Abstract:
A radio receiver comprises a first amplifier to amplify a received radio signal; a quadrature demodulator to demodulate the radio signal amplified by said first amplifier and thereby generate a baseband signal; a second amplifier to amplify the baseband signal; a demodulator to demodulate the baseband signal amplified by said second amplifier; and a gain controller to control timing of a change in a gain of said second amplifier, in case that changing the gain of said first amplifier and the gain of said second amplifier, on the basis of a gain of said first amplifier before and after the change.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-122316, filed on Apr. 25th, 2003, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a radio receiver and a radio signal processing method.  
         [0004]     2. Related Background Art  
         [0005]     In recent years, radio communication devices have widely spread. As a result, it is increasingly demanded to reduce the number of components and the manufacture cost for a radio circuit and manufacture the radio circuit as a monolithic IC. In order to cope with this demand, the direct conversion scheme is adopted for the radio circuits.  
         [0006]      FIG. 8  is a block diagram of a conventional receiver adopting the direct conversion scheme. An antenna  10  receives an RF (Radio Frequency) signal, and an LNA (Low Noise Amplifier)  20  amplifies this RF signal. A quadrature demodulator  30  multiplies the amplified RF signal by an LO (Local Oscillator) signal supplied from a local oscillator (not illustrated). As a result, the RF signal is directly converted to a baseband signal. An LPF (Low Pass Filer)  40  conducts waveform shaping on the baseband signal, and a VGA (Variable Gain Amplifier)  50  amplifies this baseband signal. In addition, a demodulator  70  demodulates this baseband signal to a digital signal. In this way, the receiver using the direct conversion scheme converts the RF signal to a digital signal, and then demodulates it by using digital signal processing.  
         [0007]     The baseband signal amplified by the VGA  50  is input not only to the demodulator  70  but also to a gain controller  60  and a DC offset canceller  94 . In the gain controller  60 , a signal strength detector  80  measures the strength of the baseband signal. A gain selector  90  decides whether to switch the gain of the LNA  20  and the gain of the VGA  50  on the basis of the measured value of the baseband signal. A gain control signal generator  92  outputs a gain control signal to the LNA  20  and the VGA  50  to switch the gain in accordance with the decision made by the gain selector  90 . In this way, the gain controller  60  effects feedback control on the strength of the baseband signal.  
         [0008]     The DC (Direct Current) offset canceller  94  removes a DC offset component from the baseband signal amplified by the VGA  50 , and feeds back this to the VGA  50 . By the way, problems concerning the DC offset component are described in “Research development tendency of mixer for direct conversion receiver (“Mission Impossible ? A Review of Mixers for Direct-Conversion Receivers”)” written by Hiroshi Tanimoto, The Transactions of the Institute of Electronics, Information, and Communication Engineers, section C, Vol. J84-C, No. 5, pp. 337-348, May 2001.  
         [0009]      FIGS. 9A and 9B  are graphs showing gains of the LNA  20  and the VGA  50 , respectively.  FIG. 9C  is a graph showing a transient response component of a DC offset in the output of the VGA  50 . The gains of the LNA  20  and the VGA  50  are simultaneously switched.  
         [0010]     When the LNA  20  is switched from a high gain to a low gain at a point t 1  in time, the VGA  50  is switched simultaneously from a low gain to a high gain. In some cases, therefore, DC offsets of both the LNA  20  and the VGA  50  overlap each other, and a very large transient response component occurs, resulting in a degraded reception performance. This is because the LNA  20  is disposed in a stage in the radio circuit preceding the VGA  50 , and consequently the transient response component of the DC offset generated in the LNA  20  is amplified by the high gain obtained after the change in the VGA  50 .  
       SUMMARY OF THE INVENTION  
       [0011]     An advantage of an aspect of the present invention is to provide a radio receiver of direct conversion scheme in which degradation of the reception performance caused by the transient response component of the DC offset is suppressed.  
