Abstract:
A code division multiplex communications system comprising: receiving circuit for receiving a radio wave and transforming the radio wave to an electric signal; delaying circuit for sequentially reading the electric signal at a timing of a clock pulse; switching circuit for shutting off a drive current of the delaying circuit at an OFF timing of the clock pulse; adding and subtracting circuit for adding and subtracting outputs of the delaying circuit in accordance with a spread code; and reproducing circuit for reproducing a transmission signal on the basis of an output of the adding and subtracting circuit. RCS95-120 is similar with respect to a point that an analog LSI matched filter is constructed by a slide capacitor system.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to spread spectrum communications and more particularly to a low power consumption code division multiplex communications system. 
     While other multiplex communication systems (FDMA, TDMA) cannot permit more than a predetermined number of users, in a code division multiple access (CDMA), since the quality of communication gradually deteriorates (graceful degradation), users can be accepted as long as the code synchronization can be set so that increase in the number of users can be expected. The CDMA has excellent interference resistance, signal concealment, and fading resistance and is being used in a wide range. 
     According to the CDMA communications system, in a transmitter, baseband data to be transmitted is multiplied by a spread code and further by a carrier, and resultant data is transmitted from an antenna. In a receiver, a spread code having the same phase as that of the spread code at the time of transmission is prepared and the baseband data is decoded by using a correlator. 
     Hitherto, sliding correlator, SAW (Surface Acoustic Wave) matched filter, digital LSI matched filter, and the like are known as correlators. 
     According to the sliding correlator, the spread code is cycled faster than a reception signal and a pull-in is performed by a discriminating circuit having a DLL (Delay Locked Loop) or the like. A signal obtained by eliminating carrier components by a sync detector or equivalent means, that is, of a frequency which is about the chip rate is inputted to the sliding correlator. The sliding correlator needs chip synchronization and has drawbacks that it takes time to capture synchronization and that the reception signal including carrier components cannot be inputted to the sliding correlator. 
     In the SAW matched filter, chip synchronization can be obtained at high speed. Although it can be used in the RF and IF bands, there are drawbacks that since the spread code is decided by a physical pattern of an SAW device, it is difficult to change the code and the filter does not easily correspond to a long spread code. 
     In the digital LSI matched filter, the chip synchronization is unnecessary. Although there is an advantage that the spread code can be easily changed, there is a drawback of a large power consumption. In the digital LSI matched filter according to conventional CMOS integrated circuit techniques, since the operating speed is slow, there is a drawback that it can be generally used only in the baseband. 
     In recent years, a mobile communication (portable telephone and the like) is being widely spread. As a communication system employed by the mobile communication, attention has been paid most to the above-mentioned CDMA. The correlator of the CDMA used in the mobile communication is requested to have programmability of the spread code and small power consumption. 
     However, the SAW matched filter has a problem regarding the programmability of the spread code. On the other hand, the digital LSI matched filter has a drawback of a large power consumption. 
     Recently, a correlator using a switched capacitor system has been developed and is being put into practical use. The correlator is accomplished by further improving the digital LSI matched filter and has power consumption of about {fraction (1/10)} of that of the digital LSI matched filter. 
     SUMMARY OF THE INVENTION 
     The invention has been made in consideration of the background and it is an object of the invention to provide a code division multiplex communications system having programmability of the spread code and the power consumption which is markedly smaller than that of the conventional technique. 
     According to the invention, there is provided a code division multiplex communications system comprising: receiving means for receiving a radio wave and transforming the radio wave to an electric signal; delaying means for sequentially reading the electric signal at a timing of a clock pulse; switching means for shutting off a drive current of the delaying means at an OFF timing of the clock pulse; adding and subtracting means for adding and subtracting outputs of the delaying means in accordance with a spread code; and reproducing means for reproducing a transmission signal on the basis of an output of the adding and subtracting means. 
     Preferably, in the code division multiplex communications system, the receiving means receives the radio wave and transforming the received signal to an intermediate frequency signal. 
     Preferably, in the code division multiplex communications system, the receiving means receives the radio wave and transforms the received radio wave to a baseband signal. 
     Preferably, in the code division multiplex communications system, the delaying means has voltage-current converting means and current delaying means, converts the electric signal to a current signal, and after that, sequentially reads the current signal by the current delaying means at the timing of the clock pulse. 
     Preferably, in the code division multiplex communications system, the current delaying means is constructed by current flip-flops of the number twice as many as the number of chips of the spread code. 
     Preferably, in the code division multiplex communications system, the current flip-flop is constructed by serially connecting a first sample and hold circuit for sampling an input current at the leading edge of a first clock pulse and holding at the trailing edge of the first clock pulse and a second sample and hold circuit for sampling an input current at the leading edge of a second clock pulse and holding at the trailing edge of the second clock pulse. 
     Preferably, in the code division multiplex communications system, the adding and subtracting means comprises: spread code output means for outputting a spread code; switching means for connecting each output of the current delaying means to a first or second current path to perform current addition on the basis of the output of the spread code output means; and subtracting means for subtracting the current of the second current path from the current of the first current path. 
     Preferably, in the code division multiplex communications system, the adding and subtracting means comprises: spread code output means for outputting the spread code; adding means for connecting outputs of the current delaying means to the first or second current path on the basis of an output of the spread code output means and adding currents; subtracting means for subtracting a current of the second current path from a current of the first current path; and switching means for turning off the operation of the adding means and subtracting means at an OFF timing of the clock pulse. 
