Abstract:
Microwave semiconductor variable attenuation circuit includes a coupler, two series circuits of inductors and a plurality of means having controllable resistance. Each Inductor of the series circuits has a defined inductance, where one end of the series circuit is connected to a terminal of the coupler. Each of means having controllable resistance respectively being connected to a junction point between two respective inductors of the series circuit. All of means having controllable resistance are controlled on the basis of a common signal is provided at a single terminal.  
     By the above features provided that inductors of defined inductance between the coupler and the means having controllable resistance (e.g. a transistor or diode), the total impedance of the attenuation circuit can be properly set and a performance degradation at high frequency can be avoided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the variable attenuation circuit used for microwave communication device, for example. Microwave communication device is controlling high frequency characteristics, such as a power gain of apparatus and an output power level by using the variable attenuation circuit. As an attenuation circuit, a variable resistor network linked to the T type or I type is constituted, and a diode or a field effect transistor is used as a variable resistor.  
           [0002]    However, it is necessary for realizing desired attenuation and desired impedance to decide each resistance of the variable resistor connected in series in parallel as any resistance according to attenuation. when above attenuation circuit is used, and the control circuit setting up resistor of a variable resistance becomes complicated.  
           [0003]    Therefore, a circuit using the directivity coupler shown in FIG. 1 or FIG. 2 is used much as a circuit of microwave. Referring now to the attenuation circuit shown in FIG. 1, a passage terminal  13 , and a coupling terminal  14  of a first directional coupler  10  connect to a coupling terminal  24 , and a passage terminal  23  of a second directional coupler  20  respectively. Field-Effect-Transistor (FET)  30   a ,  30   b  are connected in parallel between the passage terminal  13  and the coupling terminal  24 , and the coupling terminal  14  and the passage terminal  23  respectively. An input terminal  11  of the first directional coupler  10 , and an input terminal  21  of the second directional coupler  20  are a signal input, and a signal output of this attenuation circuit respectively. And isolation terminals  12  and  22  of respective directional couplers  10  and  20  terminate in termination resistors  15  and  25  respectively. Signals inputted from the input terminal  11  as signal input are distributed to the passage terminal  13  and coupling terminal  14  by first directional coupler  10 . After passing through the parallel circuit of the FETs  30   a  and  30   b , the distributed signals are inputted into the coupling terminal  24  and the passage terminal  23  of the second directional coupler  20  respectively, is compounded, and is outputted from the input terminal  21  of the second directional coupler  20  as signal output. FET  30   a  and  30   b  that voltage between drain  31   a ,  31   b  and source  32   a ,  32   b  is 0 [V] are used as variable resistor by gate bias provided for gate  33   a ,  33   b  through resistor  16   a ,  16   b  from control terminal  41 . The power absorbed by FET  30   a  and  30   b  is changed, passage loss is controlled according to changing the resistance of FET  30   a  and  30   b  compared with the characteristic impedance (for example, 50 [Ω]) of a directional coupler, as a result, the variable attenuation circuit is realized with it.  
           [0004]    Moreover, Since the reflective power produced by the mismatching with the impedance of FET and the characteristic impedance of a directional coupler is absorbed by the terminus resistance  15  connected to the isolation terminal  12 , it can realize a matching state without returning to the input terminal  11 .  
           [0005]    Next referring now to the attenuation circuit shown in FIG. 2, this circuit uses the mismatching with impedance of the directional coupler  10  and FET  30   a  or  30   b . And as a result, a variable attenuation circuit is realized by compounding the reflected signal, making it output from the isolation terminal  12 , changing the impedance of FET  30   a  and  30   b , and controlling reflection. Therefore, in the variable attenuation circuit using the directional coupler, the matching state is realized by using only gate bias of FET connected in parallel as control voltage, and using the character of a directional coupler. In the circuit shown in FIG. 1 as mentioned above, in order to obtain the desired attenuation, the resistance of variable resistor, such as FET, is changed.  
