Abstract:
The attenuation characteristics of an attenuator largely changes depending on the frequency of an input signal. Accordingly, a difference between the amounts of attenuation of gains of each two attenuators included in a communication device is not constant. In communications using the wireless USB, the difference needs to be in a range of 2 dB±1 dB. Thus, the communication device does not meet the standards of the wireless USB unless the difference between the amounts of attenuation of the attenuators is adjusted. In this regard, provided is a communication device including first and second attenuators that attenuate a signal. The second attenuator is provided with a regulator circuit that adjusts a relation between an amount of attenuation of the signal through the first attenuator and an amount of attenuation of the signal through the second attenuator.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a communication device, and more particularly to a communication device provided with an attenuator that attenuates a gain of an input signal. 
     2. Description of the Related Art 
     In an ultra-wideband (UWB) communication system that performs communications by using a wideband and high-frequency signal, signals having a wide range of frequencies can be used to perform communications. Accordingly, in this UWB communication system, communications are performed while hopping frequencies used by respective communication devices.  FIG. 1  schematically shows a state where a communication device  101  and a communication device  102  perform communications. For example, consider a case where the communication device  101  transmits a signal to the communication device  102  while hopping frequencies used. Here, assume that noise or the like is mixed in the signal from the communication device  101  due to an influence from another communication device to thereby deteriorate quality of the signal. Such a case may be found when a mobile communication terminal such as a cellular phone performs communications near any of the communication devices, for example. In that case, the communication device  102  detects the deterioration in quality of the signal transmitted from the communication device  101  by use of an amount of noise included in a received signal, and then transmits a signal indicating the information to the communication device  101 . The communication device  101  acquires the information on the deterioration in quality of the transmitted signal from the signal that is transmitted from the communication device  102 . Here, there is known a method of suppressing deterioration in quality of the signal by attenuating a gain of a signal to be transmitted in that case. In this method, the communication device  101  once attenuates the gain of the signal to be transmitted and then transmits the signal to be transmitted again to the communication device  102 . 
     For this reason, each of the communication devices  101  and  102  is provided with an attenuator that is a circuit for attenuating the gain of the signal to be transmitted. Here, http://www.mwave-lab.jp/vr_att.htm (Non-patent Document 1) discloses a technique to include multiple attenuators and switch the attenuators by a switch to adjust the gain of a signal to be transmitted. Specifically, each of the communication devices  101  and  102  includes multiple attenuators having different amounts of attenuation of the gain of a signal. The communication devices  101  and  102  each select one of the attenuators as necessary to transmit a signal to be transmitted while attenuating the gain of the signal with an appropriate amount of attenuation. 
     Meanwhile, there is also known an attenuator disclosed in Japanese Unexamined Patent Application Publication No. Hei 6-334504 (Patent Document 1), which implements a concrete circuit configuration of the above-described attenuator.  FIG. 2  shows the circuit configuration of the attenuator disclosed in Patent Document 1. This attenuator is a T-type attenuator formed of a circuit in which field effect transistors (FETs)  201  and  202  are connected in series and another FET  203  is connected thereto in shunt. When a path between a source and a drain of the FET  203  is conducted by adjusting a gate voltage, a signal inputted from an input terminal (IN) of the attenuator is shunted into a component that passes through the FET  202  and a component that passes through the FET  203 . As a result, the gain of a signal outputted from an output terminal (OUT) is attenuated. 
     Here, it is known that the amount of attenuation of the gain of the attenuator changes depending on the frequency of the input signal. Specifically, the amount of attenuation of the gain of the attenuator has a frequency characteristic. The reason that the amount of attenuation of the gain of the attenuator has the frequency characteristic is that the respective circuit elements constituting the attenuator have parasitic element components. Referring to the FET  201  and the FET  203  in  FIG. 2 , it is apparent that the FETs  201  and  203  each have a parasitic capacitor and a parasitic resistor. The input signal to the attenuator is also shunted through this parasitic capacitor. Moreover, since impedance of the parasitic capacitor changes depending on the frequency of the input signal, an amount of current to be shunted through the parasitic capacitor changes depending on the frequency of the input signal to the attenuator. As the input signal to the attenuator leaks through the parasitic capacitor in an amount corresponding to the frequency of the input signal, an amount of current to be outputted from the attenuator also varies depending on the frequency of the input signal. If the amount of current to be outputted from the attenuator varies depending on the frequency of the input signal, the gain of the signal to be outputted from the attenuator also varies depending on the frequency. As a consequence, the amount of attenuation of the gain for the input signal to the attenuator has the frequency characteristic. 