         [0012]     A radio receiver of an embodiment accordance with the instant invention comprises a first amplifier to amplify a received radio signal; a demodulation circuit line comprising a quadrature demodulator to demodulate the radio signal amplified by said first amplifier and to generate a baseband signal, a second amplifier to amplify the baseband signal, and a demodulator to demodulate the baseband signal amplified by said second amplifier; and a gain controller to control timing of a change in a gain of said second amplifier, in case that the gain of said first amplifier and the gain of said second amplifier are changed, on the basis of a gain of said first amplifier before and after the change.  
         [0013]     A radio receiver of another embodiment accordance with the instant invention comprises a radio receiver comprising: a first amplifier to amplify a received radio signal; a quadrature demodulator to demodulate the radio signal amplified by said first amplifier and to generate a baseband signal; a second amplifier to amplify, after changing a gain of said first amplifier, the baseband signal with a gain based on a gain of said first amplifier obtained before and after the change; and a demodulator to demodulate the baseband signal amplified by said second amplifier.  
         [0014]     A radio receiver of further embodiment accordance with the instant invention comprises a first amplifier to amplify a received radio signal; a quadrature demodulator to demodulate the radio signal amplified by said first amplifier and to generate a baseband signal; a second amplifier to amplify the baseband signal; a demodulator to demodulate the baseband signal amplified by said second amplifier; and a gain controller to delay timing of a change in a gain of said second amplifier as compared with timing of a change in a gain of said first amplifier, in case that the gain of said first amplifier is changed from a high gain to a low gain and the gain of said second amplifier is changed from a low gain to a high gain.  
         [0015]     A radio signal processing method of an embodiment accordance with the instant invention comprises: receiving a radio signal; amplifying the radio signal; demodulating the amplified radio signal to a baseband signal; amplifying the baseband signal; demodulating the amplified baseband signal; determining timing of a change in a gain of said first amplifier and a gain of said second amplifier, in case that the gain of said first amplifier and the gain of said second amplifier are changed, on the basis of the gain of said first amplifier obtained before and after the change; and changing the gain of said first amplifier and the gain of said second amplifier in accordance with the determined timing. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a block diagram showing an embodiment according to the present invention;  
         [0017]      FIG. 2  is a block diagram showing a concrete example of a change timing controller  196 ;  
         [0018]      FIGS. 3A  to  3 F are time charts showing gains of the LNA  120  and VGA  150 , and signal strength of a baseband signal;  
         [0019]      FIGS. 4A  to  4 H are time charts showing gains of the LNA  120  and VGA  150  during phasing and signal strengths of a received signal and a baseband signal;  
         [0020]      FIG. 5  is a flow diagram showing operation of a radio receiver in an embodiment;  
         [0021]      FIG. 6  is a flow diagram showing details of operation of a gain controller  160  at a step S 70 ;  
         [0022]      FIG. 7  is a graph showing a DC offset component induced when the gain of the LNA  120  is changed from a high gain to a low gain and the gain of the VGA  150  is changed from a low gain to a high gain;  
         [0023]      FIG. 8  is a block diagram of a conventional receiver; and  
         [0024]      FIGS. 9A  to  9 C are graphs showing gains of conventional LNA  20  and VGA  50 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Hereafter, embodiments according to the present invention will be described with reference to the drawings. These embodiments do not restrain the present invention. In a radio receiver of a direct conversion scheme according to embodiments of the present invention, the gain of the LNA and the gain of the VGA can be changed respectively at points in time that are different from each other. As a result, the transient response component of the DC offset in the output of the VGA is reduced.  
         [0026]      FIG. 1  is a block diagram of a radio receiver  100  according to an embodiment of the present invention. The radio receiver  100  is a radio receiver using the direct conversion scheme. The direct conversion scheme is a scheme in which an RF signal having a high frequency is converted to a baseband signal having a low frequency without using an intermediate frequency. The radio receiver  100  includes an antenna  110 , an LNA  120 , a quadrature demodulator  130 , an LPF  140 , a VGA  150 , a gain controller  160 , a demodulator  170  and a DC offset canceller  194 .  
