Abstract:
There is provided a communication device including: a first node connected to an antenna; a transmission unit outputting a signal to the antenna via the first node; a reception unit having a signal input thereto from the antenna via the first node; a first switch provided between the first node and the transmission unit; and a second switch provided between the first node and the reception unit, and in which the second switch is alternately turned on and off repeatedly, and the reception unit includes an amplifier amplifying a signal that the transmission unit outputs via the first and second switches and a mixer mixing a signal amplified in the amplifier and a local signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-223293, filed on Sep. 28, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present embodiments relate to a communication device. 
     BACKGROUND 
     In recent years, a demand for systems using a high frequency such as a millimeter wave radar and a broadband wireless LAN has increased. In order to provide these systems inexpensively, besides a reduction in price of a semiconductor itself, it is essential to reduce a cost necessary for inspection. The above inspection is to measure whether an IC satisfies its specification by actually inputting a high frequency signal, and all products to be shipped or a considerable number of products to be shipped are needed to be evaluated. In the evaluation, a high frequency measuring device and evaluation device are needed. Thus, money and time are necessary and it costs high for the evaluation. A system capable of evaluating products simply has been requested in order to reduce the cost for the above evaluation. 
     Further, there has been known a portable wireless terminal including: at least two antennas; a transmission/reception circuit to be used for transmitting and receiving signals; a reception circuit to be used only for receiving a signal; a signal strength detection unit detecting received signal strength of each of the above-described respective antennas; an antenna switching unit for connecting one of the above-described respective antennas to the above-described transmission/reception circuit and connecting the other of the antennas to the above-described reception circuit; and a control unit, based on received signal strength detected in the above-described signal strength detection unit, determining the antenna with the highest received signal strength of the above-described respective antennas and controlling the above-described antenna switching unit to connect the above-described antenna with the highest received signal strength to the above-described transmission/reception circuit. 
     Further, there has been known a variable attenuator being a variable attenuator attenuating a signal input to an input terminal to output the attenuated signal from an output terminal, the variable attenuator including: a plurality of transmission lines connected in series between the input terminal and the output terminal; a first resistance element connected in parallel to the transmission line connected to the input terminal; and a second resistance element connected in parallel to the transmission line connected to the output terminal. 
     Further, there has been known a high frequency switch device including first to third terminals, a first FET, a first inductor and a first capacitor connected in parallel to the first FET respectively, a first circuit having one end thereof connected to the first terminal, a second FET, a second inductor and a second capacitor connected in parallel to the second FET respectively, and a second circuit having one end thereof connected to the other end of the first circuit and having the other end thereof connected to the second terminal.
     [Patent Document 1] Japanese Laid-open Patent Publication No. 2004-363862   [Patent Document 2] International Publication Pamphlet No. WO 2006/100726   [Patent Document 3] Japanese Laid-open Patent Publication No. 11-46101   

     SUMMARY 
     A communication device includes: a first node connected to an antenna; a transmission unit outputting a signal to the antenna via the first node; a reception unit having a signal input thereto from the antenna via the first node; a first switch provided between the first node and the transmission unit; and a second switch provided between the first node and the reception unit, and in which the second switch is alternately turned on and off repeatedly, and the reception unit includes an amplifier amplifying a signal that the transmission unit outputs via the first and second switches and a mixer mixing a signal amplified in the amplifier and a local signal. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view depicting a configuration example of a communication device according to a first embodiment; 
         FIG. 2  is a circuit diagram depicting a configuration example of a first switch and a second switch according to a second embodiment; 
         FIG. 3  is a circuit diagram depicting a configuration example of a first switch and a second switch according to a third embodiment; 
         FIG. 4  is a circuit diagram depicting a configuration example of a first switch and a second switch according to a fourth embodiment; 
         FIG. 5A  and  FIG. 5B  are circuit diagrams depicting configuration examples of a first switch and a second switch according to a fifth embodiment; 
         FIG. 6  is a circuit diagram depicting a configuration example of a first switch and a second switch according to a sixth embodiment; and 
         FIG. 7  is a circuit diagram depicting a configuration example of a communication device according to a seventh embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a view depicting a configuration example of a communication device according to a first embodiment. The communication device has an integrated circuit (IC)  100  and an antenna  104 . The integrated circuit  100  has a transmission unit  101 , a reception unit  102 , and a switch unit  103 . The transmission unit  101  has an oscillator  111 , a modulation circuit  112 , and a power amplifier  113 . The reception unit  102  has a low-noise amplifier  114  and a frequency mixer (mixer)  115 . The switch unit  103  has a first switch  117 , a second switch  118 , and a switch driver  116 . 