     Preferably, in the code division multiplex communications system, in the subtracting means, first and second current mirror circuits are connected in series, a current of the second current path is supplied to an input terminal of the first current mirror circuit, a current of the first current path is supplied to an output terminal of the first current mirror circuit and an input terminal of the second current mirror circuit, and an output is obtained from an output terminal of the second current mirror circuit. 
     Preferably, in the code division multiplex communications system, the reproduction means comprises: a current-voltage converter for converting an output of the adding and subtracting means to a voltage signal; and a demodulator for reproducing the transmission signal by integrating an output of the current-voltage converter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the construction of a correlator according to an embodiment of the invention; 
     FIG. 2 is a block diagram showing the construction of a code division multiplex communications system according to an embodiment of the invention; 
     FIG. 3 is a circuit diagram showing the construction of a V/IC  101  in FIG. 1; 
     FIG. 4 is a circuit diagram showing the construction of a CDF/F  102   1  in FIG. 1; 
     FIG. 5 is a circuit diagram showing the construction of an analog switch  104   1  in FIG. 1; 
     FIGS. 6A and 6B are circuit diagrams the construction of a current adder  105  in FIG. 1; 
     FIG. 7 is a circuit diagram showing the construction of a I/VC  107  in FIG. 1; 
     FIGS. 8A,  8 B, and  8 C are timing diagrams showing the operation of the code division multiplex communications system according to the embodiment of the invention; 
     FIGS. 9A,  9 B,  9 C,  9 D and  9 E are timing diagrams showing a transmission wave of a spread spectrum communication; 
     FIGS. 10A,  10 B,  10 C,  10 D,  10 E,  10 F and  10 G are timing diagrams showing the operation of the CDF/F in FIG. 4; 
     FIG. 11 is a circuit diagram showing another construction of the CDF/F in FIG. 1; 
     FIG. 12 is a block diagram showing the construction of a code division multiplex communications system according to a second embodiment of the invention; 
     FIGS. 13A,  13 B,  13 C and  13 D are timing diagrams for explaining the operation when the phases of clock pulses W 1  and W 2  in FIG. 4 are changed; 
     FIGS. 14A,  14 B,  14 C,  14 D,  14 E and  14 F are timing diagrams showing the operation of the CDF/F shown in FIG. 11; and 
     FIGS. 15A and 15B are diagram specifically showing the construction of a current source in the embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (1) Description of the Embodiments 
     An embodiment of the invention will be described hereinbelow with reference to the drawings. FIG. 2 is a block diagram showing the construction of a code division multiplex communications system (receiving side) according to an embodiment of the invention. In the diagram, reference numeral  1  denotes an antenna for receiving waves transmitted from a transmitter which will be described hereinafter;  2  a mixer for mixing the received transmission wave and a signal outputted from a local oscillator  3  and generating an IF (intermediate frequency) signal; and  4  a carrier synchronization detector for detecting synchronization of the outputs of the mixer  2 . As a spread code, a PN (Pseudo random Noise) code is used. Reference numeral  5  denotes a correlator for obtaining the correlation between the PN code generated by a PN code generator  6  and the output of the carrier sync detector  4  and generating a correlation signal; and  7  a demodulator constructed by using an integrator and the like for demodulating baseband data on the basis of the output of the correlator  5 . 
     The construction of the correlator  5  shown in FIG. 2 will be explained with reference to FIG.  1 . The correlator  5  is different from a conventional correlator, uses a switched current method (switched current matched filter), and detects the correlation by current addition. In FIG. 1, reference numeral  101  denotes a V/IC (Voltage/Current Converter) for converting a voltage value of a signal Vin inputted from a terminal T 1  to a current value Iin and outputting the current value Iin from a terminal T 2 . 
     FIG. 3 is a diagram showing a construction example of the V/IC  101  in FIG.  1 . In FIG. 3, OP 1  denotes an operational amplifier for amplifying the voltage difference between the (−) terminal and the (+) terminal. The (+) terminal is connected to the terminal T 1  and the (−) terminal is connected to the ground via a resistor R 1 . M 15  shows an n-channel type MOS transistor which converts a voltage to a current and whose source is connected to the ground via the resistor R 1 . Its drain is connected to the terminal T 2  and its gate is connected to an output terminal of the operational amplifier OP 1 . This construction relates to a so-called sink type V/I converter. A so-called source type V/I converter may be also used. 
     In FIG. 1,  102   1 ,  102   2 , . . . ,  102   n  (n is a natural number) denote CDF/Fs (Current Delay Flip/Flops) which sample and temporarily hold currents inputted from terminals T 6   1  to T 6   n  at timings of clock pulses inputted to terminals T 7   1  to T 7   n  and output from terminals T 9   1  to T 9   n  and terminals T 10   1  to T 10   n  at timings of clock pulses inputted to terminals T 8   1  to T 8   n . 
     FIG. 4 is a diagram showing an example of the construction of the CDF/F  102   1  in FIG. 1 (each of the CDF/Fs  102   2  to  102   n  has the same construction). The CDF/F  102   1  is constructed by sample and hold circuits SH 1  and SH 2  for holding current. In the sample and hold circuit SH 1 , M 1  denotes an n-type MOS transistor whose source is connected to the ground. Its drain is connected to a power source Vdd via a constant current source A 1 , its gate is connected to the drain, and the source is connected to the ground via an MOS transistor M 2 . 
     Similarly, M 3  is an n-type MOS transistor whose source is connected to the ground. Its drain is connected to the power source Vdd via a constant current source A 2 , its gate is connected to the gate of the MOS transistor M 1  via a switch SW 1  and its source is connected to the ground via an MOS transistor M 4 . 
     The n-type MOS transistor is a so-called n-channel MOSFET. A p-type MOS transistor denotes a p-channel MOSFET. Each of those n-type MOS transistor and the p-type MOS transistor is an enhancement-type MOSFET in which a current hardly flows in the drain/source when a voltage is not applied to the gate. Although a depletion type MOSFET in which a current flows in the drain/source when no voltage is applied to the gate can be also used, there is a drawback that its performance cannot obtain the operation characteristics shown in the embodiment. 
     As a fundamental construction, in one sample and hold circuit, that is, in SH 1  in FIG. 4, current values of the current sources of A 1  and A 2  are the same. The “ratio of the gate width to the gate length” in the n-channel MOS transistor M 1  and that of M 3  in SH 1  are the same. In SH 2  in FIG. 4, the current values of current sources of A 3 , A 4 , and A 5  are the same. The “ratio of the gate width to the gate length” in each of n-channel MOS transistors M 5 , M 7 , and M 9  in SH 2  is the same. Consequently, an absolute value of the input current Iin of SH 1  and that of an output current Is of SH 1  are equal. An input current Is in SH 2 , an output current (Iout) from T 9   1 , and an output current from T 10   1  are also equal. 