         [0027]     The DC offset canceller  194  is, for example, a circuit formed by connecting an amplifier having a constant gain and an integrator (low pass filter) in cascade. Owing to such a configuration, the DC offset canceller  194  can remove the DC offset component. The DC offset canceller  194  removes a DC offset component contained in the baseband signal, and then feeds back this baseband signal to the VGA  150 . The DC offset component is induced by a component of an LO signal that leaks to the antenna  110  and the LNA  120  and undergoes frequency conversion as an input of the quadrature demodulator  130 .  
         [0028]     Each of the LNA  120  and the VGA  150  is formed so as to be able to be changed stepwise in gain. In the present embodiment, the gain of the LNA  120  can be changed to two levels, i.e., a high-gain level and a low-gain level. The gain of the VGA  150  can be changed to multiple levels between a high gain and a low gain inclusive thereof.  
         [0029]     The gain controller  160  is formed so as to effect feedback control on the gains of the LNA  120  and the VGA  150  in order to keep a baseband signal supplied from the VGA  150  at a predetermined signal strength.  
         [0030]     The configuration of the gain controller  160  will now be described in more detail. The gain controller  160  includes a signal strength detector  180 , a gain selector  190 , a change timing controller  196 , a gain control signal generator  192  and a delay controller  198 . The signal strength detector  180  detects the signal strength of the baseband signal amplified by the VGA  150 . The gain selector  190  conducts selection on the gain of the LNA  120  and the gain of the VGA  150  so as to keep the signal strength of the baseband signal detected by the signal strength detector  180  at a constant signal strength. The change timing controller  196  controls timing at which the gain of the VGA  150  should be changed, on the basis of the gain of the LNA  120  selected by the gain selector  190  and the actual gain of the LNA  120  at the current point in time.  
         [0031]     The quadrature demodulator  130 , the VGA  150  and the demodulator  170  are connected in series. Hereafter, this is referred to as a demodulation circuit line. In the present embodiment, two demodulation circuit lines are connected in parallel after the LNA  120 , and used for an I-axis component and a Q-axis component of a received signal, respectively. One gain controller  160  is connected to the two demodulation circuit lines to control the two VGAs  150  in common. For example, the gain controller  160  changes gains of the two VGAs  150  by the same period of time after changing the gain of the LNA  120 . The gain controller  160  changes gains of the two VGAs  150  by the same quantity. In this way, the gain controller  160  controls a plurality of demodulation circuit lines in common. As a result, the radio receiver  100  can demodulate the I-axis component and the Q-axis component of the received signal in common.  
         [0032]      FIG. 2  shows a concrete example of the change timing controller  196 . The change timing controller  196  includes a gain comparator  201  and a delay control signal generator  203 .  
         [0033]     The gain comparator  201  compares the actual gain of the LNA  120  at the current point in time with the gain of the LNA  120  selected by the gain selector  190 . The gain comparator  201  previously stores a certain threshold concerning the gain of the LNA  120 . The high gain of the LNA  120  is a gain larger than the threshold, and the low gain is a gain smaller than the threshold. The gain comparator  201  compares the gain of the LNA  120  before a change with the gain of the LNA  120  after the change. Then, with the comparison result, the gain comparator  201  decides whether the gain of the LNA  120  is changed from the high gain to the low gain, the gain of the LNA  120  is changed from the low gain to the high gain, or the gain of the LNA  120  is not changed. Here, the gain of the LNA  120  before the change means the actual gain of the LNA  120  at the current point in time, and the gain of the LNA  120  after the change means the gain of the LNA  120  selected by the gain selector  190 . Furthermore, in the present embodiment, changing the gain means switching the gain stepwise. By the way, the gain comparator  201  also previously stores a certain threshold concerning the gain of the VGA  150 . The high gain of the VGA  150  is a gain larger than the threshold, and the low gain of the VGA  150  is a gain smaller than the threshold.  
