Variable attenuator

A variable attenuator operable in a frequency band from at least 10 GHz is disclosed. The variable attenuator includes an input port; an output port; a first transmission line connecting the input port with the output port; an attenuating unit provided between the first transmission line and the ground; and a second transmission line. The attenuating unit includes at least one transistor with two current terminals coupled with the first transmission line and ground, respectively. The second transmission line is coupled between the two current terminals of the transistor. The second transmission line is operable as an inductor in the frequency band. A feature of the variable attenuator is that the transistor and the second transmission line cause a resonance frequency within the frequency band by a capacitor between the two current terminals and the inductance of the second transmission line.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims benefit of priority of Japanese Patent Application No. 2018-041658, filed on Mar. 8, 2018, the entire content of which is incorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a variable attenuator, in particular, the invention relates to a variable attenuator for radio frequency (RF) signals.

2. Background Arts

A Japanese Patent Application laid open No. jp2000-357927a has disclosed a linearizer configured by, what is called, a T-type attenuator. Specifically, the linearizer disclosed therein includes first and second resistors connected in series, a third resistor in one terminal thereof being connected with an intermediate terminal between the former two resistors to form a T-character, a field effect transistor (FET) connected with another terminal of the third resistor and operable as a variable resistor, and an inductor operable as a reactance element and provided between the FET and the ground. Another Japanese Patent Application laid open No. jp2005-159803a has disclosed an amplifier applicable to high frequencies. The high frequency amplifier disclosed therein includes a transistor that receives a high frequency signal in a control terminal thereof and outputs an amplified high frequency signal in one of the current terminals thereof. The high frequency amplifier further includes a variable attenuator provided between the control terminal of the transistor and an input terminal of the amplifier. The variable attenuator includes a capacitor and a switching transistor connected in series with the capacitor between a signal line and the ground. Turning on and off the switching transistor, the signal carried on the signal line may be attenuated.

A variable attenuator has been known in the field where the variable attenuator is configured with several transistors between a signal line and the ground, and the attenuation thereof may be adjusted by selecting the transistors to be turned on. Such an attenuator preferably shows no signal loss when all transistors are turned off. However, parasitic capacitance inherently attributed to a transistor, specifically, between two current terminals of the transistor, may form a leak pass from the signal line to the ground even when the transistor turns off, which makes hard to realize no-loss, or to reduce loss for the signal carried on the signal line.

SUMMARY OF INVENTION

An aspect of the present invention relates to a variable attenuator that is operable in a frequency band with a lowest range of at least 10 GHz. The variable attenuator comprises an input port, an output port, a first transmission line, an attenuating unit, and a second transmission line. The first transmission line connects the input port with the output port. The attenuating unit, which is provided between the first transmission line and the ground, and between the input port and the output port, includes at least one transistor having two current terminals coupled with the first transmission line and the ground, respectively. The second transmission line, which is connected between the two current terminals of the transistor, operates as an inductor. The transistor and the second transmission line cause a resonance frequency formed by a capacitor inherently attributed between the two current terminals of the transistor and the inductor attributed to the second transmission line A feature of the variable attenuator of the invention is that the resonance frequency is set within the frequency band.

DESCRIPTION OF EMBODIMENTS

Next, some embodiments according to the present invention will be described referring to drawings. The present invention, however, is not restricted to those embodiments, and has a scope defined in claims attached hereto and all changes and modifications of elements in the claims and equivalent thereto. Also, in the description of the drawings, numerals or symbols same with or similar to each other will refer to elements same with or similar to each other without duplicating explanations.

FIG. 1is a functional circuit diagram of a variable attenuator1according to an embodiment of the present invention. The variable attenuator1includes two hybrid couplers,2and3, an input port4a, an output port5a, two transmission lines,7and8, and two or more attenuating units,11to14and21to24, each including two or more transistors type of field effect transistor (FET), two transmission lines,19aand19b, and other two transmission lines,29aand29b.

