Abstract:
A transceiver suitable for larger scale of integration employs direct conversion reception for reducing the number of filters. Also, the number of VCOs is reduced by utilizing dividers to supply a receiver and a transmitter with locally oscillated signals at an RF band. Dividers each having a fixed division ratio are used for generating locally oscillated signals for the receiver, while a divider having a switchable division ratio are used for generating the locally oscillated signal for the transmitter. In addition, a variable gain amplifier for baseband signal is provided with a DC offset voltage detector and a DC offset canceling circuit for supporting high speed data communications to accomplish fast cancellation of a DC offset by eliminating intervention of a filter within a feedback loop for offset cancellation.

Description:
This application is a continuation application of U.S. application Ser. No. 10/742,813, filed Dec. 23, 2003, now U.S. Pat. No. 7,366,489 now allowed, which is a continuation application of U.S. application Ser. No. 09/711,105, filed Nov. 14, 2000, now U.S. Pat. No. 6,826,388. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a mobile communication apparatus which can be implemented with a less number of components, and more particularly, to a transceiver which employs a direct conversion scheme suitable for larger scale of integration. 
     2. Description of the Related Art 
     With explosive popularization of mobile communication apparatus, requirements for a reduction in size and cost have been increased. For this reason, it is desired to eliminate VCO (voltage controlled oscillator), reduce the number of filters, and apply integrated circuits with a higher degree of integration. A prior art example of a transceiver which meets such requirements is described in K. Takikawa et. al., “RF Circuits Technique of Dual-Band Transceiver IC for GSM and DCS1800 applications,” IEEE 25 th  European Solid-State Circuits Conference Preprints pp. 278-281, 1999. The configuration of this transceiver is illustrated in  FIG. 10A . The illustrated transceiver comprises an integrated circuit  1016 , and other components  1001 - 1015  which are connected external to the integrated circuit  1016 . The prior art example supports two frequency bands, i.e., 900 MHz band and 1.8 GHz band. Also, the transceiver employs a superheterodyne scheme for a receiver and an offset PLL scheme for a transmitter. The superheterodyne receiver requires two RF (high frequency) filters  1001 ,  1002  for suppressing out-of-band blocker signals; two image rejection filters  1003 ,  1004  for rejecting blocker signals in an image frequency band associated with mixing; and an IF (intermediate frequency) filter  1005  for filtering out blocker signals near a reception channel. The receiver also requires two local oscillators  1006 ,  1007  for supporting the two frequency bands, i.e., 900 MHz band and 1.8 GHz band. 
     A reception scheme which can reduce the number of externally connected components is a direct conversion scheme. A prior art example of a direct conversion receiver is described in Behzad Razavi, “A 900-MHz CMOS Direct Conversion Receiver,” IEEE Symposium on VLSI Circuits, pp. 113-114, 1997. The configuration of this receiver is illustrated in  FIG. 10B . Since no image response exists in principle, the direct conversion scheme does not require an image rejection filter. Also, an IF filter is eliminated since it can be replaced by a filter integrated in an IC. In this prior art example, a VCO  1025  oscillates at a frequency twice an input frequency of the receiver which is in a range of 1850-1920 MHz. When this receiver is applied to GSM, DCS1800 dual band receiver, the VCO  1025  must oscillate in a range of 1850 to 1920 MHz (for GSM) and in a range of 3610 to 3760 MHz (for DCS1800). However, since it is difficult for a single VCO to cover these frequency bands, two VCOs are required. 
     A widely known drawback of the direct conversion receiver is a DC offset voltage. This is generated because an input signal to mixers  1019 ,  1020  is equal to a locally oscillated signal in frequency. For example, if the locally oscillated signal leaks into an input terminal for an input signal, locally oscillated signals are mutually multiplied to generate DC offset voltage. A prior art example of a scheme for canceling the DC offset voltage is described in Asad A. Abidi et. al., “Direct-Conversion Radio Transceivers for Digital Communications,” IEEE Journal of Solid-State Circuits, pp. 1399-1410, vol. 30, no. 12. December 1995. The configuration of this transceiver is illustrated in  FIG. 11 . An output DC offset voltage of a variable gain amplifier composed of variable gain amplifiers  1101 ,  1103 ,  1105  and low pass filters  1102 ,  1104  is detected by a digital signal processor (DSP)  1106 . The DSP  1106  outputs a DC offset voltage cancel signal to an input of the variable gain amplifier  1101  based on the detected information. 