         [0034]     The delay control signal generator  203  generates a delay control signal that indicates a delay time used to delay the change in gain of the VGA  150 . In the case where the gain of the LNA  120  is changed from the high gain to the low gain, the delay control signal generator  203  generates a delay control signal when the gain of the VGA  150  is changed from the low gain to the high gain. This delay control signal is output to the delay controller  198 . On the other hand, in the case where the gain of the LNA  120  is changed from the low gain to the high gain or the gain of the LNA  120  is not changed, the delay control signal generator  203  does not generate the delay control signal when the gain of the VGA  150  is changed.  
         [0035]     The gain control signal generator  192  is supplied with the gain selected in the gain selector  190  via the change timing controller  196 . The gain control signal generator  192  outputs a gain control signal to the LNA  120  and the delay controller  198  on the basis of the gains of the LNA  120  and the VGA  150  selected by the gain selector  190 . The gain control signal is a signal indicating the gains respectively of the LNA  120  and the VGA  150  selected by the gain selector  190 .  
         [0036]     The delay controller  198  outputs the gain control signal to the VGA  150 , after a predetermined delay time has elapsed since a point in time at which the delay controller  198  receives the gain control signal, in accordance with the delay control signal. Since the gain control signal is transmitted directly to the LNA  120 , the gain of the VGA  150  is changed with a delay to the gain of the LNA  120 . On the other hand, in the case where the delay control signal is not output from the delay control signal generator  203 , the delay controller  198  outputs the gain control signal to the VGA  150  without delaying the gain control signal.  
         [0037]     In this way, the gain controller  160  is formed so as to control the timing at which the gain of the VGA  150  is changed on the basis of the gain of the LNA  120  before and after the change.  
         [0038]     When the gain of the LNA  120  is changed, it is evident in the present embodiment that the gain change of the VGA  150  is brought about. Therefore, the timing at which the gain of the VGA  150  is changed is controlled on the basis of the gain of the LNA  120  before and after the change.  
         [0039]     In the case where it is not evident that the gain change of the VGA  150  is brought about when the gain of the LNA  120  is changed, however, the timing at which the gain of the VGA  150  is changed may be controlled on the basis of the gains of the LNA  120  and the VGA  150  before and after the change.  
         [0040]      FIGS. 3A  to  3 F are time charts showing gains of the LNA  120  and the VGA  150 , and the signal strength of the baseband signal. With reference to  FIGS. 3A  to  3 F, operation of the LNA  120  and the VGA  150  will now be described in further detail.  
         [0041]     First, the gain of the LNA  120  is changed from the low gain to the high gain as shown in  FIG. 3A , and the gain of the VGA  150  is changed from the high gain to the low gain as shown in  FIG. 3B . The gains of the LNA  120  and the VGA  150  are changed at a point in time t 20 . A transient response characteristic of the DC offset induced at this time is relatively small as shown in  FIG. 3C .  
         [0042]     Subsequently, the gain of the LNA  120  is changed from the high gain to the low gain as shown in  FIG. 3D , and the gain of the VGA  150  is changed from the low gain to the high gain as shown in  FIG. 3E . The gain of the LNA  120  is changed at a point in time t 21 . If at this time the gain of the VGA  150  is changed simultaneously with the change in the gain of the LNA  120  as represented by a broken line in  FIG. 3E , a large transient response component of the DC offset is induced as represented by a broken line in  FIG. 3F .  
         [0043]     In the present embodiment, therefore, the gain of the VGA  150  is changed from the low gain to the high gain with a delay time Td after the change in the gain of the LNA  120  as represented by a solid line in  FIG. 3E . The delay time Td is represented by Td=t 31 −t 21 . As a result, the transient response component of the DC offset induced at the point in time t 21  becomes smaller than the transient response component induced in the conventional technique. The delay time Td is larger than 0, and smaller than a repetition period (Δt shown in  FIG. 9 ) of the change in gains of the LNA  120  and the VGA  150 .  
         [0044]     In this way, the gain of the VGA  150  is changed with a delay to the change in the gain of the LNA  120 , in the present embodiment. Therefore, the transient response component of the DC offset can be reduced.  