     Each of switches SW 1  and SW 2  in FIG. 4 can be constructed by the n-type MOS transistor. When the power source voltage Vdd is applied to the gate, the drain/source of the n-type MOS transistor are made conductive and the on-state is obtained. When the gate voltage is zero, the source/drain are in a shut-off state and the off-state is obtained. Similarly, each of switches SW 11 , SW 12 , SW 21 , and SW 22  in FIG. 11 which will be described hereinafter can be constructed by the n-type MOS transistor. 
     When the current values in a single CDF/F are equal as mentioned above, (n) CDF/Fs can be constructed by the same circuits, so that circuit designing is facilitated. Current values of the current sources and the “ratio of the gate width to the gate length” of each MOS transistor may be deliberately changed. In this case, since the input and output currents in SH 1  and SH 2  are changed according to the current values of the current sources and “the ratio of the gate width to the gate length” of the MOS transistor, the circuit designing is complicated. 
     The “ratio of the gate width to the gate length” of each of the n-channel type MOS transistors M 2 , M 4 , M 6 , M 8 , and M 10  does not have to be the same. However, since those MOS transistors are used as switches, in order to obtain the same on-resistance when they are conductive, it is preferable that the MOS transistors have the same “ratio of the gate width to the gate length”. 
     The switch SW 1  is constructed by an MOS transistor and is turned on when a clock pulse W 1  inputted from the terminal T 7   1  is “1” and is turned off when the clock pulse W 1  is “0”. C 1  denotes a parasitic capacitance between the gate and source of the n-type MOS transistor M 3 . 
     When the clock pulse is “1”, specifically, the voltage Vdd is applied. When the clockpulse is “0”, potential is zero. Assuming now that SW 1  and SW 2  are constructed by the n-type MOS transistors, when the clock pulse is “1”, SW 1  is ON and when the clock pulse is “0”, SW 2  is OFF. 
     In the construction of the sample and hold circuit SH 2 , M 5  denotes an n-type MOS transistor whose source is connected to the ground. Its drain is connected to the power source Vdd via the constant current source A 3 , its gate is connected to the drain, and the source is connected to the ground via an MOS transistor M 6 . M 7  denotes the n-type MOS transistor whose source is connected to the ground. Its drain is connected to the power source Vdd via the constant current source A 4 , its gate is connected to the gate of the MOS transistor M 5  via the switch SW 2 , and the source is connected to the ground via an MOS transistor M 8 . Similarly, M 9  denotes the n-type MOS transistor whose source is connected to the ground. Its drain is connected to the power source Vdd via the constant current source A 5 , its gate is connected to the gate of the MOS transistor M 7 , and the source is connected to the ground via an MOS transistor M 10 . 
     The switch SW 2  is turned on when a clock pulse W 2  inputted from the terminal T 8   1  is “1” and is turned off when the signal W 2  is “0”. The switch SW 2  is constructed by an MOS transistor. C 2  indicates a parasitic capacitance between the gate and the source of the MOS transistor M 7  and C 3  denotes a parasitic capacitance between the gate and the source of the MOS transistor M 8 . 
     The drain of the n-type MOS transistor M 7  is connected to the terminal T 9   1  and the drain of the n-type MOS transistor M 9  is connected to the terminal T 10   1 . The drain of the n-type MOS transistor M 3  and the drain of the n-type MOS transistor M 5  are connected. The gates of the MOS transistors M 2 , M 4 , M 6 , M 8 , and M 10  are commonly connected to a terminal Ts. 
     Reference numeral  103  in FIG. 1 denotes a switch circuit for switching current paths inputted to terminal T 11   1  to T 11   n  to a terminal T 13  or T 14  by signals inputted from terminals T 12   1  to T 12   n . The switch circuit  103  is constructed by analog switches  104   1 ,  104   2 , . . . ,  104   n . PN codes generated by the PN code generator  6  (FIG. 2) are applied to the terminals T 121  to T 12 n. 
     FIG. 5 is a diagram showing the construction of the analog switch  104   1  (each of  104   2  to  104   n  has the same construction) in FIG.  1 . In FIG. 5, M 20   1  denotes an n-type MOS transistor. Its drain is connected to the terminal T 11   1 , its source is connected to a terminal T 13   1 , and its gate is connected to the terminal T 12   1 . M 21   1  indicates a p-type MOS transistor. Its drain is connected to the terminal T 11   1 , its source is connected to a terminal T 14   1 , and its gate is connected to the terminal T 12   1 . 
     Outputs T 13   1  to T 13   n  of the analog switches are commonly connected to T 13  in FIG.  1 . Outputs T 14   1  to T 14   n  of the analog switches are commonly connected to T 14  in FIG.  1 . 
     Reference numeral  105  in FIG. 1 denotes a current adder for adding a current flowing in a terminal T 15  and a current obtained by inverting a current flowing in a terminal T 16  by an inverting means  106  and outputting the result of addition to an output terminal T 17 . In other words, the current flowing in the terminal T 16  is subtracted from the current flowing in the terminal T 15  and the result is outputted to the output terminal T 17 . 
     FIG. 6A is a diagram showing an example of the construction of the current adder  105  in FIG.  1 . In FIG. 6A, M 30  denotes an n-type MOS transistor whose source is connected to the ground. The drain is connected to the power source Vdd via a constant current source A 30  and is connected to the terminal T 16 . The gate is connected to the drain and the source is connected to the ground. M 31  denotes an n-type MOS transistor whose source is connected to the ground. Its drain is connected to the power source Vdd via a constant current source A 31  and is connected to the terminal T 15 . Its gate is connected to the gate of the MOS transistor M 30  and the source is connected to the ground. 
     M 32  denotes an n-type MOS transistor whose source is connected to the ground. Its drain is connected to the power source Vdd via a constant current source A 32  and is connected to the terminal T 15 . Its gate is connected to the drain and the source is connected to the ground. M 33  indicates an n-type MOS transistor whose source is connected to the ground. Its drain is connected to the power source Vdd via a constant current source A 33  and to the terminal T 17 . Its gate is connected to the gate of the n-type MOS transistor M 32  and its source is connected to the ground. The current values of the constant current sources A 30  to A 33  are the same. The circuit constructed by the MOS transistors M 30 , M 31  and the constant current sources A 30 , A 31 , and the circuit constructed by the MOS transistors M 32 , M 33  and the constant current sources A 32 , A 33  are current mirror circuits. 