     SUMMARY OF THE INVENTION 
     As described above, in the direct conversion receiver, the number of externally connected filters can be reduced. However, if the direct conversion receiver is used in place of the superheterodyne receiver in the GSM, DCS1800 dual band transceiver of  FIG. 10A , the number of local oscillators is increased. This is because the transmitter requires a locally oscillated frequency in a range of 1150 to 1185 MHz (for GSM) and in a range of 1575 to 1650 MHz (for DCS1800), and the receiver requires a locally oscillated frequency in a range of 1850 to 1920 MHz (for GSM) and in a range of 3610 to 3760 MHz (for DCS1800), but a single VCO encounters difficulties in covering a plurality of bands. For a further reduction in cost, a reduction in the number of VCOs is a primary subject. 
     Also, in GPRS (General Packet Radio Service) which implements high speed data communications based on a GSM system, a plurality of slots are assigned to reception and transmission. Thus, fast DC offset voltage cancellation is required. In addition, the DC offset voltage cancellation must be performed every operation frame. First, the necessity for the fast offset cancellation is explained with reference to  FIG. 4 . One frame of GSM is comprised of eight slots, each of which has a duration of 577 μm. Assume herein a severe condition for the DC offset voltage cancellation, in which four slots are assigned to the reception (RX), and one slot is assigned to the transmission (TX). While a transmission slot TX 1 ′ is assigned to a slot  7 , the transmission slot TX 1 ′ is transmitted at a timing of TX 1 , which is 237 μsec before the slot  7 , in consideration of a propagation delay to a base station. Also, a monitoring period of approximately 500 μsec and a PLL synchronizing period are required other than transmission and reception. Assuming that the PLL synchronizing period lasts approximately 150 μsec, a time available for canceling the DC offset voltage, in which a transceiver does not operate, is calculated as 1154−500−237−150*2=117 μsec, thus requiring fast DC offset cancellation. 
     Next, the necessity for the offset cancellation performed every frame is explained with reference to  FIG. 5 .  FIG. 5  shows a measuring circuit for measuring a received frequency dependency of an output DC offset voltage of a mixer, and the result of a measurement made thereby. The result of the measurement reveals that the output DC offset voltage has the frequency dependency. Therefore, in a system such as GSM, DCS1800, in which a received frequency is not fixed during a call but the frequency hops within a reception band, it is difficult to previously anticipate the DC offset voltage. Therefore, the DC offset voltage must be canceled every operation frame. 
     The scheme employed in the example of  FIG. 11  is not suitable for high speed data communications since a filter intervening in a feedback loop for offset cancellation make the fast offset cancellation difficult. Therefore, the realization of a fast offset canceling scheme suitable for high speed data communications is a second subject. 
     To realize the first subject, in the present invention, a receiver and a transmitter are supplied with locally oscillated signals in an RF band from a single VCO utilizing dividers. Dividers each having a fixed division ratio are used for generating the locally oscillated signals for the receiver, while a divider having a switchable division ratio is used for generating the locally oscillated signal for the transmitter. 
     To realize the second subject, in the present invention, a variable gain amplifier for baseband signal is provided with a DC offset voltage detector and a DC offset canceling circuit to accomplish fast cancellation of a DC offset by eliminating intervention of a filter within a feedback loop for offset cancellation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the configuration of a mobile communication apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram illustration a receiver of the mobile communication apparatus according to the present invention; 
         FIG. 3  is a circuit diagram illustrating in detail a circuit for removing a DC offset of a receiver according to the present invention; 
         FIG. 4  is an operation timing diagram in a GSM standard; 
         FIG. 5  shows a method of measuring a DC offset voltage generated by a mixer, and the result of a measurement made by the method; 
         FIGS. 6A ,  6 B,  6 C are diagrams illustrating the principles of the operation of a chopper amplifier which can be applied to the present invention; 