         [0045]     Furthermore, in  FIG. 3F , an area S B  of a region surrounded by a straight line L and the solid line is obviously smaller than an area S A  of a region surrounded by the straight line L and the broken line. In the present embodiment, therefore, the DC offset component per unit time is smaller than that in the conventional technique. Since, as described above, the error rate in the reception characteristics is proportionate to an accumulation value of an area S per unit time, the present embodiment has a smaller error rate in the reception performance than that of the conventional technique. In the present embodiment, therefore, the reception performance becomes better than that in the conventional technique.  
         [0046]     Although in the present embodiment the gain of the LNA  120  can be changed to two levels, it is also permissible that the gain of the LNA can be changed to three or more levels.  
         [0047]      FIGS. 4A  to  4 H are time charts showing gains of the LNA  120  and VGA  150  in the case where the received electric field strength (so called “RSSI (received signal strength indicator)) changes monotonously, and time charts showing signal strengths of the received signal and signal strengths of the baseband signal. A variant for the embodiment shown in  FIGS. 3A  to  3 F will now be described with reference to  FIGS. 4A  to  4 H.  
         [0048]     First, the case where the signal strength of the received signal supplied from the antenna  110  falls between a point in time t 10  and a point in time t 30  as shown in  FIG. 4A  will now be described. The gain of the VGA  150  gradually rises stepwise from the point in time t 10  as shown in  FIG. 4C  under the feedback control of the gain controller  160 . As a result, the amplification factor for the received signal rises even if the signal strength of the received signal falls. Therefore, the signal strength of the baseband signal is kept constant as shown in  FIG. 4D .  
         [0049]     However, there is an upper limit in the gain of the VGA  150 . If the gain of the VGA  150  arrives at a vicinity of its upper limit at the point in time t 20 , therefore, the gain of the LNA  120  is changed from the low gain to the high gain as shown in  FIG. 4B , and the gain of the VGA  150  is changed from the high gain to the low gain as shown in  FIG. 4C . The gain change widths of the LNA  120  and the VGA  150  are nearly equal to each other. As a result, the fall in the gain of the VGA  150  can be compensated by the increase in the gain of the LNA  120 . The transient response characteristic of the DC offset induced at this time is relatively small as shown in  FIG. 4D . In the present embodiment, the gain of the VGA  150  is changed in a larger number of steps as compared with the gain of the LNA  120  as shown in  FIGS. 4B, 4C ,  4 F and  4 G. Even if the received signal strength changes linearly as shown in  FIG. 4A , therefore, the signal strength of the baseband signal can be kept constant in the present embodiment.  
         [0050]     Between the points in time t 20  and t 30 , the signal strength of the received signal further continues to fall. In such a case, the signal strength of the baseband signal can be kept constant by making the gain of the VGA  150  further rise stepwise. In  FIG. 4D  and  FIG. 4H  described later, the transient response component of the DC offset caused by the stepwise gain switching of the VGA  150  is omitted, because it is small.  
         [0051]     Subsequently, the case where the signal strength of the received signal rises between a point in time t 11  and a point in time t 31  as shown in  FIG. 4E  will now be described. The gain of the VGA  150  gradually falls stepwise from the point in time t 11  as shown in  FIG. 4G  under the feedback control of the gain controller  160 . As a result, the amplification factor for the received signal falls even if the signal strength of the received signal rises. Therefore, the signal strength of the baseband signal is kept constant as shown in  FIG. 4H .  
         [0052]     However, there is a lower limit in the gain of the VGA  150 . If the gain of the VGA  150  arrives at a vicinity of its lower limit at the point in time t 21 , therefore, the gain of the LNA  120  is changed from the high gain to the low gain as shown in  FIG. 4F . If at this time the gain of the VGA  150  is changed simultaneously with the change in the gain of the LNA  120 , a large transient response component of the DC offset occurs at the point in time t 21  as represented by a broken line in  FIG. 4H .  