     In a fundamental construction, the current values of the current sources A 30  and A 31  are equal and the “ratio of the gate width to the gate length” of the MOS transistor M 30  and that of M 31  are equal. Similarly, the current values of the current sources A 32  and A 33  are equal and the “ratio of the gate width to the gate length” of the MOS transistor M 32  and that of M 33  are equal. With such a construction, the following operation is performed. 
     In the construction, assuming now that a current flowing from the terminal T 16  is Im, a current flowing from the terminal T 15  to the MOS transistor M 31  is also Im. As a result, when it is assumed that the full current flowing from the terminal T 15  is Ip, a current flowing from the terminal T 15  to the MOS transistor M 32  is (Ip−Im), and a current Iout flowing from the output terminal T 17  to the outside is −(Ip−Im). 
     When the current values of the current sources A 30  and A 31 , the “ratio of the gate width to the gate length” of the MOS transistor M 30  and that of M 31 , the current values of the current sources A 32  and A 33 , and the “ratio of the gate width to the gate length” of the MOS transistor M 32  and that of M 33  are not equal respectively, an output current is generally “−(αIp−βIm)”. α and β are values determined by the current values and the “ratio of the gate width to the gate length” of each MOS transistor. 
     FIG. 6B is a diagram showing another example of the construction of the current adder  105 . In FIG. 6B, M 70  denotes an n-type MOS transistor. Its drain is connected to the power source Vdd via a constant current source A 70  and also to the terminal T 16 , its gate is connected to the drain, and the source is connected to the ground via an MOS transistor M 74 . 
     M 71  denotes an n-type MOS transistor. Its drain is connected to the power source Vdd via a constant current source A 71  and also to the terminal T 15 , its gate is connected to the gate of the MOS transistor M 70 , and the source is connected to the ground via an MOS transistor M 75 . 
     M 72  indicates an n-type MOS transistor. Its drain is connected to the power source Vdd via a constant current source A 72  and also to the terminal T 15 , its gate is connected to the drain, and its source is connected to the ground via an MOS transistor M 76 . 
     M 73  indicates an n-type MOS transistor. Its drain is connected to the power source Vdd via a constant current source A 73  and also to the terminal T 17 , its gate is connected to the gate of the MOS transistor M 72 , and its source is connected to a transistor M 77 . 
     M 74 , M 75 , M 76 , and M 77  are the MOS transistors and the gates are connected to the terminal Ts. Those MOS transistors M 74 , M 75 , M 76 , and M 77  are of the n-type. When a voltage higher than (a threshold value voltage of the MOS transistor—the earth voltage) is applied to the gate, the transistors are turned on. 
     The current values of the constant current sources A 70  to A 73  are set to be equal. The circuit constructed by the MOS transistors M 70 , M 71 , M 74 , and M 75  and the constant current sources A 70  and A 71  and the circuit constructed by the MOS transistors M 72 , M 73 , M 76 , and M 77  and the constant current sources A 72  and A 73  are current mirror circuits when the MOS transistors M 74 , M 75 , M 76 , and M 77  are “on”, that is, in a conductive state. 
     In a fundamental construction, it is set so that the current values of the current sources A 70  and A 71  are equal and the “ratio of the gate width to the gate length” of the MOS transistor M 70  and that of M 71  are equal. Similarly, the current values of the current sources A 72  and A 73  are equal and the “ratio of the gate width to the gate length” of the MOS transistor M 72  and that of M 73  are equal. With such a construction, the following operation is performed. 
     In the construction, assuming now that a current flowing from the terminal T 16  is Im, a current flowing from the terminal T 15  to the MOS transistor M 71  is also Im. As a result, when it is assumed that the full current flowing from the terminal T 15  is Ip, a current flowing from the terminal T 15  to the MOS transistor M 72  is (Ip−Im), and a current Iout flowing from the output terminal T 17  to the outside is accordingly −(Ip−Im). 
     When the current values of the current sources A 70  and A 71 , the “ratio of the gate width to the gate length” of the MOS transistor M 70  and that of M 71 , the current values of the current sources A 72  and A 73 , and the “ratio of the gate width to the gate length” of the MOS transistor M 72  and that of M 73  are not equal, an output current is generally −(αIp−βIm). α and β are values determined by the current values and the “ratio of the gate width to the gate length” of each MOS transistor. 
     It is preferable that the “ratio of the gate width to the gate length” of each of the MOS transistors M 74 , M 75 , M 76 , and M 77  is the same so as to have the same ON resistance. 
     Reference numeral  107  in FIG. 1 denotes an I/VC (Current/Voltage Converter) for converting a current value inputted from the terminal T 18  to a voltage value and outputting the voltage value from the terminal T 19 . FIG. 7 is a diagram showing an example of the construction of the I/VC  107 . In FIG. 7, OP 2  denotes an operational amplifier and R 2  indicates a resistor interposed between the (−) terminal and the output terminal of the operational amplifier OP 2 . 
     In the above description, circuit codes are used as current sources. In an actual circuit, a current source having the construction shown in FIGS. 15A and 15B can be used. FIG. 15A shows a circuit portion including the current source in FIGS. 4,  6 A,  6 B, and  11 . In FIG. 15A, M 301  denotes an n-type MOS transistor in which the source is connected to the ground, the gate and the drain are connected, and the drain is connected to the power source Vdd via a current source A 301 . 
     FIG. 15B is a diagram showing a specific circuit of the current source A 301  shown in FIG.  15 A. In FIG. 15B, M 302  denotes an n-type MOS transistor having the same construction as that of the MOS transistor M 301  shown in FIG.  15 A. M 303  indicates a p-type MOS transistor in which the drain is connected to the drain of M 302  and the source is connected to Vdd. With such a construction, when a proper voltage VEE is applied to the gate of M 303 , the p-type MOS transistor M 303  operates as a current source. A current J of the current source is determined by the “gate length”, “ratio of the gate width to the gate length”, and the gate voltage of the p-type MOS transistor. After the circuit is constructed, the value of the current J of the current source can be controlled by varying the gate voltage VEE. 
     The operation of the embodiment will be described hereinbelow. FIG. 8 is a diagram showing a process for demodulating a spread spectrum transmission wave. The antenna  1  in FIG. 2 receives the spread spectrum modulated transmission wave multiplied by a carrier wave. The received transmission wave shown in FIG. 8A will be described with reference to FIG.  9 . FIG. 9 is a waveform chart for explaining the spread spectrum modulating process. 
     A data packet shown in FIG. 9 consists of 128 chips. In case of transmitting baseband data “1” shown in FIG. 9A, a PN code shown in FIG.  9 B and the baseband data “1” are multiplied. The PN code denotes a pseudo noise code. As the PN code, the m-series code, Gold code, orthogonal m-series code, orthogonal Gold code, orthogonal code formed from the Walsh function, and the like are known. Especially, the orthogonal code has the following characteristics. In the autocorrelation function, when the phase difference is zero, the correlation value is maximum. In the cross correlation function, when the phase difference is zero, the correlation value is zero. Since the orthogonal code has the characteristic, it can be said that the code is adapted to a channel division in the CDMA. The correlator  5  according to the embodiment can perform a correlating operation to any code by the signals from T 12   1  to T 12   n  applied to the switch matrix  103 . 
     By multiplying the signal of FIG. 9C which is spread modulated by the multiplying process with a carrier wave shown in FIG. 9E, the spread spectrum transmission wave shown in FIG. 9D can be obtained. 
     In case of transmitting, for instance, baseband data “0”, the spread modulated data has a waveform of a phase opposite to that of the waveform shown in FIG.  9 C. The waveform of the phase opposite to that of FIG. 9C is multiplied by the carrier wave shown in FIG. 9E, thereby forming a transmission wave of data “0”. 
     The transmission wave shown in FIG. 8A inputted from the antenna  1  in FIG. 2 is mixed with a signal of a frequency generated by the local oscillator  3  in the mixer  2 , thereby obtaining the IF (intermediate frequency) signal which has the frequency equal to the difference between the carrier wave and the signal. The IF signal is detected by the carrier sync detector  4  and is converted to a signal based on the PN code shown in FIG.  9 B and the baseband data. The correlator  5  obtains the correlation between the output signal of the carrier sync detector  4  and the PN code generated by the PN code generator  6 . The PN code generated by the PN code generator  6  and the PN code in the above-mentioned transmission are the same. 
     The operation of the correlator  5  shown in FIG. 1 will be described in detail. The spread modulated data (refer to FIG. 8B) outputted from the carrier sync detector  4  is inputted from the terminal T 1  to the V/IC  101 , converted to a current by the V/IC  101 , and the current is sequentially supplied to the CDF/F  102   1 . The current data outputted from the V/IC  101  is read while being sequentially shifted by the CDF/F  102   1  to CDF/F  102   n  on the basis of the clock pulses W 1  and W 2 . 
     The operation of the CDF/Fs  102   1  to  102   n  will be described in detail with reference to FIGS. 4 and 10. FIG. 10 shows an example of a timing chart showing the operation of the CDF/F  102   1 . 
     The clock pulse W 1  shown in FIG.  10 A and the clock pulse W 2  shown in FIG. 10B have the same period and duty ratio. The phases of them are deviated by an amount corresponding to “ON” time of the clock pulse W 1 . When either one of the clock pulses W 1  and W 2  is in the “1” state, a signal WS shown in FIG. 10C is in the “1” state. The signal WS is applied to the terminal Ts in FIG.  4 . Consequently, when the signal WS is “1”, the MOS transistors M 2 , M 4 , M 6 , M 8 , and M 10  are turned “ON”. 
     When the signal WS becomes “1” at a time t 1  shown in FIG. 10, all of the MOS transistors M 2 , M 4 , M 6 , M 8 , and M 10  are turned on and the circuit of FIG. 4 enters an enable state. It is assumed that the current flowing from the V/IC  101  to the CDF/F  102   1  at this time point is Iin (refer to FIG.  10 D). The current Iin is supplied from the terminal T 6   1  to the drain of the MOS transistor M 1 . When each of the current values of the constant current sources A 1  to A 5  is equal to J, the current value Ia flowing in the MOS transistor M 1  is equal to (J+Iin) (refer to FIG.  10 D). 
     When the clock pulse W 1  shown in FIG. 10A becomes “1” at this time, the switch SW 1  (FIG. 4) is closed, thereby short-circuiting the gate of the MOS transistor M 1  and the gate of the MOS transistor M 3 . The switch SW 2  is open at this time, so that the gate of the MOS transistor M 5  and the gates of the MOS transistors M 7  and M 9  are electrically disconnected. 
     When the switch SW 1  is “ON”, the MOS transistors M 1  and M 3  construct a current mirror circuit and the current (J+Iin) which is the same as that flows in the MOS transistor M 1  flows in the MOS transistor M 3 . Consequently, the current Is (refer to FIG. 4) flowing from the drain side of the MOS transistor M 3  to the drain side of the MOS transistor M 5  is equal to −Iin and the current Ib in the MOS transistor M 5  is (J−Iin) (refer to FIG.  10 F). The parasitic capacitance C 1  between the gate and the source of the MOS transistor M 3  is charged at this time. The above-mentioned steps relate to steps of current sampling. 
     When the clock pulse W 1  becomes “0” and the clock pulse W 2  becomes “1” at a time t 2 , the switch SW 1  is opened and the gate of the MOS transistor M 1  and the gate of the MOS transistor M 3  are disconnected. In this instance, the current in the MOS transistor M 3  is held by the parasitic capacitance C 1  and the value of the current Is is accordingly held at −Iin. This is the current holding step. 
     On the other hand, when the switch SW 2  is closed at the time t 2 , the gate of the MOS transistor M 5  and the gates of the MOS transistors M 7  and M 9  are short-circuited. Consequently, the currents flowing in the MOS transistors M 7  and M 9  are equal to (J−Iin) which is the same current as that flows in the MOS transistor M 5 . As a result, the current Iout (FIG. 4) is equal to the current Iin as shown in FIG.  10 G and the current Iin is out putted from the terminal T 9   1 . The current outputted from the terminal T 10   1  is the same. 
     At this time, parasitic capacitance C 2  between the gate and the source of the MOS transistor M 7  and the parasitic capacitance C 3  between the gate and the source of the MOS transistor M 9  are charged. 
     When the clock pulse W 2  becomes “0” at a time t 3 , the switch SW 2  is “OFF” and the output current Iout is held by the parasitic capacitance C 2 . The signal WS becomes “0” at this point, and after that, the circuit of FIG. 4 enters a disable state. When the signal WS becomes “1” again at a time t 4 , the circuit is in the enable state and operation similar to the above is restarted. While the circuit is in the disable state, by the gate parasitic capacitance of each of the MOS transistors M 1 , M 3 , M 5 , M 7 , and M 9 , the operation can be restarted at the time t 4  in the same state as that of time t 3 . 
     The sampling and holding processes are sequentially executed, so that the current values corresponding to chip values of the PN code inputted to the terminal T 1  are sequentially set in the CDF/Fs  102   1  to  102   n . 
     The currents outputted from the CDF/Fs  102   1  to  102   n  are collected in the terminal T 15  or T 16  of the current adder  105  by the switch circuit  103 . That is, the currents are added. 
     Assuming now that the number of CDF/Fs is 10 and the PN code is “1111110000”, the output currents of the CDF/Fs  102   1  to  102   6  flow in the terminal T 15  via the switch circuit  103  and the output currents of the CDF/Fs  102   7  to  102   10  flow in the terminal T 16  via the switch circuit  103 . 
     The current of the sum of the output currents of the CDF/Fs  102   1  to  102   6  flows in the terminal T 15  and the current of the sum of the output currents of the CDF/Fs  102   7  to  102   10  flows in the terminal T 16 . 
