Abstract:
An AGC circuit is capable of prevention of occurrence of distortion of an input signal of an A/D converter and enables use of full scale of the A/D converter even in demodulation of a signal of a modulation system, in which a frequency axis is commonly occupied by a plurality of receivers, such as CDMA. The AGC circuit includes an amplifier to amplify an input signal and a controller to compare an amplitude of the input signal and a first reference value and control a gain of the amplifier depending upon a result of the comparison. The AGC circuit further includes first reference value controller to detect a maximum value of the amplitude of the input signal amplified by the amplifier and controlling the first reference value depending upon the maximum value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an automatic gain control (AGC) circuit and control method therefor. More particularly, the invention relates to an AGC circuit and control method therefor amplifying a received multiplexed signal of a Code Division Multiple Access (CDMA) system or so forth. 
     2. Description of the Related Art 
     The conventional AGC circuit is designed to increase a gain of an amplifier circuit when an input voltage is lower than a reference voltage and to decrease the gain when the input voltage is higher than the reference voltage for the purpose of obtaining an output of a constant level and less distortion. Accordingly, the reference voltage is fixed at a predetermined value. 
     In case of a Time Division Multiple Access (TDMA) system and a Frequency Division Multiple Access (FDMA) system, not so significant problem may be encountered even in the system, in which the reference voltage is fixed. 
     The reason is that, TDMA and FDMA systems are systems, which permit one receiver to occupy a frequency axis in time sequence. In these system, a relationship between an average value of a reception voltage and a maximum value thereof can be checked in advance by a parameter (such as in a roll off ratio in a Quadrature Phase Shift Keying (QPSK)) of a modulation system. 
     With the reference voltage thus determined, a signal having substantially constant level and less distortion can be obtained as the output of the AGC circuit. Accordingly, full scale of an analog-to-digital converter (A/D converter) provided on an output side of the AGC circuit can be used effectively. 
     Next, CDMA system will be discussed briefly. FIG. 11 is a frequency spectrum chart showing a structure of a transmission signal in CDMA system. In FIG. 11, a vertical axis represents an output level (dB) and a horizontal axis represents a frequency (Hz). 
     In FIG. 11, CDMA system is a system, in which signals from a plurality of stations, e.g.  3  stations of U 1  to U 3 , are transmitted in multiplexed fashion in the same frequency band f. Namely, the signals of a plurality of stations are transmitted in multiplexed fashion at the same time and at the same frequency. 
     In such modulation system according to CDMA, in which a plurality of receivers commonly occupy the frequency axis, a relationship between the average value and the maximum value of the reception voltage is variable depending upon number of receivers at instantaneous timing to frequently cause difficulty in prevention of occurrence of distortion of the input signal of the A/D converter. 
     When a number of the receivers receiving the signals at the same timing is large, peak factor (crest factor) of the reception waveform is obviously increased. On the other hand, in order to prevent increasing of the peak factor, a reception S/N can be degraded by setting the reference voltage sufficiently low with respect to the full scale of the A/D converter. 
     On the other hand, one example of the AGC circuit variable of the reference voltage has been disclosed in Japanese Unexamined Patent Publication No. Heisei 7-226725 which will be hereinafter referred to as first prior art. The disclosed first prior art has been worked out with giving importance for amplification of a small input voltage. 
     FIG. 12 is a block diagram showing a construction of the conventional AGC circuit disclosed in the first prior art. Referring to FIG. 12, the AGC circuit converts a received orthogonal frequency division multiplexed signal S 50  into a baseband by a frequency converting portion  101  and amplifies a baseband signal S 51  after conversion by an AGC amplifier  102 , and transmits the baseband signal S 52  amplified by the AGC amplifier  102  to mixers  106  and  107  by dividing into two paths by a divider  103 . 
     The mixers  106  and  107  mix the respectively input baseband signals S 53  and S 54  with signals S 55  and S 56  output from a phase converter  105  and having phase difference of  900  and output signals as first and second baseband signals S 57  and S 58 . 
     The first baseband signal S 57  is converted into a digital signal S 59  by an A/D converter  110  via a low-pass filter  108 . Similarly, the second baseband signal S 58  is converted into a digital signal S 60  by an A/D converter  111  via a low-pass filter  109 . 
     Outputs S 59  and S 60  of the A/D converters  110  and  111  are detected voltages by a rectifier circuit  114 , and a voltage S 62  depending upon a detected value S 61  is generated by a control voltage generator  116 . 
