Patent Publication Number: US-2007123176-A1

Title: Direct conversion rf front-end transceiver and its components

Description:
BACKGROUND ART  
      1. Field of the Invention  
      The present invention relates to an RF front-end transceiver and, more particularly, to a direct conversion RF front-end transceiver and its components with which a frequency band can be reconfigured by a frequency control signal that controls an oscillator.  
      2. Discussion of Related Art  
      An RF front-end transmitter for a wireless communication is composed of a transmit mixer and a transmit amplifier. The transmit mixer serves to multiply a carrier frequency with a base band signal outputted from a base band processor and convert it into a radio frequency (RF) signal. The transmit amplifier amplifies and outputs power of an output signal of the transmit mixer. With such configuration, the RF front-end transmitter converts the inputted base band signal into the RF signal and amplifies, and outputs it. A RF front-end receiver for a wireless communication is composed of a receive amplifier and a receive mixer. The receive amplifier amplifies and outputs a small signal inputted through an antenna. The receive mixer converts the RF signal outputted from the receive amplifier into the base band signal and outputs the converted base band signal. With such configuration, the RF front-end receiver amplifies the input RF signal and converts the amplified input RF signal into the base band signal and outputs it.  
      In designing the RF front-end transceiver, impedance should be matched to transmit maximum power. Generally, in implementing a wireless communication system, 50 ohm is used as a matching point, considering power transmission of electromagnetic wave energy and distortion of a signal waveform. That is, input impedance and output impedance should be matched to 50 ohm. The impedance mentioned herein is a concept including resistance and reactance. Therefore, 50 ohm impedance matching means that the reactance is 0. That is, to achieve the 50 ohm impedance matching, resonance caused by an inductor and a capacitor is used. Therefore, a specific RF front-end transceiver transmits the maximum power over a specific frequency band where the resonance is generated by the inductor and the capacitor, while it does not transmit the maximum power over the frequency band other than the above one. In other words, the maximum power can be transmitted around the resonance frequency of the receive amplifier, the receive mixer, the transmit amplifier and the transmit mixer, while it cannot transmit over the frequency band other than the above one. Due to this feature, there are problems that the specific RF front-end transceiver can be used only for the specific RF frequency band, and that a number of RF front-end transceivers are required to process a number of RF frequency band signals. As such, when a number of RF front-end transceivers are employed, there are problems that a hardware design becomes complicated and the cost is high.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to providing a direct conversion RF front-end transceiver and its components with which a signal processing frequency band can be reconfigured by a frequency control signal.  
      To address the foregoing problems, a first aspect of the present invention provides an RF front-end transceiver comprising: an oscillator for outputting a resonant frequency signal whose frequency is controlled by a frequency control signal; a receive amplifier for amplifying and outputting a receive RF signal; a receive mixer for mixing the receive RF signal amplified and the resonant frequency signal to convert the receive RF signal into a receive base band signal; a transmit mixer for mixing a transmit base band signal and the resonant frequency signal to convert the transmit base band signal into a transmit RF signal; and a transmit amplifier for amplifying and outputting the transmit RF signal, wherein a resonant frequency of at least one of the receive amplifier, the receive mixer, the transmit mixer and the transmit amplifier is controlled by the frequency control signal.  
      A second aspect of the present invention provides an RF front-end receiver comprising: an oscillator for outputting a resonant frequency signal whose frequency is controlled by a frequency control signal; a receive amplifier for amplifying and outputting a receive RF signal; and a receive mixer for mixing the receive RF signal amplified and the resonant frequency signal to convert the receive RF signal into a receive base band signal, wherein a resonant frequency of a least one of the receive amplifier and the receive mixer is controlled by the frequency control signal.  
      A third aspect of the present invention provides an RF front-end transmitter comprising: an oscillator for outputting a resonant frequency signal whose frequency is controlled by a frequency control signal; a transmit mixer for mixing a transmit base band signal and the resonant frequency signal to convert the transmit base band signal into a transmit RF signal; and a transmit amplifier for amplifying and outputting the transmit RF signal, wherein a resonant frequency of at least one of the transmit mixer and the transmit amplifier is controlled by the frequency control signal.  
