the first embodiment of the invention is based on a concept that a stepwise dc offset component generated by digitally setting gains on the basis of pga method is digitally canceled as a system , thereby solving the problems inherent to analog offset cancel operations to realize ideal agc control . the first embodiment of the invention will be described in further detail by way of example with reference to the accompanying drawings . now , referring to fig6 , there is shown a block diagram of an exemplary configuration of a receiver apparatus in which a pga - based direct conversion receiver is used in a ds - cdma mobile communication system , practiced as the first embodiment of the invention . as shown in fig6 , as described in the related - art section hereof , a signal received by a terminal at its external antenna is differentially amplified by a low - noise amplifier 301 to be divided into two . each differentially amplified signal is c - cut of its direct - current component , subsequently down - converted from rf signal to baseband signal by a quadrature mixer 302 at a stretch . at this moment , to mix with a signal obtained by dividing an lo signal from a local oscillator 304 into an in - phase component and a quadrature component by a divider 303 , an in - phase component signal and a quadrature component signal are generated in the baseband . these baseband signals are removed of an interference wave signal power of an adjacent channel for example by an lpf 305 , thereby extracting only a desired wave signal . next , 3 - wire serial digital data 312 are decoded by the pga control circuit 311 and the baseband signals are amplified by discretely switching the gains of a baseband amplifier 306 . an lpf 307 is a filter inserted to convert a 50 % roll - off characteristic downlink signal at the sender side into a 100 % roll - off characteristic downlink signal as a total transfer function . the s / n of the baseband signals are maximized by the lpf 307 and then the maximized signals are quantized by an a / d converter 308 from analog to digital . the quantized signals are sent to a finger processing block and path search block 319 which constitutes a baseband signal processing block . in the finger processing block and path search block 319 , decode processing including rake synthesis and error correction is performed on the quantized signals , which is not described in detail herein , thereby correctly decoding the downlink signal received from a base station . the following describes the details of the agc control method based on the pga method according to the invention . concurrently with the processing by the finger processing block and path search block 319 , digital signal processing is performed on the iq baseband signal quantized by the a / d converter 308 by a receive signal level computing unit 314 . in computation of the receive signal level , the receive signal level computing unit 314 executes averaging processing to remove the influence of the instantaneous change in the receive envelope level which is rayleigh - scattered by phasing . this processing is executed , by use of an averaged time as a parameter which can be set as desired by a pga control processing block 315 . the pga control processing block 315 may be constituted by a digital signal processor ( dsp ) for example . this parameter is set on the basis of a simulation or actual measurement data . the pga control processing block 315 makes a comparison between the value of the receive signal level preset to get the optimum input level in the a / d converter 308 and the value of the current receive level obtained in the receive signal level computing unit 314 . on the basis of the result of the comparison , the pga control processing block 315 selects the optimum pga data from a pga control data table 317 and sends the selected optimum pga data to a pga data generator 318 . the pga data generator 318 adds other bits such as address bits to the received pga data to generate 3 - wire serial digital data 312 which is supplied to the pga control circuit 311 . fig9 shows an exemplary configuration of this pga control data table 317 . this table defines the pga data to be used for the difference between the current value of the receive signal level and the optimum value thereof . the table values in the figure are indicated by variables for the sake of convenience ; actually , however , they are particular values ( the same holds true with other tables to be described later ). suppose here that the maximum gain of the baseband amplifier 306 be 80 db for example , then the operation of the first initial agc pull - in from the power - on operation is as follows . in this case , the initial cell search operation is performed , so that it is necessary to be able to receive p - sch , s - sch , and p - cpich which are always transmitted from a base station . for the initial value of pga data , the pga control processing block 315 sets 40 db which is half the maximum gain of the baseband amplifier 306 . at this moment , if the receive signal level obtained by the receive signal level computing unit 314 is found to be greater than the preset receive signal