Balun circuit and integrated circuit device

A balun circuit comprising first through third CPW lines becoming signal I/O ports, a first differential transmission line for linking the central conductor of the second CPW line and the ground conductor of the first CPW line and for linking the ground conductor of the second CPW line and the central conductor of the first CPW line, a second differential transmission line for linking the central conductors of the first and third CPW lines and for linking the ground conductors of the first and third CPW lines, and a joint for connecting at least two ground conductors of the first through third CPW lines. The differential transmission line has a first line formed in a dielectric layer on a substrate, a second line arranged in the underlying layer, and an underlying line at a fixed potential arranged between the substrate and the second line.

This application is the National Phase of PCT/JP2008/054354, filed Mar. 11, 2008, which is based upon and claims the benefit of priority from Japanese patent application No. 2007-068426, filed on Mar. 16, 2007, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a balun circuit suitable for an integrated circuit device and an integrated circuit device having the balun circuit.

BACKGROUND ART

In general, a wireless communication apparatus uses a mixer circuit for frequency conversion to a RF (Radio Frequency) signal for communication, which is a relatively high frequency, from an IF (Intermediate Frequency) signal for signal processing, which is a relatively low frequency, or frequency conversion to an IF signal from a RF signal.

FIG. 1is a circuit diagram showing a structure of a single balance-type mixer circuit that is used in a wireless communication apparatus and the like.

As shown inFIG. 1, a single balance-type mixer circuit has two mixer elements51and 180-degree phase combination circuit52. Mixer elements51mix two IF signals of reverse-phase, which are differential signals, and two local oscillation signals of in-phase (hereinafter, referred to as LO signals) and output an upper sideband signal and a lower sideband signal that are necessary for communication. 180-degree phase combination circuit52combines the two inputted signals so that they have a phase difference of 180 degrees and outputs a signal after the combination. Hence, the upper sideband signal and lower sideband signal generated from mixer elements51are combined to be in-phase by 180-degree phase combination circuit52and are then outputted as a RF signal that is used in the communication.

At this time, although the LO signals, which are unnecessary for communication, are outputted from mixer elements51, the two LO signals inputted in-phase to mixer elements51are outputted in-phase without change. Thus, the LO signals are combined to become reverse-phase by 180-degree phase combination circuit52, so that they are cancelled and removed.

In the meantime, 180-degree phase combination circuit52shown inFIG. 1can be used as a 180-degree phase splitter when a signal is inputted from the output port (Output) thereof and when it is taken out from the input ports0,180. In this case, it is possible to obtain two IF signals having a phase difference of 180 degrees by inputting a RF signal and a LO signal to the mixer elements. The circuit that splits or combines the signals to have a phase difference of 180 degrees is used as a circuit that converts a differential signal into a non-differential signal or a non-differential signal into a differential signal, a circuit that splits a differential signal to a plurality of active elements, a circuit that combines a differential signal, and the like. Due to this, there has been a recent increase in demand such that the 180-degree phase combination circuit (180-degree phase splitting circuit) should be used in a microwave IC that is used in a wireless communication apparatus and the like. Meanwhile, in the microwave IC, a CPW (Coplanar Waveguide) line is used as a transmission line because the processing of the underside surface of a substrate is unnecessary.

In the meantime, in order to split a high frequency signal and to enable two signals after splitting to have a phase difference of 180 degrees, a rat race circuit is generally used. The rat race circuit splits a signal by branching a signal line into two lines and provides the two signal lines after the branching with a length difference corresponding to a ½ wavelength of a signal frequency to be transmitted, thereby enabling the two split signals to have a phase difference of 180 degrees.

However, the line length corresponding to a ½ wavelength of a signal frequency is about several mm or several cm even for a high frequency signal of GHz or more and requires a large circuit area. Due to this, it is difficult to incorporate the rat race circuit into the microwave IC.