     A first node N 1  is connected to the antenna  104  via an antenna terminal  107 . A transmission unit terminal  105  is connected to an output terminal of the transmission unit  101 . A reception unit terminal  106  is connected to an input terminal of the reception unit  102 . The first switch  117  is connected between the first node N 1  and the transmission unit terminal  105 . The second switch  118  is connected between the first node N 1  and the reception unit terminal  106 . The oscillator  111  oscillates a high frequency (RF) signal at a first frequency f1. The first frequency f1 is, for example, 77 GHz. The modulation circuit  112 , based on a transmission signal DTx, modulates an oscillation signal from the oscillator  111  to output a signal LO. The transmission signal DTx is, for example, 10 MHz. The power amplifier  113  amplifies an output signal from the modulation circuit  112  to output an amplified signal to the transmission unit terminal  105 . The low-noise amplifier  114  amplifies a signal input from the reception unit terminal  106  to output a signal RF. The frequency mixer  115  mixes the signal RF and the signal LO to output a signal IF. 
     The communication device has a transmission mode, a reception mode, and an inspection mode. First, the transmission mode is explained. In the transmission mode, by control of the switch driver  116 , the first switch  117  is turned on and the second switch  118  is turned off. Concretely, the oscillator  111  oscillates a high frequency signal. The modulation circuit  112  modulates an oscillation signal from the oscillator  111  based on the transmission signal DTx. The power amplifier  113  amplifies an output signal from the modulation circuit  112  to wirelessly transmit an amplified signal via the first switch  117  and the antenna  104 . The signal to be transmitted is attenuated by the wireless transmission to thus be amplified in the power amplifier  113 . As described above, the transmission unit  101  outputs a signal to the antenna  104  via the first switch  117  and the first node N 1 . 
     Next, the reception mode is explained. In the reception mode, by control of the switch driver  116 , the first switch  117  is turned off and the second switch  118  is turned on. The reception unit  102  has a signal input thereto from the antenna  104  via the first node N 1  and the second switch  118 . The low-noise amplifier  114  has the signal that is wirelessly received via the antenna  104  and the second switch  118  input thereto and amplifies the input signal to output a signal. The modulation circuit  112  outputs an oscillation signal from the oscillator  111  as it is as the signal LO. The frequency mixer  115  mixes the signal RF output from the low-noise amplifier  114  and the signal (local signal) LO output from the modulation circuit  112  to output the intermediate frequency signal IF. For the output signal from the frequency mixer  115 , processing such as demodulation is performed in a demodulation circuit. 
     Next, the inspection mode is explained. In the inspection mode, a signal is input to the reception unit  102 , and based on a ratio of power of the input signal to the reception unit  102  and power of an output signal from the reception unit  102 , an inspection of a gain of the reception unit  102  is performed. In the inspection mode, by control of the switch driver  116 , the first switch  117  is turned off (fixed), and the second switch  118  is alternately turned on and off repeatedly by a second frequency f2. The oscillator  111  oscillates a high frequency signal at the first frequency f1, (which is 77 GHz, for example). In the inspection mode, the modulation circuit  112  does not perform modulation to output an oscillation signal from the oscillator  111  as it is as the signal LO. The power amplifier  113  amplifies the signal LO output from the modulation circuit  112 . 
     The first switch  117  is off. The first node N 1  has a leakage signal input thereto from the power amplifier  113  via the first switch  117 . Note that in the reception mode, the signal that the antenna  104  receives wirelessly is a weak signal. Thus, the signal that the first node N 1  receives from the antenna  104  in the reception mode and the leakage signal that the first node N 1  receives from the power amplifier  113  in the inspection mode are signals of substantially the same power, and an inspection on substantially the same condition as that of the reception mode can be performed. When the first switch  117  is turned on tentatively, the signal that the first node N 1  receives from the power amplifier  113  in the inspection mode is more increased in power than the signal that the first node N 1  receives from the antenna  104  in the reception mode, which is inappropriate. The first switch  117  is in an off state and thereby functions as an attenuator attenuating a signal. 
