2  connected with a signal input and output terminal P 2 , a transmission line L 4  connected between the two transmission lines L 2 , a field effect transistor (FET) T whose gate is connected with a control voltage application terminal V 1  through a bias resistor R, whose drain is connected with the transmission lines L 2  and L 4 , and whose source is grounded, and a field effect transistor (FET) T whose gate is connected with the control voltage application terminal V 1  through a bias resistor R, whose drain is connected with the transmission lines L 2  and L 4 , and whose source is grounded. Note that the transmission lines expressed by the same reference symbol have the same characteristic impedance. 
         [0007]    When an FET parameter is changed in the known millimeter-band SPST switching circuit, it is necessary to redesign a line length of the transmission line L 4  located between the two FETs.  FIG. 8  shows a characteristic variation example in a case of a 77 GHz-band switching circuit. In  FIG. 8 , Loss (dotted line) indicates an insertion loss before variation and ISO (dotted line) indicates an isolation characteristic before variation. When an off capacitance of an FET is increased by, for example, 20%, the insertion loss becomes Loss′ (alternate long and short dash line) shown in  FIG. 8  and the isolation becomes ISO′ (alternate long and short dash line) shown therein. Therefore, there is a problem that the characteristics are shifted to a low-frequency side. In order to solve such a problem, it is necessary to shorten the line length of the transmission line L 4 , thereby shifting the positions of the FETs. 
         [0008]    A known millimeter-band single-pole double-throw (SPDT) switching circuit will be described with reference to  FIG. 9  (see, for example, JP 2003-179402 A).  FIG. 9  is an equivalent circuit diagram showing a structure of the known millimeter-band SPDT switching circuit. 
         [0009]    The known millimeter-band SPDT switching circuit shown in  FIG. 9  includes a transmission line L 2  connected with a signal input and output terminal P 1 , a transmission line L 2  connected with a signal input and output terminal P 2 , an inductor LL connected between the input and output terminal P 1  and a control voltage application terminal V 1 , an inductor LL connected between the input and output terminal P 2  and a control voltage application terminal V 2 , a transmission line L 1 ′ connected with one of the transmission lines L 2 , a transmission line L 1 ′ connected with the other of the transmission lines L 2 , two capacitors C 1 , each of which is connected between corresponding one of the transmission lines L 1 ′ and a branch point, and a transmission line L 5  connected between a signal input and output terminal P 0  and the branch point. The switching circuit further includes a first diode D whose anode is connected with the transmission lines L 2  and L 1 ′ and cathode is grounded, a second diode D whose anode is connected with transmission line L 1 ′ and a capacitor C 1  and cathode is grounded, a third diode D whose anode is connected with the transmission lines L 2  and L 1 ′ and cathode is grounded, and a fourth diode D whose anode is connected with transmission line L 1 ′ and a capacitor C 1  and cathode is grounded. 
         [0010]    When the diodes Dare used as the switching elements, a control voltage is applied thereto through the inductor LL for cutting off an RF signal. However, it is necessary to provide the capacitor C 1  for cutting off the voltage in the branch point. Therefore, there is a problem that the insertion loss is increased by the capacitor C 1 . 
         [0011]    A millimeter-band switching circuit in which a coupling line is provided between two switching elements has been proposed (see, for example, JP2003-224404 A). In the conventional millimeter-band switching circuit, two structures in each of which the coupling line is provided between the switching elements are symmetrically arranged and a capacitor or an inductor is connected between the structures. 
         [0012]    As described above, in the conventional millimeter-band SPST switching circuit, when the off capacitance of the FET (switching element) increases, there is a problem that the insertion loss and the isolation characteristic are shifted to the low-frequency side. In order to solve such a problem, it is necessary to shorten the line length of the transmission line L 4  between the two FETs, which leads to another problem in that the FETs need to be shifted in position. 
         [0013]    As described above, in the known millimeter-band SPDT switching circuit, when the diodes D are used as the switching elements, it is necessary to provide the capacitor C 1  for cutting off the voltage in the branch point. Therefore, there is a problem that the insertion loss is increased by the capacitor C 1 . 
         [0014]    The conventional millimeter-band switching circuit using the coupling line has a problem that the number of parts is large, thereby increasing cost. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention has been made to solve one of the above-mentioned problems and an object of the present invention is to obtain a millimeter-band switching circuit capable of easily correcting a varied off capacitance of a switching element without shifting a position of the switching element. 