      A fourth aspect of the present invention provides an amplifier comprising: an amplification unit for amplifying a signal inputted to an input unit and outputting the amplified signal to an output unit; and an input resonant unit connected to the input unit, and for changing a resonant frequency in accordance with a frequency control signal, wherein the frequency control signal is used to control a frequency of a resonant frequency signal outputted from an oscillator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a structure diagram of a direct conversion RF front-end transceiver according to a first embodiment of the present invention;  
       FIG. 2  is a structure diagram of a direct conversion RF front-end receiver according to a first embodiment of the present invention;  
       FIG. 3  is a structure diagram of a direct conversion RF front-end transmitter according to a first embodiment of the present invention;  
       FIGS. 4 and 5  are diagrams showing examples of amplifiers that can be employed in the direct conversion RF front-end transceiver, transmitter and receiver of FIGS.  1  to  3 ;  
       FIGS. 6 through 9  are diagrams for illustrating a resonant circuit (an LC tank) controlled by a digital control signal and an analog control signal;  
       FIG. 10  is a structure diagram of a direct conversion RF front-end transceiver according to a second embodiment of the present invention;  
       FIG. 11  is a structure diagram of a direct conversion RF front-end receiver according to a second embodiment of the present invention;  
       FIG. 12  is a structure diagram of a direct conversion RF front-end transmitter according to a second embodiment of the present invention;  
       FIG. 13  is a structure diagram of a direct conversion RF front-end transceiver according to a third embodiment of the present invention;  
       FIG. 14  is a structure diagram of a direct conversion RF front-end receiver according to a third embodiment of the present invention;  
       FIG. 15  is a structure diagram of a direct conversion RF front-end transmitter according to a third embodiment of the present invention;  
       FIG. 16  is circuit diagram showing an example of a switched capacitor LC tuned VCO that is frequency-variable by a digital control signal and an analog control signal;  
       FIG. 17  is a diagram showing an amplifier that can be used in an RF front-end transceiver according to a third embodiment of the present invention; and  
       FIG. 18  is a diagram showing a mixer according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.  
       FIGS. 1 through 3  are diagrams for illustrating a direct conversion RF front-end transceiver, receiver and transmitter according to a first embodiment of the present invention.  
       FIG. 1  is a structure diagram of a direct conversion RF front-end transceiver according to a first embodiment of the present invention. In  FIG. 1 , the direct conversion RF front-end transceiver is composed of an RF front-end receiver  100  and an RF front-end transmitter  200 . The RF front-end receiver  100  is composed of a receive amplifier  110 , a receive mixer  120  and a voltage controlled oscillator (VCO)  130 . The RF front-end transmitter  200  is composed of a transmit mixer  210  and a transmit amplifier  220 .  
      The receive amplifier  110  amplifies and outputs a receive RF signal inputted through an antenna (not shown). The receive mixer  120  mixes the receive RF signal outputted from the receive amplifier  110  and the output resonant frequency f LO  outputted from the VCO  130  to convert the receive RF signal into a receive base band signal. In the receive amplifier  110  and the receive mixer  120 , a resonant frequency is controlled by a resonant frequency control signal. The VCO  130  outputs the output resonant frequency signal f LO  whose frequency is controlled by the resonant frequency control signal. The output resonant frequency f LO  corresponds to a carrier frequency. The resonant frequency control signal can be provided from the base band processor  300  or a frequency synthesizer. The transmit mixer  210  mixes a base band signal outputted from the base band processor  330  and the resonant frequency f LO  outputted from the VCO  130  to convert the base band signal into an RF signal. The transmit amplifier  220  amplifier and outputs the output signal power of the transmit mixer  210 . The resonant frequency of the transmit mixer  210  and the transmit amplifier  220  is controlled by the resonant frequency control signal.  