         FIG. 7  is a block diagram illustrating an embodiment in which a chopper amplifier is applied to the receiver of the present invention; 
         FIG. 8  is a block diagram illustrating a circuit for canceling a DC offset voltage for a variable gain amplifier without influence of a previous circuit in the receiver of the present invention; 
         FIG. 9  is a block diagram showing that a timing signal for removing a DC offset can be provided from a baseband circuit; 
         FIG. 10A  is a block diagram illustrating the configuration of a mobile communication apparatus to which a conventional superheterodyne scheme is applied; 
         FIG. 10B  is a block diagram illustrating the configuration of a prior art direct conversion receiver; 
         FIG. 11  shows a prior art DC offset voltage canceling technique; 
         FIG. 12  is a table showing IF frequencies of a transmitter for use in a GSM operation; 
         FIG. 13  is a table showing IF frequencies of the transmitter for use in a DCS1800 operation; 
         FIG. 14  shows a method of decoupling a filter capacitance to accelerate a DC offset removing operation; 
         FIG. 15  shows a method of decoupling a filter capacitance to simplify a DC offset removing circuit; 
         FIG. 16  is a block diagram illustrating a GSM/DCS1800 dual band transmitter; 
         FIG. 17  is a table listing spurious in GSM transmission; 
         FIG. 18  is a table listing spurious in DSC1800 transmission; 
         FIG. 19  shows a GSM spurious standard; 
         FIG. 20  shows allocation of VCO oscillating frequencies in which locally oscillated frequency bands are coincident for transmission and reception; 
         FIG. 21  shows allocation of VCO oscillating frequencies in which locally oscillated frequency bands do not overlap for transmission and reception; 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A first embodiment of the present invention will be described with reference to  FIG. 1 . The first embodiment uses an exemplary mobile communication apparatus, as an application, which supports the European cellular telephone GSM (900 MHz band) and DCS1800 (1800 MHz band). 
     A direct conversion scheme is applied to a receiver for converting an RF signal directly to a baseband signal, and the offset PLL scheme, previously shown in the prior art example, is applied to a transmitter. The receiver comprises low noise amplifiers  101 ,  102 ; mixers  103 ,  104 ; and a variable gain low pass filter  139 . Each of the mixers converts a signal frequency band from the RF band to the baseband, and simultaneously performs demodulation for separating an RF signal into a sine component and a cosine component. For this purpose, the mixers  103 ,  104  must be applied with locally oscillated signals which are shifted in phase by 90°. The locally oscillated signals are generated using dividers  105 ,  115 . The locally oscillated signals are generated by a PLL loop formed of a VCO  111  and a PLL  112 . When a VCO for oscillating in a 3600 MHz band is used for the VCO  111 , the output of the divider  115  is in a 1800 MHz band and provides a locally oscillated signal for DCS1800. Also, as the divider  116  is positioned prior to the divider  105 , an output frequency of the divider  105  is in a 900 MHz band and provides a locally oscillated signal for GSM. Output baseband signals of the mixers  103 ,  104  are inputted to a variable gain low pass filter  139  for level adjustment and blocker signal rejection. The variable gain low pass filter  139  comprises low pass filters  106 ,  107 ,  137 ,  138 , and variable gain amplifiers  108 ,  109 . Also, for suppressing a DC offset voltage at the output of the variable gain low pass filter  139 , a DC offset voltage canceling circuit  110  is provided. The DC offset voltage canceling circuit  110  has a DC offset voltage detecting unit and a DC offset canceling unit. 
     For reducing the number of externally connected components, the transmitter also uses the same VCO  111  as the receiver. How to determine an IF frequency (fIF) used in the transmitter is explained below. Assume that reception frequencies received at an antenna  136  are fr G  (for GSM) and fr D  (for DCS1800); and a transmission frequencies are ft G  (for GSM) and ft D  (for DCS1800). As described above, since the oscillating frequency of the VCO  111  is four times the GSM reception frequency and twice the DSC1800 reception frequency, the oscillating frequency of the VCO  111  can be expressed as 4·fr G =2·fr D . When signals generated by dividing this oscillating frequency by  m  (GMS) and by  n  (DCS1800) are used as locally oscillated signals for a mixer  126  of an offset PLL, an IF frequency fIF G  for GSM is expressed by the following equation 1: 
     