         [0016]    The present invention has been made to solve another one of the above-mentioned problems and another object of the present invention is to obtain a millimeter-band switching circuit capable of suppressing an increase in insertion loss without providing a capacitor in a branch point. 
         [0017]    Further, the present invention has been made to solve another one of the above-mentioned problems and another object of the present invention is to obtain a millimeter-band switching circuit in which the number of parts can be decreased to reduce cost. 
         [0018]    A millimeter-band switching circuit of the present invention includes: a coupling line whose line length can be changed; a first input and output terminal; a second input and output terminal; a first transmission line connected between the first input and output terminal and a first end of the coupling line; a second transmission line connected between the second input and output terminal and a second end of the coupling line; a first switching element connected in parallel with the first transmission line; and a second switching element which is connected in parallel with the second transmission line and turned ON/OFF simultaneously with turning ON/OFF the first switching element. 
         [0019]    According to the present invention, the millimeter-band switching circuit has an effect that the varied off capacitance of the switching element can be easily corrected without shifting the position of the switching element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In the accompanying drawings: 
           [0021]      FIG. 1  is an equivalent circuit diagram showing a structure of a millimeter-band SPST switching circuit according to Embodiment 1 of the present invention; 
           [0022]      FIG. 2  shows an insertion loss and an isolation characteristic of the millimeter-band SPST switching circuit according to Embodiment 1 of the present invention; 
           [0023]      FIG. 3  is an equivalent circuit diagram showing a structure of a millimeter-band SPDT switching circuit according to Embodiment 2 of the present invention; 
           [0024]      FIG. 4  is an equivalent circuit diagram showing a structure of a millimeter-band switching circuit according to Embodiment 3 of the present invention; 
           [0025]      FIG. 5  is an equivalent circuit diagram showing a structure of a millimeter-band SPDT switching circuit according to Embodiment 4 of the present invention; 
           [0026]      FIG. 6  is an equivalent circuit diagram showing a structure of a millimeter-band SPDT switching circuit according to Embodiment 5 of the present invention; 
           [0027]      FIG. 7  is an equivalent circuit diagram showing a structure of a known millimeter-band SPST switching circuit; 
           [0028]      FIG. 8  shows an insertion loss and an isolation characteristic of the known millimeter-band SPST switching circuit; and 
           [0029]      FIG. 9  is an equivalent circuit diagram showing a structure of a known millimeter-band SPDT switching circuit. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       [0030]    A millimeter-band SPST switching circuit according to Embodiment 1 of the present invention will be described with reference to  FIGS. 1 and 2 .  FIG. 1  is an equivalent circuit diagram showing a structure of the millimeter-band SPST switching circuit according to Embodiment 1 of the present invention. Hereinafter, in each of the drawings, the same reference symbols indicate the same or corresponding portions. 
         [0031]    The millimeter-band SPST switching circuit according to Embodiment 1 of the present invention as shown in  FIG. 1  includes a transmission line (first transmission line) L 2  connected with a signal input and output terminal P 1 , a transmission line (second transmission line) L 2  connected with a signal input and output terminal P 2 , a coupling line L 1  which is connected between the two transmission lines L 2  and has a length L, a field effect transistor (FET) (first switching element) T whose gate is connected with a control voltage application terminal V 1  through a bias resistor R, whose drain is connected with the transmission line L 2  and the coupling line L, and whose source is grounded, and a field effect transistor (FET) (second switching element) T whose gate is connected with the control voltage application terminal V 1  through a bias resistor R, whose drain is connected with the coupling line L 1  and the transmission line L 2 , and whose source is grounded. 
         [0032]    The millimeter-band SPST switching circuit is manufactured using a monolithic microwave integrated circuit (MMIC). 
         [0033]    Next, the operation of the millimeter-band SPST switching circuit according to Embodiment 1 of the present invention will be described with reference to the drawings.  FIG. 2  shows an insertion loss and an isolation characteristic of the millimeter-band SPST switching circuit according to Embodiment 1 of the present invention. 
         [0034]    In  FIG. 2 , the abscissa indicates a frequency F (GHz) and the ordinate indicates an insertion loss and an isolation (dB). In addition, Loss (dotted line) indicates an insertion loss before variation and ISO (dotted line) indicates an isolation characteristic before variation. Further, Loss″ (solid line) indicates an insertion loss after variation and ISO″ (solid line) indicates an isolation characteristic after variation. 