level , then the gain is decreased by that difference ; if smaller , then the gain is increased by that difference . namely , the pga data in the pga control data table 317 can be restricted , at maximum , to the pga data equivalent to that the gain variation is half ( 40 db ) the above - mentioned maximum gain . on the other hand , in the actual mobile communication environment , the instantaneous receive level varies by approximately 20 to 30 db due to rayleigh phasing , which presents no problem because the receive signal level computing unit 314 executes averaging to remove the influence of the variation . a dc offset cancel table 316 contains , as shown in fig1 , the digital data corresponding to the dc offset voltages at the final stage which are generated when discretely switching gains from currently set pga data to the pga data to be set next . the pga control processing block 315 recognizes both the currently set pga data and the pga data to be set next , so that the pga control processing block 315 selects the optimum dc offset cancel data by referencing the dc offset cancel table 316 and sends the selected dc offset cancel data to a digital - to - analog converter 313 . the digital - to - analog converter 313 converts the received dc offset cancel data into an analog voltage and adds it to an iq output signal of the direct conversion receiver . the pga control processing block 315 controls the timing by use of a receive slot timing obtained by the finger processing block and path search block 319 such that the pga data are switched at the beginning of the slot . however , since the sequence of operations of decoding the 3 - wire serial digital data 312 by the pga control circuit 311 to switch the gains of the baseband amplifier 306 in a discrete manner are executed in analog circuits , the timing of the dc offset voltage occurring at the iq output delicately varies . thus , it is difficult to provide complete synchronization with the dc offset cancel voltage supplied from the digital - to - analog converter 313 . consequently , there occurs a slight but uncancellable glitch component . this problem may be overcome by capturing the receive data after several tens ns so that this glitch component is not included in averaging the receive level in the receive signal level computing unit 314 . if a dpch signal having spreading factor ( sf )= 4 has been received for example , the stepwise dc offset component due to the pga method which deteriorates s / n can be removed without losing the receive data , as compared with the related - art technology , because the data length of one symbol is about 1 microsecond or higher . it should be noted that the value of the elapsed time before starting this receive data capturing may be set as a parameter to the pga control processing block 315 as desired in advance . fig7 shows the iq output waveforms in the above - mentioned sequence of operations . the following examines the dc offset cancel table 316 again . as described above , it is necessary , as described above , for the dc offset cancel table 316 to write beforehand the digital data corresponding to the dc offset voltages at the final stage which occurs when discretely switching gains from the currently set pga data to the pga data to be set next . the data amount of this table would reach a considerable amount if all cases of variations from particular pga data to another particular pga data are written to the table . in addition , a considerable amount of measurement would be required to make the table . actually , however , the amount of the data which must be written beforehand can be reduced significantly , which will be described below . as described above , a stepwise dc offset occurs due to the fluctuation in the transistor pairs in the differential amplifier sequence constituting the baseband amplifier . therefore , if the total gain lowers as shown in the graph of fig8 , there is little change in the variation of the dc offset voltage at the final stage . namely , the case in which the total gain increases may only be considered . consequently , the dc offset cancel table 316 shown in fig1 may be written only when the next pga data are greater than the current pga data . further , as with the pga control data table 317 , the pga data to be written to the table may be restricted , at maximum , to the data equivalent to the value ( 40 db ) half the above - mentioned maximum gain . fig1 a and 11b show another exemplary configuration of the dc offset cancel table 316 . in this example , both table 316 a and table 316 b are used . as seen from the graph of fig8 , if the dc offset is determined dependent on the increase in gain , then the dc offset cancel table 316 a may be configured such that it is not referenced by the current pga data and the next pga data as with the dc offset cancel table 316 in fig1 but it is referenced by the difference between the current pga data and the next pga data . in this case , the number of table entries can be reduced significantly . however , it should be noted that combinations of particular current pga data and next pga data , though small in number as described above , may