Abstract:
A frequency converter includes: a first terminal through which a local oscillator signal is input; a second terminal through which an input signal with a frequency to be converted is input; a third terminal through which an output signal with a different frequency resulting from the conversion is output; and a field effect transistor with gate, source and drain terminals for converting the frequency of the input signal and outputting the signal with the different frequency as the output signal. The gate terminal is connected to the first terminal, while the drain terminal is connected to the second and third terminals. The frequency converter further includes a trap circuit, which resonates at a frequency of a harmonic of the local oscillator signal to substantially eliminate the harmonic.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a frequency converter using a field effect transistor (FET). 
     A transceiver for use in mobile telecommunications sets or TV receivers needs a frequency converter. The frequency converter converts a radio frequency (RF) signal with a high frequency ranging from 1 to 20 GHz into an intermediate-frequency (IF) signal with a frequency ranging from 10 to 1,000 MHz, or vice versa, using a local oscillator (LO) signal. 
     To cope with recent upsurge in number of cellular phone users, demand for digital signal transmission and reception techniques has been steeply rising in the field of mobile telecommunications. Also, digital broadcasting has been rapidly popularized to meet the strong demand for multi-channel satellite or ground wave telecasting. For these purposes, the distortion involved with a frequency converter should be reduced as much as possible using FET&#39;S. 
     Frequency converters with an FET are classified into several types depending on the combination of LO, RF and IF signals with the three input terminals of the FET, namely, source, drain and gate terminals. 
     Among these various types, a type of frequency converter, in which the LO and RF signals are input to the gate and drain terminals of an FET and the IF signal is output through its drain terminal, is most preferable, because such a converter attains presently lowest possible distortion. 
     Hereinafter, a prior art frequency converter of this type will be described with reference to FIG.  6 . 
     FIG. 6 illustrates a circuit configuration for the prior art frequency converter. As shown in FIG. 6, the gate terminal  1   a  of an FET  1  is connected to a first terminal  3 , to which an LO signal is input, via an LO matching circuit  2 . The drain terminal  1   b  of the FET  1  is connected to not only a second terminal  5  through an RF matching circuit  4  but also a third terminal  7  by way of an IF matching circuit  6 . And the source terminal  1   c  of the FET  1  is grounded. The impedances of the LO, RF and IF matching circuits  2 ,  4  and  7  have been optimized in accordance with the frequencies of their associated LO, RF and IF signals, respectively. 
     Suppose this frequency converter is applied to downconversion, version, i.e., to convert a signal with a relatively high frequency into a signal with a relatively low frequency. In that case, the RF signal, which has been input to the second terminal  5 , is converted into the IF signal using the LO signal that has been input through the first terminal  3 , and then output through the third terminal  7 . Conversely, suppose this frequency converter is applied to upconversion, i.e., to convert a signal with a relatively low frequency into a signal with a relatively high frequency. In that case, the IF signal, which has been input to the third terminal  7 , is converted into the RF signal using the LO signal that has been S input through the first terminal  3 , and then output through the second terminal  5 . 
     Next, it will be described how the conventional frequency converter operates as a downconverter. 
     First, an LO signal with an alternating voltage, which has been input through the first terminal  3 , is passed through the LO matching circuit  2  and then input to the gate terminal  1   a  of the FET  1 . The FET  1  serves as a switch, which turns ON when the LO signal is positive and turns OFF when the LO signal is negative. Also, there is a channel resistor R d▪  (not shown) inside the FET  1 . The channel resistor R d▪  functions as a nonlinear resistor having a resistance changing nonlinearly with time. Accordingly, when a relatively high alternating voltage (i.e., the LO signal) is applied to the gate terminal  1   a  of the FET  1 , the RF signal, which has been input to the drain terminal  1   b  of the FET  1 , is converted into the IF signal due to the existence of the nonlinear channel resistor R d▪ . Then, the IF signal is output through the third terminal  7 . Suppose the frequencies of the RF, LO and IF signals are represented as f RF , f LO  and f IF , respectively. Since f IF , represents a difference between f RF , and f LO , f IF =|f RF −f LO |. 
     On the other hand, when the frequency converter functions as an upconverter, the IF signal input through the third terminal  7  is converted into the RF signal with a frequency represented as the sum of the frequencies f IF  and f LO  of the IF and LO signals; |f IF +f LO |=f RF . Then, the RF signal is output through the second terminal  5 . 