         [0035]    When a control voltage is applied to the control voltage application terminal V 1 , the two FETs are simultaneously turned ON and become a high resistance. Therefore, for example, a millimeter-band signal inputted from the input and output terminal P 1  is outputted from the input and output terminal P 2  through the transmission line L 2 , the coupling lines L 1 , and the transmission line L 2 . In contrast to this, a millimeter-band signal inputted from the input and output terminal P 2  is outputted from the input and output terminal P 1 . This corresponds to an ON state of the millimeter-band SPST switching circuit. 
         [0036]    When the control voltage applied to the control voltage application terminal V 1  becomes an OFF state, the two FETs are simultaneously turned OFF and each become lower in resistance. Therefore, for example, the millimeter-band signal inputted from the input and output terminal P 1  flows from the FET (left side in  FIG. 1 ) to a ground terminal. The millimeter-band signal inputted from the input and output terminal P 2  flows from the FET (right side in  FIG. 1 ) to the ground terminal. This corresponds to an OFF state of the millimeter-band SPST switching circuit. 
         [0037]    As shown in  FIG. 1 , the coupling line L 1  has two lines and a length of an overlapped portion of the two lines is the length L. When the coupling line L 1  having the length L is connected between the two FETs and the length L of the coupling line L 1  is changed, the frequency characteristics of the switching element can be adjusted. At the time of manufacturing, one of the two lines of the coupling line L 1  is moved to change the length L of the overlapped portion of the two lines. 
         [0038]      FIG. 2  shows an effect obtained by the change of the length L. As described in the known example, in a case where the off capacitance of an FET is increased by, for example, 20%, when the line length L of the coupling line L 1  shown in  FIG. 1  is shortened, the frequency characteristics can be improved. In  FIG. 2 , Loss″ and ISO″ indicate an insertion loss and an isolation, respectively, which are improved by shortening the line length L of the coupling line L 1 . In this case, only the line length L of the coupling line L 1  is adjusted, with the result that the frequency characteristics can be adjusted without a change in FET position. 
       Embodiment 2 
       [0039]    A millimeter-band SPDT switching circuit according to Embodiment 2 of the present invention will be described with reference to  FIG. 3 .  FIG. 3  is an equivalent circuit diagram showing a structure of the millimeter-band SPDT switching circuit according to Embodiment 2 of the present invention. 
         [0040]    The millimeter-band SPDT switching circuit according to Embodiment 2 of the present invention as shown in  FIG. 3  includes a transmission line (first transmission line) L 5  connected between a signal input and output terminal (first input and output terminal) P 0  and a branch point PP, the coupling line (first coupling line) L 1  which has the length L and is located on the left side, the transmission line (second transmission line) L 2  connected with the signal input and output terminal (second input and output terminal) P 1 , a transmission line (third transmission line) L 3  which is connected with the branch point PP and located on the left side, the field effect transistor (FET) (first switching element) T whose gate is connected with the control voltage application terminal V 1  through the bias resistor R, whose drain is connected with the transmission line L 2  and the coupling line L 1 , and whose source is grounded, and the field effect transistor (FET) (second switching element) T whose gate is connected with the control voltage application terminal V 1  through the bias resistor R, whose drain is connected with the coupling line L 1  and the transmission line L 3 , and whose source is grounded. 
         [0041]    The millimeter-band SPDT switching circuit further includes a coupling line (second coupling line) L 1  which has the length L and is located on the right side, a transmission line (fourth transmission line) L 2  connected with the signal input and output terminal (third input and output terminal) P 2 , a transmission line (fifth transmission line) L 3  which is connected with the branch point PP and located on the right side, a field effect transistor (FET) (third switching element) T whose gate is connected with a control voltage application terminal V 2  through a bias resistor R, whose drain is connected with the transmission line L 2  and the coupling line L 1 , and whose source is grounded, and a field effect transistor (FET) (fourth switching element) T whose gate is connected with the control voltage application terminal V 2  through a bias resistor R, whose drain is connected with the coupling line L 1  and the transmission line L 3 , and whose source is grounded. 
         [0042]    The millimeter-band SPDT switching circuit is manufactured using a monolithic microwave integrated circuit (MMIC). 