The input port4areceives a radio frequency (RF) signal subject to the variable attenuator1, where the RF signal has a primary frequency component exceeding 10 GHz with a preset frequency bandwidth. For instance, the RF signal may have a lowest frequency of 10 to 44 GHz, while, a highest frequency of 11 to 45 GHz. Further specifically, the RF signal has a frequency range within, what is called, the K-band used in the communication satellite system with the lowest frequency of 18 GHz and the highest frequency of 23 GHz. The RF signal entering the input port4amay have power of 0.01 to 10 W.

The transmission lines,7and8, which are a pair of first transmission lines carrying the RF signal from the input port4ato the output port5a, may be micro-strip lines. Specifically, the first transmission lines,7and8, in respective one ends thereof are coupled with the input port4athrough the hybrid coupler2, while, in respective other ends thereof couple with the output port5athrough the other hybrid coupler3. One of the first transmission lines7includes some transmission elements, where three transmission elements,7ato7c, are provided in the first transmission line7, each of which have respective electrical lengths and connected in series. Similarly, the other first transmission line8includes three transmission elements,8ato8c, with respective electrical lengths and connected in series.

The hybrid coupler2, which has a type of 90° hybrid coupler with two input ports and two output ports, where one of the two input ports couples with the input port4aof the variable attenuator1and the other of the input ports is terminated with a resistive element with impedance of 50Ω, which is not illustrated in the figures; while, the two output ports of the hybrid coupler2couple with the first transmission lines,7and8. The hybrid coupler2may split the RF signal evenly to the two output ports thereof. Thus, the input RF signal may be evenly split into the respective first transmission lines,7and8, with power just half of the power of the input RF signal. The split RF signals output from the output ports of the hybrid coupler2have respective phases with a difference therebetween of 90°. The other input port may output an RF signal entering the two output ports of the hybrid coupler2that are reflected in the respective first transmission lines,7and8. The other input port terminated with the resistive element may reduce an RF signal outgoing the input port4afrom the hybrid coupler2.

The other hybrid coupler3, which is set between the transmission lines,7and8, and the output port5a, may also have a type of 90° coupler with two input ports coupling with the transmission lines,7and8, and two output ports, one of which couples with the output port5a, while, the other output port is terminated by a resistive element with impedance of 50Ω, which is not illustrated in the figures. The hybrid coupler3may combine RF signals each carried on the transmission lines,7and8, and having respective phases with a difference of 90°, and outputs a combined RF signal to the output port5a.

The attenuating units,11to14, are connected in parallel to each other between the transmission line7and the ground31. The attenuating unit11provided closest to the hybrid coupler2includes three transistors,11ato11c, that are connected in series between the transmission line7and the ground31. Specifically, the transistor11ain one of current terminals thereof, for instance, a drain thereof, couples with the transmission line7, the other of the current terminals, for instance, a source thereof, couples with one of the current terminals of the next transistor11b, the other of the current terminals of the transistor11bcouples with one of the current terminals of the next transistor11c, and the other of the current terminals of the transistor11cis grounded. The transistors,11ato11c, in respective control terminals, namely, respective gates thereof, are connected with a control port6through respective transmission lines,15ato15c. The transmission lines,15ato15c, provided between the control terminals of the transistors,11ato11c, and the common control port6may suppress or prevent the RF signal from leaking to the control port6, which may be replaced with resistors. The attenuating unit11including three transistors,11ato11c, may be set closest to the hybrid coupler2among the attenuating units,11to14, subject to the transmission line7.

The other attenuating units,12to14, include respective two transistors,12aand12b,13aand13b, and14aand14b. Thus, the attenuating units,12to14, include lesser count of transistors compared with the attenuating unit11provided closest to the hybrid coupler2, which means that the attenuating unit11possibly receives an RF signal with power greater than power which the rest attenuating units,12to14, possibly receive. The attenuating units,12to14, receive RF signals attenuated by attenuating units provided closer to the hybrid coupler2.

The attenuating unit14, which may be the last attenuating unit provided closest to the hybrid coupler3, includes two transistors,14aand14b, connected in series between the transmission line7and the ground. Specifically, the upper transistor in one of the current terminals (drain) thereof couples with the transmission line7, in another of the current terminals (source) thereof couples with one of the current terminal (drain) of the lower transistor, and in another of the current terminals (source) is grounded. The transistors,14aand14b, in respective control terminals (gate) are connected with the common control port6through respective transmission lines,18aand18b.