     The prior art frequency converter, however, has various shortcomings. Firstly, the frequency conversion performed by the converter is affected by the nonlinear channel resistor R d▪  to generate second and third harmonics with twice and thrice the frequencies of the fundamental frequency f LO  of the LO signal, thus interfering with the frequency conversion by the FET  1 . 
     Accordingly, when the frequencies of the LO, RF and IF signals are 2.2 GHz, 2.0 GHz and 200 MHz, respectively, the conventional frequency converter results in a conversion loss as high as about 7 dB. 
     Secondly, an LO signal amplifier including another FET usually precedes the first terminal  3  in a telecommunications system and those second and third harmonics are also generated during amplification by the LO signal amplifier. And those harmonics are input to the FET  1 , too. 
     That is to say, the FET  1  is further affected by the additional harmonica produced by the FET on the previous stage. Accordingly, when the frequencies of the LO, RF and IF signals are 2.2 GHz, 2.0 GHz and 200 MHz, respectively, the conversion version loss involved with the conventional frequency conversion is as high as about 8 dB. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to reduce a conversion loss caused by a frequency converter. 
     A first inventive frequency converter includes: a first terminal through Which a local oscillator signal is input; a second terminal through which an input signal with a frequency to be converted is input; a third terminal through which an output signal with a different frequency resulting from the conversion is output; and a field effect transistor with gate, source and drain terminals for converting the frequency of the input signal and outputting the signal with the different frequency as the output signal. The gate terminal is connected to the first terminal, while the drain terminal is connected to the second and third terminals. The frequency converter further includes a trap circuit, which is connected to the source terminal of the field effect transistor and resonates at a frequency of a harmonic of the local oscillator signal, thereby substantially eliminating the harmonic. 
     In the first frequency converter, the trap circuit resonates at a frequency of a harmonic of the local oscillator signal, thereby substantially eliminating the harmonic. That is to say, the frequency conversion by the field effect transistor is much less interfered with by the harmonic, thus attaining reduced conversion loss and improved conversion efficiency. Accordingly, supposing the first inventive frequency converter results in a conversion loss at the same level as the prior art converter, the inventive converter can greatly reduce the power level of the LO signal. As a result, this converter can greatly contribute to reduction in power dissipated by a wireless communications system. 
     In one embodiment of the present invention, the trap circuit preferably includes: an LC serial circuit consisting of an inductor and a capacitor that are connected in series to each other; and a resistor connected in parallel to the LC serial circuit. In the trap circuit, f=1/(2π×(LC) ½ ) is preferably met, where f is the frequency of the harmonic of the local oscillator signal, L is an inductance of the inductor and C is a capacitance of the capacitor. And one terminal of the trap circuit is preferably connected to the source terminal of the field effect transistor, while the other terminal of the trap circuit is preferably grounded. 
     In such an embodiment, the trap circuit resonates at a frequency of a second harmonic of the local oscillator signal. Accordingly, the frequency conversion performed by the field effect transistor is much less interfered with by the second harmonic. 
     A second inventive frequency converter includes: a first terminal through which a local oscillator signal is input; a second terminal through which an input signal with a frequency to be converted is input; a third terminal through which an output signal with a different frequency resulting from the conversion is output; and a field effect transistor with gate, source and drain terminals for converting the frequency of the input signal and outputting the signal with the different frequency as the output signal. The gate terminal is connected to the first terminal, while the drain terminal is connected to the second and third terminals. The converter further includes a trap circuit, which is connected to the gate terminal of the field effect transistor and resonates at a frequency of a harmonic of the local oscillator signal, thereby substantially eliminating the harmonic. 
     In the second frequency converter, the trap circuit resonates at a frequency of a harmonic of the local oscillator signal, thereby substantially eliminating the harmonic. That is to say, the frequency conversion performed by the field effect transistor is much less interfered with by the harmonic. In addition, the amplification performed by another field effect transistor, which is provided at a stage preceding the first terminal for amplifying the LO signal, is also much less interfered with by the harmonic, thus attaining far lower conversion loss and much higher conversion efficiency. Accordingly, supposing the second inventive frequency converter results in a conversion loss at the same level as the prior art converter, the inventive converter can greatly reduce the power level of the LO signal. As a result, this converter significantly contributes to further reduction in power dissipated by a wireless communications system. 