     On the other hand, a voltage generator  120  generates a constant voltage S 63 . A voltage adder  118  adds the constant voltage S 63  to a voltage S 62  generated by the control voltage generator  116  to control a gain of the AGC amplifier  102  by a resultant voltage S 64 . 
     The voltage S 63  to be added is a voltage for controlling the gain of the AGC amplifier  102  so that input waveform to the A/D converters  110  and  111  exceeds allowable inputs of the A/D converters  110  and  111 . 
     When input waveform to the A/D converters  110  and  111  exceeds allowable inputs of the A/D converters  110  and  111 , the quantization error for the small amplitude portion of the input waveform to the A/D converters  110  and  111  would be reduced. 
     The large amplitude portion of the input waveforms to the A/D converters  110  and  111  are clipped exceeding an allowable voltage range to cause influence of distortion or so forth. What is demodulated by a demodulation device is an Orthogonal Frequency Division Multiplexing (OFDM) wave. Considering OFDM wave, occurrence probability of the large amplitude portion is quite small not to cause significant problem in comparison with improvement effect. 
     FIG. 13 is a frequency spectrum chart of OFDM wave. As shown in FIG. 13, OFDM wave causes mutual interference of a signal at a center frequency f 1  and a signal at a center frequency f 2  within a frequency region f 12 , for example, and causes mutual interference of a signal at a center frequency f 2  and a signal at a center frequency f 3  within a frequency region f 23 . 
     However, mutually interfering signals are only two signals and is completely different from the case of CDMA. In the present invention generally handles CDMA mutually interfering three or more signals. 
     Namely, when the AGC circuit as disclosed in the first prior art is employed in CDMA, the large amplitude portion is clipped exceeding the allowable voltage range of the A/D converter to encounter a problem to cause large distortion or so forth. 
     Other example of this kind of AGC circuit has been disclosed in Japanese Unexamined Patent Publication No. Heisei 8-331192 (hereinafter referred to as second prior art). The disclosed invention is directed to a Quadrature Amplitude Modulation (QAM) demodulation device which demodulates a modulated wave signal by performing phase correction based on accurate synchronization detection by simple construction and simple calculation. 
     The disclosed AGC circuit performs gain adjustment by extracting upper M in number of values of the amplitude values of symbol signals. However, an object of the invention disclosed in the second prior art is to demodulate the modulated wave signal by performing phase correction on the basis of accurate synchronization detection, which is far different from an object of the present invention. 
     Accordingly, even in the second prior art, there has not been disclosed means for preventing occurrence of distortion in the input signal to the A/D converter in a CDMA demodulation device, as a task of the present invention. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an AGC circuit and control method therefor which can prevent occurrence of distortion of an input signal of an A/D converter and whereby to enable use of full scale of the A/D converter even upon demodulation of signals of modulation system, in which a frequency axis is occupied by a plurality of receivers, such as CDMA. 
     According to the first aspect of the present invention, an AGC circuit comprises: 
     amplifying means for amplifying an input signal; 
     control means comparing an amplitude of the input signal and a first reference value and controlling a gain of the amplifying means depending upon a result of comparison; and 
     first reference value control means for detecting a maximum value of the amplitude of the input signal amplified by the amplifying means and controlling the first reference value depending upon the maximum value. 
     According to the second aspect of the present invention, a control method of an AGC circuit comprises: 
     a first process of amplifying an input signal, comparing an amplitude of the amplified input signal with a first reference value and controlling an amplitude of the input signal depending upon the result of comparison; and 
     a second process of detecting a maximum value of an amplitude of the amplified input signal with a predetermined period and controlling the first reference value depending upon the maximum value. 
     With the present invention set forth above, the first reference value, to be compared with the input signal is varied depending upon the maximum value of the amplitude of the input signal to control the gain of the amplifying means. 
     By the construction set forth above, the AGC circuit is capable of prevention of occurrence of distortion of an input signal of an A/D converter and enables use of full scale of the A/D converter even in demodulation of a signal of a modulation system, in which a frequency axis is commonly occupied by a plurality of receivers, such as CDMA. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken to be limitative to the invention, but are for explanation and understanding only. 