           [0006]    However, since reactance component of the impedance by influence of the parasitic capacity of FET or a parasitic inductance becomes large According frequency becomes high, even if gate bias changes, it is not able to change enough in the impedance of FET. Referring now to FIG. 3, FIG. 3 is a passage characteristic diagram of the variable attenuator in consideration of parasitic capacity of FET. FIG. 3 shows the passage characteristic of the case, for example, in the composition of FIG. 1, used High-Electron-Mobility-Transistor (HEMT) that gate length is 0.3 micrometers and gate width is 300 micrometers. Moreover, it uses four fingers Lange couplers of main frequency 25 GHz as directional coupler. There is a problem that the variable range becomes small remarkably, in high frequency domain, as passage loss becomes large.  
           [0007]    Accordingly, it is an object of the present invention to provide a microwave variable attenuation circuit, in the variable attenuation circuit using a coupler, preventing increase of the passage loss and decrease of the variable attenuation by the parasitic capacity of a variable resistor, and having good transmission characteristic. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a circuit diagram of a variable attenuator according to the prior art:  
         [0009]    [0009]FIG. 2 is a circuit diagram of a variable attenuator according to the prior art; and  
         [0010]    [0010]FIG. 3 is a passage characteristic diagram of a variable attenuator according to the prior art.  
         [0011]    [0011]FIG. 4 is a circuit diagram of first embodiment of a variable attenuator according to the present invention;  
         [0012]    [0012]FIG. 5 is an equivalent circuit diagram of a Field-Effect-Transistor;  
         [0013]    [0013]FIG. 6 is a passage characteristic diagram of variable attenuator according to the present invention;  
         [0014]    [0014]FIG. 7 is a circuit diagram of second embodiment of a variable attenuator according to the present invention;  
         [0015]    [0015]FIG. 8 is a circuit diagram of third embodiment of a variable attenuator according to the present invention;  
         [0016]    [0016]FIG. 9 is a circuit diagram of fourth embodiment of a variable attenuator according to the present invention;  
         [0017]    [0017]FIG. 10 is a circuit diagram of fifth embodiment of a variable attenuator according to the present invention;  
         [0018]    [0018]FIG. 11 is a monolithic microwave integrated circuit diagram of fifth embodiment of a variable attenuator according to the present invention;  
         [0019]    [0019]FIG. 12 is a circuit diagram of sixth embodiment of a variable attenuator according to the present invention;  
         [0020]    [0020]FIG. 13 is a circuit diagram of seventh embodiment of a variable attenuator according to the present invention;  
         [0021]    [0021]FIG. 14 is a circuit diagram of eighth embodiment of a variable attenuator according to the present invention; 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    This invention will be described in further detail with reference to the accompanying drawings.  
         [0023]    (First Embodiment)  
         [0024]    Referring now to FIG. 4, there is shown a variable attenuator circuit using Field-Effect-Transistors (referred to as FET below). Inductors  51   a ,  51   b ,  52   a , and  52   b  are respectively equal inductance, the inductors  51   a  and  52   a  are connected in series to a passage terminal  13  of first directional coupler  10  (for example, 3 dB directional coupler) and a coupling terminals  24  of second directional coupler  20 , and the inductors  51   b  and  52   b  are connected in series to a coupling terminal  14  of first directional coupler  10  and a passage terminal  23  of second directional coupler  20 . Drain terminals  31   a  and  31   b  of FET  30   a  and  30   b  are respectively connected to connection parts of the inductors  51   a  and  52   a  and inductors  51   b  and  52   b , gate terminals  33   a  and  33   b  connect to the control terminal  41  through resistors  16   a  and  16   b , and source terminals  32   a  and  32   b  are grounded. Moreover, in this circuit, an input terminal  11  of the first directional coupler  10  is as a signal input, an input terminal  21  of the second directional coupler  20  is as a signal output, and isolation terminals  12  and  22  of directional couplers  10  and  20  terminate termination resistors  15  and  25  respectively.  