      With this configuration, the RF front-end transceiver amplifies the inputted RF signal and converts it into the base band signal to output to the base band processor  300 , and converts the base band signal outputted from the base band processor  300  into the RF signal and amplifies and outputs the converted RF signal. Further, the same resonant frequency control signal controls the resonant frequency f LO  outputted from the VCO  130  as well as the resonant frequency of the receive amplifier  110 , the receive mixer  120 , the transmit mixer  210  and the transmit amplifier  220 , so that the maximum power can be transmitted even when the signal processing frequency band of the RF front-end transceiver is changed. This direct conversion RF front-end transceiver uses a fact that the frequency of the RF signal f RF  is equal to the output resonance frequency f LO  of the VCO where each of the receive amplifier  110 , the receive mixer  120 , the transmit mixer  210  and the transmit amplifier  220  includes a replica LC resonant circuit similar to an LC resonant circuit. However, the replica LC resonant circuit has a parasitic inductor or a parasitic capacitor, etc., so that it is not the exactly same one as the LC resonant circuit used in the VCO  130 .  
       FIG. 2  is a structure diagram of the direct conversion RF front-end receiver according to a first embodiment of the present invention. In  FIG. 2 , a direct conversion RF front-end receiver is composed of a receive amplifier  110 , a receive mixer  120 , a voltage controlled oscillator (VCO)  130  and an Base band (BB)  140 . The BB  140  is composed of a VGA (Variable Gain Amplifier), a Filter and an analog to digital converter (ADC).  
      The receive amplifier  110  amplifies and outputs a small signal inputted through an antenna (not shown). The receive mixer  120  mixes the receive RF signal outputted from the receive amplifier  110  and the resonant frequency f LO  outputted from the VCO  130  to convert the receive RF signal into a receive base band signal. In the receive amplifier  110  and the receive mixer  120 , a resonant frequency is controlled by the resonant frequency control signal. The VCO  130  outputs the output resonant frequency f LO  where the resonant frequency is controlled by the resonant frequency control signal. The resonant frequency control signal can be provided from the base band processor (not shown) or a frequency synthesizer (not shown). The BB  140  amplifies and filters the analog base band signal outputted from the receive mixer  120 , and converts the analog base band signal into a digital signal.  
      With this configuration, the RF front-end receiver amplifies the inputted RF signal and converts it into a digital base band signal to output to the base band processor  300 . Further, the resonant frequency f LO  outputted from the VCO  130  as well as the resonant frequency of the receive amplifier  110  and the receive mixer  120  are controlled by the same resonant frequency control signal, so that the maximum power can be transmitted even when the signal processing frequency band of the RF front-end receiver is changed. This direct conversion RF front-end receiver uses a fact that the RF signal frequency f RF  is equal to the output frequency f LO  of the VCO,where each of the receive amplifier  110  and the receive mixer  120  includes a replica LC resonant circuit similar to an LC resonant circuit. However, the replica LC resonant circuit has a parasitic inductor or a parasitic capacitor, etc., so that it is not the exactly same one as the LC resonant circuit used in the VCO  130 .  
       FIG. 3  is a structure diagram of a direct conversion RF front-end transmitter according to a first embodiment of the present invention. In  FIG. 3 , a direct conversion RF front-end transmitter is composed of a transmit mixer  210 , a transmit amplifier  220 , a voltage controlled oscillator (VCO)  230  and a Base band (BB)  240 . The BB  240  is composed of a VGA (Variable Gain Amplifier), a Filter and an digital to analog converter (DAC).  
      The BB  240  converts a digital base band signal into an analog base band signal, and amplifies and filters the digital base band signal. The transmit mixer  210  mixes a base band signal outputted from the base band processor  330  and the resonant frequency f LO  outputted from the VCO  230  to convert the base band signal into an RF signal. The transmit amplifier  220  amplifies and outputs the output signal power of the transmit mixer  210 . The resonant frequency of the transmit mixer  210  and the transmit amplifier  220  are controlled by the resonant frequency control signal. The VCO  230  outputs the resonant frequency signal f LO  whose frequency is controlled by the resonant frequency control signal. The resonant frequency control signal can be provided from the base band processor (not shown) or a frequency synthesizer (not shown).  