Two attenuating units,12and13, provided midway between the two attenuating units,11and14, interposing respective transmission elements,7ato7c, therebetween, have arrangements substantially same with that of the last attenuating unit14; that is, the attenuating units,12and13, each include two transistors,12aand12b, and13aand13b, connected in series between the transmission line7and the ground. The former attenuating unit12is provided in downstream the first attenuating unit11interposing the transmission element7a, the latter attenuating unit13locates in downstream the former attenuating unit12interposing the transmission element7b, and the last attenuating unit14is provided in downstream the latter attenuating unit13interposing the transmission element7c. The two transistors,12aand12b, and13aand13b, in respective control terminals (gate) are connected with the common control port6through respective transmission lines,16ato16b, and17aand17b.

Three transistors,11ato11c, in the first attenuating unit11in sizes thereof, namely, gate widths thereof are equal to or smaller than widths of the two transistors,12aand12b, in the second attenuating unit12which are equal to or smaller than gate widths of the two transistors,13aand13b, in the third attenuating unit13which are equal to or smaller than gate widths of the two transistors,14aand14b, in the last attenuating unit14. In an example, the three transistors,11ato11c, in the first attenuating unit11and the two transistors,12aand12bin the second attenuating unit12have the gate width of 200 μm; while, the two transistors,13aand13b, in the third attenuating unit13and the two transistors,14aand14b, in the last attenuating unit14have the gate width of 800 μm.

One of reasons why the transistors provided in downstream attenuating units have gate widths greater than the gate widths of the transistors in upstream attenuating units is to reduce resistance Rdsbetween the drain and the source of the transistors. That is, the transistors,11ato14b, in the respective attenuating units,11to14, are controlled by a control signal provided from the control port6common to the transistors,11ato14b; specifically, a gate bias intermediate between those to fully turn-off and to fully turn-on the transistors,11ato14b, are provided from the control port6. Specifically, the control port6provides a gate bias under an attenuating condition such that the gate bias turns on the transistor but does not fully turns on the transistor not to increase distortions; that is, the gate bias flows a drain current that is about a half of a maximum drain current when the transistor fully turns on. Accordingly, the transistors,11ato14b, leave substantial drain-source resistance even when the gate bias turns on the transistor. The transistors,13ato14b, in the downstream attenuating units,13and14, with greater gate widths may reduce the drain-source resistance when the transistors are moderately biased to flow the drain current that is about a half of the maximum drain current.

Two transmission lines,19aand19b, which are the second transmission lines in the present embodiment, may have a type of micro-strip line and behave as inductors at the frequency subject to the variable attenuator1. Widths and lengths of the transmission lines,19aand19b, may determine inductance thereof.

The transmission line19ais provided in the first attenuating unit11closest to the hybrid coupler2and in a topmost transistor11aclosest to the transmission line7. That is, the transmission line19ais provided in parallel to the transistor11a, namely, between the two current terminals of the transistor11a. The transmission line19ain the inductance L thereof may be determined such that the inductance thereof and capacitance Cdsinherently attributed to the transistor11abetween the two current terminals thereof cause a resonance frequency ωr1, exactly, ωr12·Cds·L=1, within the frequency band subject to the variable attenuator1.

The transmission line19bis provided in the attenuating unit14closest to the output port5aand the topmost transistor14aclosest to the transmission line7in the attenuating unit14. Specifically, the transmission line19bis provided in parallel to the transistor14a, namely, in parallel to the parasitic capacitance Cdsof the transistor14abetween two current terminals thereof. The inductance L attributed to the transmission line19band the capacitance Cdsof the transistor14amay satisfy the relation of ωr22·Cds·L=1, where ωr2is the resonance frequency that is set within the frequency band subject to the variable attenuator1to the present embodiment.

Two resonance frequencies, ωr1and ωr2, may be set within the frequency band subject to the variable attenuator1, or, preferably, at the highest frequency thereof or closer to the highest frequency within the band. Specifically, when the frequency band is 18 to 23 GHz, the resonance frequencies, ωr1and ωr2, may be around 22.9 GHz.

The attenuating units,21to24, which are provided between the transmission line8and the ground31, may have arrangements same with those of the aforementioned attenuating units,11to14. Specifically, the attenuating unit21provides three transistors21ato21c, configured in series between the transmission line8and the ground31such that the source electrodes are directly connected with the drain electrodes of the downward transistors. Also, three transistors,21ato21c, in respective gates thereof are connected to the common control port6through respective transmission lines,25ato25c. The attenuating unit21is provided closest to the input port4a, or the hybrid coupler2.

The second and third attenuating units,22and23, include respective two transistors,22aand22b, and23aand23b, connected in series between the transmission line8and the ground31. Thus, the second and third attenuating units,22and23, include a lesser count of transistors compared with the first attenuating unit21by the reason same with that for the attenuating units,11to14.

The last attenuating unit24also includes two transistors,24aand24b, connected in series between the transmission line8and the ground31, where the gate electrodes thereof are connected with the common control port6through the respective transmission lines,28aand28b. The last attenuating unit24is provided closest to the output port5a, or the hybrid coupler3.

Those attenuating units,21to24, are connected in parallel between the transmission line8and the ground31interposing the transmission elements,8ato8c; that is, the first attenuating unit21is provided closest to the hybrid coupler2without interposing any transmission elements, the second attenuating unit22is next provided interposing the transmission element8aagainst the first attenuating unit21, the third attenuating unit23is next provided interposing the transmission element8bagainst the second attenuating unit22, the fourth attenuating unit24is next provided interposing the transmission element8cand closest to the output port5a, namely, the hybrid coupler3interposing no transmission line.

The transistors,21ato21c, in the first attenuating unit21, the two transistors,22aand22b, in the second attenuating unit22, the two transistors,23aand23b, in the third attenuating unit23, and the two transistors,24aand24b, in the last attenuating unit24have the gate width same with those of the transistors,11ato11c, in the other first attenuating unit11, those of the transistors,12aand12b, in the other second attenuating unit12, those of the transistors,13aand13b, in the other third attenuating unit13, and those of the transistors,14aand14b, in the other last attenuating unit14, respectively.

Two transmission lines,29aand29b, which are the third transmission line in the present embodiment, may be a type of micro-strip line to be operable as an inductor with the frequency band subject to the variable attenuator1of the present embodiment. Widths and lengths of the transmission lines,29aand29b, may determine the inductance thereof.

The transmission line29ais provided in the first attenuating unit21provided closest to the input port4aand in topmost transistor21aclosest to the transmission line8. Specifically, the transmission line29ais connected in parallel to the transistor21a, namely, connected between two current terminals, the drain and the source, of the transistor21a. The inductance L of the transmission line29amay be set so as to set the resonance frequency ωr3determined by the equation of ωr32·L·Cds=1 within the frequency band.

The transmission line29bis provided in the last attenuating unit24closest to the output port5a, namely, closest to the hybrid coupler3, and in the topmost transistor24aclosest to the transmission line8. Specifically, the transmission line29bis provided in parallel to the transistor24a, that is, connected in parallel between two current terminals of the transistor24a. The transmission line29bis operable as an inductor with inductance L in the frequency band subject to the variable attenuator1, which is set so as to satisfy the equation of ωr42·L·Cds=1, where Cdsis parasitic capacitance between the drain and the source of the transistor24a.

Two resonance frequencies, ωr3and ωr4, similar to the resonance frequencies, ωr1and ωr2, for the aforementioned transmission lines,19aand19b, may be set within the frequency band subject to the variable attenuator1, or, preferably, at the highest frequency thereof or closer to the highest frequency of the band. Specifically, when the frequency band is 18 to 23 GHz, the resonance frequencies, ωr3and ωr4, may be around 22.9 GHz.

In the variable attenuator1, setting a control signal provided to the control port6in an level to turn off the transistors,11ato14band21ato24b, the drain-source resistance of the transistors,11ato14band21ato24bbecomes high impedance and the RF signal entering the input port4atransmits on the two transmission lines,7and8, showing smaller attenuation and reaches the output port5a. Contrary, when the control signal provided to the control port6is set in a level to turn on the transistors,11ato14band21ato24b, the RF signal entering the input port4aflows in the ground31during the transmission on the transmission lines,7and8, and only a limited portion thereof reaches the output port5a.