     In the drawings: 
     FIG. 1 is an illustration showing a construction of the first embodiment of an AGC circuit according to the present invention; 
     FIG. 2 is a waveform chart showing a relationship between a reception data and a diffusion sign; 
     FIG. 3 is an illustration showing one example of a structure of the reception data; 
     FIG. 4 is a diagrammatic explanatory illustration showing an I component and a Q component by an orthogonal coordinates; 
     FIG. 5 is a diagrammatic explanatory illustration showing an I component and a Q component by an orthogonal coordinates in the case where a signal is 16 QAM; 
     FIG. 6 is a waveform chart showing a relationship between an amplified value Pt and a maximum value V 10 ; 
     FIG. 7 is a diagrammatic explanatory illustration showing structures of a reception voltage V 3  and a reference voltage V 1  input to a comparator  11 ; 
     FIG. 8 is a flowchart showing operation of an overall AGC circuit; 
     FIG. 9 is a flowchart showing operation of an overall AGC circuit; 
     FIG. 10 is an illustration showing an overall construction of a transmission and reception equipment including the AGC circuit according to the present invention; 
     FIG. 11 is a frequency spectrum chart showing a structure of a transmission signal of CDMA system; 
     FIG. 12 is an illustration showing a construction of the AGC circuit disclosed in Japanese Unexamined Patent Publication No. Heisei 7-226725; and 
     FIG. 13 is a frequency spectrum chart of OFDM wave. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be discussed hereinafter in detail in terms of the preferred embodiment of the present invention with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to those skilled in the art that the present invention may be practiced without these specific details. In other instance, well-known structures are not shown in detail in order to avoid unnecessarily obscure the present invention. 
     At first, discussion will be given for an overall construction of a mobile station in a mobile radio communication system according to the present invention, and preferably, to a transmission and reception equipment including an AGC circuit. FIG. 10 is an illustration showing the overall construction of the transmission and reception equipment including the AGC circuit according to the present invention. The transmission and reception equipment is constituted with an antenna  51 , a changeover switch  52 , a receiving portion  53 , a transmitting portion  54  and a digital signal processing portion  13 . 
     Furthermore, the receiving portion  53  is constructed with band-pass filters  56  to  60 , amplifiers  61  to  64 , an AGC amplifier  1 , a Quadrature demodulator (Q-DEM)  2 , A/D converters  7  and  8  and a Phase Locked Loop (PLL)  67  including a Voltage Controlled Oscillator (VCO) and a mixer  68 . The transmitting portion  54  is constructed with band-pass filters  71  to  74 , amplifiers  75  to  78 , an AGC amplifier  79 , a quadrature modulator (Q-MOD)  80 , a digital-to-analog (D/A) converter  81 , a Phase Locked Loop (PLL)  83 ,  84  including respective Voltage Controlled Oscillators (VCOs), a Temperature Compensated Xtal Oscillator (TCXO)  85 , a mixer  86  and an isolator  87 . 
     Next, discussion will be given for operation. The changeover switch  52  is a switch for switching between an antenna and an external RF (ext RF) terminal. The external RF terminal is used for measurement and vehicle mounting for the purpose of production and maintenance. 
     At first, discussion will be given for operation of the receiving portion  53 . The CDMA signal received by the antenna is processed for frequency conversion by the mixer  68  via the changeover switch  52 , the band-pass filter  56 , the amplifier  61  and the band-pass filter  57 . 
     The signal from the VCO in the PLL  83  is input to the mixer  68  via the amplifiers  63  and  62  and the band-pass filter  60  as a local signal. 
     The CDMA signal frequency converted by the mixer  68  is input to the AGC amplifier  1  via the band-pass filter  58 . The CDMA signal is amplified by the AGC amplifier  1  and thereafter is demodulated into I (in-phase) component and Q (quadrature) component having phase shifted for 90° relative to the I component by the quadrature demodulator  2 . Respective components are converted into digital values through the A/D converters  7  and  8 . 
     On the other hand, the signal from the VCO in the PLL  67  is input to the quadrature demodulator  2  via the amplifier  64 . 
     Then, the I component and the Q component converted into the digital values by the A/D converters  7  and  8  subject to a predetermined digital signal process by the digital signal processing portion  13 . 
     The digital signal processing includes a process for despreading or reverse-spreading by a despreader (not shown) to extract only desired signal. 
     It should be noted that the AGC amplifier discussed later is constructed by a portion including the AGC amplifier  1 , the band-pass filter  59 , the quadrature demodulator  2 , the A/D converters  7  and  8  and the digital signal processing portion  13 , among the foregoing construction. 