     The second switch  118  has a control signal at the second frequency f2, (which is, for example, 1 kHz), input thereto and is turned on and off repeatedly at the second frequency f2. As a result, the second switch  118  has a signal at the first frequency f1 input thereto from the first node N 1  to output a signal at a third frequency f3 to the low-noise amplifier  114 . The third frequency f3 is a frequency of f1±f2. The low-noise amplifier  114  amplifies the signal input from the second switch  118  to output the signal RF at the third frequency f3. 
     The frequency mixer  115  mixes the signal RF output from the low-noise amplifier  114  and the signal (local signal) LO output from the modulation circuit  112  to output the intermediate frequency signal IF. Concretely, the frequency mixer  115  outputs the intermediate frequency signal IF at the second frequency f2 being a difference frequency between the signal RF at the third frequency f3 (=f1±f2) and the local signal LO at the first frequency f1. For example, the first frequency f1 is 77 GHz, and the second frequency f2 is 1 kHz. Since the frequency mixer  115  outputs the signal IF at the low frequency f2, the inspection can be performed by an inexpensive device. 
     As described above, the frequency mixer  115  has the signal RF and the local signal LO that is generated in the transmission unit  101  input thereto to generate the signal IF at the difference frequency therebetween. A power detection unit detects power of the frequency mixer  115 . Further, the power detection unit detects power of an input signal to the low-noise amplifier  114  in advance. An inspection device can inspect a gain of the reception unit  102  (low-noise amplifier  114  and frequency mixer  115 ) by taking a ratio of the power of the signal IF output from the frequency mixer  115  and the power of the input signal to the low-noise amplifier  114 . 
     Even when the first switch  117  is turned off, part of the signal at the first frequency f1 actually passes through the first node N 1  as leakage power. Normally, power of −20 dB to −30 dB or so ( 1/100 to 1/1000) passes through the first node N 1 . In this embodiment, the above leakage power is used for the inspection of the reception unit  102 . By using the power leaked to the first node N 1 , on/off switching in the second switch  118  is performed. When a frequency of the switching thereof is set as f2 and modulation is performed in the second switch  118 , a signal having a frequency component of f1±f2 is input to the reception unit  102 . In the reception unit  102 , the modulated signal is amplified in the low-noise amplifier  114  to then be input to the frequency mixer  115 . Part of the signal LO generated in the transmission unit  101 , (of which the frequency is f1), is also input to the frequency mixer  115 . The frequency mixer  115  outputs the signal IF at the frequency f2 being a difference component between the two frequencies f1 and f3. By obtaining a ratio of strength of the signal to be input to the reception unit  102  and strength of the signal to be output from the reception unit  102 , a gain of the reception unit  102  can be obtained. A gain of the reception unit  102 , which has not been able to be evaluated unless a high frequency signal at a frequency different from the first frequency f1 is externally input to the antenna terminal  107  so far, can be obtained by employing a system as above. 
     By detecting the power of the input signal to the low-noise amplifier  114  and the power of the output signal from the frequency mixer  115 , a gain of the reception unit  102  can be evaluated. Since the signal LO from the transmission unit  101  is used, without using an external high frequency signal source, gain performance of the reception unit  102  can be inspected inexpensively and an inspection cost can be reduced. 
     Note that the local signal LO to be input to the frequency mixer  115  does not have to be an output signal from the modulation circuit  112  as long as it is a signal at the same frequency as the first frequency f1. 
     Second Embodiment 
       FIG. 2  is a circuit diagram depicting a configuration example of a first switch  117  and a second switch  118  according to a second embodiment. 
     The first switch  117  has a plurality of first inductors  201 , a plurality of first n-channel field-effect transistors  202 , and a plurality of resistances  203 . The plural first inductors  201  are connected in series between a first node N 1  and a transmission unit terminal  105 . Note that the first inductors  201  may also be inductor components of a transmission line. The plural first n-channel field-effect transistors  202  have drains thereof connected to interconnection points of the plural first inductors  201 , have sources thereof connected to reference potential (ground potential) nodes, and have a voltage Vc 1  higher than a threshold voltage applied to gates thereof via the resistances  203  respectively. The voltage Vc 1  is a fixed voltage at a high level, and is supplied by the switch driver  116  in  FIG. 1 . As a result, the first n-channel field-effect transistors  202  are turned on to function as on-resistances  204  between the interconnection points of the first inductors  201  and the reference potential nodes. 