     In one embodiment of the present invention, the trap circuit preferably includes an LC serial circuit consisting of an inductor and a capacitor that are connected in series to each other. In the trap circuit, f=1/(2π×(LC) ½  is preferably met, where f is the frequency of the harmonic of the local oscillator signal, L is an inductance of the inductor and C is a capacitance of the capacitor. And one terminal of the trap circuit is preferably connected to the gate terminal of the field effect transistor, while the other terminal of the trap circuit is preferably grounded. 
     In such an embodiment, the trap circuit resonates at a frequency of a second harmonic of the local oscillator signal. Thus, both the frequency conversion by the field effect transistor and the amplification by another field effect transistor are much less interfered with by the second harmonic. 
     In an alternative embodiment, the trap circuit may includes an LC parallel circuit consisting of an inductor and a capacitor that are connected in parallel to each other. In the trap circuit, f=1/(2π×(LC) ½ ) is also preferably met, where f is the frequency of the harmonic of the local oscillator signal, L is an inductance of the inductor and C is a capacitance of the capacitor. And one terminal of the trap circuit is preferably connected to the gate terminal of the field effect transistor, while the other terminal of the trap circuit is preferably connected to the first terminal. 
     In such an embodiment, the trap circuit resonates at a frequency of a second harmonic of the local oscillator signal. Thus, both the frequency conversion by the field effect transistor and the amplification by another field effect transistor are much less interfered with by the second harmonic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram schematically illustrating a frequency converter according to a first embodiment of the present invention. 
     FIG. 2 is a circuit diagram illustrating a specific implementation of the frequency converter according to the first embodiment. 
     FIG. 3 is a circuit diagram schematically illustrating a frequency converter according to a second embodiment of the present invention. 
     FIG. 4 is a circuit diagram illustrating a specific implementation of the frequency converter according to the second embodiment. 
     FIG. 5 is a circuit diagram illustrating another specific implementation of the frequency converter according to the second embodiment. 
     FIG. 6 is a circuit diagram schematically illustrating a prior art frequency converter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EMBODIMENT 1 
     Hereinafter, a frequency converter according to a first embodiment of the present invention will be described with reference to FIGS. 1 and 2. 
     FIG. 1 illustrates a schematic circuit configuration for the frequency converter according to the first embodiment. As shown in FIG. 1, the gate terminal  10   a  of an FET  10  is connected to a first terminal  12 , to which an LO signal is input, via an LO matching circuit  11 . The drain terminal  10   b  of the FET  10  is connected to not only a second terminal  14  through an RP matching circuit  13  but also a third terminal  16  by way of an IF matching circuit  15 . 
     The frequency converter according to the first embodiment is characterized by grounding the source terminal  10   c  of the FET  10  via a trap circuit  17 . The trap circuit resonates at respective frequencies of the second and third harmonics of the LO signal, thereby substantially eliminating these harmonic components. The impedances of the LO, RF and IF matching circuits  11 ,  13  and  15  have been optimized in accordance with the frequencies of their associated LO, RF and IF signals, respectively. 
     Suppose the frequency converter of the first embodiment is applied to downconversion, to convert a signal with a relatively high frequency into a signal with a relatively low frequency. In that case, the RF signal, which has been input to the second terminal  14 , is converted into the IF signal using the LO signal that has been input through the first terminal  12 , and then output through the third terminal  16 . Conversely, suppose this frequency converter is applied to upconversion, i.e., to convert a signal with a relatively low frequency into a signal with a relatively high frequency. In that case, the IF signal, which has been input to the third terminal  16 , is converted into the RP signal using the LO signal that has been input through the first terminal  12 , and then output through the second terminal  14 . 
     Next, it will be described how the frequency converter of the first embodiment operates as a downconverter. 
     When the RF signal with a frequency f RF  is input through the second terminal  14  to the drain terminal  10   b  of the FET  10 , the frequency f RF  of the RF signal is reduced by the frequency f LO  of the LO signal, which has been input through the first terminal  12 , due to the existence of a nonlinear channel resistor R d┘ in the FET  10 . Thus, the RF signal is converted into the IF signal with a frequency f IF (=|f RF −f LO |), which is output through the third terminal  16 . 