exceptionally cause a large dc offset . namely , in the method in which the gains of the baseband amplifier 306 are discretely switched by decoding through the pga control circuit 311 , a case may occur in which a fairly large dc offset is generated at the final stage even when the gain is varied by 1 db . this phenomenon takes place only when the gains of the amplifiers constituting the pga - based baseband amplification block discretely vary largely ; for example , this phenomenon takes place in a restricted case where the pga amplifier of the first stage shown in fig3 is switched in gain from 0 db to 20 db . this problem may be overcome by the table 316 b which separately specifies the offset cancel amounts for the above - mentioned combinations . these combinations of particular current pga data and next pga data and the corresponding dc offset amounts have been known by the actual measurement and therefore can be incorporated in the table 316 b in advance . thus , the amount of the data to be written beforehand to the dc offset cancel table 316 can be reduced significantly . the second embodiment of the invention is a method for overcoming the problems involved in the related - art analog dc offset cancel operation performed on the stepwise dc offset component caused by digitally setting gains based on the pga method . the following describes the second embodiment of the invention with reference to accompanying drawings . now , referring to fig1 , there is shown a block diagram illustrating an exemplary configuration of a receiver apparatus in which a pga - based direct conversion receiver is used in a ds - cdma mobile communication system , practiced as the second embodiment of the invention . as shown in fig1 , as described in the section regarding the related - art , a signal received by a terminal at its external antenna is differentially amplified by a low - noise amplifier 301 to be divided into two . each differentially amplified signal is c - cut of its direct - current component , subsequently down - converted from rf signal to baseband signal by a quadrature mixer 302 at a stretch . at this moment , to mix with a signal obtained by dividing an lo signal from a local oscillator 304 into an in - phase component and a quadrature component by a divider 303 , an in - phase component signal and a quadrature component signal are generated in the baseband . these baseband signals are removed of an interference wave signal power of an adjacent channel for example by an lpf 305 , thereby extracting only a desired wave signal . next , 3 - wire serial digital data 312 are decoded by the pga control circuit 311 and the baseband signals are amplified by discretely switching the gains of a baseband amplifier 306 . an lpf 307 is a filter inserted to convert a 50 % roll - off characteristic downlink signal at the sender side into a 100 % roll - off characteristic downlink signal as a total transfer function . the s / n of the baseband signals are maximized by the lpf 307 and then the maximized signals are quantized by an a / d converter 308 from analog to digital . the quantized signals are sent to a finger processing block and path search block 319 which constitutes a baseband signal processing block . in the finger processing block and path search block 319 , decode processing including rake synthesis and error correction is performed on the quantized signals , which is not described in detail herein , thereby correctly decoding the downlink signal received from a base station . concurrently with the processing by the finger processing block and path search block 319 , digital signal processing is performed on the iq baseband signal quantized by the a / d converter 308 by a receive signal level computing unit 314 . in computation of the receive signal level , the receive signal level computing unit 314 executes averaging processing to remove the influence of the instantaneous change in the receive envelope level which is rayleigh - scattered by phasing . this processing is executed , by use of an averaged time as a parameter which can be set as desired by a pga control processing block 315 . this parameter is set on the basis of a simulation or actual measurement data . the operations so far are the same as those of the first embodiment of the invention . it should be noted that the 3 - wire serial digital data 312 are generated by a pga data generator 318 as instructed by a pga control processing block 315 . in the second embodiment , a pga control data table 317 is also used as with the first embodiment . the circuitry shown in fig1 also has a dc offset canceller circuit 310 as with the related - art technology and includes a mask hold circuit 320 behind the baseband amplifier 306 . this mask hold circuit 320 masks the iq signal data outputted from the baseband amplifier 306 to hold the dc level . problems involved in the pga - based direct conversion receiver are , as described above , the stepwise dc offset component which deteriorates s / n and the large glitch component which saturates the a / d converter 308 to cause an erroneous agc control operation . on the other hand , if the lpf cutoff frequency is temporarily