Hence, instead of obtaining a phase difference by using a difference of the line lengths, a method has been suggested in which a phase difference of 180 degrees is obtained by using a balun circuit that converts a non-differential transmission line such as CPW line or micro strip line into a differential transmission line such as slot line or CPS (Coplanar Strips) line, or a differential transmission line into a non-differential transmission line (for example, Yasuhiro Hamada, Kenichi Maruhashi, Masaharu Ito, Shuya Kishimoto, Takao Morimoto, and Keiichi Ohata, “A60-GHz-band Compact IQ Modulator MMIC for Ultra-high-speed Wireless Communication,” 2006 IEEE MTT-S International Microwave Symposium Digest, pp. 1701-1704, June 2006 (Non-Patent Document 1)).

First CPW line61, second CPW line62a, third CPW line62band FCPW line63are non-differential transmission lines having a central conductor and two ground conductors arranged to sandwich the central conductor therebetween. The two ground conductors of first CPW line61, second CPW line62a, third CPW line62band FCPW line63are connected by air bridges68, respectively.

In the balun circuit shown inFIG. 2, first CPW line61is converted into FCPW line63by CPW-FCPW conversion section64and FCPW line63is branched and converted into first CPS line65aand second CPS line65bby FCPW-CPS conversion branch section66. In addition, first CPS line65ais converted into second CPW line62aby first CPS-CPW conversion section67aand second CPS line65bis converted into third CPW line62bby second CPS-CPW conversion section67b. Here, the central conductor of second CPW line62ais connected to the ground conductor of FCPW line63and the central conductor of third CPW line62bis connected to the central conductor of FCPW line63. In addition, the ground conductor of second CPW line62ais connected to the central conductor of FCPW line63and the ground conductor of third CPW line62bis connected to the ground conductor of FCPW line63.

Like this, the relation between the connection of the central and ground connectors of second CPW line62ato the central and ground connectors of FCPW line63, and the connection of the central and ground connectors of third CPW line62bto the central and ground connectors of FCPW line63is reversed. Thus, when a signal is inputted from first CPW line61, differential signals having a phase difference of 180 degrees are outputted from second CPW line62aand third CPW line62b. Since such structure does not use a method that obtains a phase difference of 180 degrees by an electrical length, it is possible to appropriately shorten the length of the CPS line and to make a circuit size small. Further, since the ground conductors of the respective CPW lines are connected to each other, the ground potential of each CPW line is same and the above structure can be easily applied to an integrated circuit. By connecting the ground conductors, the phase difference of the signals outputted from second CPW line62aand third CPW line62bis not always 180 degrees. However, it is possible to compensate for a deviation of the phase difference by making the lengths of first CPS line65aand second CPS line65bdifferent.

However, the balun circuit of the Non-Patent Document 1 has the following problems.

A first problem is that when the balun circuit is formed on a conductive substrate made of silicon, for example, the insertion loss of the balun circuit is increased. This is caused by a substrate loss. That is, this occurs because the electromagnetic fields occurring in the CPW, FCPW and CPS lines are spread into the substrate and a signal is attenuated by a resistance component of the substrate. Hence, in high frequency lines formed on a conductive substrate, a conductive layer referred to as a ground shield is generally arranged in an underlying layer, which is connected to a ground potential to shield an electric field and thus to prevent a loss by the substrate. However, even when the ground shield is applied to the balun circuit, the power is not equally split and a phase difference is not 180 degrees. In other words, it is impossible to operate as a balun circuit.

This is because the coupling between the ground shield and each of two strip-shaped conductors constituting the CPS line is predominant over the coupling between the strip-shaped conductors, so that a micro strip line mode becomes a main transmission mode and the CPS line section does not resultantly operate a differential transmission line.

A second problem is that it is difficult to reduce the circuit size.

This is caused by the CPS lines of the balun circuit. To be more specific, since the CPS lines are such that the two strip-shaped conductors are arranged in a line, the CPS lines occupy an area obtained by adding at least a gap of the conductors and a conductor width corresponding to two conductors. Furthermore, since the spread of the electromagnetic fields in the horizontal direction is large, it is necessary to keep another circuit including the ground conductors at a distance.