     The present inventor has found out the following problems of the above-mentioned conventional techniques. In recent years, communications using a wireless USB (universal serial bus) applying a UWB communication system has been drawing attention. In the communications using the wireless USB, in accordance with the standard thereof, a difference between the amounts of attenuation of gains of each two attenuators included in a communication device needs to satisfy a range of 2 dB±1 dB within a frequency band used by the wireless USB. For example, assume a case where a communication device is provided with four attenuators ATT 1  to ATT 4  having different amounts of attenuation of gains and the communication device uses these attenuators ATT 1  to ATT 4  by switching these attenuators with one another. In this case, a difference between the amounts of attenuation of the gains that the attenuators ATT 1  and ATT 2  respectively have, a difference between the amounts of attenuation of the gains that the attenuators ATT 2  and ATT 3  respectively have, and a difference between the amounts of attenuation of the gains that the attenuators ATT 3  and ATT 4  respectively have need to satisfy the range of 2 dB±1 dB within the frequency band used by the wireless USB. However, as described previously, each attenuator has the considerably variable amount of attenuation of the gain depending on the frequency of the input signal. Accordingly, the difference in the amount of attenuation of the gains between each two attenuators is generally not constant within the frequency band used by the wireless USB. A concrete example is shown in  FIG. 3 .  FIG. 3  shows frequency dependency of the amount of attenuation of the gain of the attenuator. The longitudinal axis indicates the amount of attenuation while the lateral axis indicates the frequency of the input signal. Meanwhile, a “to-be-used frequency band” in the graph indicates the frequency range used by the wireless USB. Here, a curved line C shows an attenuation characteristic of the attenuator ATT 1  included in the communication device while a curved line D shows an attenuation characteristic of the attenuator ATT 2  included in the communication device, for example. It is apparent that the curved line C and the curved line D have different change ratios for the change in the frequency of the input signal. As a consequence, the difference between the amount of attenuation of the gain of the attenuator ATT 1  and the amount of attenuation of the gain of the attenuator ATT 2  at a point A diverges considerably from the difference between the amount of attenuation of the gain of the attenuator ATT  1  and the amount of attenuation of the gain of the attenuator ATT 2  at a point B. Accordingly, in the example shown in  FIG. 3 , the difference between the amounts of attenuation of the gains of the attenuators ATT 1  and ATT 2  is not constant in the to-be-used frequency band, the attenuator ATT 1  having the attenuation characteristic indicated with the curved line C and the attenuator ATT 2  having the attenuation characteristic indicated with the curved line D. If these attenuators ATT 1  and ATT 2  are used in the communication device based on the wireless USB, there arises a problem that the communication device does not meet the standards of the wireless USB. 
     SUMMARY 
     A communication device according to the present invention includes first and second attenuators that attenuate a signal. The second attenuator includes a regulator circuit to adjust a relation between an amount of attenuation of the signal through the first attenuator and an amount of attenuation of the signal through the second attenuator. When designing the communication device, a circuit designer is able to adjust a difference in the amount of attenuation of the signal through the first and second attenuators by using this regulator circuit. 
     According to the present invention, the circuit designer is able to adjust the difference between the amount of attenuation of the signal of an attenuator and that of another attenuator, the attenuators being included in a communication device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view showing multiple communication devices performing communications. 
         FIG. 2  is a view showing a circuit configuration of a T-type attenuator according to Patent Document 1. 
         FIG. 3  is a graph showing a difference in attenuation characteristics between two attenuators. 
         FIG. 4  is a view showing a configuration of a communication device of the present invention. 