         [0053]     In the present variant, the gain of the VGA  150  is changed from the low gain to the high gain with a delay time Td after the change in the gain of the LNA  120  as represented by a solid line in  FIG. 4G . At this time, the gain change widths of the LNA  120  and the VGA  150  are nearly equal to each other. The delay time Td is represented by Td=t 31 −t 21 . As a result, a transient response component of the DC offset induced at the point in time t 21  becomes smaller as compared with the transient response component in the conventional technique as shown in  FIG. 4H . The delay time Td is a value that is larger than 0 and that is smaller than a repetition period (Δt shown in  FIG. 9 ) of the change in gains of the LNA  120  and the VGA  150 .  
         [0054]     Between the points in time t 21  and t 31 , the signal strength of the received signal further continues to rise. In this case, the signal strength of the baseband signal can be kept constant by making the gain of the VGA  150  fall gradually.  
         [0055]     Thus, in the present variant, the gain of the VGA  150  is changed with a delay to the change in the gain of the LNA  120 , and consequently effects similar to those of the embodiment shown in  FIGS. 3A  to  3 F can be obtained.  
         [0056]     In the conventional technique, the gain of the VGA  150  is changed between the points in time t 21  and t 31  as represented by a broken line in  FIG. 4G . This means that the gain of the VGA  150  is being controlled when the transient response component of the DC offset is occurring. Therefore, the gain of the VGA  150  is changed largely at the point in time t 21 .  
         [0057]     On the other hand, in the present variant, the gain of the VGA  150  is not changed during the delay time Td, i.e., between the points in time t 21  and t 31 . As a result, the width of the gain of the VGA  150  changed at the point in time t 31  in the present variant is smaller than that at the point in time t 21  in the conventional technique. Therefore, the transient response component of the DC offset induced in the present variant is relatively small. Furthermore, according to the embodiment shown in  FIGS. 3A  to  3 F, the width of the gain of the VGA  150  changed at the point in time t 31  is equal to that changed at the point in time t 21  in the conventional technique. Therefore, a peak P 2  Of the transient response component of the DC offset induced at the point in time t 31  in the present variant becomes further smaller than a peak P 1  of the transient response component induced at the point in time t 31  in the embodiment shown in  FIGS. 3A  to  3 F.  
         [0058]     In the present variant, the gain of the VGA  150  is changed singly without changing the gain of the LNA  120  in some cases. In this case, however, the transient response component of the DC offset is relatively small similarly as in  FIG. 4D , and consequently no problems are posed.  
         [0059]      FIG. 5  is a flow diagram showing operation of the radio receiver  100  in the embodiment. An RF signal is received by the antenna  110  (S 10 ). The received signal is amplified by the LNA  120  (S 20 ). The quadrature demodulator  130  converts the RF signal having a high frequency to the baseband signal (S 30 ). The baseband signal is subjected to waveform shaping in the LPF  140  (S 40 ), and amplified in the VGA  150  (S 50 ). The DC offset canceller  194  removes the DC offset component from this baseband signal (S 60 ). The gain controller  160  is supplied with the baseband signal output from the VGA  150 , and the gain controller  160  effects feedback control on the LNA  120  and the VGA  150  (S 70 ). In addition, the demodulator  170  demodulates the baseband signal to the digital signal (S 80 ).  
         [0060]      FIG. 6  is a flow diagram showing details of operation conducted by the gain controller  160  at the step S 70 . If the baseband signal is input to the gain controller  160 , the signal strength detector  180  detects the signal strength of the baseband signal (S 70 - 1 ).  
         [0061]     Subsequently, the gain selector  190  selects gains of the LNA  120  and the VGA  150  so as to keep the signal strength of the baseband signal constant (S 70 - 3 ). Subsequently, the change timing controller  196  compares the actual gain state of the LNA  120  at the current point in time with the selected gain of the LNA  120  (S 70 - 5 ). As a result of this comparison, it is determined whether the gain of the LNA  120  passes through a threshold previously stored in the change timing controller  196  before and after a change (S 70 - 6 ).  
         [0062]     If the gain of the LNA  120  passes through this threshold before and after the change, the change timing controller  196  furthermore judges the gain state of the LNA  120  at the current point in time (S 70 - 7 ). Judgment on the gain state of the LNA  120  can be conducted by determining whether the gain of the LNA  120  is higher than this threshold (S 70 - 8 ). If the gain of the LNA  120  at the current point in time is the high gain as a result of this decision, the change timing controller  196  outputs the delay control signal to the delay controller  198  (S 70 - 9 ).  