     The voltage Vc 1  is a voltage higher than a threshold voltage of the transistors  202 , which is, for example, 1 V. When the voltage Vc 1  is applied, equivalent circuits between the drains and the sources of the transistors  202  are represented as the resistances  204 . Thus, the resistances  204  are connected in parallel to the interconnection points of the inductors  201  between the transmission unit terminal  105  and the first node N 1 . When the resistances  204  are connected in parallel, impedances are reduced and power to be input from the transmission unit terminal  105  is short-circuited to the reference potential nodes substantially. As a result, the first switch  117  is turned off. Leakage power of the first switch  117  is −20 dB to −30 dB or so as described previously. 
     The second switch  118  has a plurality of second inductors  211 , a plurality of second n-channel field-effect transistors  212 , and a plurality of resistances  213 . The plural second inductors  211  are connected in series between the first node N 1  and a reception unit terminal  106 . Note that the second inductors  211  may also be inductor components of a transmission line. The plural second n-channel field-effect transistors  212  have drains thereof connected to interconnection points of the plural second inductors  211 , have sources thereof connected to reference potential nodes, and have a pulse voltage Vc 2  in which a voltage higher than a threshold voltage and a voltage lower than the threshold voltage are repeated alternately applied to gates thereof via the resistances  213  respectively. The voltage Vc 2  is a pulse voltage at a high level and a low level at a second frequency f2, and is supplied by the switch driver  116  in  FIG. 1 . When the voltage Vc 2  turns to a high level, the second re-channel field-effect transistors  212  are turned on to function as on-resistances  214  between the interconnection points of the second inductors  211  and the reference potential nodes. Further, when the voltage Vc 2  turns to a low level, the second re-channel field-effect transistors  212  are turned off to function as capacitances  215  between the interconnection points of the second inductors  211  and the reference potential nodes. 
     The voltage Vc 2  is a pulse voltage in which a voltage higher than a threshold value of the transistors  212 , (which is, for example, 1 V), and a voltage lower than the threshold value of the transistors  212 , (which is, for example, −1 V), are repeated alternately. The operation in the case when a voltage higher than the threshold value is applied is described previously in the first switch  117 , so that it is omitted. When a voltage lower than the threshold value is applied, the drains-to-sources of the transistors  212  are represented as equivalent circuits of the capacitances  215 . A capacitance value of each of the capacitances  215  is selected to be a value to math with an inductance of each of the inductors  211 , and thereby a passed state is achieved. For example, when the capacitance value of each of the capacitances  215  is set as C and the inductance of each of the indictors  211  is set as L, L and C are selected so that a characteristic impedance √{square root over ( )}(L/C) becomes 50Ω. By impedance matching, the second switch  118  is turned on. Since the voltage Vc 2  is a pulse voltage at the second frequency f2, the second switch  118  is alternately turned on and off repeatedly at the second frequency f2. 
     Power to be input from the transmission unit terminal  105  is tentatively set to be −10 dBm (10 mW) and leakage power of the first switch  117  is tentatively set to be −30 dB. Then, when the first switch  117  is turned off as described above, power of the first node N 1  becomes −20 dBm or so. Further, when the second switch  118  is turned on, power of the reception unit terminal  106  becomes −20 dBm or so, and when the second switch  118  is turned off, power of the reception unit terminal  106  becomes −50 dBm or so. The second switch  118  is turned on/off, and thereby modulation signals of −20 dBm/−50 dBm are output to the reception unit terminal  106 . Note that P [dBm] is expressed as 10 P [mW]/10 . 
     Third Embodiment 
       FIG. 3  is a circuit diagram depicting a configuration example of a first switch  117  and a second switch  118  according to a third embodiment. 
     The first switch  117  has a plurality of first inductors  201 , a plurality of first n-channel field-effect transistors  202 , and a plurality of resistances  203 . The plural first inductors  201  are connected in series between a first node N 1  and a transmission unit terminal  105 . The plural first n-channel field-effect transistors  202  have sources and drains thereof connected in series between the plural first inductors  201 . A voltage Vc 1  higher than a threshold voltage is applied to respective gates of the plural first n-channel field-effect transistors  202  via the resistances  203 . The voltage Vc 1  is a fixed voltage at a high level, and is supplied by the switch driver  116  in  FIG. 1 . As a result, the first n-channel field-effect transistors  202  are turned on to function as on-resistances  204  between the sources and the drains. 