      With this configuration, the RF front-end transmitter converts a digital base band signal into an RF signal and amplifies and outputs it. Further, the resonant frequency f LO  outputted from the VCO  130  as well as the resonant frequency of the transmit mixer  210  and the transmit amplifier  220  are controlled by the same resonant frequency control signal, so that the maximum power can be transmitted even when the signal processing frequency band of the RF front-end transmitter is changed. This direct conversion RF front-end transmitter uses a fact that the RF signal frequency f RF  is equal to the output frequency f LO  of the VCO, where each of the transmit mixer  210  and the transmit amplifier  220  includes a replica LC resonant circuit similar to an LC resonant circuit. However, the replica LC resonant circuit has a parasitic inductor or a parasitic capacitor, etc., so that it is not the exactly same one as the LC resonant circuit used in the VCO  230 .  
       FIGS. 4 and 5  are diagrams for illustrating an amplifier that can be employed in the direct conversion RF front-end transceiver, transmitter and receiver of  FIGS. 1 through 3 .  
      The amplifier shown in  FIG. 4  is a common gate amplifier in which the resonant frequency of an input and an output is variable. This amplifier is composed of an input capacitor C C , first and second NMOS transistors MN 1  and MN 2 , first and second resistors R 1  and R 2 , an input resonant circuit L T1  and C V1  and an output resonant circuit L T2  and C V2 . Both ends of the input capacitor C C  are connected to an input RF signal RF IN  and a source of the first NMOS transistor MN 1 , respectively, and serves to transmit only an alternating current signal of the input RF signal RF IN  to the source of the first NMOS transistor MN 1 . The input resonant circuit L T1  and C V1  includes a variable capacitor C V1  and an inductor L T1  connected in parallel with the variable capacitor C V1 , where both ends of the input resonant circuit L T1  and C V1  are connected to the source of the first NMOS transistor MN 1  and the ground voltage. The capacitance of the variable capacitor C V1  is changed according to a frequency control signal, so that an input resonant frequency, that is, the resonant frequency of the input resonant circuit L T1  and C V1  is changed according to the frequency control signal. Gates of the first and second NMOS transistors MN 1  and MN 2  are connected to a bias voltage V BIAS  through a first resistor and a second resistor. Each of the first and second NMOS transistors MN 1  and MN 2  amplifies the source signal and transmit it to a drain. A net resistance of 50 ohm for input matching can be obtained using gm (transconductance) of the first NMOS transistor MN 1 . The output resonant circuit L T2  and C V2  includes a variable capacitor C V2  and an inductor L T2  connected in parallel with the variable capacitor C V2 , where both ends of an output resonant circuit L T2  and C V2  are connected to the power supply voltage and the drain of the second NMOS transistor MN 2 , respectively. The capacitance of the variable capacitor C V2  is changed according to the frequency control signal, so that the resonant frequency of the output resonant circuit L T2  and C V2  (an output resonant frequency) is changed according to the frequency control signal. With this configuration, the amplifier amplifies and outputs the input RF signal RF IN , where the input resonant frequency and the output resonant frequency are controlled by the frequency control signal.  