SUMMARY

It is an exemplary object of the present invention to provide a balun circuit which is capable of splitting or combining signals having a phase difference of 180 degrees, easily incorporated into an integrated circuit device while realizing a desired circuit performance and having a small loss even when a conductive substrate is used, and an integrated circuit device having the balun circuit.

In order to achieve the above object, the exemplary aspect of the invention provides a balun circuit comprising:

a first CPW line, a second CPW line and a third CPW line that become signal I/O ports;

a first differential transmission line that links a central conductor of the second CPW line and a ground conductor of the first CPW line and links a ground conductor of the second CPW line and a central conductor of the first CPW line;

a second differential transmission line that links a central conductor of the third

CPW line and the central conductor of the first CPW line and links a ground conductor of the third CPW line and the ground conductor of the first CPW line; and

a joint that connects two or more among the ground conductor of the first CPW line, the ground conductor of the second CPW line and the ground conductor of the third CPW line,

wherein the first differential transmission line and the second differential transmission line have a first line, a second line and an underlying layer connected to a fixed potential arranged between a substrate and the second line, the first line and the second line being formed in a dielectric layer on the substrate and the second line being formed at a position nearer to a substrate than the first line and being electromagnetically coupled to the first line.

Alternatively, a balun circuit of the present invention comprises:

a first CPW line, a second CPW line and a third CPW line that become signal I/O ports;

a first differential transmission line that links a central conductor of the second CPW line and a central conductor of the first CPW line and links a ground conductor of the second CPW line and a ground conductor of the third CPW line;

a second differential transmission line that links a central conductor of the third CPW line and the ground conductor of the first CPW line and links the ground conductor of the third CPW line and the ground conductor of the second CPW line; and

a joint that connects two or more among the ground conductor of the first CPW line, the ground conductor of the second CPW line and the ground conductor of the third CPW line,

wherein the first differential transmission line and the second differential transmission line have a first line, a second line and an underlying layer connected to a fixed potential arranged between a substrate and the second line, the first line and the second line being formed in a dielectric layer on the substrate and the second line being formed at a position nearer to a substrate than the first line and being electromagnetically coupled to the first line.

Alternatively, a balun circuit of the invention comprises:

a first CPW line, a second CPW line and a third CPW line that become signal I/O ports;

a first differential transmission line that links a central conductor of the second CPW line and a central conductor of the third CPW line and links a ground conductor of the second CPW line and a central conductor of the first CPW line;

a second differential transmission line that links the central conductor of the third CPW line and the central conductor of the second CPW line and links a ground conductor of the third CPW line and a ground conductor of the first CPW line; and

a joint that connects two or more among the ground conductor of the first CPW line, the ground conductor of the second CPW line and the ground conductor of the third CPW line,

where the first differential transmission line and the second differential transmission line have a first line, a second line and an underlying layer connected to a fixed potential arranged between a substrate and the second line, the first line and the second line being formed in a dielectric layer on the substrate and the second line being formed at a position nearer to a substrate than the first line and being electromagnetically coupled to the first line.

In the above structure, since it is possible to strongly electromagnetically couple the first and second lines by adjusting the line width and gap of the first and second lines, it is possible to enlarge the coupling between the lines even when there is a ground shield. Due to this, the coupling of the first and second lines becomes predominant, not a micro strip mode occurring between the underlying conductor connected to the fixed potential and each of the strip-shaped conductors, so that a differential transmission mode becomes a main transmission mode. Hence, the CPS line section operates as a differential transmission line. As a result, it is possible to realize a balun circuit capable of splitting or combining the signals having a phase difference of 180 degrees.

In addition, since the underlying line connected to the fixed potential shields the electric field, the electric field occurring in the differential transmission line consisting of the first line and the second line does not reach the substrate. Due to this, it is possible to make a balun circuit that has low loss in which no loss is caused by the resistance component of the substrate.

Furthermore, since the differential transmission line does not require a wide line area, such as slot line or CPS line of arranging and coupling two lines on a same plane, it is possible to reduce the circuit size. Due to this, it is possible to easily incorporate the balun circuit into an integrated circuit device and to downsize a circuit of the integrated circuit device having the balun circuit.