     In this case, the second and third harmonics of the LO signal, which have been produced due to the existence of the channel resistor R d▪  in the FET, are substantially eliminated by the tap circuit  17 . That is to say, the frequency conversion performed by the FET  10  is much less interfered with by the harmonics. As a result, this frequency converter attains lower conversion loss and higher conversion efficiency. 
     For example, suppose the frequencies of the LO, RF and IF signals are 2.2 GHz, 2.0 GHz and 200 MHz, respectively, and the trap circuit  17  substantially eliminates the second and third harmonics. In that case, the conversion loss involved with the frequency converter is about 4 dB, which is about 3 dB lower than that involved with the prior art frequency converter. 
     FIG. 2 illustrates a specific implementation of the frequency converter according to the first embodiment. As shown in FIG. 2, the LO matching circuit  11  is an LC serial circuit consisting of an inductor  11   a  and a capacitor  11   b  that are connected in series to each other. A bias circuit  18  is provided between the LO matching circuit  11  and the FET  10 . One terminal of the bias circuit  18  is grounded via a resistor, while the other terminal thereof is connected to the gate terminal  10   a  of the FET  10  to apply a bias voltage thereto. It should be noted that the LO matching circuit  11  and bias circuit  18  may have any configurations other than those illustrated in FIG.  2 . 
     The trap circuit  17  is implemented as an LC resonator, which includes: an LC serial circuit consisting of an inductor  17   a  and a capacitor  17   b  that are connected in series to each other; and a resistor  17   c  connected in parallel to the LC serial circuit. One terminal of the LC resonator  17  is connected to the source terminal  10   c  of the FET  10 , while the other terminal thereof is grounded. 
     Suppose a resonant frequency condition given by f=1/(2π×(LC) ½ ) is met, where f is the frequency of the harmonic of the LO signal, L is an inductance of the inductor  17   a  and C is a capacitance of the capacitor  17   b . In that case, the trap circuit  17  resonates at a frequency of the second harmonic of the LO signal. Thus, the second harmonic component of the LO signal can be eliminated. Specifically, where the frequency of the LO signal is 2.2 GHz, the second harmonic component of the LO signal can be removed by setting the inductance L of the inductor  17   a  and capacitance C of the capacitor  17   b  to 1.3 nH and 1 pF, respectively. 
     In this manner, the trap circuit  17  can eliminate the second harmonic component of the LO signal with much more certainty. As a result, the conversion loss involved with this frequency converter is about 4.5 dB, which is about 2.5 dB lower than that of the prior art frequency converter. 
     EMBODIMENT 2 
     Next, a frequency converter according to a second embodiment of the present invention will be described with reference to FIGS. 3,  4  and  5 . 
     FIG. 3 illustrates a circuit configuration for the frequency converter according to the second embodiment. As shown in FIG. 3, the gate terminal  20   a  of an FET  20  is connected to a first terminal  23 , to which an LO signal is input, via an LO matching circuit  21  and a trap circuit  22 . The drain terminal  20   b  of the FET  20  is connected to not only a second terminal  25  through an RF matching circuit  24  but also a third terminal  27  by way of an IF matching circuit  26 . The impedances of the LO, RF and IF matching circuits  21 ,  24  and  26  have been optimized in accordance with the frequencies of their associated LO, RF and IF signals, respectively. In the example illustrated in FIG. 3, the LO matching circuit  21  precedes the trap circuit  22 . Alternatively, the trap circuit  22  may precede the LO matching circuit  21 . 
     The trap circuit  22  resonates at respective frequencies of the second and third harmonics of the LO signal to substantially eliminate these harmonics. Thus, the second and third harmonics of the LO signal, which have been produced due to the existence of a channel resistor R d▪  in the FET  20 , are substantially removed by the tap circuit  22 . That is to say, the frequency conversion performed by the FET  20  is much less interfered with by the harmonics. 
     In addition, according to the second embodiment, the trap circuit  22  is connected to the gate terminal  20   a  of the FET  20 . Accordingly, even if a second FET is connected as a preceding-stage amplifier to the first terminal  23  to amplify the LO signal and if second and third harmonics of the LO signal are produced during the amplification by the second FET, these harmonics are also removable by the trap circuit  22 . That is to say, the trap circuit  22  according to the second embodiment can eliminate not only the harmonics produced in the FET  20  but also those produced in the second FET functioning as an amplifier. As a result, the conversion loss involved with this frequency converter can be greatly reduced and the conversion efficiency can be considerably increased. 