     In the second embodiment, the transistors  202  are connected in parallel. This embodiment is configured in a manner that the transistors  202  are connected in series. The case when the voltage Vc 1  is fixed at a voltage higher than a threshold value is explained. As described previously, in the above case, the drains-to-sources of the transistors  202  are represented as the resistances  204  in equivalent circuits. When the resistances  204  are connected in series, impedances are increased, and the first switch  117  turns to an off (open) state. 
     The second switch  118  has a plurality of second inductors  211 , a plurality of second n-channel field-effect transistors  212 , and a plurality of resistances  213 . The plural second inductors  211  are connected in series between the first node N 1  and a reception unit terminal  106 . The plural second re-channel field-effect transistors  212  have sources and drains thereof connected in series between the plural second inductors  211 . A pulse voltage Vc 2  in which a voltage higher than a threshold voltage and a voltage lower than the threshold voltage are repeated alternately is applied to respective gates of the plural second n-channel field-effect transistors  212 . The voltage Vc 2  is a pulse voltage at a second frequency f2, and is supplied by the switch driver  116  in  FIG. 1 . When the voltage Vc 2  turns to a high level, the second n-channel field-effect transistors  212  are turned on to function as on-resistances  214  between the sources and the drains. Further, when the voltage Vc 2  turns to a low level, the second re-channel field-effect transistors  212  are turned off to function as capacitances  215  between the sources and the drains. 
     The voltage Vc 2  is a voltage in which a voltage higher than a threshold value of the transistors  212  and a voltage lower than the threshold value of the transistors  212  are alternately repeated. The operation in the case when a voltage higher than the threshold value is applied to turn the second switch  118  off is similar to that of the above-described first switch  117 . The case of the second switch  118  being in an on state is explained. When a voltage lower than the threshold value is applied to the gates of the transistors  212 , equivalent circuits of the transistors  212  turn to the capacitances  215 . By selecting an appropriate value as a size of each of the transistors  212 , each of the capacitances  215  to have a value to cancel an impedance (L) of each of the inductors  211  is achieved, and then a synthesized impedance Z of the inductor  211  and the transistor  212  (capacitance  215 ) is expressed by the following expression.
 
 Z=jΩL+ 1/( jΩC )
 
     Here, j is a symbol of an imaginary number, w is an angular frequency (=2×π×f), f is a frequency, L is an inductance of the inductor  211 , and C is a capacitance value of the equivalent capacitance  215  between the drain and the source of the transistor  212 . The inductance L and the capacitance value C (size of the transistor  212 ) are selected so that the value of the synthesized impedance Z becomes 0, and thereby the impedance Z of the second switch  118  is reduced, (which is 0 ideally), and the second switch  118  turns to an on (continuity) state. That is, L and C are selected to be f=1/{2×π×√{square root over ( )}(L×C)}. 
     Since the voltage Vc 2  is a pulse voltage at the second frequency f2, the second switch  118  is alternately turned on and off repeatedly at the second frequency f2. By using the second switch  118 , modulation signals at the second frequency f2 can be generated and a gain of a reception unit  102  can be simply measured. 
     Fourth Embodiment 
       FIG. 4  is a circuit diagram depicting a configuration example of a first switch  117  and a second switch  118  according to a fourth embodiment. This embodiment is the combination of the second embodiment and the third embodiment. This embodiment ( FIG. 4 ) is such that n-channel field-effect transistors  401  and  411  and resistances  402  and  412  are added to the second embodiment ( FIG. 2 ). Hereinafter, points where this embodiment differs from the second embodiment are explained. 
     The first switch  117  has the third n-channel field-effect transistor  401  and the resistance  402  besides first inductors  201 , first n-channel field-effect transistors  202 , and resistances  203 . The third n-channel field-effect transistor  401  has a source and a drain thereof connected in series between the plural first inductors  201 . A voltage Vc 1  higher than a threshold voltage is applied to a gate of the third n-channel field-effect transistor  401  via the resistance  402 . The third n-channel field-effect transistor  401 , similarly to the re-channel field-effect transistors  202  in the third embodiment, is turned on to function as a resistance  403  between the source and the drain. This brings the first switch  117  into an off state. 