raised up to around 100 to 200 khz for about 10 microseconds to speed up the convergence of dc offset and the iq signal data for this 10 - microsecond period are masked at the output , the problem of the drop of receive data occurs when spreading factor ( sf )= 4 . to circumvent this problem , when a dc offset component large enough for causing s / n deterioration is not generated , the second embodiment turns off the mask hold circuit 320 to output the iq signal without change and , only when a dc offset component large enough for causing s / n deterioration is generated , the lpf cutoff frequency is temporarily raised to around 100 to 200 khz for about 10 microseconds as with the related - art technology to speed up the convergence of the dc offset and , at the same time , masks the iq signal data for this 10 - microsecond period at the output . consequently , the frequency of the drop of receive data can be lowered , thereby preventing the ber / bler deterioration of the receiver in total . as described above , the occurrence of a large stepwise dc offset is a phenomenon that takes place in limited cases when the total gain rises and the gains of amplifiers constituting the pga - based baseband amplification block are discretely switched largely . therefore , the actual measurement in advance allows the understanding of stepwise dc offset variation data and a dc offset threshold value which deteriorates s / n due to the stepwise dc offset . since the pga control processing block 315 recognizes both the currently set pga data and the pga data to be set next , the combinations of current pga data and next pga data that cause such dc offsets exceeding this threshold value are stored in a mask enabling table 321 as shown in fig1 . when changing the pga data , the pga control processing block 315 references the mask enabling table 321 to turn on / off the related - art analog dc offset cancel operation . at the same time , an on / off flag is attached by the pga data generator 318 to generate the 3 - wire serial digital data 312 . the 3 - wire serial digital data 312 are then decoded by the pga control circuit 311 incorporated in the direct conversion receiver to turn on the on / off flag only for the combination stored in the mask enabling table 321 , thereby enabling the mask processing ; for other combinations , the on / off flag is turned off to disable the mask processing . instead of the mask enabling table 321 , the dc offset cancel table 316 shown in fig1 may be used for the pga control processing block 315 to compare the offset cancel amount ( equivalent to the offset amount ) obtained by the table reference with the threshold value , thereby determining whether to perform masking processing . in this case , to be specific , the related - art analog dc offset cancel operation is turned on / off by the threshold value determination by the pga control processing block 315 . at the same time , the on / off flag is attached by the pga data generator 318 to generate pga data . further , the value ( 1 or 0 ) of the flag may be held in the table in accordance with the current pga data and the next pga data to be referenced by the pga control processing block 315 , thereby obtaining the flag value . further , in the configuration described above , the mask enabling table 321 stores the combination of current pga data and next pga data for enabling the mask processing ; conversely , a mask disabling table ( not shown ) for storing the combination of the pga data for disabling the mask processing may be arranged . herein , the mask enabling table and the mask disabling table are generically referred to as a mask table . as described above , a large glitch component causing a trouble in an agc control operation usually occurs with a large stepwise dc offset , so that this can be solved by masking the iq signal data at the output for the period of 10 microseconds , while speeding up the dc offset convergence by temporarily raising the lpf cutoff frequency up to around 100 to 200 khz for this period of about 10 microseconds as with the related - art technology as described earlier . the following describes the third embodiment of the invention . in this embodiment , a mechanism for lowering the frequency of the occurrence of a stepwise dc offset component originally causing s / n deterioration is introduced . the concept of the third embodiment is as follows . in the actual mobile communication environment , a large increase in the total gain of the baseband amplification block by one session of pga data setting is not frequent because the averaging processing is executed in the receive signal level computing unit 314 as described earlier . therefore , an actual problem is a variation of about several db as a total gain . the case in which this variation causes a stepwise dc offset component that deteriorating s / n is the discrete and large switching of the gain of each amplifier constituting the pga - based baseband amplification block described with reference to fig3 . therefore , to prevent the switching of each amplifier from going up to a larger gain variation than that of the related - art technology when the total gain