       
         
           
             
               
                 
                   
                     fIF 
                     G 
                   
                   = 
                   
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                           4 
                           · 
                           
                             fr 
                             G 
                           
                         
                         m 
                       
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                         f 
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                         ⁢ 
                         
                           t 
                           G 
                         
                       
                     
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                   equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     Similarly, an IF frequency fIF D  for DCS1800 is expressed by the following equation 2: 
     
       
         
           
             
               
                 
                   
                     fIF 
                     D 
                   
                   = 
                   
                      
                     
                       
                         
                           2 
                           · 
                           
                             fr 
                             D 
                           
                         
                         n 
                       
                       - 
                       
                         f 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           t 
                           D 
                         
                       
                     
                      
                   
                 
               
               
                 
                   equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     Assume herein that fr G =925 MHz, ft G =880 MHz, fr D =1805 MHz, and ft D =1710 MHz. The IF frequency fIF G  calculated for  m  is shown in  FIG. 12 , and the IF frequency fIF D  calculated for  n  is shown in  FIG. 13 . Since a divide-by-two divider is used for the frequency division, 2 to the i-th power ( i  is an integer number) is used for  m  and  n . For employing a single VCO for generating the IF frequencies,  m  and  n  cannot be freely selected but must be selected such that fIF G  is substantially equal to fIF D . Also, when a divide-by-two divider is used, the ratio of fIF G  to fIF D  may be substantially equal to 2 to the j-th power (j is an integer number). Here, “substantially equal” means that the two frequencies may be included within an oscillating frequency range of the VCO even if they are not exactly the same. In  FIGS. 12 and 13 , combinations of  m  and  n  which satisfy the foregoing condition are (m, n)=(2, 1) and (4, 2). The IF frequencies fIF are eventually determined from such combinations of  m  and  n  in consideration of power consumption, the presence or absence of generation of an unwanted spurious signal, and so on. In this embodiment, a combination (m, n)=(4,2) is chosen. Dividers  117 ,  118  and a change-over switch  121  are provided subsequent to the VCO  111 , and are controlled to divide the output frequency of VCO  111  by four for GSM and by two for DCS1800. Next, the oscillating frequency of the VCO  114  is determined depending on the power consumption, the scale of passive elements contained in IC, and so on. In this embodiment, the oscillating frequency of the VCO  114  is divided by eight for GSM and by four for DCS1800 to generate fIF G =45 MHz and fIF D =95 MHz by selecting the oscillating frequency in a 300 MHz band and providing dividers  119 ,  120  and a change-over switch  122  subsequent to the VCO  114 . 
     Next, the problem of spurious will be explained in a more specific manner.  FIGS. 17 ,  18  show the spurious when the IF frequency is fixed, and the locally oscillated frequency is changed.  FIGS. 17 ,  18 , which correspond to GSM and DSC1800, show the spurious which results from the difference between an integer multiple (multiplied by  m ) of the IF frequency and the locally oscillated frequency when transmission signals are generated from transmission oscillators  128 ,  124 . In the tables, fIF represents the IF frequency, and fVCO the transmission frequency. A numerical value written in each field shows the difference between a spurious signal and the transmission frequency in units of MHz. Hatched fields indicate the spurious generated nearby within 10 MHz, which are not easily removed by a loop filter  127  of the transmitter. As will be understood from  FIGS. 17 ,  18 , when the IF frequency is fixed, it is difficult to avoid a region in which the spurious appears near the transmission frequency within the transmission band, and the effectiveness of varying the IF frequency depending on the transmission frequency is understandable. For example, in the example of GSM shown in  FIG. 17 , the spurious can be avoided by selecting the IF frequency at 45 MHz from 880 MHz to 888 MHz and selecting the IF frequency at 46 MHz from 888 MHz to 914 MHz. 
     In this embodiment, the locally oscillated signal applied to the mixer  126  in the transmitter exists within the reception band.  FIG. 16  illustrates the transmitter of this embodiment in an enlarged view. The locally oscillated signal existing within the reception band leaks through a path designated by  2309 , and amplified by an amplifier at a later stage and irradiated. A standard related to the irradiation of the spurious in GSM is summarized in  FIG. 19 . While the spurious within the reception band is allowed only at five points and at a level of −36 dBm or lower, it is desired to suppress the spurious to −79 dBm/100 kHz in principle.  FIG. 20  summarizes the oscillating frequencies of the VCO so far explained in the foregoing embodiment. A reception band  2701  and a transmission band  2703  for DCS1800 are coincident, and a reception band  2702  and a transmission band  2704  for GSM are coincident as well. For shifting these bands from each other, consider a frequency allocation as shown in  FIG. 21 . A transmission band  2705  shifted from the reception band  2701  for DCS1800 does not overlap the reception band  2701 , so that the leakage of the locally oscillated frequency within the reception band during transmission can be avoided. The same is applied to GSM as well. 
     Next, a receiver according to a second embodiment of the present invention will be described with reference to  FIG. 2 . 
     The illustrated receiver comprises a low noise amplifier  102 ; a mixer  104 ; a divider  105 ; low pass filters  106 ,  137 ; variable gain amplifiers  108 ,  201 ; DC offset voltage canceling circuits  110 ; and a decoder  205 . The low noise amplifier  102  in turn comprises a load resistor  207 ; a transistor  208 ; and a capacitance  209 , while the DC offset voltage canceling circuit  110  comprises a digital-to-analog converter (DAC)  202 ; an analog-to-digital converter (ADC)  203 ; and a controller  204 . The mixer  104  comprises mixers  210 ,  206 . 
     An output DC voltage of the variable gain amplifier  108  is converted to a digital signal by the ADC  203 , and inputted to the controller  204 . The controller  204  measures the DC offset voltage at the output of the variable gain amplifier  108  to output a cancel signal for canceling the DC offset voltage. The cancel signal is converted from a digital signal to an analog signal by the DAC  202 , and an output signal of the DAC  202  cancels the DC offset voltage of the variable gain amplifier  108 . The decoder  205  selects one of the DC offset voltage canceling circuits  110 , such that only the selected circuit is operative. In this way, since no filter intervenes within a feedback loop comprised of the variable gain amplifier and the DC offset voltage canceling circuit, a delay otherwise caused by the filter is eliminated, thereby making it possible to realize fast offset cancellation. Here, it is also possible to employ a one-bit ADC, i.e., a simple comparator. 
     Next, a variable gain amplifier and a DC offset voltage canceling circuit according to a third embodiment of the present invention will be described with reference to  FIG. 3 . 
     The variable gain amplifier comprises resistors  307 ,  308 ,  312 ; and transistors  309 ,  310 ,  311 . The transistors  309 ,  310  are applied with an input voltage at their bases to deliver output voltages from their collectors. The gain can be controlled, for example, by a base voltage of the transistor  311 . A DAC  313  comprises transistors  301 ,  302 ,  303 ; and resistors  304 ,  305 ,  306 . Since the output of a controller  204  is connected to bases of the transistors  301 ,  302 ,  303 , the controller  204  can control collector DC currents of the transistors  301 ,  302 ,  303 . The collector DC currents are summed with a collector current of the transistor  309 , and then converted to a voltage by the resistor  307 . Assume now that a DC offset voltage ΔV (=V 2 −V 1 ) exists, and the resistance of the resistors  307 ,  308  are represented by R L ; an output DC current of the DAC  313  by I DAC1 ; and an output DC current of the DAC  314  by I DAC2 . In this event, the controller  204  controls the DACs  313 ,  314  to satisfy the relationship expressed by equation 3:
 