     Next, the operation of the transmitting portion  54  will be discussed. The operation of the transmitting portion  54  is the same operation as the receiving portion  53  effected in reversed sequential order. 
     At first, the signal is generated by a spreading process by a spreader (not shown) in the digital signal processing portion  13 . Thereafter, the spread signal is converted into an analog signal of the I component and the Q component by a D/A converter  81 . 
     The analog converted I component and Q components are processed for quadrature modulation by the quadrature modulator  80  and then input to the AGC amplifier  79  via the band-pass filter  74  for amplification. 
     The quadrature modulation signal amplified by the AGC amplifier  79  is converted into a predetermined frequency by the mixer  86 , and thereafter transmitted from the antenna  51  via the band-pass filter  73 , the amplifier  76 , the band-pass filter  72 , the amplifier  75 , the isolator  87 , the band-pass filter  71  and the changeover switch  52 . 
     On the other hand, the signal from the VCO in the PLL  83  is input to the mixer  86  via the amplifiers  63  and  77  and the signals from the PLLs  67  and  83  are input to the quadrature modulator  80  via the TCXO  85 , the PLL  84  including the VCO and the amplifier  78 . 
     Next, the first embodiment of the AGC amplifier according to the present invention will be discussed. FIG. 1 is an illustration showing a construction of the first embodiment. It should be noted that similar or common components to those shown in the overall construction shown in FIG. 10 will be identified by like reference numerals and detailed description for such components will be omitted to avoid redundant disclosure and to keep the disclosure simple enough so as to facilitate clear understanding of the present invention. 
     Referring to FIG. 1, the first embodiment of the AGC amplifier according to the present invention is constructed with the AGC amplifier  1 , to which a reception wave is input, the quadrature demodulator  2  demodulating the output signal of the AGC amplifier  1 , low-pass filters  3  and  4  extracting a low frequency component of the output signal of the quadrature demodulator  2 , the amplifiers  5  and  6  amplifying the output signals of the low-pass filters  3  and  4 , the A/D converters  7  and  8  performing analog-to-digital conversion for the output signal of the amplifiers and  5  and  6 , route Nyquist filters (RNFs)  9  and  10 , performing wave shaping of the output data of the A/D converters  7  and  8 , a reception data processing circuit  13  processing the output data from the route Nyquist filters  9  and  10  to take out a desired demodulated output signal, a reception voltage measuring circuit  12  measuring output voltages of the A/D converters  7  and  8  and outputting an average value within a predetermine period, converging as an AGC amplifier loop, a comparator  11  compares an output of the reception voltage measuring circuit  12  and an output of the reference voltage generator  15  for outputting a difference of both outputs, and a maximum voltage detector  16  controlling a reference voltage generated by the reference voltage generator  15 . 
     Next, operation will be discussed. A reception wave S 1  is input to the AGC amplifier  1  and is amplified with a gain corresponding to a gain control voltage V 2  output from the comparator  11 . 
     The AGC amplifier  1  has a variable gain amplification control terminal la to linearly vary amplification by a gain controlling voltage V 2  input via the variable gain amplification control terminal  1   a.    
     The quadrature demodulator  2  takes out in-phase I component and quadrature Q component of the baseband signal from the reception wave S 1 . Respective components I and Q pass respective low-pass filters  3  and  4 , amplified by the amplifiers  5  and  6 , A/D converted by the A/D converters  7  and  8 , wave shaped by the digital filters (route Nyquist filters in the shown embodiment)  9  and  10  so as not to cause intersymbol interference, and output as the demodulated output signal S 2  by the reception data processing circuit  13 . 
     It should be noted that the band-pass filter  59  between the AGC amplifier  1  and the quadrature demodulator  2  is neglected in FIG.  1 . Also, the reception data processing circuit  13  in FIG. 1 is identified by the same reference numeral with the digital signal processing portion of FIG. 10 which means that the reception data processing circuit  13  of FIG. 1 is included in the digital signal processing portion  13  of FIG.  10 . 
     Returning to FIG. 1, the output voltages V 5  and V 6  output from the A/D converters  7  and  8  are measured by the reception voltage measuring circuit  12 . 
     Then, an average voltage V 3  of the output voltage data V 5  and V 6  in a predetermined period is measured. 
     The average value V 3  and the reference voltage V 1  output from the reference voltage generator  15  are input to the comparator  11  for comparison. 