Accordingly, it is possible to obtain a balun circuit capable of splitting or combining signals having a phase difference of 180 degrees, and that is easily incorporated into an integrated circuit device while realizing a desired circuit performance and having a small loss even when a conductive substrate is used.

EXEMPLARY EMBODIMENT

Hereinafter, the invention will be described with reference to the drawings.

First Exemplary Embodiment

FIG. 3is a plan view showing a structure of a balun circuit of a first exemplary embodiment andFIG. 4is a sectional view showing the balun circuit shown inFIG. 3, taken along a line A-A′.

As shown inFIG. 3, a blaun circuit of a first exemplary embodiment has first CPW line1, second CPW line2aand third CPW line2b, which are signal input/output ports, FCPW line3, which is a non-differential transmission line, CPW-FCPW line conversion section4that converts first CPW line1into FCPW line3, first differential transmission line5ahaving a length of L1, second differential transmission line5bhaving a length of L1+L2, line conversion branch section6that converts FCPW line3into first differential transmission line5aand second differential transmission line5b, first conversion section7athat converts first differential transmission line5ainto second CPW line2a, second conversion section7bthat converts second differential transmission line5binto third CPW line2b, and underlying conductor8that is connected to a ground potential, which are formed on substrate9.

The respective ground conductors of first CPW line1, second CPW line2aand third CPW line2bare arranged to surround the respective elements formed on substrate9. In addition, the ground conductor of first CPW line1, the ground conductor of second CPW line2a, the ground conductor of third CPW line2band the ground conductor of FCPW line3are connected to underlying conductor8by ground via12.

The conductors of one side of each of first differential transmission line5aand second differential transmission line5bare connected by via13between the strip-shaped conductors, so that first differential transmission line5aand second differential transmission line5bare commonly used. The common conductor is connected to the ground conductor of second CPW line2aby ground via12and also to the central conductor of third CPW line2b.

Further, the conductor of the other side of first differential transmission line5a, which is not the common conductor, is connected to the central conductor of second CPW line2a, and the conductor of the other side of second differential transmission line5bis connected to ground conductor of third CPW line2bby ground via12. In line conversion branch section6, the central conductor of FCPW line3is branched into two. The one is connected to second strip-shaped conductor11bof first differential transmission line5athrough via13between the strip-shaped conductors and the other is connected to first strip-shaped conductor11aof second differential transmission line5b.

The ground conductor of the one of FCPW line3is connected to underlying conductor8by ground via12and first strip-shaped conductor11aof first differential transmission line5a. In addition, the ground conductor of the other of FCPW line3is connected to underlying conductor8and second strip-shaped conductor11bof second differential transmission line5bby ground via12.

First differential transmission line5ais comprised of first strip-shaped conductor (first line)11aand second strip-shaped conductor (second line)11barranged in an underlying layer of first strip-shaped conductor11a, which are arranged in dielectric layer10formed on substrate9, and underlying conductor (underling line)8connected to a fixed potential arranged between substrate9and the second strip-shaped conductor, as shown inFIG. 4. Underlying conductor8is provided at a location in dielectric layer10near to substrate9and a transmission line using first strip-shaped conductor11aand second strip-shaped conductor11bis formed. Second differential transmission line5bhas the same structure as first differential transmission line5a.

In general, the structure as shown inFIG. 4is formed by using a multi-layer line process in which a sheet resistance becomes smaller the nearer it gets to the upper surface layer. Hence, a line width of second strip-shaped conductor11bformed in the underlying layer of first strip-shaped conductor11ais larger than that of first strip-shaped conductor11aso that the loss by the resistances of each conductor is same.

Here, when a microwave signal is inputted into the differential transmission line of the structure shown inFIG. 4, a micro strip mode is generated by first strip-shaped conductor11aand underlying conductor8and is generated by second strip-shaped conductor11band underlying conductor8. In the mean time, first strip-shaped conductor11aand second strip-shaped conductor11bare electromagnetically coupled.

In the CPS line described in the Non-Patent Document 1, since the micro strip mode is predominant, the differential transmission is impossible. Meanwhile, in the structure shown inFIG. 4, two strip-shaped conductors11a,11bare sufficiently electromagnetically coupled by arranging them high and low, and a differential transmission line operates even when the micro strip modes are mixed.

At this time, the electric field generated in the differential transmission line and the electric field in the micro strip mode are shielded by underlying conductor8and do not reach substrate8. Due to this, the signal passing through the transmission line is not influenced by the resistance component of substrate8, so that the loss in transmitting a signal is decreased. In addition, since underlying conductor8is arranged below each of first CPW line1, second CPW line2a, third CPW line2band FCPW line3, the same effect will occur.

Hence, according to the balun circuit of this exemplary embodiment, the signal is transmitted in a differential mode by differential transmission lines5a,5bin the circuit, contrary to the CPS line that does not transmit a signal in a differential mode when there is underlying conductor8. Due to this, it is possible to obtain differential signals having a phase difference of 180 degrees from second CPW line2aand third CPW line2b.

Further, it is possible to compensate for the phase difference of the signals outputted from second CPW line2aand third CPW line2bby adjusting L2that is a difference of the lengths of first differential transmission line5aand second differential transmission line5b.

In addition, since first strip-shaped conductor11aand second strip-shaped conductor11bare arranged to be overlapped, it is possible to make first differential transmission line5aand second differential transmission line5bsmaller than the CPS line of the prior art. Additionally, it is possible to make the differential transmission lines smaller by arranging the lines in a crank shape, meander shape or spiral shape. Furthermore, it is possible to freely set the length (L1) of first differential transmission line5aand second differential transmission line5bwithin a range in which a layout area is permissible.

Hence, it is possible to downsize the balun circuit and to easily incorporate it to an integrated circuit.

In addition, since the ground conductors of first CPW line1, second CPW line2aand third CPW line3a, which are signal I/O ports, are at the same potential, first CPW line1, second CPW line2aand third CPW line3aoperate in the same condition even when a 3-terminal active element and the like are connected to first CPW line1, second CPW line2aand third CPW line3a. Accordingly, it is possible to realize a desired circuit performance.

Second Exemplary Embodiment

As shown inFIG. 5, a balun circuit of a second exemplary embodiment has first CPW line21, second CPW line22aand third CPW line22b, which are signal I/O ports, first differential transmission line25ahaving a length of L3, second differential transmission line25bhaving a length of L3+L4, third differential transmission line23, line conversion section27cthat converts first CPW line21into third differential transmission line23, differential transmission line branch section24that branches third differential transmission line23into first differential transmission line25aand second differential transmission line25b, first line conversion section27athat converts first differential transmission line25ainto second CPW line22a, second conversion section27bthat converts second differential transmission line25binto third CPW line22band underlying conductor28that is connected to a ground potential, which are formed on substrate29.

The conductors of one side of each of first differential transmission line25aand second differential transmission line25bare common. The common conductor is connected to the ground conductor of second CPW line22aand the ground conductor of third CPW line22bby ground via12.

In addition, the conductor of the other side of first differential transmission line25a, which is not the common conductor, is connected to the central conductor of second CPW line22aand the conductor of the other side of second differential transmission line25bis connected to the central conductor of third CPW line22b.

The ground conductors of first CPW line21, second CPW line22aand third CPW line22bare arranged to surround the respective elements formed on substrate29. In addition, the ground conductors of each of first CPW line21, second CPW line22aand third CPW line22bare connected to underlying conductor28by ground via12.

Third differential transmission line23is branched into first differential transmission line25aand second differential transmission line25bby line branch section24. To be more specific, first strip-shaped conductor11aof third differential transmission line23is connected to first strip-shaped conductor11aof second differential transmission line25band second strip-shaped conductor11bof third differential transmission line23is connected to first strip-shaped conductor11aof first differential transmission line25aby via13between the strip-shaped conductors.

Differential transmission lines25a,25b,23of the second exemplary embodiment have the same structure as the differential transmission lines of the first exemplary embodiment and the same effect as the first exemplary embodiment can be made.