     For example, suppose the frequencies of the LO, RF and IF signals are 2.2 GHz, 2.0 GHz and 200 MHz, respectively, and the trap circuit  22  substantially eliminates the second and third harmonics. In that case, the conversion loss involved with this frequency converter is about 4.5 dB, which is about 3.5 dB lower than that involved with the conventional frequency converter including an amplifier on a stage preceding the first terminal. 
     FIG. 4 illustrates a specific implementation of the frequency converter according to the Second embodiment. As shown in FIG. 4, the LO matching circuit  21  is an LC serial circuit consisting of an inductor  21   a  and a capacitor  21   b  that are connected in series to each other. A bias circuit  28  is provided between the LO matching circuit  21  and the FET  20 . One terminal of the bias circuit  28  is grounded via a resistor, while the other terminal thereof is connected to the gate terminal  20   a  of the FET  20  by way of the trap circuit  22  to apply a bias voltage to the FET  20 . It should be noted that the LO matching circuit  21  and bias circuit  28  may have any configurations other than those illustrated in FIG.  4 . 
     The trap circuit  22  is implemented as an LC resonator, in which an inductor  22   a  and a capacitor  22   b  are connected in series to each other. One terminal of the LC resonator  22  is connected to the gate terminal  20   a  of the FET  20 , while the other terminal thereof is grounded. 
     Suppose a resonant frequency condition given by f=1/(2π×(LC) ½ ) is met, where f is the frequency of the harmonic of the LO signal, L is an inductance of the inductor  22   a  and C is a capacitance of the capacitor  22   b . In that case, the trap circuit  22  resonates at a frequency of the second harmonic of the LO signal. Thus, the second harmonic component of the LO signal can be eliminated. Specifically, where the frequency of the LO signal is 2.2 GHz, the second harmonic component of the LO signal is removable by setting the inductance L of the inductor  22   a  and capacitance C of the capacitor  22   b  to 1.3 nH and 1 pF, respectively. 
     In this manner, the trap circuit  22  can eliminate the second harmonic component of the LO signal with much more certainty. As a result, the conversion loss involved with this frequency converter is about 5 dB, which is about 3 dB lower than that of the conventional frequency converter including an amplifier at a stage preceding the first terminal. 
     FIG. 5 illustrates another specific implementation of the frequency converter according to the second embodiment. As shown in FIG. 5, the LO matching circuit  21  is an LC serial circuit consisting of an inductor  21   a  and a capacitor  21   b  that are connected in series to each other. A bias circuit  28  is provided between the LO matching circuit  21  and the FET  20 . One terminal of the bias circuit  28  is grounded via a resistor, while the other terminal thereof is connected to the gate terminal  20   a  of the FET  20  by way of a trap circuit  22  to apply a bias voltage to the FET  20 . It should be noted that the LO matching circuit  21  and bias circuit  28  may have any configurations other than those illustrated in FIG.  5 . 
     The trap circuit  22  is implemented as an LC resonator, in which an inductor  22   a  and a capacitor  22   b  are connected in parallel to each other. One terminal of the LC resonator  22  is connected to the gate terminal  20   a  of the FET  20 , while the other terminal thereof is connected to the first terminal  23  via the Lo matching circuit  21 . 
     Suppose a resonant frequency condition given by f=1/(2π×(LC) ½ ) is met, where f is the frequency of the harmonic of the LO signal, L is an inductance of the inductor  22   a  and C is a capacitance of the capacitor  22   b . In that case, the trap circuit  22  resonates at a frequency of the second harmonic of the LO signal. Thus, the second harmonic component of the LO signal can be eliminated. Specifically, where the frequency of the LO signal is 2.2 GHz, the second harmonic component of the LO signal is removable by setting the inductance L of the inductor  22   a  and capacitance C of the capacitor  22   b  to 1.3 nH and 1 pF, respectively. 
     In this manner, the trap circuit  22  can eliminate the second harmonic component of the LO signal with much more certainty. AS a result, the conversion loss involved with this frequency converter is about 5 dB, which is about 3 dB lower than that of the conventional frequency converter including an amplifier at a stage preceding the first terminal.