         [0063]     Subsequently, the gain control signal generator  192  outputs the gain control signal to the LNA  120  and the delay controller  198  on the basis of the gains of the LNA  120  and the VGA  150  selected by the gain selector  190  (S 70 - 11 ). The delay controller  198  is supplied with the gain control signal and the delay control signal, and the delay controller  198  delays the gain control signal and outputs the delayed gain control signal to the VGA  150 . As a result, the gain of the VGA  150  is changed with a delay to the change in the gain of the LNA  120  (S 70 - 13 ).  
         [0064]     If the gain of the LNA  120  at the current point in time is the low gain at the step S 70 - 8 , the change timing controller  196  does not output the delay control signal. Since the delay control signal is not output, the delay controller  198  outputs the gain control signal to the VGA  150  without delaying it. As a result, the gain of the VGA  150  is changed simultaneously with a change in the gain of the LNA  120  (S 70 - 15 ).  
         [0065]     If the gain of the LNA  120  does not pass through the threshold before and after the change at the step S 70 - 6 , the delay control signal is not output. The change timing controller  196  further makes a decision whether to change the gain of the VGA  150  (S 70 - 17 ). In this decision, the gain comparator  201  previously stores a threshold located between the low gain and the high gain of the VGA  150 , and judges a gain higher than the threshold to be the high gain and judges gain lower than the threshold to be the low gain. If the gain of the VGA  150  passes through this threshold before and after the change, the change timing controller  196  decides to change the gain of the VGA  150  (S 70 - 18 ).  
         [0066]     If the gain of the VGA  150  is decided to be changed, the gain control signal generator  192  outputs the gain control signal to the VGA  150  via the delay controller  198  to change the gain of the VGA  150 . Since the delay control signal is not issued from the change timing controller  196 , the delay controller  198  passes the gain control signal to the VGA  150  without delaying it. As a result, the gain of the VGA  150  is changed (S 70 - 19 ).  
         [0067]     If the gain of the VGA  150  is decided not to be changed, the delay controller  198  does not change the gain of VGA  150 .  
         [0068]      FIG. 7  is a graph showing a DC offset component actually measured when the gain of the LNA  120  is changed from the high gain to the low gain and the gain of the VGA  150  is changed from the low gain to the high gain.  FIG. 7  corresponds to the graph shown in  FIG. 4H  in which the signal strength of the baseband signal has been obtained by actually measuring it. A curve A shows a DC offset component measured when the gain of the LNA  120  and the gain of the VGA  150  are changed simultaneously in the same way as the conventional technique. A curve B shows a DC offset component measured when the gain of the VGA  150  is changed with a delay to a change in the gain of the LNA  120  according to the embodiment. These graphs are data showing actual measured results obtained when an RF signal from a signal generator is input to the LNA  120  and a baseband signal output from the VGA  150  is observed on a digital oscilloscope.  
         [0069]     In the curve A, the transient response component of the DC offset output from the VGA  150  is approximately 70 mV maximum. On the other hand, in the curve B, the transient response component of the DC offset output from the VGA  150  is approximately 30 mV maximum. Therefore, the transient response component in the curve B is obviously lower than the transient response component in the curve A.  
         [0070]     In the case where a threshold is provided for the DC offset component, the probability in the embodiment that the DC offset component exceeds the threshold becomes lower as compared with the conventional technique.  
         [0071]     An area S B  of a region surrounded by a curve B and a broken line is obviously smaller than an area S A  of a region surrounded by a curve A and the broken line. In the embodiment, therefore, the DC offset component per unit time is smaller as compared with the conventional technique. Since the error rate in the reception characteristic is proportionate to the accumulation value of the area S per unit time as described earlier, the error rate in the reception characteristic in the embodiment is smaller as compared with the conventional technique. As a result, the embodiment becomes better in reception performance than the conventional technique.  
         [0072]     Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and example embodiments will be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following.