     The comparator  11  generates the gain controlling voltage V 2  corresponding to a comparison differential voltage to input the gain controlling voltage V 2  to the AGC amplifier  1  to control own gain so that the difference between the reference voltage V 1  and the average value V 3  becomes “0”. 
     The AGC amplifier  1  controls the AGC loop to maintain the input level of the A/D converters  7  and  8  constant and without causing distortion. 
     The AGC loop is a loop returning to the AGC amplifier  1  again via the quadrature demodulator  2 , the A/D converters  7  and  8 , the reception voltage measuring circuit  12  and the comparator  11 . 
     Next, discussion will be given for the maximum voltage detector  16 . The maximum voltage detector  16  is connected between the outputs of the A/D converters  7  and  8  and an input of the reference voltage generator  15 . 
     The maximum voltage detector  16  detects a maximum voltage V 10  of the outputs of the A/D converters  7  and  8  to control the reference voltage V 1  generated by the reference voltage generator  15  depending upon how large and/or small the maximum voltage V 10  is. 
     In case of the signal other than the CDMA system, the reference voltage V 1  can be derived from the average voltage and the maximum voltage of the outputs V 5  and V 6  of the A/D converters  7  and  8  relatively easily. 
     However, in case of the signal of the CDMA system, the maximum voltage is increased and decreased depending upon number of signals to be multiplexed, and is varied as time goes. 
     Accordingly, if the reference voltage V 1  is set similarly to the signal other than the CDMA system, for example, when the signal having instantaneous high voltage is input to the A/D converters  7  and  8 , the reference voltage V 1  exceeds the full scale of the A/D converters  7  and  8  making it impossible to convert such high voltage into the digital value. 
     Then, it is assumed that the reference voltage V 1  is preliminarily set relatively low expecting input of a high voltage signal to the A/D converters  7  and  8  to lower the gain of the AGC amplifier  1 . Then, even when the high voltage signal is input to the A/D converters  7  and  8 , such voltage can be converted into the digital signal by the A/D converters  7  and  8 . 
     However, when the high voltage signal is not input to the A/D converters  7  and  8 , all of the voltages as converted output of the A/D converters  7  and  8  inherently have low voltage. Assuming that the A/D converters  7  and  8  have performance to output data at  8  bits, only lower several bits are effectively used when all of the converted outputs are low voltage to make it impossible to effectively use the full scale of the A/D converters  7  and  8 . 
     Therefore, by detecting the maximum values of the voltages input to the A/D converters  7  and  8  (equal to the voltages output from the A/D converters  7  and  8 ), if the maximum value does not reach the full-scale voltage of the A/D converters  7  and  8 , the reference voltage V 1  is raised to increase the gain of the AGC amplifier  1 . 
     On the other hand, when the occurrence that the maximum value becomes full scale of the A/D converters  7  and  8 , exceeds the predetermined reference value, the reference voltage V 1  is lowered to decrease the gain of the AGC amplifier  1 . 
     By this, full scale of the A/D converters  7  and  8  can be used effectively. 
     Next, detailed operation of respective parts will be discussed. At first, a relationship between the reception data and spread sign will be discussed briefly. FIG. 2 is a waveform chart showing a relationship between the reception data and the spread sign. 
     As shown in FIG. 2, the spread sign in a chip zone Tc and the reception data in one bit zone width T are multiplied by a not shown multiplier to generate a transmission signal. A signal, on which a plurality of transmission signals are multiplexed, is input to the A/D converters  7  and  8 . 
     On the other hand, in the A/D converters  7  and  8 , per chip zone Tc, the reception data per Tc*N (N is greater than or equal to 2) is preferably sampled. 
     FIG. 3 is an illustration showing one example of a structure of the reception data. As shown in FIG. 3, for example, one slot is formed with a pilot signal  31 , a transmitter power control (TPC)  32  and a reception data  33 . The reception data  33  is sampled by the A/D converter  7  and  8  and is converted into the digital data V 5  and V 6 . 
     The digital data V 5  and V 6  output from the A/D converters  7  and  8  are input to the reception voltage measuring circuit  12 . Next, reception voltage measuring circuit  12  will be discussed. 
     To the reception voltage measuring circuit  12 , the digital data V 5  of the I component and the digital data V 6  of the Q components are input. 