         [0043]    Next, the operation of the millimeter-band SPDT switching circuit according to Embodiment 2 of the present invention will be described with reference to the drawing. 
         [0044]    For example, a control voltage is applied to the control voltage application terminal V 1  to simultaneously turn ON the two FETs located on the input and output terminal P 1  side. On the other hand, a control voltage applied to the control voltage application terminal V 2  is set to an OFF state to simultaneously turn OFF the two FETs located on the input and output terminal P 2  side. Then, a millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 1 . 
         [0045]    The control voltage is applied to the control voltage application terminal V 2  to simultaneously turn ON the two FETs located on the input and output terminal P 2  side. On the other hand, the control voltage applied to the control voltage application terminal V 1  is set to an OFF state to simultaneously turn OFF the two FETs located on the input and output terminal P 1  side. Then, the millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 2 . 
         [0046]    Even in a case of the two-branch switching circuit shown in  FIG. 3 , when not the FET positions but the lengths L of the two coupling lines L 1  are changed, the frequency characteristics can be adjusted. 
       Embodiment 3 
       [0047]    A millimeter-band switching circuit according to Embodiment 3 of the present invention will be described with reference to  FIG. 4 .  FIG. 4  is an equivalent circuit diagram showing a structure of the millimeter-band switching circuit according to Embodiment 3 of the present invention. 
         [0048]    The millimeter-band switching circuit according to Embodiment 3 of the present invention as shown in  FIG. 4  includes the transmission line L 5  connected between the signal input and output terminal P 0  and the branch point PP, the first coupling line L 1  having the length L, the transmission line L 2  connected with the signal input and output terminal P 1 , the first transmission line L 3  connected with the branch point PP, the field effect transistor (FET) T whose gate is connected with the control voltage application terminal V 1  through the bias resistor R, whose drain is connected with the transmission line L 2  and the coupling line L 1 , and whose source is grounded, and the field effect transistor (FET) T whose gate is connected with the control voltage application terminal V 1  through the bias resistor R, whose drain is connected with the coupling line L 1  and the transmission line L 3 , and whose source is grounded. 
         [0049]    The millimeter-band switching circuit further includes the second coupling line L 1  having the length L, the transmission line L 2  connected with the signal input and output terminal P 2 , the second transmission line L 3  connected with the branch point PP, the field effect transistor (FET) T whose gate is connected with the control voltage application terminal V 2  through the bias resistor R, whose drain is connected with the transmission line L 2  and the coupling line L 1 , and whose source is grounded, and the field effect transistor (FET) T whose gate is connected with the control voltage application terminal V 2  through the bias resistor R, whose drain is connected with the coupling line L 1  and the transmission line L 3 , and whose source is grounded. 
         [0050]    The millimeter-band switching circuit further includes a third coupling line (third coupling line) L 1  having the length L, a transmission line (sixth transmission line) L 2  connected with a signal input and output terminal (fourth input and output terminal) P 3 , a third transmission line (seventh transmission line) L 3  connected with the branch point PP, a field effect transistor (FET) (fifth switching element) T whose gate is connected with a control voltage application terminal V 3  through a bias resistor R, whose drain is connected with the transmission line L 2  and the coupling line L 1 , and whose source is grounded, and a field effect transistor (FET) (sixth switching element) T whose gate is connected with the control voltage application terminal V 3  through a bias resistor R, whose drain is connected with the coupling line L 1  and the transmission line L 3 , and whose source is grounded. 
         [0051]    The millimeter-band switching circuit further includes an nth coupling line L 1  having the length L, a transmission line L 2  connected with a signal input and output terminal Pn, an nth transmission line L 3  connected with the branch point PP, a field effect transistor (FET) T whose gate is connected with a control voltage application terminal Vn through a bias resistor R, whose drain is connected with the transmission line L 2  and the coupling line L 1 , and whose source is grounded, and a field effect transistor (FET) T whose gate is connected with the control voltage application terminal Vn through a bias resistor R, whose drain is connected with the coupling line L 1  and the transmission line L 3 , and whose source is grounded. 
         [0052]    The millimeter-band switching circuit is manufactured using a monolithic microwave integrated circuit (MMIC). 
         [0053]    Next, the operation of the millimeter-band switching circuit according to Embodiment 3 of the present invention will be described with reference to the drawing. 
