Abstract:
A spread spectrum receiver using a correlator is disclosed, in which first and second comparing means comparing a correlation spike coming from the correlator with a first and a second reference voltage are used, the reference voltages being regulated, depending on the output of the first comparing means so that variations of the correlation spikes are followed precisely to obtain a two-valued correlation pulse.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a spread spectrum receiver and in particular to an improvement of a method, by which a two-valued correlation pulse is obtained from a correlation spike. 
     BACKGROUND OF THE INVENTION 
     In the spread spectrum communication using a convolver, there is known e.g. a circuit indicated in FIG. 6, by which a correlation pulse is obtained from a correlation spike outputted by the convolver. In the figure, reference numerals 21 to 23 are a first, a second and a third threshold detector, respectively; 24 and 25 are monostable multi-vibrators; 26 is an up-down counter of N bits; 27 is a D/A converter; and 28 is a voltage divider. 
     In the circuit described above, voltages V A , V B  and V C  at outputs A, B and C of the voltage divider 28 are given to the detectors 21, 22 and 23, respectively, as threshold voltages, and the circuit works as indicated in the following table. 
     
         __________________________________________________________________________OUTPUT      OUTPUT            STATE OF UP-STATEa     b     DOWN COUNTER 26                        V.sub.A, V.sub.B, V.sub.C__________________________________________________________________________1    PRESENT      PRESENT            COUNT UP    INCREASE2    ABSENT      PRESENT            COUNT STOP  MAINTAINED3    ABSENT      ABSENT            COUNT DOWN  DECREASE__________________________________________________________________________ 
    
     As the result of the operation described above, a relationship between a correlation spike S 1  and the threshold voltages V A , V B  and V C , as indicated in FIG. 7(a), is valid and thus a threshold voltage V C  suitable for restoring data so as to obtain a correlation pulse, as indicated in FIG. 7(b), is obtained. 
     However the prior art circuit construction has problems as described below. 
     1. Three threshold detectors are necessary. 
     2. ΔV=V A  -V B  should be sufficiently great so that the output of the D/A converter 27 is not varied by noise, for the case where the S/N ratio of the received signal is low and noise is superposed on the correlation spike, which worsens the reliability. 
     3. In the case where FSK or CSK (Code Shift Keying) modulation is effected on the transmitter side and the receiver side is operated so as to follow the frequency (for FSK) or the PN code (for CSK), a control delay τ is produced, as indicated in FIG. 8, and a time zone where the frequency or the PN code is instantaneously in disaccordance that, is produced. In this time zone, since the output of the convolver disappears, the threshold levels V A  to V C  are decreased and therefore, when the output of the convolver appears again, the threshold level can be suitable no more. 
     In FIG. 8, PN represents a time-inverted code of the PN code. 
     OBJECT OF THE INVENTION 
     Consequently the object of the present invention is to provide a spread spectrum receiver having correlation pulse generating mean having a good response, capable of obtaining a suitable threshold level, by which the threshold detection is simple. 
     SUMMARY OF THE INVENTION 
     In order to achieve the above object, a spread spectrum receiver according to the present invention is characterized in that it comprises a D/A converter, a correlator; first comparing means for comparing a first voltage corresponding to the output of the D/A converter with a correlation spike coming from the correlator; second comparing means for comparing a second voltage corresponding to the output of the D/A converter with the correlation spike coming from the correlator; switching means for outputting an inputted clock while switching over a first output and a second output therefor, depending on the output of the first comparing means; and counting means, whose count value varies, depending on the first and the second output described above, and which outputs the count value to the D/A converter described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an embodiment of the present invention; 
     FIGS. 2A and 2B to 5 are schemes for explaining the operation thereof; 
     FIG. 6 is a block diagram of a prior art correlation pulse generating circuit; 
     FIGS. 7 and 8 are schemes for explaining the operation thereof. 
    