     The current from the terminal T 15  and the current obtained by inverting the current from the terminal T 16  are added by the current adder  105  and the result is outputted from the terminal T 17 . According to the example, when the current data “1111110000” which is the same as that of the PN code is set in the CDF/Fs  102   1  to  102   10 , the output current of the current adder  105  reaches a peak value (refer to FIG.  8 C). Thus, a peak voltage is outputted from the I/VC  107 . 
     The correlator  5  in FIG. 1 outputs a positive peak value when the data of the same phase as that of the PN code generated from the PN code generator  6  (FIG. 2) is set in the CDF/Fs  102   1  to  102   n . The correlator  5  outputs a negative peak value when the data of the opposite phase is set. That is, the positive peak is outputted when the PN spread modulated baseband data “1” is received by the CDF/Fs  102   1  to  102   n  and the negative peak is outputted when the baseband data “0” is received. The peak value is integrated by the demodulator  7  (FIG.  2 ), thereby obtaining the original baseband data. 
     (2) Another Embodiment 
     FIG. 11 is a circuit diagram showing another construction example of the CDF/Fs  102   1  to  102   n  in FIG.  1 . In FIG. 11, M 50  denotes an n-type MOS transistor in which the drain is connected to the power source Vdd via the constant current source A 51 , the gate is connected to the drain via the switch SW 12 , and the source is connected to the ground via the MOS transistor M 51 . The drain of the n-type MOS transistor M 50  is connected to the terminal T 6   1  via the switch SW 11 . 
     M 52  denotes an n-type MOS transistor in which the drain is connected to the power source Vdd via the constant current source A 52 , the gate is connected to the drain via the switch SW 22 , and the source is connected to the ground via the MOS transistor M 53 . The drain of the n-type MOS transistor M 52  is connected to the drain of the n-type MOS transistor M 50  and to the terminal T 9   1  via the switch SW 21 . 
     M 54  denotes an n-type MOS transistor in which the drain is connected to the power source Vdd via the constant current source A 3 , the gate is connected to the gate of the n-type MOS transistor M 52 , and the source is connected to the ground via the MOS transistor M 55 . The drain of the n-type MOS transistor M 54  is connected to the terminal T 10   1 . The gates of the MOS transistors M 51 , M 53 , and M 55  are connected to the terminal Ts. 
     The operation of the CDF/F shown in FIG. 11 will be described with reference to FIG.  14 . It is assumed that the current in each of the constant current sources A 51  to A 53  is J. When the signal WS shown in FIG. 14C becomes “1” at a time t 1 , the MOS transistors M 51 , M 53 , and M 55  are turned on, and the circuit of FIG. 11 enters an enable state. When the clock pulse W 1  shown in FIG. 14A simultaneously becomes “1” at this time, the switches SW 11  and SW 12  are closed and the current Iin inputted from the terminal T 6   1  is supplied to the drain of the n-type MOS transistor M 50 . 
     The current flowing in the n-type MOS transistor M 50  is equal to (J+Iin) which is the sum of the current supplied from the constant current source A 51  and the current Iin. 
     When the clock pulse W 1  becomes “0” and the clock pulse W 2  becomes “1” at a time t 2 , the switches SW 11  and SW 12  are opened and the switches SW 21  and SW 22  are closed. 
     The current of the n-type MOS transistor M 50  is held at (J+Iin) by the parasitic capacitance of the gate/source of the n-type MOS transistor M 50 . The current Is is accordingly −Iin. As a result, the current in the n-type MOS transistor M 52  is (J−Iin). Similarly, the current of the n-type MOS transistor M 54  is (J−Iin). 
     When the clock pulse W 2  becomes “0” at a time t 3 , the switches SW 21  and SW 22  are opened. 
     The current (J−Iin) of the MOS transistor M 52  is held by the parasitic capacitance between the gate and the source. As a result, the current Iin flows as the current Iout from the constant current source A 52  to the terminal T 9   1 . At this time, similarly, the current Iin flows from the drain of the MOS transistor M 54  to the terminal T 10   1 . Simultaneously, the signal WS becomes “0” so that the MOS transistors M 51 , M 53 , and M 55  are turned off and the circuit of FIG. 11 enters the disable state. Although the state continues until a time t 4 , by the charges stored in the parasitic capacitance between the gate and the source of each of the MOS transistors M 50 , M 52 , and M 54 , the operation can be restarted at the time t 4  in the same state as that at the time t 3 . 
     According to the circuit of FIG. 11, the number of constant current sources can be reduced as compared with the circuit of FIG.  4 . 
     FIG. 12 is a block diagram showing the construction of a code division multiplex communications system (receiving side) according to another embodiment of the invention. In FIG. 12, reference numeral  201  denotes an antenna for receiving a transmission wave from a transmitter (not shown);  202  a mixer for mixing the received transmission wave and a signal wave oscillated by the local oscillator  3  and outputting an IF signal;  204  a correlator having the construction similar to that of the correlator  5  shown in FIG. 1 for obtaining the correlation between the PN code generated by a programmable PN code generator  205  and the IF signal and outputting a correlation signal; and  206  a demodulator for reproducing a baseband signal on the basis of the inputted correlation signal. 
     According to the embodiment, the correlation peak appears in two pulses as shown in FIG.  13 C. In order to prevent this, it is sufficient to set so that the clock pulses W 1  and W 2  have opposite phases and the phase of W 2  is advanced more than that of W 2 . In this case, the correlation peak is as shown in FIG.  13 D. 
     The duty ratios of the clock pulses W 1  and W 2  are set to the same in the foregoing. If the clock pulse WS is in a state of “J” when the clock pulses W 1  and W 2  are “1”, the operation can be performed even if the duty ratios of the clock pulses W 1  and W 2  are different to each other. 
     (3) Effects of the Embodiments 
     As obviously understood from the above description, in the current adding type correlator  5  according to the embodiment, the circuit is in a disable state per period of the clock pulse, thereby realizing the low power consumption. The effect of the low power consumption will be described hereinbelow. 
     (a) to (d) relate to cases of the following circuits. 
     a: a case where the transistors M 2 , M 4 , M 6 , M 8 , and M 10  are eliminated in the circuit of FIG.  4  and the circuit shown in FIG. 6A is used as the current adder  105 . 
     b: a case where the transistors M 51 , M 53 , and M 55  are eliminated in the circuit of FIG.  11  and the circuit shown in FIG. 6A is used as the current adder  105 . 
     c: a case where the circuit of FIG. 4 is used and the circuit shown in FIG. 6B is used as the current adder  105 . 
     d: a case where the circuit of FIG. 11 is used and the circuit shown in FIG. 6B is used as the current adder  105 . 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 the case of baseband correlation 
               