      The amplifier shown in  FIG. 5  is a cascode amplifier where the resonant frequency of the input and output is variable. This amplifier is composed of an input capacitor C C , a gate inductor Lg, a gate-source capacitor Cgs, a source inductor Ls, first and second NMOS transistors MN 1  and MN 2 , first and second resistors R 1  and R 2 , and an output resonant circuit L d  and C v . The RF input signal RF IN  is inputted to a gate of the first NMOS transistor MN 1  via the input capacitor C C  and the gate inductor Lg. An input resonant circuit is composed of the gate inductor Lg, the gate-source capacitor Cgs and the source inductor Ls connected in series. The capacitance of the gate-source capacitor Cgs is changed according to the frequency control signal, so that a resonant frequency of the input resonant circuit (an input resonant frequency) is changed according to the frequency control signal. The gate of the first NMOS transistor MN 1  is connected to the bias voltage V BIAS  via the first resistance R 1 . The first NMOS transistor MN 1  amplifies the gate signal and outputs it to the drain. The gate of the second NMOS transistor MN 2  is connected to the bias voltage V BIAS  via the second resistor R 2 . The second NMOS transistor MN 2  amplifies the source signal and outputs it to the drain. The output resonant circuit L d  and C V  includes a variable capacitor C V  and an inductor L d  connected in parallel with the variable capacitor C V , where both ends of the output resonant circuit L d  and C V  are connected to the drain of the second NMOS transistor MN 2  and the power supply voltage, respectively. The capacitance of the variable capacitor C V  is changed according to the frequency control signal, so that the resonant frequency of the output resonant circuit L d  and C V  (the output resonant frequency) is changed according to the frequency control signal. With this configuration, the amplifier amplifies and outputs the input RF signal RF IN , where the input resonant frequency and the output resonant frequency are controlled by the frequency control signal.  
      Using the direct conversion RF front-end transceiver according to the first embodiment of the present invention, a system that can change the resonant frequency can be implemented, but there occurs a new serious problem in that the resonant frequency is changed using the variable capacitor. This will significantly degrade the signal linearity due to the nonlinear characteristic. This capacitive non-linearity is in proportion to the gain of the variable capacitor indicating a change ratio of the input controlled voltage change to the output capacitance for the used variable capacitor. Therefore, in order to obtain the desired system performance without signal distortion, the gain of the variable capacitor should be very small. Thus, in the present invention, the resonant circuit is controlled using a digital control signal and an analog control signal, to reduce the capacitive non-linearity, so that a wide-band of variable frequency band can be obtained, and also, the low frequency gain of the resonant circuit (the low capacitive non-linearity) can be obtained.  
       FIGS. 6 through 8  are diagrams for illustrating a resonant circuit (an LC tank) controlled by a digital control signal and an analog control signal.  
       FIG. 6  illustrates a method of implementing an LC tank circuit with a digital control signal VDT and an analog control signal VAT. The LC tank (A) controls an inductor with the digital control signal, so that the inductance is discretely tuned, and a variable capacitor is tuned with an analog control signal. There is a drawback that the planar inductor should be integrated into this LC tank using a silicon process, and the fine-tuning is more difficult relative to tuning the capacitor. Further, using an inductor with the switch gives a bad impact on Q of the resonant circuit. However, with regard to the overall current consumption, it is advantageous for the large frequency tuning. An LC tank (B) uses a typical switched capacitor. This LC tank uses a fixed inductor, a variable capacitor and a switched capacitor. An LC tank (C) adds a digitally tuned inductor to the circuit of the LC tank (B). This LC tank can achieve a large frequency change by tuning the inductor, so that the current consumption suitable to the variable frequency range can be obtained. Therefore, this LC tank can be used for a multi-band system where the large frequency tuning is required. For example, when operated in a low frequency ranges of the entire variable frequency range, the inductor is tuned, so that the current consumption can be reduced relative to tuning only with the reduced capacitor, and in the given frequency band, the tuning can be finely performed with the switched capacitor and the variable capacitor. An LC tank (D) shows a case where a fixed capacitor and an inductor whose inductance is changed by the digital control and the analog control are used.  
       FIG. 7  is a diagram showing a resonant circuit where a variable capacitor Cv, switched capacitors C 1 , SW 1 ˜C N , SW N , and an inductor L T  are connected in parallel. The capacitance of the variable capacitor Cv is controlled by the analog control signal. The switches SW 1 ˜SW N  are controlled by the digital control signal. This resonant circuit corresponds to the LC tank (B) of  FIG. 6 .  