When the differential transmission lines are branched, they are respectively split in a reverse phase. Hence, the signals of reverse phase and the same power are transmitted to first differential transmission line25aand second differential transmission line25b. At this time, the common conductor of first differential transmission line25aand second differential transmission line25bis connected to the ground conductors in second CPW line22aand third CPW line22b. Thus, when the lengths of first differential transmission lines25aand second differential transmission line25bare the same (L4=0), the signals having a phase difference of 180 degrees will be outputted from second CPW line22aand third CPW line22b.

However, for a case where first CPW line21is used as an input port and second CPW line22aand third CPW line22bare used as output ports, when the respective ground conductors of first CPW line21, second CPW line22aand third CPW line22bare connected and when another integrated circuit device having CPW lines and the like is connected to the respective CPW lines, a case may occur where a phase difference of the signals outputted from second CPW line22aand third CPW line22bis not 180 degrees and the same signal power is not split to first differential transmission line25aand second differential transmission line25b, as in the first exemplary embodiment.

Due to this, in this exemplary embodiment, in order to output the signals having a phase difference of 180 degrees from second CPW line22aand third CPW line22b, the length of first differential transmission line25aand the length of second differential transmission line25bare changed to compensate for the phase difference. In this exemplary embodiment, since the phase difference of the signals outputted from second CPW line22aand third CPW line22bis compensated for by the length of L4, it is possible to freely set the length L3within a range in which a layout area is permissible.

Meanwhile, the splitting ratio of the signal power for first differential transmission line25aand second differential transmission line25bcan be corrected by optimizing the shape of the ground conductors arranged on the periphery, as in the prior art.

FIG. 5shows the structure in which the common conductors of first differential transmission line25aand second differential transmission line25bare connected to the ground conductors of second CPW line22aand third CPW line22b. However, a structure is possible in which the common conductors of first differential transmission line25aand second differential transmission line25bare connected to the central conductors of second CPW line22aand third CPW line22b. In this case, it is preferable that the conductor of the other side of first differential transmission line25a, which is not the common conductor, be connected to the ground conductor of second CPW line22a, and that the conductor of the other side of second differential transmission line25bbe connected to ground conductor of third CPW line22b.

According to the balun circuit of the second exemplary embodiment, by providing first differential transmission line25afor linking first CPW line21and second CPW line22aand second differential transmission line25bfor linking first CPW line21and third CPW line22b, it is possible to obtain differential signals having a phase difference of 180 degrees from second CPW line22aand third CPW line22b. In addition, since the area of the balun circuit can be downsized, it is possible to easily incorporate the balun circuit into an integrated circuit device.

Furthermore, since the ground conductors of first CPW line21, second CPW line22aand third CPW line22b, which are signal I/O ports, are at the same potential, first CPW line21, second CPW line22aand third CPW line22boperate in the same condition even when a 3-terminal active element and the like is connected to first CPW line21, second CPW line22aand third CPW line22b. Accordingly, it is possible to realize a desired circuit performance.

Third Exemplary Embodiment

As shown inFIG. 6, a balun circuit of a third exemplary embodiment is different from the first exemplary embodiment in that the lengths of first differential transmission line35aand second differential transmission line35bare the same (L5) and the lengths of second CPW line32aand third CPW line32bare different (a difference thereof is L6). Since the other structures are the same as the balun circuit of the first exemplary embodiment, descriptions thereof will be omitted.

In the balun circuit of the third exemplary embodiment, in order to output the signals having a phase difference of 180 degrees from second CPW line32aand third CPW line32b, a phase difference is compensated for by making the lengths of second CPW line32aand third CPW line32bdifferent.

In this exemplary embodiment, since the phase difference of the signals outputted from second CPW line32aand third CPW line32bis compensated for by a value of L6, it is possible to freely set the length (L5) of first differential transmission line35aand second differential transmission line35bwithin a range in which a layout area is permissible.

To be more specific, the balun circuit shown inFIG. 6can be downsized and easily incorporated into an integrated circuit device, as in the first and second exemplary embodiments. Due to this, the balun circuit of the third exemplary embodiment can realize the same effect as the first and second exemplary embodiments.