     The second switch  118  has the fourth re-channel field-effect transistor  411  and the resistance  412  besides second inductors  211 , second n-channel field-effect transistors  212 , and resistances  213 . The fourth n-channel field-effect transistor  411  has a source and a drain thereof connected in series between the plural second inductors  211 . A pulse voltage Vc 2  in which a voltage higher than a threshold voltage and a voltage lower than the threshold voltage are alternately repeated is applied to a gate of the fourth n-channel field-effect transistor  411  via the resistance  412 . When the voltage Vc 2  is at a high level, the fourth n-channel field-effect transistor  411 , similarly to the n-channel field-effect transistors  212  in the third embodiment, is turned on to function as a resistance  214  between the source and the drain. When the voltage Vc 2  is at a low level, the fourth re-channel field-effect transistor  411  is turned off to function as a capacitance  215  between the source and the drain. This alternately brings the second switch  118  into an on state and an off state repeatedly at a second frequency f2. 
     Fifth Embodiment 
       FIG. 5A  and  FIG. 5B  are circuit diagrams depicting configuration examples of a first switch  117  and a second switch  118  according to a fifth embodiment. This embodiment enables a level of a signal to be input to a reception unit  102  to change arbitrarily. 
       FIG. 5A  is such that a switch  501  is added to the second embodiment ( FIG. 2 ). The switch  501  selectively supplies a voltage Vc 1  or Vc 2  to a gate of one transistor  212  of the plural second n-channel field-effect transistors  212 . That is, the voltage Vc 1  higher than a threshold voltage or the voltage Vc 2  in which a voltage higher than the threshold voltage and a voltage lower than the threshold voltage are alternately repeated is selectively applied to the gate of one of the second n-channel field-effect transistors  212 . The operation in the case when the switch  501  supplies the voltage Vc 2  to the gate of the transistor  212  is the same as that of the second embodiment. In the case when the switch  501  supplies the voltage Vc 1  to the gate of the transistor  212 , it is possible that the transistor  212  to which the voltage Vc 1  is supplied is turned on, and it functions as a resistance to attenuate a signal. 
     In the first to fourth embodiments, for example, the case when the modulation signals in which −20 dBm and −50 dBm appear alternately are output to the reception unit  102  is explained. In the case when the level of a signal to be input to the reception unit  102  is large, which is −20 dBm, it can be lowered in this embodiment. It is set in a manner that the voltage Vc 1  higher than a threshold value is applied to one of the gates of the second re-channel field-effect transistors  212 . In the case when the voltage Vc 1  higher than the threshold value is applied, the transistor  212  is regarded as the resistance, and thus the level of a signal to pass is attenuated. Thus, the level of a signal at the time of on, which is −20 dBm in the second embodiment, is attenuated. The number of the transistors  212  to which the fixed voltage Vc 1  is applied is increased and decreased by the switch  501 , and thereby the level of a signal to be input to the reception unit  102  can be set to be an arbitrary value. 
       FIG. 5B  is such that a switch  502  is added to the second embodiment ( FIG. 2 ). The switch  502  selectively supplies the voltage Vc 1  or Vc 2  to a gate of one transistor  202  of the plural first n-channel field-effect transistors  202 . That is, the voltage Vc 1  higher than the threshold voltage or the voltage Vc 2  in which a voltage higher than the threshold voltage and a voltage lower than the threshold voltage are alternately repeated is selectively applied to the gate of one of the first n-channel field-effect transistors  202 . The operation in the case when the switch  502  supplies the voltage Vc 1  to the gate of the transistor  202  is the same as that of the second embodiment. In the case when the switch  502  supplies the voltage Vc 2  to the gate of the transistor  202 , it is possible that the transistor  202  to which the voltage Vc 2  is supplied is turned on and off repeatedly to strengthen a signal. 
     In the second embodiment, when the level of a signal to be input to the reception unit  102  is not adequate, the number of the transistors  202  to which the voltage Vc 2  is supplied is increased, and thereby an amount of power to pass can be increased. 
     Note that this embodiment can be applied to the third and fourth embodiments besides the second embodiment. 
     Sixth Embodiment 
       FIG. 6  is a circuit diagram depicting a configuration example of a first switch  117  and a second switch  118  according to a sixth embodiment. This embodiment ( FIG. 6 ) is such that a fifth n-channel field-effect transistor  601  and resistances  602  and  603  are added to the second embodiment ( FIG. 2 ). Hereinafter, points where this embodiment differs from the second embodiment are explained. The fifth n-channel field-effect transistor  601  has a drain and a source thereof connected between a first node N 1  and a reference potential node. In the transmission mode and the reception mode, a voltage Vc 5  lower than a threshold voltage is applied to a gate of the fifth n-channel field-effect transistor  601  via the resistance  603 , and in the inspection mode, the voltage Vc 5  higher than the threshold voltage is applied to the gate of the fifth n-channel field-effect transistor  601  via the resistance  603 . The resistance  602  is connected between the source of the transistor  601  and the reference potential node. 