       FIG. 8  is a resonant circuit controlled only by the digital control signal. This resonant circuit cannot be used in the VCO, while can be used in the receive amplifier, the receive mixer, the transmit mixer and the transmit amplifier. These are not required to exactly match the resonant frequency with the VCO, so that the resonant frequency may be controlled only by the digital control signal as illustrated in  FIG. 8 . When such resonant circuit is used, the minimum unit of the resonant frequency that is discretely changed by the digital control should be small in order not to have a large frequency difference with the VCO.  
      The existing resonant circuit used for the direct conversion RF front-end transceiver according to the first embodiment of the present invention can be replaced with the resonant circuit shown in  FIGS. 6 through 8 . That is, the resonant circuit shown in  FIGS. 6 and 7  can be used in the VCO, the receive amplifier, the receive mixer, the transmit mixer and the transmit amplifier, and the resonant circuit shown in  FIG. 8  can be used in the receive amplifier, the receive mixer, the transmit mixer and the transmit amplifier. With this, the linearity degradation due to the variable capacitor, arisen as a new issue in the direct conversion RF front-end transceiver according to the first embodiment of the present invention, can be blocked.  
       FIG. 9  shows a frequency synthesizer ( 410  to  450 ) and a digital analog tuning VCO (DAT-VCO)  460  that can generate the digital control signal and the analog control signal available in the resonant circuit shown in  FIGS. 6 through 8 .  
      In  FIG. 9 , the frequency synthesizer is composed of a phase frequency detector (hereinafter, referred to as a “PFD”)  410 , a current pump (hereinafter, referred to as a “CP”)  420 , a low pass filter (hereinafter, referred to as a “LPF”)  430 , a digital tuner (hereinafter, referred to as a “DT”)  440  and an N divider  450 . The PFD  410  compares the frequency and phase of a reference frequency f REF  with that of an output frequency f DIV  of the N divider  450  and outputs their differences. The CP  420  flows the charge that corresponds to the output of the PFD  410  into the LPF  430  of the next stage. The LPF  430  serves as a loop filter of the overall frequency synthesizer and provides the DAT-VCO  460  of the next stage with the analog control signal VAT. The DT  440  measures the analog control signal VAT periodically, and accordingly, changes the digital control signal value inputted to the DAT-VCO. When the value of the analog control signal VAT is above a predetermined upper limit at the time of a periodic measurement, the DT  440  changes the value of the digital control signal to discretely increase the frequency of the DAT-VCO, while the value of the analog control signal VAT is below a predetermined lower limit, the DT  440  changes the value of the digital control signal to discretely reduce the frequency of the DAT-VCO. When the value of the analog control signal VAT value is between the upper limit and the lower limit, the value of the digital control signal outputted from the DT  440  remains unchanged. The N divider  450  divides and outputs the DAT-VCO output frequency with a frequency ratio N. The DAT-VCO  460  controls the output frequency f LO  using the analog control signal VAT and the digital control signal VDT. With this configuration, the frequency synthesizer ( 410  to  450 ) outputs the analog control signal VAT and the digital control signal VDT, and the DAT-VCO  460  outputs the output frequency f LO  controlled by the analog control signal VAT and the digital control signal VDT.  
       FIGS. 10 through 12  are diagrams showing a direct conversion RF front-end transceiver according to a second embodiment of the present invention.  
       FIG. 10  is a structure diagram showing a direct conversion RF front-end transceiver according to a second embodiment of the present invention. The transceiver shown in  FIG. 10  is similar to that shown in  FIG. 1 , but is different in that a receive amplifier  510 , a receive mixer  520 , a DAT-VCO  530 , a transmit mixer  610  and a transmit amplifier  620  are controlled by the digital control signal VDT and the analog control signal VAT.  
       FIG. 11  is a structure diagram showing a direct conversion RF front-end receiver according to a second embodiment of the present invention. The receiver shown in  FIG. 11  is similar to that shown in  FIG. 2 , but is different in that a receive amplifier  510 , a receive mixer  520 , and a DAT-VCO  530  are controlled by the digital control signal VDT and the analog control signal VAT.  
       FIG. 12  is a structure diagram showing a direct conversion RF front-end transmitter according to the second embodiment of the present invention. The transmitter shown in  FIG. 12  is similar to that shown in  FIG. 3 , but is different in that a transmit mixer  610 , a transmit amplifier  620 , and a DAT-VCO  630  are controlled by the digital control signal VDT and the analog control signal VAT.  
       FIGS. 13 through 15  are diagrams showing a direct conversion RF front-end transceiver according to a third embodiment of the present invention.  
       FIG. 13  is a structure diagram showing a direct conversion RF front-end transceiver according to a third embodiment of the present invention. The transceiver shown in  FIG. 13  is similar to that shown in  FIG. 1 , but is different in that a DAT-VCO  730  is controlled by the digital control signal VDT and the analog control signal VAT, and a receive amplifier  710 , a receive mixer  720 , a transmit mixer  810  and a transmit amplifier  820  are controlled by the digital control signal VDT.  
       FIG. 14  is a structure diagram showing a direct conversion RF front-end receiver according to the third embodiment of the present invention. The receiver shown in  FIG. 14  is similar to that shown in  FIG. 2 , but is different in that a DAT-VCO  730  is controlled by the digital control signal VDT and the analog control signal VAT, and a receive amplifier  710  and a receive mixer  720  are controlled by the digital control signal VDT.  
       FIG. 15  is a structure diagram showing a direct conversion RF front-end transmitter according to a third embodiment of the present invention. The transmitter shown in  FIG. 15  is similar to that shown in  FIG. 3 , but is different in that a DAT-VCO  830  is controlled by the digital control signal VDT and the analog control signal VAT, and a transmit mixer  810  and a transmit amplifier  820  are controlled by the digital control signal VDT.  
      The direct conversion RF front-end transceiver according to the second and third embodiment of the present invention shown in  FIGS. 10 through 15  acts to blocking the linearity degradation due to the inductor and the capacitor having a nonlinear characteristic in the resonant circuit of the direct conversion RF front-end transceiver according to the first embodiment of the present invention shown in  FIGS. 3 through 5 . Therefore, the resonant circuit used in  FIGS. 10 through 15 , allows the frequency to be changed continuously or discontinuously using a digital control signal and an analog control signal, so that the variable capacitor gain is reduced while the variable frequency range is widened. Further, this control signal is controlled using the frequency synthesizer shown in  FIG. 9 .  
       FIG. 16  is a circuit diagram showing an example of a switched capacitor LC tuned VCO where a frequency is changed by the digital control signal and the analog control signal. In  FIG. 16 , the resonant circuit of the VCO is composed of an inductor L T  and a variable capacitor C TV . The variable capacitor C TV  is controlled by the analog control signal VAT and the digital control signal VDT. First and second NMOS transistors MN 1  and MN 2  and first and second PMOS transistors MP 1  and MP 2  have -Gm that compensates for the loss of the resonant circuit. The bias current sources MNc 1  through MNcn are the bias current source for the VCO. The bias current sources MNc 1  through MNcn in the drawings are set to be under the control of the VDT. When the variable frequency band of the VCO is significantly wide, the required current is variable to make the signal amplitude of the VCO large in outputting at low frequency, so that a phase noise can remain constant to some degree in the overall variable frequency band. However, when the variable frequency range of the VCO is narrow, the control for the bias current source is not required.  