         FIG. 5  is a view showing a configuration of an attenuator including a capacitance element serving as a regulator circuit. 
         FIG. 6  is a graph showing variation in attenuation characteristics in the case of using the attenuator including the capacitance element serving as the regulator circuit. 
         FIG. 7  is a view showing another configuration of an attenuator including a capacitance element serving as the regulator circuit. 
         FIG. 8  is a graph showing variation in attenuation characteristics in the case of changing a capacitance value of the capacitance element serving as the regulator circuit. 
         FIG. 9  is a view showing another configuration of an attenuator including capacitance elements serving as the regulator circuits. 
         FIG. 10  is a view showing still another configuration of an attenuator including capacitance elements serving as the regulator circuits. 
         FIG. 11  is a view showing yet another configuration of an attenuator including capacitance elements serving as the regulator circuits. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
     An embodiment of the present invention will be described below with reference to the accompanying drawings.  FIG. 4  shows a configuration of a communication device  400  according to an embodiment of the present invention. The communication device  400  includes: a transmission signal processing unit  401  that performs signal processing such as digital baseband processing or modulation processing on a transmission side; a driver amplifier  402  that amplifies a signal outputted from the transmission signal processing unit  401 ; and a reception signal processing unit  411  that performs signal processing such as digital baseband processing or modulation processing on a reception side. The signal outputted from the driver amplifier  402  is either outputted directly from an antenna  407  or outputted from the antenna  407  after being passed through any of attenuators  403  to  406  (ATT 1  to ATT 4 ). For example, it is possible to design the attenuators ATT 1  to ATT 4  to have various amounts of attenuation of gains of the signal in the descending order. A selector  408  determines a passage of transmitting, to the antenna  407 , the signal outputted from the driver amplifier  402  on the basis of an instruction from a controller  409 . When the controller  409  receives a notification signal that notifies the controller  409  of deterioration in quality of the signal transmitted from the controller  409  through a reception passage and the reception signal processing unit  411 , the controller  409  determines the amount of attenuation of the gain of the signal to be transmitted on the basis of the output from the driver amplifier  402  and the amounts of attenuation of the gains of the attenuators ATT 1  to ATT 4 , which are written in a memory  410  in advance. Then, the controller  409  sends an instruction to the selector  408  so that the selector  408  can select an appropriate attenuator to achieve the necessary amount of attenuation of the gain. The antenna  407  receives the signal with the attenuated gain from any one of the attenuators ATT 1  to ATT 4  and the outputs the signal. Here, the memory  410  stores information as to which attenuator has how much amount of attenuation at which frequency. Note that, whether or not the quality of the signal deteriorates is judged by a different communication device serving as a communication counterpart of the communication device  400 . The notification signal mentioned above is also transmitted from the above-described different communication device to the communication device  400 . One method of judging the deterioration in quality is to use a signal-to-noise ratio of a reception signal, for example. The different communication device transmits the notification signal to the communication device  400  when the signal-to-noise ratio deteriorates to a predetermined degree. 
     Next,  FIG. 5  shows an attenuator used in the communication device  400  according to this embodiment.  FIG. 5  shows an attenuator  500  used in the communication device. The attenuator  500  is a two-terminal pair network (two-port circuit) which includes circuit elements between ports formed of terminals  501  and  503  and ports formed of terminals  502  and  504 . In the attenuator  500 , metal-oxide-semiconductor (MOS) transistors  505  and  506 , which are a type of field-effect transistors, are connected in series between the terminal  501  and the terminal  502  as an example of a first circuit and a second circuit. Moreover, the attenuator  500  includes a MOS transistor  507  connected in shunt between the MOS transistors  505  and  506 . Here, a circuit  509  formed of the MOS transistors  505  to  507  is a general T-type attenuator formed of the MOS transistors. Moreover, a capacitance element  508  is connected in shunt between the MOS transistors  505  and  506  relative to the T-type attenuator. Since the MOS transistor  507  and the capacitance element  508  serve as shunt components in the two terminal pair network, they are also connected to an interconnection that connects between the terminal  503  and the terminal  504 . A value of the capacitance element  508  may be set to 20 [fF], for example. These MOS transistors are usually formed of n-type MOS transistors. However, p-type MOS transistors are also applicable. Moreover, each of the MOS transistors  505  to  507  includes a gate terminal. 