Meanwhile, in the third exemplary embodiment, it is possible to compensate for the phase difference of the signals outputted from second CPW line32aand third CPW line32bby changing the lengths of second CPW line32aand third CPW line32b. Due to this, it is not necessary to make the lengths of first differential transmission line35aand second differential transmission line35bsame. That is, the lengths of these lines may be different.

In addition,FIG. 6shows an example where the phase difference of the signals outputted from the second CPW line and the third CPW line is compensated for by changing the lengths of the second CPW line and the third CPW line of the balun circuit shown in the first exemplary embodiment. However, such a structure can be also applied to the balun circuit of the second exemplary embodiment. In other words, even when the lengths of the first and second differential transmission lines shown inFIG. 5are set to be the same and even when the lengths of the second and third CPW lines are changed, it is possible to compensate for a phase difference of the signals outputted from the second and third CPW lines.

Fourth Exemplary Embodiment

The fourth exemplary embodiment is an example where the balun circuit of the first exemplary embodiment is used as the 180-degree phase combiner of the single balance-type mixer circuit shown inFIG. 1.

As shown inFIG. 7, the integrated circuit device of this exemplary embodiment has the balun circuit shown inFIG. 3, two FETs41a,41b, which are mixer elements, capacitors42a,43aconnected to FET41aand capacitors42b,43bconnected to FET41b. In the branch section of the CPW line, the ground conductors of the underlying conductor and the CPW line are connected by using the ground via.

A source electrode of FET41athat is the mixer element is connected to the ground conductor of the second CPW line of the balun circuit of the first exemplary embodiment and a source electrode of FET41bis connected to the ground conductor of the third CPW line of the balun circuit of the first exemplary embodiment. Gate electrodes of FETs41a,41bare connected to a LO signal source and a gate bias source by first input ports44a,44b.

In addition, a drain electrode of FET41ais connected to the central conductor of the second CPW line through capacitor42aand a drain electrode of FET41bis connected to the central conductor of the third CPW line through capacitor42b. Furthermore, to the drain electrode of FET41aare connected capacitor43ahaving the other end connected to the ground conductor and a stub having a predetermined length. Through the stub, an IF signal is inputted from second input port45a.

Likewise, to the drain electrode of FET41bare connected capacitor43bhaving the other end connected to the ground conductor and a stub having a predetermined length. Through the stub, an IF signal of a reverse phase is inputted from second input port45b.

In the meantime, the capacitance of capacitors43a,43bis set to a value that impedance is open in the frequency of a RF signal when seen from the drain electrode and an insertion loss is lowest in the frequency of an IF signal.

In the above structure, an upper sideband signal, a lower sideband signal and a LO signal are outputted from the drain electrodes of FETs41a,41bthat are mixer elements, the upper sideband signal and the lower sideband signal are combined to be in-phase by the balun circuit and the LO signal is combined to be a reverse phase by the balun circuit.

In the integrated circuit device of this exemplary embodiment, the source electrodes of two FETs41a,41bthat are mixer elements are connected to the ground conductors by joints46a,46b. As described in the first exemplary embodiment, since the potentials of the respective ground conductors are the same, the operating conditions of two FETs41a,41bare the same and the powers of the LO signals outputted from FETs41a,41bare the same.

Hence, the LO signals outputted from two FETs41a,41bare combined to be a reverse phase and are thus cancelled by the balun circuit shown inFIG. 7, and the power of the LO signal included in the signal of output port47is reduced.

In addition, according to this exemplary embodiment, since the balun circuit can be downsized, the integrated circuit device having the balun circuit can be downsized.

Meanwhile, the fourth exemplary embodiment shows an example where the balun circuit of the first exemplary embodiment is used as a 180-degree phase combiner of the single balance-type mixer circuit. However, the balun circuits shown in the second and third exemplary embodiments can be also used as a 180-degree phase combiner of the single balance-type mixer circuit.