       FIG. 17  is a diagram showing an amplifier that can be used in an RF front-end transceiver according to a third embodiment of the present invention.  FIG. 17  is a cascode amplifier where input and output resonant frequencies are variable. This amplifier is composed of an input capacitor C C , a gate inductor Lg, a gate-source capacitor Cgs, a source inductor Ls, first and second NMOS transistors MN 1  and MN 2 , first and second resistors R 1  and R 2  and an output resonant circuit L d  and C v . An RF input signal RF IN  is inputted to the gate of the first NMOS transistor MN 1  via the input capacitor C C  and the gate inductor Lg. The gate inductor Lg, the gate-source capacitor Cgs and the source inductor Ls, connected in series, constitute the input resonant circuit. The capacitance of the gate-source capacitor Cgs is changed according to the digital control signal VDT. The gate of the first NMOS transistor MN 1  is connected to the first bias voltage V BIAS1  via the first resistor R 1 . The first NMOS transistor MN 1  amplifies a gate signal and outputs it to the drain. The gate of the second NMOS transistor MN 2  is connected to the second bias voltage V BIAS2  via the second resistor R 2 . The second NMOS transistor MN 2  amplifies the source signal and output it to the drain. The output resonant circuit L d  and C V  includes an inductor L d  in parallel with a variable capacitor C V , where both ends of the output resonant circuit L d  and C V  are connected to the power supply voltage and the drain of the second NMOS transistor MN 2 , respectively. The capacitance of the variable capacitor C V  is changed according to the digital control signal VDT. With this configuration, the amplifier amplifies and outputs the input RF signal RF IN , and the input resonant frequency and the output resonant frequency are controlled by the digital control signal VDT.  
      Input impedance Zin of this amplifier is expressed in Equation. 1.  
             Zin   =         (       ω   ⁢           ⁢     L   g       -     1     ω   ⁢           ⁢     C   c         -     1     ω   ⁢           ⁢     C     g   ⁢           ⁢   s           +     ω   ⁢           ⁢     L   s         )     ·   j     +         g   m     ⁢     L   s         C     g   ⁢           ⁢   s                   &lt;     Equation   ⁢           ⁢   1     &gt;             
 
      It can be found that when the gate-source capacitor Cgs is increased in Equation. 1, the net resistance of the input impedance is reduced. Therefore, when the net resistance (impedance) is increased by the digital control signal VDT, if the gm value is also increased, the net resistance can remain constant. The gm value is increased when the first bias voltage V BIAS1  is increased, so that when the gate-source capacitor Cgs is increased, if the first bias voltage V BIAS1  is designed to increase, the net resistance can remain constant.  FIG. 17  also shows an example of the circuit that supplies the first bias voltage V BIAS1 . This circuit is composed of an inverter, n switches (sw 1  through swn), n bias NMOS transistors (MN B1  through MN Bn ), a load resistor R LOAD , an output resistor R B  and a capacitor C B . When the digital control signal VDT is increased, the output of the inverter is reduced, so that the number of short switches (sw 1  through swn) is also reduced. The voltage drop of the load resistor is then reduced, resulting in increasing the first bias voltage V BIAS1  outputted. With this configuration, when the digital control signal VDT is increased, the net resistance can remain constant by increasing the gate-source capacitor Cgs as well as the gm.  
       FIG. 18  shows a mixer according to a second embodiment of the present invention. Referring to  FIG. 18 , the mixer is composed of six NMOS transistors MN 1 ˜MN 6 , four PMOS transistors MP 1 ˜MP 4 , two resistors R 1  and R 2 , a capacitor C, an inductor L and a variable capacitor C TV /2. The mixer multiplies and outputs the signals Ina+ and Ina− inputted to the gate of the first and the second NMOS transistors MN 1  and MN 2  with the output signals of the frequency oscillator inputted to the gates of the third to sixth NMOS transistor MN 3 ˜MN 6 .  
      Although the present invention has been specifically described with reference to the preferred embodiments, it should be noted that these embodiments are not restrictive but just illustrative. Further, those skilled in the art will appreciate that a variety of modification can be made without departing from the scope of the present invention.  
      According to the present invention, the direct conversion RF front-end transceiver and its components can change the resonant frequency over several frequency bands inputted from an antenna. Therefore, it has an advantage that a multi-band or wideband of signal frequency can be processed with one system hardware.  
      Further, the direct conversion RF front-end transceiver and its components according to the present invention can change the resonant frequency and determine the resonant frequency through programming. Therefore, it has an advantage that the resonant frequency can be determined irrespective of the process change and a platform of RF blocks or reconfigurable RF blocks can be configured.  
      Further, the direct conversion RF front-end transceiver and its components according to the present invention can be designed with a significantly reduced area, so that it is very competitive with respect to the costs.