         [0054]    For example, a control voltage is applied to the control voltage application terminal V 1  to simultaneously turn ON the two FETs located on the input and output terminal P 1  side. On the other hand, each of control voltages applied to the other control voltage application terminals V 2 , V 3 , . . . , Vn is set to an OFF state to simultaneously turn OFF all the FETs located on the sides of the input and output terminals P 2 , P 3 , . . . , Pn. Then, a millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 1 . 
         [0055]    A control voltage is applied to the control voltage application terminal V 2  to simultaneously turn ON the two FETs located on the input and output terminal P 2  side. On the other hand, each of control voltages applied to the other control voltage application terminals V 1 , V 3 , . . . , Vn is set to an OFF state to simultaneously turn OFF all the FETs located on the sides of the input and output terminals P 1 , P 3 , . . . , Pn. Then, the millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 2 . 
         [0056]    A control voltage is applied to the control voltage application terminal V 3  to simultaneously turn ON the two FETs located on the input and output terminal P 3  side. On the other hand, each of control voltages applied to the other control voltage application terminals V 1 , V 2 , . . . , Vn is set to an OFF state to simultaneously turn OFF all the FETs located on the sides of the input and output terminals P 1 , P 2 , . . . , Pn. Then, the millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 3 . 
         [0057]    A control voltage is applied to the control voltage application terminal Vn to simultaneously turn ON the two FETs located on the input and output terminal Pn side. On the other hand, each of control voltages applied to the other control voltage application terminals V 1 , V 2 , V 3 , . . . , Vn is set to an OFF state to simultaneously turn OFF all the FETs located on the sides of the input and output terminals P 1 , P 2 , P 3 , . . . , Pn. Then, the millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal Pn. 
         [0058]    Even in the case of the n-branch switching circuit shown in  FIG. 4 , an effect of the frequency characteristic adjustment is identical to that obtained in each of the embodiments. 
       Embodiment 4 
       [0059]    A millimeter-band SPDT switching circuit according to Embodiment 4 of the present invention will be described with reference to  FIG. 5 .  FIG. 5  is an equivalent circuit diagram showing a structure of the millimeter-band SPDT switching circuit according to Embodiment 4 of the present invention. 
         [0060]    The millimeter-band SPDT switching circuit according to Embodiment 4 of the present invention as shown in  FIG. 5  includes the transmission line L 5  connected between the signal input and output terminal P 0  and the branch point PP, a coupling line L 3 ′ which has the length L, is connected with the branch point PP, and is located on the left side, the transmission line L 2  connected with the signal input and output terminal P 1 , a transmission line L 1 ′ which is connected between the transmission line L 2  and the coupling line L 3 ′ and located on the left side, the switching element T such as the field effect transistor (FET) whose gate is connected with the control voltage application terminal V 1  through the bias resistor R, whose drain is connected with the transmission lines L 2  and L 1 ′, and whose source is grounded, and the switching element T such as the field effect transistor (FET) whose gate is connected with the control voltage application terminal V 1  through the bias resistor R, whose drain is connected with the transmission line L 1 ′ and the coupling line L 3 ′, and whose source is grounded. 
         [0061]    The millimeter-band SPDT switching circuit further includes a coupling line L 3 ′ which has the length L, is connected with the branch point PP, and is located on the right side, a transmission line L 2  connected with the signal input and output terminal P 2 , a transmission line L 1 ′ which is connected between the transmission line L 2  and the coupling line L 3 ′ and located on the right side, the switching element T such as the field effect transistor (FET) whose gate is connected with the control voltage application terminal V 2  through the bias resistor R, whose drain is connected with the transmission lines L 2  and L 1 ′, and whose source is grounded, and the switching element T such as the field effect transistor (FET) whose gate is connected with the control voltage application terminal V 2  through the bias resistor R, whose drain is connected with the transmission line L 1 ′ and the coupling line L 3 ′, and whose source is grounded. 
         [0062]    The millimeter-band SPDT switching circuit is manufactured using a monolithic microwave integrated circuit (MMIC). 
         [0063]    Next, the operation of the millimeter-band SPDT switching circuit according to Embodiment 4 of the present invention will be described with reference to the drawing. 