In addition, the balun circuit shown in the first to third exemplary embodiments is not limited to the single balance-type mixer circuit shown in this exemplary embodiment. In other words, the balun circuit can be used as any circuit as long as it is a circuit enabling two signals to have a phase difference of 180 degrees, such as multiplication circuit, differential amplification circuit and the like. When using the balun circuit shown in the first to third exemplary embodiments, it is possible to reduce the overall size of an integrated circuit device comprising the balun circuit.

Additionally, in the differential transmission lines shown in the first to fourth embodiments, it is possible to adjust the degree of coupling of the differential transmission lines by appropriately adjusting the gap and conductor width of first strip-shaped conductor11aand second strip-shaped conductor11b. In addition, it is possible to adjust the degree of coupling of the differential transmission lines by offsetting the central positions of first strip-shaped conductor11aand second strip-shaped conductor11b.

In addition, the differential transmission line is not limited to the structure shown inFIG. 4. That is, it may have a sectional structure as shown inFIGS. 8 to 10.

The differential transmission line shown inFIG. 8has a structure such that two first strip-shaped conductors11aare comprised and one of the conductors is connected to second strip-shaped conductor11bby via13between the strip-shaped conductors, thereby making the effective sheet resistances the same.

The differential transmission line shown inFIG. 9has a structure such that second strip-shaped conductor11bis added to the differential transmission line shown inFIG. 8and two sets of first strip-shaped conductors11aand second strip-shaped conductors11bare respectively connected by via13between the strip-shaped conductors. The transmission line shown inFIG. 9obtains the same effects as the differential transmission line shown inFIG. 8and the coupling with underlying conductor8can be easily adjusted.

The differential transmission line shown inFIG. 10has a structure such that third strip-shaped conductor11cis arranged in an upper layer of first strip-shaped conductor11aand third strip-shaped conductor11cand second strip-shaped conductor11bare connected by via13between the strip-shaped conductors. The transmission line shown inFIG. 10obtains the same effects as the differential transmission lines shown inFIGS. 8 and 9. Further, the coupling between the strip-shaped conductors is strong and it is possible to easily make a design in which a desired coupling is made by adjusting the line width and gap of the respective strip-shaped conductors.

In the meantime, the first to third exemplary embodiments show an example where underlying conductor8is connected to the ground potential. However, underlying conductor8may be at a fixed potential and may be connected to a power supply voltage, for example.

Further, although a dielectric substrate, a semiconductor substrate and the like are used as the substrate on which the balun circuit shown in the first to third exemplary embodiments is incorporated, the material of the substrate is not limited thereto. For example, the substrate may be a conductive substrate of silicon, a semi-insulating substrate of gallium arsenide or an insulating substrate.

Additionally, although the first to third exemplary embodiments show an example where a plate-shaped conductor is used for underlying conductor8, underlying conductor8may have a stripe shape or a lattice shape.

Furthermore, underlying conductor8may be arranged on only a part of the balun circuit as long as it has sufficient size to shield the electric field. In addition, the underlying conductor may not be comprised when the substrate loss is not problematic.

In addition, although the first to third exemplary embodiments show an example where the ground conductors of all the CPW lines and FCPW lines are connected to underlying conductor8by using the ground via and the ground conductors of each line are connected to each other through underlying conductor8, such a connection is made so as to stabilize the signal transmission mode in the CPW lines. Thus, if a signal is without fail transmitted without loss, it is not necessary to connect the ground conductors of all the CPW lines and FCPW lines. Further, it is not necessary to use the ground via and the underlying conductor so as to connect the ground conductors of each of the CPW lines and FCPW lines. For example, an air bridge may be used.

Furthermore, although the first to third exemplary embodiments show an example where the CPW lines are used as signal I/O ports, it is possible to replace at least one thereof with a FCPW line having a definite ground conductor width.

In addition, according to the first to fourth exemplary embodiments, each of the following the ground conductor of the first CPW line, the ground conductor of the second CPW line and the ground conductor of the third CPW line are connected by the surrounding conductor. However, it is sufficient if at least two ground conductors are connected to each other.

This application is based upon and claims the benefit of priority from Japanese patent application No. 2007-068426, filed on Mar. 16, 2007, the disclosure of which is incorporated herein in its entirety by reference.