    
     DETAILED DESCRIPTION 
     Hereinbelow an embodiment of the present invention will be explained, referring to the drawings. 
     FIG. 1 shows an embodiment of the correlation pulse generating circuit in a spread spectrum receiver according to the present invention, in which reference numerals 1 and 15 are first and second comparing means, each of which is composed of a threshold detector; 2 and 16 are monostable multi-vibrators; 3 is an SR flipflop; 4, 5 and 10 are AND gates; 6 and 12 are up-down counters; 7 and 8 are magnitude comparators; 9 and 11 are OR gates; 13 is a D/A converter; 14 is a voltage divider; 17 is a counter effecting counting operation, when the input is at the low level; and 18 is an inverter. The up-down counter 7, the magnitude comparators 7 and 8, and the OR gate 9 constitute a well-known random walk filter as count comparing means. Further the AND gate 4, the SR flipflop 3, the monostable multi-vibrator 2 and the inverter 18 constitute switching means. Still further the AND gate 10 and the OR gate 11 constitute a gate means. 
     At first, the operation will be explained for the case of state, where a correlation spike a coming from the convolver is low with respect to the output p of the D/A converter 13, as indicated in FIG. 2A. 
     In this case, b is continuously at the &#34;L level&#34; and u is continuously at the &#34;H level&#34;. Consequently the mono-multi-vibrator 2 is not triggered so that the output c thereof is at the &#34;L level&#34;. Therefore the Q output d of the SR flipflop 3 is at the &#34;H level&#34; and the Q output e is kept at the &#34;L level&#34;. If the output of the OR gate 11 is at the &#34;H level&#34;, CLOCK 1 passes through the AND gates 10 and 4 and is given to the up-count input U of the up-down counter 6, which effects up-count. When the value of the count output h increases and reaches the value of the upper limit data D2, the output of the magnitude comparator 7 is changed to the &#34;H level&#34; and at the same time a load instruction signal k is given to the up-down counter 6 so that the count value is equal to the central value data D1. The up-down counter 12 counts down pulses i form the magnitude comparator 7. Therefore the value of the count output o decreases and the output voltage p of the D/A converter 13 is lowered so as to approach finally the peak value of the correlation spike a. As far as the state indicated in FIG. 2A, i.e. the state, where the peak value of the correlation spike a is lower than the voltage p described above, continues, this operation is repeated so that the voltage p described above continues to approach the peak value o the correlation spike a. 
     Next the operation in the state indicated in FIG. 2B will be explained. In this case, pulses appear periodically at the output b of the threshold detector 1, which repeats to trigger the mono-multi-vibrator 2. Consequently since the output c of the monostable multi-vibrator 2 is at the &#34;H level&#34; and the output u of the inverter 18 is periodically at the &#34;L level&#34;, the Q output d of the SR flipflop 3 is at the &#34;L level&#34; and the Q output e is at the &#34;H level&#34;. CLOCK 1 passes the AND gates 10 and 5 and the up-down counter 6 effects down-count. When the value of the count output h reaches the lower limit value date D3 of n, the outputs of the magnitude comparator 8 is at the &#34;H level&#34; and the count value of the up-down counter 6 is equal to the central value data D1. Consequently the up-down counter 12 effects up-count so that the value of the count output o increases and the voltage p described above is raised. That is, the voltage p increases so as to approach the peak value of the correlation spike a. 
     As explained above, the voltage p described above is controlled so as to approach the peak value of the correlation spike a. In the case where the voltage p is almost in accordance with the correlation spike a, the voltage p repeats to be finely raised and lowered so as to follow the neighborhood of the peak value. Since the voltage p follows the peak value of the correlation spike a, it is possible to obtain a correlation pulse r by using an always suitable threshold voltage, even if the peak value of the correlation spike varies, by forming the threshold value by using a voltage of obtained by suitably dividing the voltage p. 
     Now the operation of the random walk filter described previously will be explained. The data set D1, D2 and D3 given to the random walk filter is chosen, in general, so as to satisfy ##EQU1## As an example, they are so chosen that D1=8, D2=15 and D3=0. By inserting such a random filter, in the case where the S/N ratio of the received signal is low and remarkable noise is mixed in the correlation spike, it can be alleviated that the voltage p described above is varied by noise. In the case where the voltage p is in accordance with the peak value of the correlation spike, the up-down counter 6 repeats up and down by noise. It is only occasionally that the count value reaches the upper limit data D2 or the lower limit data D3. This aspect is indicated in FIG. 3. In this way variations in the voltage p are reduced. 
     Next the the operation of the embodiment described above against an instantaneous disappearance of the correlation spike will be explained. When the correlation spike disappears, the correlation pulse r disappears and thus it does not trigger the multivibrator 16. Then, the output s thereof is changed from the &#34;H level&#34; to the &#34;L level&#34;. At this time, since the output t of the counter 17 is also at the &#34;L level&#34;, the output v of the OR gate 11 is at &#34;L level&#34; and CLOCK 1 cannot pass through the AND gate 10 and the counting operation of the up-down counter 6 is stopped. That is, the output voltage p of the D/A converter varies no more and keeps the directly preceding state. When the correlation spike a of the convolver appears again, the correlation pulse r appears, and the output s of the monostable multi-vibrator 16 is turned to the &#34;H level&#34;, the up-down counter 6 begins again the counting operation to start the peak value following operation. 
     As described above, since the directly preceding threshold voltage is kept during the instantaneous disappearance indicated in FIG. 5, a suitable threshold level can be obtained, also when the correlation spike appears again. 
     However, in the case where this disappearance is not due to the normal operation, but for example the reception level is lowered rapidly, the counting operation of the up-down counter 6 remains stopped, until the reception level is restored again. Therefore the peak value cannot be followed and as a result long burst errors would be produced. For this reason, in the embodiment described above, when the correlation pulse r disappears, the output s of the multivibrator 16 is turned from the &#34;H level&#34; to the &#34;L level&#34;. The counter 17 carries out the counting operation, when the output s stated above is at the &#34;L level&#34;, and the count value is &#34;0&#34;, when the output s is at the &#34;L level&#34;. It is a counter which is so constructed that it carries out the counting operation, when the output s is at the &#34;L level&#34;, and the count t thereof is turned from the &#34;L level&#34; to the &#34;H level&#34;, when the count value reaches a predetermined value C. When the output s of the multi-vibrator 16 is turned to the &#34;L level&#34;, the counter 17 begins to count CLOCK 2 and after a predetermined period of time T, the count output t is turned to the &#34;H level&#34;. At this time, the disappearance continues yet and even if the output s stated above is at the &#34;L level&#34;, the output v of the OR gate 11 is at the &#34;H level&#34;. Thus the up-down counter 6 begins again the counting operation. FIG. 5 indicates the operation described above. 
     As explained above, according to the present invention, it is possible to simplify the construction of the threshold detectors, to improve response characteristics and to obtain a threshold level suitable for transforming a correlation spike into a correlation pulse.