varies by these several db , the applicant of the present invention conceptualizes that the point at which the pga gain of each amplifier is discretely switched is provided with hysteresis characteristic . fig1 shows an exemplary configuration of a pga baseband amplification block according to the third embodiment . in this example , the pga amplification block is constituted by four stages of amplifiers , a total gain range being 72 db . for example , it is assumed that pga 1 discretely switches between gains 10 , 15 , and 20 db , pga 2 discretely switches between gains − 20 , − 10 , 1 , 10 , and 15 db , pga 3 discretely switches between gains 1 to 24 db in step of 1 db , and pga 4 discretely switches between gains 1 , − 14 , − 1 , − 15 , 9 , and 4 db . in this case , the related - art control method executes the gain switching on each stage as shown in fig1 a for the total gain variation of 0 to 72 db in step of 1 db . the horizontal axis of the graph indicates the control words equivalent to pga data , one word being in unit of 1 db . now , suppose that the gain range of pga 3 which is switched every 1 db step be expanded by 6 db to 1 to 30 db in the baseband amplifier 306 and a hysteresis width of 6 db is provided to the point at which the gain of each pga is discretely switched , for example . in this case also , as shown in fig1 b , the total gain range is configured as a 1 - db step variable baseband amplification block like that shown in fig1 a . hysteresis denotes that there is a difference in the gain switching control path between going to and coming back ; therefore , control is executed such that the path of the side that rejects as much as possible the change from the current status . for example , in the case of pga 2 , if the total gain is raised from the 0 db side , the gain is switched from − 20 db to − 10 db at 12 db in related - art technologies . with gpa 1 , gpa 3 , and gpa 4 , the gains are switched in conjugation therewith . ( it should be noted that , because pga 3 takes charges of the variation in unit of 1 db among the amplifiers , it is always switched at the switching of the total gain .) the same holds true with the variation in the reverse direction . namely , if the total gain decreased from 24 db for example , the gains of all pgas are switched at 12 db . therefore , if a relatively small variation occurs at around 12 db , the gain switching of all stages takes place frequently . the same holds true with other points of the total gain at which simultaneous switching of plural stages takes place . however , in hysteresis control , if the total gain increases from the side of 0 db to over 12 db in the example of fig1 b , the gain switching of pga 1 , pga 2 , and pga 4 does not take place up to 18 db of the total gain thanks to hysteresis . the gain rise ( 6 db ) for that period is carried by pga 3 having the expanded gain variation range . the sequential 1 db gain variation of only pga 3 does not generate a dc offset large enough for causing a problem . obviously , if the total gain further rises over 18 db , it causes the gain variation of all stages , but the frequency at which the simultaneous variation of plural pgas takes place is reduced because of an allowance of 6 db . the same holds true with the variation in the reverse direction . for example , if the total gain lowers from 24 db to 18 db , the gain switching of pga 1 , pga 2 , and pga 4 does not occur ; only when the total gain lowers below 12 db , the gain switching of all stages occurs . thus , the introduction of hysteresis for the gain switching of each stage significantly lowers the number of points at which the stepwise dc offset for deteriorating s / n takes place , namely the frequency at which the pga gains are discretely switched . the setting of the hysteresis width of the pga baseband amplification block is not restricted to 6 db ; for example , the hysteresis width can be set to a desired value by the pga control processing block 315 in advance . the hysteresis setting is sent with the 3 - wire serial digital data 312 as a setting parameter value . the pga control circuit 311 decodes this pga data sequence to recognize the parameter value . on the basis of this parameter value , the pga control circuit 311 can discretely switch the gain of the baseband amplification block 306 by use of the hysteresis width setting . execution of the above - mentioned sequence of operations allows the lowering of the frequency at which the stepwise dc offset component which originally causes s / n deterioration occurs . as described and according to the third embodiment of the invention , the mask processing lowers the frequency at which the stepwise dc offset component for originally deteriorating s / n occurs , thereby preventing the deterioration of the ber / bler of the receiver in total . the third embodiment can be employed independently of or in combination with the above - mentioned first and second embodiments . while the preferred embodiments of the present invention have been described using specific terms , such description is for illustrative purposes only , and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims .