A Microwave or Millimetre Wave Circuit

Abstract: A microwave element in which the impedance control is performed by using the capacitance of a FET at off state, includes an impedance control circuit for controlling the capacitance between the source and the drain of the FET at off state which is constructed by a series connection circuit comprising a diode and a capacitance, connected in parallel therewith between a source and a drain of the FET, and means for applying variable bias voltage to said diode. A microwave switch is constructed by two FETs (6, 7), two impedance control circuits for controlling the capacitance of the FETs (6, 7) at off state, an input terminal (1), and two output terminals (2, 3). A microwave switched line phase shifter is constituted by two transmission lines, with different electrical lengths, four FETs, two of them are connected in series between the both ends of the two transmission lines, respectively, impedance control circuits for controlling the capacitances of the FETs, an input terminal, and an output terminal. A microwave loaded-line type phase-shifter is constituted by a main transmission line, two loaded lines having the same electrical length connected in parallel with each other, connected with the front and rear end of the main line, impedance control circuits for controlling the capacitance of the FETs at off state.

Description: FIELD OF THE INVENTION The present invention relates to microwave circuits, and more particularly to control circuits utilized in a switch or a phase shifter in microwave or millimeter wave band. BACKGROUND OF THE INVENTION Figure 4(a) shows a circuit construction of a prior art microwave switch as e.g. known from figure 2 of patent document FR-A-2607643. In figure 4(a), denoted at 1 is an RF input terminal. Denoted at 2 and 3 are a first and a second RF terminals, respectively. A first and a second FET 6, 7 are connected between the RF input terminal 1 and the RF output terminals 2, 3, respectively. Denoted at 4 are gate terminals of the FET 6 and the FET 7, respectively. Inductances 5 are connected in parallel with the FET 6 and the FET 7, respectively. Figure 4(b) shows an equivalent circuit of an FET in a state where the FET is off and figure 4(c) shows an equivalent circuit in a state where the FET is on. This switch operates as follows. In order to make an RF signal input from the input terminal 1 and output from the first output terminal 2, 0V is applied to the gate terminal 4 of the FET 6 thereby to turn on the FET 6. A voltage not exceeding the FET pinch-off voltage, for example, -5 V, is applied to the gate terminal 4 of the FET 7 thereby to turn off the FET 7. Then, the FET 6 and the FET 7 are thought to be equivalent to the circuits of figures 4(c) and 4(b), respectively. Since only a resistance is inserted between the terminals 1 and 2, the RF signal transmits from the terminal 1 to the terminal 2. On the other hand, at between the terminals 1 and 3, a resonance circuit comprising the inductance 5 connected with the FET 7 and the capacitance of the FET 7 resonates at a desired frequency, and the electrical state thereof becomes similar to a state where infinite impedance is inserted thereat, therefore the RF signal does not transmit from the terminal 1 to the terminal 3. In addition, when the voltages applied to the gate terminals 4 of FETs 6 and 7 are reversed, the RF signal transmits to the second output terminal 3. Figure 5 shows a prior art microwave switched- line type phase shifter. In figure 5, the same reference numerals denote the same elements as those shown in figure 4. Reference numerals 8 and 9 designate a third and a fourth FET, respectively. Denoted at 10 and 11 are a first and a second transmission line, respectively. This phase shifter operates as follows. In a case where the FET 6 and the FET 7 are turned on and the FET 8 and the FET 9 are turned off by controlling the voltages applied to the gate terminal 4, the RF signal which is input from the input terminal 1 is output to the output terminal 2 through the first transmission line 10. To the contrary, in a case where the FET 6 and the FET 7 are turned off, and the FET 8 and the FET 9 are turned on, the RF signal input from the input terminal 1 transmits through the second transmission line 11. In these cases, since the transmission lines 10 and 11 have a required length difference, the phase shift amount between the two states can be varied. Figure 6 shows a prior art loaded line type phase shifter. In figure 6, the same reference numerals denote the same elements as those shown in figures 4 and 5. Denoted at 12, is a third transmission line. This loaded line type phase shifter is constructed of a transmission line 12 of a quarter wavelength and two pairs of series connection of transmission line and FET. The parameters of the series connection pair of transmission line and FET (a pair of 10 and 6, and that of 11 and 7) are set such that the susceptance value viewed from the main line 12 is -jB in a state where the FETs 6 and 7 are on and that the susceptance value is + jB in a state where the FETs 6 and 7 are off. Then, the susceptance value B is related to the phase shift amount by, B = tan (A/2). A: required phase shift quantity Then, the length of the main line 12 is set to produce such a required difference between the phase of the input signal and that of the output signal of the main line 12. In the above-described impedance control circuit used in such as a microwave switch, a switched-line type phase shifter or a loaded-line type phase shifter, the control of the circuit is executed by using the capacitance of FET in its off state. However, this capacitance value of FET varies to a large extent due to variations in the FET fabrication process, thereby varying the circuit characteristics to a great extent. To be concrete, figures 7(b) and 7(c) show the transmission characteristics of the resonance circuit of figure 7(a) in case where the capacitance of the FET is 0.15 pF and 0.1 pF, respectively, with using the inductance 5 of 2 nH. As is apparent from these figures, when the capacitance of the FET 5 at, off state is 0.15 pF, the attenuation amount S₂₁ of the output terminal 2 viewed from the input terminal 1 takes the maximum at about 9 GHz, and when it is 0. 15 pF, the attenuation amount S₂₁ takes the maximum at about 11 GHz. Thus, the resonance frequency at which the impedance becomes infinite varies to a great extent due to the variations in the characteristics of the FET 5. SUMMARY OF THE INVENTION It is an object of the present invention to provide microwave circuits capable of controlling the capacitance of the FET at off state, and capable of obtaining constant uniform circuit characteristics independently of the variations in the FET characteristics. Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only. A microwave or millimetre-wave circuit in accordance with the invention is defined in claim 1 of the claims appended. In the preferred embodiment of this circuit a series connection of a diode and a capacitance is loaded in parallel to between the source and the drain of FET, and the bias voltage of the diode is controlled to adjust the capacitance between the source and the drain of FET at off state. Therefore, even when the capacitance of FET at off state varies due to the variations in the FET characteristics, the constant uniform circuit characteristics can be obtained, and the impedance control by FET can be performed at high precision. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1(a) is a diagram showing a circuit of a microwave switch according to a first embodiment of the present invention; Figures 1(b) and 1(c) are diagrams showing an equivalent circuit of a portion of the microwave switch when the FET is off and on, respectively; Figure 2 is a diagram showing a circuit construction of microwave switched- line type phase shifter according to a second embodiment of the present invention; Figure 3 is a diagram showing a circuit construction of microwave loaded-line type phase shifter according to a third embodiment of the present invention; Figure 4(a) is a diagram showing a circuit construction of a prior art microwave switch; Figures 4(b) and 4(c) are diagrams showing an equivalent circuit of the microwave switch of figure 4(a), when the FET is off and on, respectively; Figure 5 is a diagram showing a circuit construction of a prior art microwave switched-line type phase shifter; Figure 6 is a diagram showing a circuit construction of a prior art microwave loaded-line type phase shifter; and Figures 7(a) to 7(c) are diagrams for explaining variations in the characteristics of the prior art microwave switch. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings. Figure 1(a) is a diagram showing a circuit construction of a microwave switch according to a first embodiment of the present invention. In figure 1(a), denoted at 1 is an RF input terminal. Denoted at 2 and 3 are a first and a second RF output terminals, respectively. A first and a second FET 6 and 7 are connected between the RF input terminal 1 and the RF output terminals 2, 3, respectively. Denoted at 4 are gate terminals of the FETs 6 and 7. Inductances 5 are connected in parallel with the FETs 6 and 7, respectively. Serial connections of a capacitor 14 and a diode 15, 16 are connected in parallel with the FET 6 and FET 7, respectively. Figures 1(b) and 1(c) are diagrams showing an equivalent circuit of a portion of the microwave switch in a state where the FET is off and on, respectively. This microwave switch operates as follows. In order to make an RF signal input to the input terminal 1 and output from the output terminal 2, OV is applied to the gate terminal 4 of the FET 6 to turn on the FET 6. A voltage not exceeding an FET pinch-off voltage, for example, -5 V, is applied to the gate terminal 4 of the FET 7 thereby to turn off the FET 7. Then the FET 6 and FET 7 are thought to be equivalent to the circuits of figures 1(c) and 1(b), respectively. As in the prior art, since only a resistance is inserted between the terminals 1 and 2 in that state, the RF signal transmits from the input terminal 1 to the first output terminal 2. On the other hand, at the path between the input terminal 1 and the second output terminal 3, the inductance 5 connected with the FET 7 and the capacitance of the FET 7 resonate at a desired frequency, and the electrical state thereof becomes similar to a state where infinite impedance is inserted thereat. Therefore, the RF signal does not transmit from the input terminal 1 to the second output terminal 3. When the capacitance of the FET at off state is changed dependent upon the process variation, a voltage is applied to the diode-bias terminal 13 of the diode 16, to change the capacitance of the FET. For example, in figure 1(b), in a case where the capacitance of the FET at off state is 0.1 pF, the capacitance of the capacitor 5 pF, and the capacitance of the diode can vary from 0.05 pF to 0.,1 pF by adjusting the bias, a capacitance value from 0.15 pF to 0.198 pF is obtained by this entire control circuit. Therefore, by varying the bias voltage of the diode, the capacitance of the FET at off state can be made constant and an elements having a constant uniform characteristics can be obtained. In addition, when the voltages applied to the gate terminals 4 of the FETs 6 and 7 are reversed, the RF signal transmits to the second output terminal 3. In the above-described embodiment, the capacitance of the diodes 15 and 16 are adjusted so that the FETs 6 and 7 resonate with respective inductances 5 at the off states thereof, thereby to make the impedance viewed from the respective end of the FET infinite at a desired frequency, whereby a switch with constant uniform characteristics is obtained. From a different point of view, in this embodiment, there is an advantage in that the capacitance of the diode can be varied by adjusting the bias voltage of the diode and therefore in that microwave switches with the same circuit construction can be provided at different frequency bands. While in the above-described embodiment, an impedance control circuit is used for a microwave switch, this can be used for a switched-line type phase shifter. Figure 2 is a diagram showing a circuit construction of a microwave switched- line type phase shifter according to a second embodiment of the present invention. In figure 2, the same reference numerals denote the same elements as those shown in figure 1. Denoted at 8 and 9 are a third and a fourth FET, respectively. Denoted at 10 and 11 are a first and a second transmission line, respectively. Denoted at 17 and 18 are a third and a fourth diode, respectively. This phase shifter operates as follows. In a state where the FETs 6 and 7 are on and the FETs 8 and 9 are off, the RF signal is input from the input terminal 1 and is output to the first output terminal 2 through the first transmission line 10. To the contrary, in a state where the FETs 6 and 7 are off and the FETs 8 and 9 are on, the RF signal transmits through the transmission line 11. In these cases, since the transmission lines 10 and 11 have a required length difference, the phase shift amount between the two states can be varied. As is the case with the switch in the above-described embodiment, even when the capacitance of the FET at off state varies, the capacitance can be varied by applying a voltage to the diode bias terminal 13 of the respective diodes, thereby resulting in a uniform element characteristics. Figure 3 is a circuit construction of loaded-line type phase shifter according to a third embodiment of the present invention. In figure 3, the same reference numerals, denote the same elements as those shown in figures 1 and 2. Denoted at 12 is a third transmission line. A loaded-line type phase shifter as this third embodiment operates in the same way as the prior art one. That is, the capacitance of the diode can be varied by varying the diode bias voltage similarly as in the switch of the first embodiment and the switched line type phase shifter of the second embodiment. Therefore, even when the element characteristics of the FET is different dependent upon the production process, the capacitance between the source and drain of the FET at off state is made uniform, and the element characteristics is greatly enhanced. As is evident from the foregoing description, the capacitance of an FET at off state is controlled by adjusting an impedance (e.g. a diode and series capacitance) loaded in parallel with the FET. Therefore, microwave elements with constant uniform characteristics can be obtained, and furthermore, a microwave element with the same circuit construction and a different operation frequency band can be obtained.

Claims: A microwave or millimeter-wave circuit, being either a switch or a phase shifter, which circuit includes a control circuit comprised of a field-effect transistor (6,7,8,9) switchable between a resistive ON state and a capacitive OFF state, and control means, connected to the gate (4) of said field-effect transistor (6,7,8,9), to apply thereto a control signal to switch said field-effect transistor (6,7,8,9) between said resistive ON state and said capacitive OFF state; which microwave or millimeter-wave circuit is characterised by: impedance means (13,14,15; 13,14,16; 13,14,17; 13,14,18) connected between the source and the drain of said field-effect transistor (6,7,8,9), which impedance means (13,14,15; 13,14,16; 13,14,17; 13,14,18) is adjustable to compensate for process variance in the OFF state capacitance of said field-effect transistor (6,7,8,9). A microwave or millimeter-wave circuit as claimed in claim 1 wherein: said impedance means (13,14,15; 13,14,16; 13,14,17; 13,14,18) comprises: a diode (15;16;17;18); a capacitor (14) connected in series with said diode (15;16;17;18);and bias means (13,-), connected to said diode (15;16; 17;18), to apply an adjustable bias voltage thereto, to adjust the capacitance of said diode (15;16;17;18). A microwave or millimeter-wave circuit as claimed in either preceding claim wherein: said control circuit includes an inductor (5) connected in parallel with said impedance means (13,14,15; 13,14,16; 13,14,17; 13,14,18) between the source and the drain of said field-effect transistor (6,7,8,9). A microwave or millimeter-wave circuit as claimed in claim 3 depending from claim 2, which circuit consists in a switch which is comprised of: first and second field-effect transistors (6,7), connected at a connection node, in series between first and second output terminals (2,3), each transistor being switchable between a resistive ON state and a capacitive OFF state; an input terminal (1) connected to said connection mode; first and second inductors (5) each connected between the source and the drain of respective first and second field effect transistors (6,7); first and second impedance means (13,14,15; 13,14,16) connected in parallel with said first and second inductors (5) respectively, each of which impedance means (13,14,15; 13,14,16) comprises: control means, connected to respective gates (4) of said first and second field-effect transistors (6,7), to apply respective control signals to switch said first and second field-effect transistors (6,7) to ON state and OFF state respectively, and conversely; and a respective diode (15;16); a respective capacitor (14) connected in series with said respective diode (15;16); and bias means (13,-) connected to said respective diode (15;16) to apply a respective adjustable bias voltage thereto, to adjust the respective capacitance of said respective diode (15;16). A microwave or millimeter-wave circuit as claimed in claim 3 depending from claim 2, which circuit consists in a switched-line type phase shifter which is comprised of: first and second transmission lines (10,11) of different electrical length; first and second field effect transistors (6,8) connected between a common input terminal (1) and one end of each respective first and second transmission line (10,11); third and fourth field effect transistors (7,9) connected between the other end of each respective first and second transmission line (10,11) and a common output terminal (2); first to fourth inductors (5) each connected between the source and the drain of respective first to fourth field effect transistors (6 to 9); control means, connected to respective gates (4) of said first to fourth field-effect transistors (6 to 9) to switch said first and third field-effect transistors (6,7) to ON state, and to switch said second and fourth field-effect transistors (8,9) to OFF state, and conversely; and first to fourth impedance means (13,14,15; 13,14,17; 13,14,16; 13,14,18) connected in parallel with said first to fourth inductors (5), respectively, each of which impedance means (13,14,15; 13,14,17; 13,14,16; 13,14,18) comprises: a respective diode (15,17,16,18); a respective capacitor (14) connected in series with said respective diode (15,17,16,18); and bias means (13,-) connected to said respective diode (15,17,16,18) to apply a respective adjustable bias voltage thereto, to adjust the respective capacitance of said respective diode (15;16). A microwave or millimeter-wave circuit, as claimed in claim 2, which circuit consists in a loaded-line type phase shifter which is comprised of: a transmission line (12); first and second loaded lines (10,11), each of the same electrical length, connected at one end each to respective ends of said transmission line (12); first and second field effect transistors (6,7) connected between ground and the other end of each respective first and second loaded line (10,11); gating means connected to respective gate (4) of said first and second field-effect transistors (6,7) to apply respective control signals to switch said first and second field-effect transistors (6,7) both to one state and conversely both to OFF state; and first and second impedance means (13,14,15; 13,14,16;) connected between the source and the drain of respective first and second field effect transistors (6,7), each of which impedance means (13,14,15; 13,14,16) comprises: a respective diode (15;16); a respective capacitor (14) connected in series with said respective diode (15;16) and bias means (13,-) connected to said respective diode (15;16) to apply a respective adjustable voltage thereto, to adjust the respective capacitance of said respective diode (15;16).

Inventors: SASAKI YOSHINOBU C O MITSUBISH, SASAKI, YOSHINOBU, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H03K17
IPC codes: H01P1, H03K17