     FIG. 4 is a diagrammatic explanatory illustration showing the I component and the Q component in a form of quadrature coordinates. FIG. 4 shows the case where the reception signal is a quadrature phase shift keying (QPSK). 
     Referring to FIG. 4, a value derived by composing the I component and the Q components is an amplitude value P of the reception signal. 
     Namely, the amplitude value of the reception signal is expressed by (I 2 +Q 2 ) ½ . Accordingly, expressing the reception signal by (I component, Q component), the amplitude value P 1  of ( 1 ,  1 ) is 2 ½ . Similarly, when amplitude values P 2  to P 4  of (− 1 ,  1 ), (− 1 , − 1 ) and ( 1 , − 1 ) become 2 ½ . 
     On the other hand, to the reception voltage measuring circuit  12 , signals arrive from a plurality of stations at the same timing. Accordingly, for example, when signals of P 1  to P 3  arrive simultaneously, a total Pt of the amplitude value becomes 3×2 ½ . The value Pt is variable depending upon number of arriving signals. 
     FIG. 5 shows the case where the signal is 16-positions Quadrature Amplitude Modulation (16QAM). In this case, the amplitude values P 1  and P 3  takes the same values but the amplitude value P 2  takes smaller value than the amplitude values P 1  and P 3 . 
     The reception voltage measuring circuit  12  derives the average value V 3  of the amplitude value Pt per a given period. The given period is per N slots (N is positive integer), for example. 
     Next, discussion will be given for the maximum voltage detector  16 . The maximum voltage detector  16  detects the maximum value V 10  of the amplitude value Pt per N slots. 
     FIG. 6 is a waveform chart showing a relationship between the amplitude value Pt and the maximum value V 10 . 
     FIG. 6 shows variation of the levels of the signal (amplitude value Pt) input to the reception voltage measuring circuit  12  and the maximum voltage detector  16  to V 21  to V 29  according to elapsing of time T. 
     The maximum value V 10  is expressed by the 8 bit outputs from the A/D converters  7  and  8 , and the average value V 3  (V 21 ) is expressed by the lower 4 bits. 
     Next, discussion will be given with respect to the comparator  11 . FIG. 7 is a diagrammatic explanatory illustration showing structures of the reception voltage V 3  and the reference voltage V 1  input to the comparator  11 . 
     Referring to FIG. 7, the lower 4 bit reception voltage (average value) V 3  and 8 bit reference voltage V 1  are input to the comparator  11 . On the other hand, the reference voltage V 1  is input from the reference voltage generator  15  and the reception voltage V 3  is input from the reception voltage measuring circuit  12 , set forth above. 
     Among  8  bit of the reference voltage V 1 , the leading one bit is a sign bit (expressing sign of + or −), and remaining 7 bits express reception voltage. On the other hand, the reception voltage V 3  is set at a voltage which can be expressed by lower 4 bits. 
     The comparator  11  controls gain of the AGC amplifier  1  to make the reception voltage V 3  and the reference voltage V 1  consistent with each other. 
     Next, overall operation of the AGC circuit will be discussed. FIGS. 8 and 9 are flowcharts showing overall operation of the AGC circuit. 
     Referring to FIG. 8, at first, the initial value V 1  of the reference voltage is set at the reference voltage generator  15  (ST 1 ). 
     Next, an initial control voltage V 2  is set to the AGC amplifier  1  (ST 2 ). 
     Then, a current reception voltage (average value) V 3  is measured by the reception voltage measuring circuit  12  (ST 3 ). 
     Thereafter, the reference voltage V 1  and the reception voltage (average value) V 3  are compared by the comparator  11  (ST 4 ). If the reference voltage V 1  and the reception voltage (average value) V 3  are not consistent, the comparator  11  applies a new voltage V 2  to the gain control terminal  1   a  of the AGC amplifier  1  so as to cause variation of gain corresponding to differential voltage in the AGC amplifier  1  (ST 5 ). 
     Next, returning to step ST 4 , steps ST 4  and ST 5  are repeated until the reference voltage V 1  and the reception voltage (average value) V 3  match with each other. 
     On the other hand, if the reference voltage V 1  and the reception voltage (average value) V 3  match with each other at ST 4 , the maximum value Vmax (V 10 ) of the reception voltage is detected at the maximum voltage detector  16  (ST 6 ). Next, the maximum voltage detector  16  checks whether the maximum value Vmax of the reception voltage matches with the voltage Vad corresponding to full scale of the A/D converters  7  and  8  or not (ST 7  of FIG.  9 ). If matched with each other, the reference voltage V 1  is held at the current value and the process from ST 3  is executed again. 