       FIG. 6  shows how the attenuation characteristic changes when the attenuator  500  shown in  FIG. 5  is used. The longitudinal axis shows an amount of attenuation while the lateral axis shows the frequency. Here, a range of the frequency used in communications is indicated as a “to-be-used frequency band”. Now, among the attenuators included in the communication device  400  shown in  FIG. 4 , the attenuator ATT 1  is assumed to have an attenuation characteristic C. Then, a T-type attenuator formed of the circuit  509  in  FIG. 5  is assumed to have an attenuation characteristic D. In this case, if the T-type attenuator formed of the circuit  509  is applied to the attenuator ATT 2  in the communication device  400 , the difference in the amount of attenuation between the attenuators ATT 1  and ATT 2  significantly varies in the to-be-used frequency band. Accordingly, assume that the attenuator  500  shown in  FIG. 5 , in which the shunt capacitor  508  is added to the T-type attenuator formed of the circuit  509 , is used as the attenuator ATT 2 . This attenuator  500  has an attenuation characteristic E. In the attenuator  500 , the shunt capacitor  508  is connected between the MOS transistors  505  and  506  in the T-type attenuator formed of the circuit  509 . For this reason, in the case of the attenuator  500 , a shunt component of an input signal is increased whereas an amount of current of an output signal is decreased in comparison with the T-type attenuator formed of the circuit  509 . As a consequence, the attenuator  500  attenuates the gain of the input signal more greatly than the T-type attenuator formed of the circuit  509 . Moreover, the attenuation of the gain of the input signal by this shunt capacitor  508  becomes greater as the input signal has a higher frequency. This is attributable to a fact that impedance of the shunt capacitor  508  is attenuated in accordance with the increase in the frequency of the input signal whereby the current component to be shunted through the shunt capacitor  508  is increased. In other words, it is possible to say that the shunt capacitor  508  is a regulator circuit for adjusting the amount of attenuation of the gain of the input signal. A circuit designer evaluates the attenuation characteristic while changing the capacitance value of the shunt capacitor  508 , and determines the capacitance value that achieves the attenuation characteristic by which the difference in the amount of attenuation relative to the attenuation characteristic C is made constant within the to-be-used frequency band. Thereafter, the circuit designer is able to use the attenuator  500  including the inserted shunt capacitor  508  having the determined capacitance value as the attenuator ATT 2  in the communication device  400 . As a result, the difference in the amount of attenuation becomes constant between the attenuators ATT 1  and ATT 2  within the to-be-used frequency band. 
     The description has been made above regarding the concept of applying the attenuator  500  shown in  FIG. 5  to the attenuator ATT 2  in order to achieve the constant difference in the amount of attenuation of the gains between the attenuators ATT 1  and ATT 2  on the assumption that the attenuator ATT 1  included in the communication device  400  has the attenuation characteristic C. Similarly, in order to achieve the constant difference in the amount of attenuation of the gains between the attenuators ATT 2  and ATT 3  included in the communication device  400 , it is also possible to further apply the attenuator  500  shown in  FIG. 5  to the attenuator ATT 3 . In that case, the shunt capacitor  508  is appropriately determined to achieve the constant difference in the amount of attenuation of the gains between the attenuators ATT 2  and ATT 3  in the to-be-used frequency band, and the attenuator  500  applying the shunt capacitor  508  having that capacitance value may be used as the attenuator AAT 3 . Similarly, it is also possible to use the attenuator  500  as the attenuator ATT 4  when achieving the constant difference in the amount of attenuation of the gains between the attenuators ATT 3  and ATT 4 . 