             
          
           
               
                   
                 (a) 
                 (b) 
                 (c) 
                 (d) 
               
               
                   
                   
               
             
          
           
               
                 Chip length 
                 128 
                   
                 128 
                   
                 128 
                   
                 128 
                   
               
               
                 Chip rate 
                 *1 
                   
                 *2 
                   
                 14 
                 Mcps 
                 14 
                 Mcps 
               
             
          
           
               
                 Sampling 
                 double 
                 ← 
                 ← 
                 ← 
               
             
          
           
               
                 Sampling 
                 28 
                 MHz 
                 ← 
                 ← 
                 ← 
               
               
                 frequency 
               
             
          
           
               
                 The number of 
                 256 
                   
                 256 
                   
                 256 
                   
                 256 
                   
               
               
                 CDF/Fs 
               
               
                 The number of 
                 12 
                   
                 10 
                   
                 17 
                   
                 13 
                   
               
               
                 Trs/CDF/Fs 
               
               
                 The number of 
                 5 
                   
                 3 
                   
                 5 
                   
                 3 
                   
               
               
                 current sources/ 
               
               
                 CDF/Fs 
               
               
                 Current per 
                 150 
                 μA 
                 150 
                 μA 
                 150 
                 μA 
                 152 
                 μA 
               
               
                 current source 
               
               
                 of CDF/F 
               
               
                 The number of 
                 512 
                   
                 512 
                   
                 512 
                   
                 512 
                   
               
               
                 Trs of switch 
               
               
                 matrix 
               
               
                 The number of 
                 8 
                   
                 8 
                   
                 12 
                   
                 12 
                   
               
               
                 Trs of current 
               
               
                 adding circuit 
               
               
                 The number of 
                 4 
                   
                 4 
                   
                 4 
                   
                 4 
                   
               
               
                 current sources 
               
               
                 of current 
               
               
                 adding circuit 
               
               
                 Current per 
                 2.56 
                 mA 
                 2.56 
                 mA 
                 2.56 
                 mA 
                 2.56 
                 mA 
               
               
                 current source 
               
               
                 in the current 
               
               
                 adding circuit 
               
               
                 Tr total number 
                 3592 
                   
                 3080 
                   
                 4876 
                   
                 3852 
                   
               
               