         [0025]    By the way, an equivalent circuit of FET  30  (Both  30   a  and  30   b ) in FIG. 5( a ) is shown a parallel circuit having a variable resistor  34  controlled by control voltage V C  and a parasitic capacitor (capacity C) in FIG. 5( b ). In present invention, when the characteristic impedance of directional couplers  10  and  20  is Z 0 , the inductance C of the inductors  51   a ,  52   a ,  51   b , and  52   b  is defined by equation 1. 
         Z 0   2 =2L/C  (1) 
         [0026]    A signal inputted from the signal input (the input terminal  11 ) is distributed by the first directional coupler  10  to the passage terminal  13  and the coupling terminal  14 , distributed signals are passing through the parallel circuits of FET  30   a  and  30   b  through the inductors  51   a ,  52   a  and inductors  51   b ,  52   b , the distributed signals are inputted into the coupling terminal  24  and the passage terminal  23  of the second directional coupler  20  respectively, and a signal is outputted from the signal output (the input terminal  21 ). The power absorbed by the FETs  30   a  and  30   b  is changed, passage loss is controlled according to changing the resistance of the FETs  30   a  and  30   b  controlled by voltage impressed to the control terminal  41 , as a result, this variable attenuation circuit is realized with it. Moreover, since the reflective power produced by the mismatching with the impedance of FET  30  and the characteristic impedance of directional couplers  10 ,  20  is absorbed by the termination resistor  15  connected to the isolation terminal  12 , it can realize a matching state without returning to the input terminal  11 .  
         [0027]    Accordingly, since inductance L of the inductors  51   a ,  52   a ,  51   b , and  52   b  satisfy Formula 1, a parallel circuit can be regarded as a circuit comprising a transmission line with same characteristic impedance as the directional couplers  10 ,  20  and a variable resistor. Thus since the impedance by the influence of the parasitic capacity  35  of the FET  30  can be made small in high frequency domain, the variable range of attenuation increases as the passage loss decreases.  
         [0028]    Referring now to FIG. 6, there is shown a passage characteristic drawing of variable attenuator. Compared with FIG. 6 of the prior art, the passage loss is small, the variable range is large, and the frequency band which can be used as an attenuator becomes large.  
         [0029]    (Second Embodiment)  
         [0030]    Referring now to second embodiment of the present invention shown in FIG. 7, the same part as FIG. 7 is shown in the same mark.  
         [0031]    A coupling terminal  14  and a passage terminal  13  of a directional coupler  10  are connected to ends of in-series circuits which connected in-series to inductors  61   a  and  52   a , and inductors  51   b  and  52   b , respectively. Another ends of in-series circuits connect termination resistors  17   a  and  17   b , respectively. Inductance of the inductors  51   a  and  52   a , and inductors  61   b  and  52   b  is equal respectively, connection parts of the inductors  51   a  and  52   a , and inductors  51   b  and  52   b  are connected to drain terminals  31   a  and  31   b  of FET  30 . and source terminals  32   a  and  32   b  of FET  30  are grounded. A control terminal is connected to gate terminals  32   a  and  32   b  of FET  30  through resistors  16   a  and  16   b , and FET  30   a  and  30   b  are controlled by voltage impressed to the control terminal  41 . Accordingly, reflected signals are outputted from a isolation terminal  12  by using mismatching with impedance of the directional coupler  10  and FET  30 . and it is realized a variable attenuation circuit to control reflection by changing impedance of FET 30 .  
         [0032]    In above variable attenuation circuit, there is the same effect as description of first embodiment. Since inductors  51   a  and  52   a , and inductor  51   b  and  52   b  of second embodiment also satisfy Formula 1, the passage loss of a reflective signal decreases in a high frequency domain, the variable range of attenuation can enlarge, and the frequency range which can use as an attenuator becomes large.  