     In the transmission mode and the reception mode, the voltage Vc 5  turns to a voltage lower than a threshold value (low level). Then, the transistor  601  is turned off to function as a capacitance. A capacitance value of the above capacitance is set to be small, and thereby the above capacitance does not affect the operation. That is,  FIG. 6  becomes equivalent to the circuit in  FIG. 2 . 
     In the inspection mode, the voltage Vc 5  turns to a voltage higher than the threshold value (high level). Then, the transistor  601  is turned on to function as a resistance. The above resistance can serve as a function of absorbing leakage power of a signal to be input from a transmission unit terminal  105  and can lower the level of a signal to be output to a reception unit terminal  106 . 
     Note that this embodiment can be applied to the third to fifth embodiments besides the second embodiment. 
     Seventh Embodiment 
       FIG. 7  is a circuit diagram depicting a configuration example of a communication device according to a seventh embodiment. This embodiment ( FIG. 7 ) is such that power detection units  701  and  702  and a control unit  703  are added to the first embodiment ( FIG. 1 ). Hereinafter, points where this embodiment differs from the first embodiment are explained. The first power detection unit  701  has a first detector for rectifying a signal at a third frequency (RF frequency) f3 and detects power B of an input signal to a low-noise amplifier  114 . The second power detection unit  702  has a second detector for rectifying a signal at a second frequency (intermediate frequency) f2 and detects power A of an output signal from a frequency mixer  115 . The low-noise amplifier  114  is a variable gain amplifier. The control unit  703  controls a gain of the low-noise amplifier  114  in accordance with a ratio A/B of the power A of the output signal from the frequency mixer  115  to the power B of the input signal to the low-noise amplifier  114 . 
     In the inspection mode, gain performance is measured in order to select whether performance of an integrated circuit  100  satisfies its specification in a factory. Besides at the time of shipment, the gain performance is measured periodically (for example, every 30 minutes) to grasp a temperature fluctuation of a reception unit  102  and a gain performance fluctuation by a time-dependent change. In the case when the gain performance deteriorates, for example, a result thereof is fed-back on hardware or software to compensate the gain of the low-noise amplifier  114 . The control unit  703  increases the gain of the low-noise amplifier  114  when the ratio A/B of the powers is smaller than a reference value. The control unit  703  reduces the gain of the low-noise amplifier  114  when the ratio A/B of the powers is larger than the reference value. This makes it possible to prevent the temperature fluctuation or the gain fluctuation by a time-dependent change and to maintain a gain of the reception unit  102  in a fixed value. 
     As described above, the communication devices in the first to seventh embodiments can evaluate the performance of the reception unit  102  by a simple method. Items to be evaluated are the gains of the low-noise amplifier  114  and the frequency mixer  115 . In the case when a high frequency signal is externally input to the reception unit terminal  106  and power of the intermediate frequency signal IF is measured tentatively, an external high frequency signal source is needed, and thereby an inspection cost is increased. In the first to seventh embodiments, the signal LO to be generated in the transmission unit  101  is input to the reception unit  102 , and the signal LO is modulated by turning the second switch  118  on/off and modulation signals are input to the reception unit  102 . An external high frequency signal source is not needed, thus enabling the gain performance to be measured by an inexpensive device for low frequency and enabling an inspection cost to be reduced. 
     The communication devices in the first to seventh embodiments can be used for, for example, a millimeter wave radar for collision avoidance, a broadband wireless LAN, or the like. 
     By measuring power of an input signal to an amplifier and power of an output signal from a mixer, a gain of a reception unit can be evaluated. Since a signal from a transmission unit is used, without using an external high frequency signal source, performance of the reception unit can be inspected inexpensively and an inspection cost can be reduced. 
     Note that the above-described embodiments merely illustrate concrete examples of implementing the present embodiments, and the technical scope of the present embodiments is not to be construed in a restrictive manner by these embodiments. That is, the present embodiments may be implemented in various forms without departing from the technical spirit or main features thereof. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention has(have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.