 R   L ·( I   DAC1   −I   DAC2 )=Δ V   equation 3
 
     Next, a variable gain amplifier according to a fourth embodiment of the present invention will be described with reference to  FIGS. 6A-6C .  FIG. 6A  illustrates an ideal variable gain amplifier  603  free from a DC offset voltage, and an input conversion DC offset voltage source  606  for the variable gain amplifier  603 . In this configuration, since no means is provided for suppressing an offset voltage, an output voltage of the offset voltage source  606  multiplied by the gain of the variable gain amplifier  603  appears as an offset between output terminals  604 ,  605 . Next,  FIGS. 6B ,  6 C illustrate configurations in which change-over switches  607 ,  608 , i.e., the third embodiment according to the present invention, are connected to the input and output of the variable gain amplifier  603 . Since  FIG. 6B  is opposite to  FIG. 6C  in the connection relationship of the change-over switches  607  and  608 , an output voltage of the offset voltage source  606  is transmitted to an opposite terminal, i.e., to  604  in  FIG. 6B  and to  605  in  FIG. 6C , while maintaining the connection relationship between the input and output terminals. Therefore, when the aforementioned change-over switches  607 ,  608  are periodically changed over, the output voltage of the offset voltage source  606  appears at the output terminals  604 ,  605  for the same time, so that the offset voltage between the output terminals becomes zero. 
     Next, a receiver according to a fifth embodiment of the present invention will be described with reference to  FIG. 7 . This embodiment shows a receiver characterized in that the variable gain amplifier  609  shown in the third embodiment is employed in place of the variable gain amplifier  201  and the DC offset voltage canceling circuit  206  in the second embodiment, and a low pass filter  702  and a buffer amplifier  701  are connected subsequent to the variable gain amplifier  609 . 
     Next, a receiver according to a sixth embodiment of the present invention will be described with reference to  FIG. 8 . This embodiment shows a receiver characterized in that a switch  801  is connected between the low pass filter  140  and the variable gain amplifier  201  in the second embodiment. For canceling a DC offset voltage, the switch  801  is turned on to short-circuit inputs of the variable gain amplifier  201 , and the switch  801  is maintained off when the cancellation is not performed. By turning on the switch  801  upon starting the cancellation, the variable gain amplifier  201  can perform the cancellation without being affected by a DC offset voltage from the previous stage. 
     Next, a mobile communication apparatus according to a seventh embodiment of the present invention will be described with reference to  FIG. 9 . This embodiment shows a mobile communication apparatus characterized in that a baseband circuit  901  is added to the first embodiment. A block  907  includes all circuits other than an antenna  139  and a circuit  143  contained in IC in the first embodiment. The baseband circuit  901  performs signal processing such as conversion of received baseband signals  902 ,  903  to audio signals, conversion of audio signals to transmission baseband signals  905 ,  906 , and so on. Further, the baseband circuit  901  outputs a DC offset cancel starting signal  904  for determining the timing at which a DC offset voltage is canceled in the circuit  143 , and inputs the signal  904  to the circuit  143 . The starting signal is sent before a receiver starts receiving a signal, such that a DC offset generated in the circuit  143  is removed before receiving the signal. 
     Next, a mobile communication apparatus according to an eighth embodiment of the present invention will be described with reference to  FIG. 14 . Switches  1401 ,  1402  are inserted between a capacitance  1403  and resistors  1404 ,  1405  in a filter  140  to reduce the time constant during DC offset cancellation. Since this can reduce a propagation delay through the filter  140 , the DC offset can be canceled faster without using the input short-circuiting switch  801  illustrated in  FIG. 8 . Also, when respective amplifiers  108 ,  201  are comprised of bipolar transistors as illustrated in  FIG. 3 , the bases of the transistors are biased through filter resistors  1404 ,  1405 . Therefore, the DC offset voltage can be canceled, including a bias offset due to variations in base current and variations in filter resistor. On the other hand, the sixth embodiment employing the short-circuiting switch  801  cannot cancel the bias offset. In addition, when the DC offset is removed in order from former stages, a residual error is removed by the DC offset canceling function at a later stage, so that a more accurate DC offset removal can be achieved. 
     Next, a mobile communication apparatus according to a ninth embodiment of the present invention will be described with reference to  FIG. 15 . When a propagation delay through the filter is reduced as in the eighth embodiment, the filter can be interposed in a feedback loop for canceling the DC offset voltage. Therefore, this embodiment can reduce the number of ADCs and accordingly the scale of the circuit, as compared with the eighth embodiment. 
     According to the present invention, three externally connected filters and one externally connected VCO can be saved as compared with the conventional superheterodyne receiver. In addition, it is possible to realize a mobile communication apparatus which can support a high speed packet transmission mode with a reduced number of parts by removing fast a DC offset voltage which causes a problem in the direct conversion receiver.