                 Power of 
                 192.0 
                 mW 
                 115. 
                 mW 
                 4.3 
                 mW 
                 2.58 
                 mW 
               
               
                 CDF/F 
               
               
                 Power of 
                 10.2 
                 mW 
                 10.2 
                 mW 
                 0.23 
                 mW 
                 0.23 
                 mW 
               
               
                 current adding 
               
               
                 circuit 
               
               
                 Total power 
                 202.2 
                 mW 
                 125.4 
                 mW 
                 4.5 
                 mW 
                 2.8 
                 mW 
               
               
                   
               
               
                 (*1, *2): In case of (a) and (b), the total power does not depend on the chip rate.  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 the case of IF correlation 
               
             
          
           
               
                   
                 (a) 
                 (b) 
                 (c) 
                 (d) 
               
               
                   
                   
               
             
          
           
               
                 Chip length 
                 128 
                   
                 128 
                   
                 128 
                   
                 128 
                   
               
               
                 Chip rate 
                 14 
                 Mcps 
                 14 
                 Mcps 
                 14 
                 Mcps 
                 14 
                 Mcps 
               
             
          
           
               
                 Sampling 
                 double 
                 (140 MHz) 
                 ← 
                 ← 
                 ← 
               
               
                 Sampling frequency 
                 280 
                 MHz 
                 ← 
                 ← 
                 ← 
               
             
          
           
               
                 The number of CDF/Fs 
                 2560 
                   
                 2560 
                   
                 2560 
                   
                 2560 
                   
               
             
          
           
               
                 The number of Trs/CDF/Fs 
                 10 &amp; 12 
                 8 &amp; 10 
                 14 &amp; 17 
                 10 &amp; 13 
               
               
                 The number of current 
                 4 &amp; 5 
                 2 &amp; 3 
                 4 &amp; 5 
                 2 &amp; 3 
               
               
                 sources/CDF/Fs 
               
             
          
           
               
                 Current per current source 
                 150 
                 μA 
                 150 
                 μA 
                 150 
                 μA 
                 150 
                 μA 
               
               
                 of CDF/F 
               
               
                 The number of Trs of switch 
                 512 
                   
                 512 
                   
                 512 
                   
                 512 
                   
               
               
                 matrix 
               
               
                 The number of Trs of current 
                 8 
                   
                 8 
                   
                 12 
                   
                 12 
                   
               
               
                 adding circuit 
               
               
                 The number of current 
                 4 
                   
                 4 
                   
                 4 
                   
                 4 
                   
               
               
                 sources of current adding 
               
               
                 circuit 
               
               
                 Current per current source 
                 2.56 
                 mA 
                 2.56 
                 mA 
                 2.56 
                 mA 
                 2.56 
                 mA 
               
               
                 in the current adding 
               
               
                 circuit 
               
               
                 Tr total number 
                 26632 
                   
                 21512 
                   
                 37132 
                   
                 26892 
                   
               
               
                 Power of CDF/F 
                 1574.4 
                 mW 
                 806.4 
                 mW 
                 352.8 
                 mW 
                 180.7 
                 mW 
               
               
                 Power of current adding 
                 10.2 
                 mW 
                 10.2 
                 mW 
                 2.3 
                 mW 
                 2.3 
                 mW 
               
               
                 circuit 
               
               
                 Total power 
                 1584.6 
                 mW 
                 816.6 
                 mW 
                 355.1 
                 mW 
                 183.0 
                 mW 
               
               
                   
               
             
          
         
       
     
     In Table 1, the sampling is a double sampling. That is, an input signal to a match filter is sampled at a frequency twice as high as the chip rate. Since the double sampling is performed in this case, the number of CDF/Fs is equal to a number of twice as long as the chip length. 
     In the example of Table 1, since the chip length is 128, the number of CDF/Fs is (2×128=) 256. The number of sampling can be also integer times as many as the number of chips. The operation can be executed even if it is not exactly the integer times. 
     In the case of the IF band correlation of Table 2, the number of CDF/Fs can be determined as follows. That is, when the IF frequency is fIF, the chip length is N. the chip rate is Cchip, and the sampling coefficient is Ms, the number of CDF/Fs is given by: 
     
       
         The number of CDF/Fs=(N×fIF×Ms)÷Cchip 
       
     
     The sampling coefficient Ms is  2  in case of double sampling. 
     In the current adding type correlator, the operating speed is controlled by a circuit response time of the CDF/F. The response speed (τ) of the CDF/F is 0.0357 nsec in case of using a 0.2μm Si process. That is, the maximum operating frequency [fmax=1/(2πτ)] is 4.46 GHz. Simulation was made by assuming the ON time of the clock pulses W 1  and W 2 , that is, “t 2 −t 1 ” and “t 3 −t 2 ” in FIGS. 10 and 13 are 0.4 nsec which is about 10 times as high as τ. 
     When used for the correlation of the PN data in Tables 1 and 2 as mentioned above, the power consumption of the correlator of (c) having the transistor for the disable state is largely reduced as compared with the correlator of (a). Similarly, the power consumption of the correlator (d) having the transistor for the disable state is largely reduced as compared with the correlator of (b). 
     Consequently, the transistors for the disable state are so controlled as to supply the drive current to the CDF/Fs  101   1  to  101   n  only at the time of sampling and holding operation of the current of the CDF/Fs  101   1  to  101   n , thereby enabling the power consumption of the correlator to be largely reduced. 
     Since the correlators are of the current adding type, the maximum operating frequency of the circuit is 4 GHz or higher and the high speed operation can be performed. 
     As mentioned above, according to the invention, since the switching means for shutting off the drive current of the delay means at the OFF timing of the clock pulse is provided, there is the effect that the power consumption can be largely reduced as compared with the conventional technique. 
     In case of using the current delay means as the delay means, there is an effect that the code division multiplex communications system having the high operating speed and the small power consumption can be provided.