       FIG. 7  shows an attenuator  700  which is formed by replacing the capacitance element  508  in the attenuator  500  according to  FIG. 5  with a variable capacitance element  708 . Among circuit elements in the attenuator  700 , the configurations other than the variable capacitance element  708  are similar to those in the attenuator  500 . The circuit designer applies the attenuator  700 , for example, to the attenuator ATT 2  out of the attenuators ATT 1  and ATT 2  included in the communication device  400 . Then, the difference in the attenuation characteristic between the attenuators ATT 1  and ATT 2  is evaluated while changing the capacitance value of the variable capacitance element  708 . The circuit designer determines an appropriate capacitance value that achieves the constant difference in the to-be-used frequency band that covers the frequencies used in the communications. Similarly, it is also possible to adjust the difference in the amount of attenuation between the attenuators ATT 3  and ATT 4  included in the communication device  400  by using the attenuator  700 . 
       FIG. 8  shows variation in the attenuation characteristic of the attenuator  700  in the case of setting each gate width of MOS transistors  705  and  706  to 13.5 [μm], setting a gate width of a MOS transistor  707  to 18.2 [μm], and changing the capacitance value of the variable capacitance element  708  to 0 [fF], 20 [fF], 50 [fF], and 100 [fF], the MOS transistors  705  to  707  and the variable capacitance element  708  constituting the attenuator  700  according to  FIG. 7 . As apparent from  FIG. 8 , it is possible to adjust the attenuation characteristic of the attenuator  700  by changing the capacitance value of the variable capacitance element  708 . 
       FIG. 9  shows an attenuator  900  in which two more capacitance elements are further added to the attenuator  700  shown in  FIG. 7 . This attenuator  900  is applicable to each of the attenuators in the communication device  400  shown in  FIG. 4 . The attenuator  900  includes MOS transistors  905 ,  906 , and  907  as well as a variable capacitance element  908 , which are circuit elements similar to those in the attenuator  700  in  FIG. 7 . Moreover, the attenuator  900  includes additional capacitance elements  909  and  910 . The attenuator  900  includes the capacitance elements  909  and  910  in addition to the capacitance element  908 . Accordingly, an influence on the attenuation characteristic, which is caused when the capacitance values of these capacitance elements vary at the time of manufacturing the attenuator  900 , can be reduced. Technically, the attenuation characteristic of the attenuator can be obtained by analyzing the frequency characteristic of S 12  and S 21  that are diagonal elements of a scattering matrix (S matrix). Here, one of the parameters that exert a large influence on the values of these S 12  and S 21  is the capacitance value of the capacitance element. Therefore, the attenuator  900  is provided with multiple capacitance elements in the circuit so that a change caused by variation in the capacitance elements of S 12  or S 21  that are the diagonal elements of the scattering matrix can be reduced. Specifically, S 12  and S 21  are fractional parameters. Thus, by providing the multiple capacitance elements in the circuit, terms that change depending on the capacitance value are included in both a denominator and a numerator of S 12  or S 21 . Accordingly, even when the capacitance elements  908 ,  909 , and  910  vary at the time of manufacturing the respective elements, the change in S 12  or S 21  is canceled out by a change in the denominator and a change in the numerator. As a consequence, the attenuation characteristic that can be obtained from S 12  or S 21  does not largely change by the variation of the capacitance values of the capacitance elements  908 ,  909 , and  910  at the time of manufacturing. 
       FIG. 10  shows an attenuator  1000  having a different circuit configuration. The circuit designer can also apply this attenuator  1000  to each of the attenuators included in the communication device  400 . In this attenuator  1000 , resistors  1011 ,  1012 , and  1013  are respectively connected to gate electrodes of MOS transistors  1005 ,  1006 , and  1007  which constitute the attenuator  1000 . Resistance values of the resistors  1011 ,  1012 , and  1013  are respectively set to 1 [kΩ], for instance. By connecting the resistors respectively to the gate electrodes of the MOS transistors  1005 ,  1006 , and  1007 , it is possible to prevent leakage of the input signal received at a terminal  1001  respectively through gate interconnections of the MOS transistors  1005 ,  1006 , and  1007 . This is because values of leak currents can be suppressed to low levels by high resistance values of the resistors  1011 ,  1012 , and  1013 . Note that, the attenuator  1000  shown in  FIG. 10  includes capacitance elements  1009  and  1010 . However, these capacitance elements exert similar effects to those of the capacitance elements  909  and  910  included in the attenuator  900  according to  FIG. 9 . Thus, these elements are not essential components for preventing the leakage of the input signal received at a terminal  901  respectively through the gate lines of the MOS transistors  905 ,  906 , and  906 . 