FIG. 2schematically shows an operation of a transistor, where the horizontal axis corresponds to a drain bias, while, the vertical axis shows a drain current. A behavior G11corresponds to a condition of the drain current against the drain bias when the transistor turns on by setting a gate bias of 0V; while, another behavior G12corresponds to a condition where the transistor turns off by supplying a gate bias Vgsof −5 V. As shown inFIG. 1, in the variable attenuator1, all transistors are biased in the drain thereof to be 0 V and only supplied with the gate biases. Accordingly, the attenuation of the variable attenuator1may be determined by a difference between slopes ΔId/ΔVdsof the behaviors, G13and G12, at no drain bias Vds=0 and at two gate biases to turn on and off the transistor, respectively.

Next, advantages according to the variable attenuator1of the present embodiment will be described.FIG. 3shows the attenuations of a variable attenuator when the transmission lines,19ato29b, in parallel to the transistors in the attenuating units,11and14, and21and24, are removed; while,FIG. 4shows the attenuations of the variable attenuator1when the transmission lines,19ato29b, are implemented. Behaviors, G21and G31are obtained when all transistors,11ato14band21ato24b, turn off by supplying the control signal of −5V, while behaviors G22and G32correspond to a status when all transistors,11ato14band21ato24b, turn on by setting the control signal to be 0 V. A frequency range A, 17.5˜23.5 GHz, inFIG. 3andFIG. 4correspond to the frequency band subject to the variable attenuator1of the present embodiment.

As the behavior G22indicates, the attenuation exceeds −30 dB when the transistors turn on, which becomes −32 to −36 dB within the frequency range A. However, the behavior G21clearly shows substantial loss of −1.6 to −2.8 dB within the frequency range A when the transistors turn off, which is unacceptable loss for an apparatus ordinarily installed in the field. This is because of the parasitic capacitance Cdsbetween the drain and the source of the transistors, which splits the RF signal carried on the signal line to the ground.

The variable attenuator1of the present embodiment provides the transmission lines,19ato29b, between two current terminals in some of the transistors, where the transmission lines,19ato29b, operate as inductors with the inductance by which the resonance frequencies, ωr1to ωr4, are fallen within the frequency range A. A parallel circuit of a capacitor and an inductor theoretically shows infinite impedance at the resonance frequency, but resistive elements attributed to the transmission lines,19ato29b, and those between the two current terminals of the transistors restrict the impedance thereof in definite value. Accordingly, the arrangement of a parallel circuit of a transistor and a transmission line between two current terminals of the transistor may set the impedance between the current terminals in substantial value, a large enough even when the transistor turns off.

InFIG. 4where the behaviors, G31and G32, correspond to statuses when the transistors turn off and turn on, respectively, for the variable attenuator implementing the transmission lines,19ato29b, where the those transmission lines,19ato29b, are set in the widths and the lengths thereof such that the resonance frequencies, ωr1to ωr4, become 29 GHz; specifically, the transmission lines,19aand29a, have the width of 10 μm and the length of 220 μm, while, the transmission lines,19band29b, have the width of 10 μm and the length of 320 μm, which sets the inductance of the former transmission lines,19aand29a, to be 0.18 nH, while, that of the latter transmission lines,19band29b, to be 0.26 nH, respectively. Because the transistors,14aand24a, in the respective final attenuating units,14and24, have the gate width greater than that of the transistors,11aand21a, in the initial attenuating units,11and21; the former transistors,14aand24a, inherently have the parasitic capacitance Cdsbetween the two current terminals thereof that is greater than that of the latter transistors,11aand21a; accordingly, the inductance attributed to the transmission lines,19band29b, connected in parallel to the former two transistors,14aand24a, become small compared with that attributed to the transmission lines,19aand29a, in order to set the resonance frequencies of the respective parallel circuits equal to each other.