         [0064]    For example, a control voltage is applied to the control voltage application terminal V 1  to simultaneously turn ON the two FETs located on the input and output terminal P 1  side. On the other hand, a control voltage applied to the control voltage application terminal V 2  is set to an OFF state to simultaneously turn OFF the two FETs located on the input and output terminal P 2  side. Then, a millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 1 . 
         [0065]    The control voltage is applied to the control voltage application terminal V 2  to simultaneously turn ON the two FETs located on the input and output terminal P 2  side. On the other hand, the control voltage applied to the control voltage application terminal V 1  is set to an OFF state to simultaneously turn OFF the two FETs located on the input and output terminal P 1  side. Then, the millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 2 . 
         [0066]      FIG. 5  shows a modified example of the switching circuit according to Embodiment 2 as shown in  FIG. 3 . Even in a case where the coupling lines L 3 ′ are used as the transmission lines connected with the branch point for the two-branch switching circuit, when the lengths L of the coupling lines L 3 ′ are changed according to variations in FET characteristics, there is the same effect of frequency characteristic adjustment. 
       Embodiment 5 
       [0067]    A millimeter-band SPDT switching circuit according to Embodiment 5 of the present invention will be described with reference to  FIG. 6 .  FIG. 6  is an equivalent circuit diagram showing a structure of the millimeter-band SPDT switching circuit according to Embodiment 5 of the present invention. 
         [0068]    The millimeter-band SPDT switching circuit according to Embodiment 5 of the present invention as shown in  FIG. 6  includes the transmission line L 5  connected between the signal input and output terminal P 0  and the branch point, the coupling line L 3 ′ which has the length L, is connected with the branch point, and is located on the left side, the transmission line L 2  connected with the signal input and output terminal P 1 , an inductor LL connected between the input and output terminal P 1  and the control voltage application terminal V 1 , the transmission line L 1 ′ which is connected between the transmission line L 2  and the coupling line L 3 ′ and located on the left side, a first diode (first switching element) D whose anode is connected with the transmission lines L 2  and L 1 ′ and cathode is grounded, and a second diode (second switching element) D whose anode is connected with transmission line L 1 ′ and the coupling line L 3 ′ and cathode is grounded. 
         [0069]    The millimeter-band SPDT switching circuit further includes the coupling line L 3 ′ which has the length L, is connected with the branch point, and is located on the right side, the transmission line L 2  connected with the signal input and output terminal P 2 , an inductor LL connected between the input and output terminal P 2  and the control voltage application terminal V 2 , the transmission line L 1 ′ which is connected between the transmission line L 2  and the coupling line L 3 ′ and located on the right side, a third diode (third switching element) D whose anode is connected with the transmission lines L 2  and L 1 ′ and cathode is grounded, and a fourth diode (fourth switching element) D whose anode is connected with transmission line L 1 ′ and the coupling line L 3 ′ and cathode is grounded. 
         [0070]    The millimeter-band SPDT switching circuit is manufactured using a monolithic microwave integrated circuit (MMIC). 
         [0071]    Next, the operation of the millimeter-band SPDT switching circuit according to Embodiment 5 of the present invention will be described with reference to the drawing. 
         [0072]    For example, a control voltage is applied to the control voltage application terminal V 1  to simultaneously turn ON the two diodes D located on the input and output terminal P 1  side. On the other hand, a control voltage applied to the control voltage application terminal V 2  is set to an OFF state to simultaneously turn OFF the two diodes D located on the input and output terminal P 2  side. Then, a millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 1 . 
         [0073]    The control voltage is applied to the control voltage application terminal V 2  to simultaneously turn ON the two diodes D located on the input and output terminal P 2  side. On the other hand, the control voltage applied to the control voltage application terminal V 1  is set to an OFF state to simultaneously turn OFF the two diodes D located on the input and output terminal P 1  side. Then, the millimeter-band signal inputted from the input and output terminal P 0  is outputted from the input and output terminal P 2 . 
         [0074]      FIG. 6  shows the two-branch switching circuit using the diodes D instead of the FETs of the millimeter-band SPDT switching circuit shown in  FIG. 5 . An effect obtained in Embodiment 5 is identical to that obtained in Embodiment 4. That is, when not the positions of the diodes D but the lengths L of the coupling lines L 3 ′ are changed, the frequency characteristics can be adjusted. It is unnecessary to provide capacitors in the branch point, so an increase in insertion loss which is caused by the capacitors can be suppressed.