       FIG. 11  shows an attenuator  1100  in which the function of a variable capacitance element  1008  in the attenuator  1000  according to  FIG. 10  is achieved by a MOS transistor. The circuit designer can apply this attenuator  1100  to each of the attenuators in the communication device  400 . The attenuator  1100  shown in  FIG. 11  includes MOS transistors  1107 ,  1110 , and  1113  and capacitance elements  1108 ,  1111 , and  1114  that are respectively connected in series to these MOS transistors  1107 ,  1110 , and  1113 . The voltage to be applied to each of the gate electrodes of the MOS transistors  1107 ,  1110 , and  1113  is controlled to thereby change the number of the MOS transistors in which the source and the drain are conducted with each other, thus realizing the variable capacitance element. The large number of the MOS transistors among the MOS transistors  1107 ,  1110 , and  1113  in which the source and the drain are conducted with each other, makes the input signal component to be shunted large. This equivalently means that the capacitance value of the variable capacitance element  1008  in  FIG. 10  increases. On the contrary, the small number of the MOS transistors among the MOS transistors  1107 ,  1110 , and  1113  in which the source and the drain are conducted with each other, makes the input signal component to be shunted small. This equivalently means that the capacitance value of the variable capacitance element  1008  in  FIG. 10  decreases. Note that, in  FIG. 11 , the three MOS transistors  1107 ,  1110 , and  1113  contribute to realize the equivalent variable capacitance element, and the three capacitance elements  1108 ,  1111  and  1114  contribute to realize the equivalent variable capacitance element. However, the number of the MOS transistors and that of the capacitance elements are not limited to three. For example, the circuit designer can select the number of the MOS transistors and that of the capacitance elements appropriately depending on the range of the capacitance value to be changed. Further, capacitance elements  1116  and  1117  exhibit the same effect as that of the capacitance elements  909  and  910  included in the attenuator  900  according to  FIG. 9 , and are not essential components to realize the equivalent variable capacitance. Furthermore, resistances  1118  and  1119  exhibit the same effect as that of the resistances  1011  and  1013  included in the attenuator  1000  according to  FIG. 10 , and are not essential components to realize the equivalent variable capacitance. 
     By using the attenuator described above, the circuit designer can adjust the amounts of attenuation by the respective attenuators included in the communication device and also to adjust the differences in the amounts of attenuation among the respective attenuators. Note that, in  FIG. 10 , the resistors  1011 ,  1012 , and  1013  are provided together with the capacitors  1009  and  1010  that are provided in order to reduce the effect of the amount of attenuation caused by the variation at the time of manufacturing the capacitors. Here, it is also possible to apply only the resistors  1011 ,  1012 , and  1013  to the attenuator  500  shown in  FIG. 5  or the attenuator  700  shown in  FIG. 7 . Meanwhile, in  FIG. 11 , in addition to the capacitors  1116  and  1117  provided for reducing the effect of the amount of attenuation caused by the variation at the time of manufacturing the capacitors, and the resistors  1109 ,  1112 ,  1113 ,  1118 , and  1119  for preventing the leak current of the MOS transistors to the gate interconnections, there are provided the MOS transistors  1107 ,  1110 , and  1113 , and the capacitance elements  1108 ,  1111 , and  1114  that are connected in series to the respective MOS transistors  1107 ,  1110 , and  1113 . Alternatively, it is also possible to apply the MOS transistors  1107 ,  1110 , and  1113  as well as the capacitance elements  1108 ,  1111 , and  1114  respectively connected in series to these MOS transistors  1107 ,  1110 , and  1113  to the shunt capacitor  508  of the attenuator  500  shown in  FIG. 5  or to the shunt capacitor  708  of the attenuator  700  shown in  FIG. 7 . 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.