         [0033]    (Third and Fourth Embodiments)  
         [0034]    The present invention is embodied the variable attenuation circuit used the diode instead of FET, but the attenuation variable circuit used FET as a variable resistor is explained in above first and second embodiments.  
         [0035]    Referring now to FIG. 8 and FIG. 9 of third and fourth embodiments, diodes  60   a  and  60   b  are used instead of FET  30   a  and  30   b  of FIG. 4 or FIG. 7. Anode terminals of diodes  60   a  and  60   b  connect to connection parts of inductors  51   a  and  52   a , and inductor  51   b  and  52   b  respectively, and cathode terminals of  60   a  and  60   b  are grounded. One ends of choke coils  61   a  and  61   b  connect anode terminals of diodes  60   a  and  60   b  respectively, another ends connect control terminal  41 . Moreover, resistance of diodes  60   a  and  60   b  are controlled by voltage impressed to choke coil  61   a  and  61   b  (control terminal  41 ).  
         [0036]    Accordingly, since inductance L of inductor  51   a ,  52   a ,  51   b , and  52   b  and connecting capacity C p  of diode  60   a  and  60   b  are decided as satisfy Formula. 2, the passage loss of a reflective signal decreases in a high frequency domain, the variable range of attenuation can enlarge, and the frequency range which can use as an attenuator becomes large. 
         Z 0   2 =2L/C p   (2) 
         [0037]    (Fifth Embodiment)  
         [0038]    Referring now to FIG. 10 of fifth embodiment, ends of first and second ladder type circuits are connected to a coupling terminal  14  and a passage terminal  13  of the first directional coupler respectively, and another ends are connected a passage terminal  23  and coupling terminals  24  of the second directional coupler respectively. A First ladder type circuit is in-series circuit connected in-series to inductors  70   a - 7   na  (natural numbers of arbitrary n) of at least two or more, and drain terminals of FET  301   a - 30   na  are connected to each connecting point of inductors  70   a - 7   na  respectively. A Second ladder type circuit is also in-series circuit connected in-series to inductors  70   b - 7   nb  of at least two or more, and drain terminals of FET  301   b - 30   nb  are connected to each connecting point of inductors  70   b - 7   nb  respectively. All of source terminals of FET  301   a - 30   na  and FET  301   b - 30   nb  are grounded, and gate terminals of FET  301   a - 30   na  and FET  301   b - 30   nb  are connected to a control terminal  41  through resistor  161   a - 16   na  and resistor  161   b - 16   nb  respectively. When characteristic impedance of directional couplers  10  and  90  are Z 0  respectively and each parasitic capacity of FET  301   a - 30   na  and FET  301   b - 30   nb  is C, inductors  70   a ,  7   na,    70   b , and  7   nb  set as inductance L, and other inductors set as inductance 2L. These characteristic impedance Z 0 , parasitic capacity C, and inductance L, 2L satisfy Formula. 1. Moreover, referring now to FIG. 11 of fifth embodiment, there is shown a Monolithic Microwave Integrated Circuit (MMIC) formed on a half-insulation semiconductor board. MMIC in FIG. 11 shows the case where n is three, source terminals of FET  301   a  and  301   b , FET  302   a  and  302   b , and FET  303   a  and  303   b  are connected to common through holes  81 - 83  respectively, and the common through holes  81 - 83  are grounded. FET and through holes of MMIC are symmetrical with a line A-A′ passing through the through holes  81 - 83  by a and b side.  
         [0039]    Accordingly, in above variable attenuation circuit, the passage loss of a reflective signal decreases in a high frequency domain, the variable range of attenuation can be enlarged, and the frequency ranges which can use as an attenuator becomes large. Since number of components of MMIC can be reduced by communalizing through holes  81 - 83 , the size of MMIC can become small. And since FET of MMIC is set up symmetry, characteristic of the whole attenuator is improved.  
         [0040]    Moreover, since each FET can be set up small by using two or more FET, parasitic capacity and inductance of inductors become small. Therefore as the whole circuit, the variable range of attenuation can be enlarged according as the minimum insertion loss can become smaller.  