     On the other hand, when the maximum value Vmax and the voltage Vad do not match with each other, the maximum voltage detector  16  sets new reference voltage V 1  to the reference voltage generator  15  (ST 8 ). 
     Then, the process from ST 3  is executed again. 
     Here, discussion will be given as to how the maximum voltage detector  16  detects maximum value Vmax=voltage Vad (ST 7 ). 
     For example, when the A/D converters  7  and  8  are 8 bit quantization and output coding in complement of 2, the lower 7 bits (0 to 127 in decimal number) become an absolute value of the measured amplitude. 
     At this time, when the maximum voltage detector  16  detects M times (“127 times”) in N slots, the maximum voltage detector  16  makes judgment that input causing overflow in the A/D converters  7  and  8  is present. 
     Namely, assuming that B oversampling (B is positive integer) is caused in the A/D converters  7  and  8  in the system where A (A is positive integer) chips are present during one slot, A×N×B in number of sampling data are observed in N slots. 
     When M in number of data correspond “127” among A×N×B in number of data, the maximum voltage detector  16  modifies the reference voltage V 1  in the reference voltage generator  15 . 
     In this case, the reference voltage V 1  is lowered. By this, the gain of the AGC amplifier  1  is also lowered. 
     Conversely, if all of A×N×B sampling data do not cause overflow and the maximum value Vmax among all data is less than Vad, the maximum voltage detector  16  modifies the reference voltage V 1  of the reference voltage generator  15 . 
     In this case, as new reference voltage V 1 , a voltage derived by multiplying the reference voltage by Vad/Vmax is set. 
     Accordingly, the voltage corresponding to “127” detected in N slots is more than or equal to one time and less than or equal to M times, the current reference value V 1  is maintained (corresponding to the case where the answer at ST 7  is YES). On the other hand when the voltage corresponding to “127” detected in N slots is zero times (not detected) or more than M times, the current reference value V 1  is modified (corresponding to the case where answer at step ST 7  is NO). 
     By employing the present invention not only in CDMA but also other modulation system, such as DMA or TDMA, distortion due to exceptional interference wave can be avoided. On the other hand, in the base station, the present invention is applicable. 
     Next, discussion will be given for the second embodiment of the present invention. The second embodiment provide a circuit for predicting number of stations (number of users) from level of the reception signal. 
     Namely, by convergence of the reference voltage V 1  generated by the reference voltage generator  15  toward the average voltage V 3  in N slots, number of users can be predicted from a ratio of Vmax/V 1 . 
     Namely, returning to flowcharts of FIGS. 8 and 9, when Vmax=Vad is not satisfied at step ST 7 , the new reference voltage v 1  is set at ST 8 . Next, process is advanced to step ST 3  and ST 4 , the reference voltage V 1  converges to the average voltage V 3  when V 3 =V 1 . 
     At this time, from a ratio between V 1  and the maximum value Vmax obtained at ST 7 , number of users can be predicted. 
     However, the foregoing is the case when transmission powers provided to respective users from the base station are the same and slot times and so forth are synchronized. 
     According the present invention, since the AGC circuit including the amplifying means for amplifying the input signal and control means for controlling gain of the amplifying means depending upon the result of comparison, in which the amplitude of the input signal amplified by the amplifying means is compared with the first reference value, is constructed with incorporating the first reference value control means detecting the maximum value of the amplitude within a predetermined period of the input signal amplified by the amplifying means and controlling the first reference value depending upon the maximum value. Even upon demodulation of the signal of the modulation type, in which the frequency axis are used in common by a plurality of receivers, such as CDMA, occurrence of distortion of the input signal of the A/D converter can be prevented to enable use of full scale of the A/D converter. 
     According to another invention, the control method of the AGC circuit including a first process of amplifying of the input signal, comparing of the amplitude of the input signal with the first reference value and controlling amplification of the input signal depending upon result of comparison, includes a second process of detecting the maximum value of the amplitude of the amplified input signal in the predetermined period and controlling the first reference value depending upon the maximum value. Even upon demodulation of the signal of the modulation type, in which the frequency axis are used in common by a plurality of receivers, such as CDMA, occurrence of distortion of the input signal of the A/D converter can be prevented to enable use of full scale of the A/D converter.