As the behavior G32shows that the attenuation becomes −32 dB to −36 dB in the frequency range A when the transistors turn on, which is comparable to the variable attenuator without the transmission lines,19ato29b, indicated by the behavior G22inFIG. 3. However, the attenuation in the frequency range A when the transistors turn off, as shown by the behavior G31, becomes −1.6 to −2.1 dB, which becomes closer to the theoretical, or ideal, amount of 0 dB compared with the behavior G21shown inFIG. 3. Thus, the transmission lines,19ato29b, may reduce the loss when that variable attenuator1is set in a pass status.

The attenuating units,11to14and21to24, may include two or more transistors each connected in series between the respective transmission lines,7and8, and the ground31, which may suppress signal amplitudes caused in the respective transistors. That is, the amplitude of the RF signal transmitting on the transmission lines,7and8, are split by the transistors connected in series between the transmission lines,7and8, and the ground. Accordingly, one transistor receives the split amplitude of the RF signal.

The transistors,11ato14band21ato24b, preferably accompany with transmission lines operable as inductors. However, those transmission lines occupy substantial areas to makes the variable attenuator large. Accordingly, the variable attenuator1of the invention provides the transmission lines,19ato29b, only in the transistors,11a,21a,14aand24a, closest to the transmission lines,7and8, and closest to the hybrid couplers,2and3, respectively. Those transistors closest to the transmission lines,7and8, receive the largest power from the RF signal. Accordingly, the transmission lines,19ato29b, provided only in the transistors,11a,14a,21a, and24a, may show the function of eliminating the signal loss most effectively.

Also, in the variable attenuator1, only the limited transistors,11aand21a, may accompany with the transmission lines,19aand29a, because those transistors,11aand21a, are arranged closest to the hybrid coupler2and exposed to the respective output terminals of the hybrid coupler2. On the other hand, the transistors,12a,13a,22a, and23a, are indirectly exposed to the output terminals of the hybrid coupler2masked by the transmission lines,7a,7b,8a, and8b, respectively, which may moderate the function of transmission lines connected in parallel to transistors. The transmission lines,19aand29a, are preferably accompanied with the transistors,11aand21a, in the uppermost one in the attenuating units,11and21, closest to the hybrid coupler2.

Also, in the variable attenuator1, only the limited transistors,14aand24a, may provide the transmission lines,19band29b, because those transistors,14aand24a, are disposed closest to the transmission lines,7and8, in the attenuating units,14and24, arranged closest to the hybrid coupler3. As described, an attenuating unit provided closer to the hybrid unit3usually includes transistors with larger sizes, which means that those transistors inherently attribute to large parasitic capacitance Cdsand cause substantial leak paths for the RF signal. Accordingly, the arrangement where the transistors,14aand24a, disposed closest to the transmission lines,7and8, in the attenuating units,14and24, disposed closest to the hybrid coupler3accompany the transmission lines,19band29b, may show most effective function to reduce the signal loss when the transistors,11ato24b, turn off.

The embodiment of the variable attenuator1thus described provides the transmission lines,19ato29b, in both of the attenuating units,11and14, and21and24, disposed closest to the hybrid couplers,2and3. However, a variable attenuator may provide a transmission line only one of the attenuating units,11and14, and21and24. Also, the transistors,11ato24b, accompany the transmission lines,15ato28b, in the control terminals thereof to prevent the RF signal from leaking from the current terminal to the control port6. However, in an alternative, those transmission lines,15ato28b, may be replaced to resistors because substantially no currents leak from the current terminals to the control terminals of the transistors,11ato24b.

The variable attenuator according to the present invention is not restricted to the embodiment described above, and may have various changes and modifications. For instance, the attenuating units,11to24, of the embodiment include two or more transistors connected in series between the transmission lines,7and8, and the ground31. However, the attenuating units,11to24, may include only one transistor with the current terminals connected to the transmission lines,7and8, and the ground31. Also, the transmission lines operable as an inductor may be provided in transistors not closest to the transmission lines,7and8, and in all transistors in the attenuating units,11,14,21, and24, disposed closest to the hybrid couplers,2and3. Also, the transistors in the attenuating units,12,13,22, and23, disposed intermediate to the hybrid couplers,2and3, may accompany with the transmission lines. Accordingly, the present invention has a scope defined in the claims attached below and all modifications and the changes for elements recited in the claims and equivalents thereto.