         [0041]    (Sixth Embodiment)  
         [0042]    Referring now to sixth embodiment of the present invention shown in FIG. 12, ends of first and second ladder type circuits are connected to a coupling terminal  14  and a passage terminal  13  of the first directional coupler respectively, and another ends are connected to termination resistors  17   a  and  17   b . The first ladder type circuit is an in-series circuit connected in-series to inductors  70   a - 7   na  of at least two or more, and drain terminals of FET  301   a - 30   na  are connected to each connecting point of inductors  70   a - 7   na  respectively. The second ladder type circuit is also an in-series circuit connected in-series to inductors  70   b - 7   nb  of at least two or more, and drain terminals of FET  301   b - 30   nb  are connected to each connecting point of inductors  70   b - 7   nb  respectively. All of source terminals of FET  301   a - 30   na  and FET  301   b - 30   nb  are grounded, and gate terminals of FET  301   a - 30   na  and FET  301   b - 30   nb  are connected to a control terminal  41  through resistor  161   a - 16   na  and resistor  161   b - 16   nb  respectively. When characteristic impedance of directional couplers  10  is Z 0  and each parasitic capacity of FET  301   a - 30   na  and FET  301   b - 30   nb  is C, inductors  70   a ,  7   na ,  70   b , and  7   nb  set as inductance L, and other inductors set as inductance 2L. These characteristic impedance Z 0 , parasitic capacity C, and inductance L, 2L satisfy Formula. 1.  
         [0043]    In above variable attenuation circuit, reflected signals are outputted from a isolation terminal  12  by using mismatching with impedance of the directional coupler  10  and FET  301   a - 30   na ,  301   b - 30   nb,  and it is realized a variable attenuation circuit to control reflection by changing impedance of FET  301   a - 30   na ,  301   b - 30   nb.    
         [0044]    Moreover, when the circuit of this embodiment forms as MMIC as shown in FIG. 11, the common through hole is not connected to only the gate terminal of FET but also the termination resistor. In this embodiment, FET and through holes of MMIC are also symmetrical with a passing line through the through holes.  
         [0045]    Accordingly, since number of components of MMIC can be reduced by communalizing through holes, the size of MMIC can become small. And since the FET of the MMIC is set up symmetry, characteristic of the whole attenuator is improved.  
         [0046]    Moreover, since each FET can be set up small by using two or more FET, parasitic capacity and inductance of inductors become small. Therefore for the whole circuit, the minimum insertion loss can become smaller, and the variable range of attenuation can be enlarged.  
         [0047]    (Seventh and Eighth Embodiments)  
         [0048]    Referring now to FIG. 13 and FIG. 14, seventh and eighth embodiments of the present invention use diodes  601   a - 60   na ,  601   b - 60   nb  instead of FET  301   a - 30   na ,  301   b - 30   nb  used fifth and sixth embodiments shown in FIG. 10 and FIG. 12. The resistance of the diodes is controlled by voltage impressed to Choke coils  61   a - 6   na ,  61   b - 6   nb  connected to anode terminals of the diode. When characteristic impedance of the directional couplers is Z 0  and each connection capacity of the diode is C p , inductors  70   a ,  7   na ,  70   b , and  7   nb  set as inductance L, and other inductors set as inductance 2L.  
         [0049]    In above embodiments like the fifth and sixth embodiments, the passage loss of a reflective signal decreases in a high frequency domain, the variable range of attenuation can be enlarged, and the frequency range that can use as an attenuator becomes large. When these embodiments form MMIC like FIG. 11, the size of MMIC can reduce by using common through holes. Since diodes of MMIC are set up symmetry, characteristic of the whole attenuator is improved.  
         [0050]    Accordingly, since each diode can be set up small by using two or more diodes, connection capacity of the diode and inductance of the inductor become small. Therefore as the whole circuit, the minimum insertion loss can be reduce, and the variable range of attenuation can be enlarged.