Abstract:
An apparatus for measuring jitter in a digital signal that includes an offset unit arranged to form an offset reference clock signal, being offset by a predetermined frequency amount from the digital signal. The apparatus also includes a sampler arranged to sample the digital signal at sampling times determined by the offset reference clock signal such that, in the absence of jitter and the offset by a predetermined frequency, there are a predetermined number of sampling times in each bit of the digital signal. The apparatus further includes at least one detector arranged to detect occasions when the number of sampling items in any bit of the digital signal is different from the predetermined number, and a counter arranged to count the occasions over a predetermined time. Also the apparatus includes an analyzer arranged to derive at least one measure of jitter from the counting of the occasions.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the measurement of jitter in a digital signal. In theory, the spacing of the transitions between levels of a digital signal have a completely uniform spacing. In practice, particularly during transmission, there may be minute variations in the actual time of the transition, relative to the theoretical transition time defined by an absolute reference clock. These variations are referred to as jitter, and may be considered to be a spurious phase modulation of the signal. 
   2. Summary of the Prior Art 
   Known systems for measuring jitter involve a very stable phase-locked loop which compares the pulse train containing jitter with an internally generated, jitter-free reference clock. The phase-locked loop has a generator for generating the reference clock, the output of which is fed to the input of a phase demodulator which also receives a digital signal containing jitter. The phase demodulator converts the signal to pulse duration modulation, which is output to a low pass filter, the output of which gives the jitter measurement, and also is fed back to the input of the reference clock generator, to form the loop. The low pass filter has cut off frequency of 5-10% of the bit rate. But since the digital signal being investigated may contain long sequences of digital zeros, a pattern/clock converter may be used to convert the digital signal into a continuous pulse train with the same jitter as the original signal, which pulse train then forms the input to the phase demodulator. Analysis of the output may involve peak value rectification before the results are displayed, and/or analysis with a spectrum analyser. 
   As mentioned above, such a jitter measurement system involves a low pass filter, and this has a significant influence on the greatest measurable jitter frequency component. The known systems also involve many analog circuits, which are more expensive than digital components. 
   SUMMARY OF THE INVENTION 
   Therefore, the present invention seeks to provide a system for measuring jitter in a digital signal, in which a clock signal is extracted from the original digital signal, offset by a predetermined frequency, and smoothed to eliminate jitter therefrom. This gives an offset reference clock signal which is then used to sample the original input signal. Preferably, that offset clock signal is frequency multiplied by an integer factor before it is used for timing the sampling. 
   The effect of the offset of the reference clock signal is that the sampling point is not fixed relative to the transition point over the bits of the input signal, but instead moves relative thereto. The sampling points are then arranged such that, in the absence of the offset and in the absence of jitter, there is a predetermined number of sampling points (normally only one, but this is not essential) in each successive bit. The present invention then proposes that the occasions when a bit of said digital signal contains other than the predetermined number of sampling points are detected. The occasions when the number of sampling points differs from the predetermined number occur because of the offset of the clock, but also due to jitter when the sampling point approaches the theoretical (absolute) transition point of the bits, being the transition point that would occur in the absence of jitter. The count of the number of occasions a bit has more sampling points than the predetermined number for a suitable measuring duration then gives a measure of the jitter. 
   Note that a bit may have more samplings than the predetermined number and a later bit may have fewer samplings than the predetermined number and both are occasions to be counted. For simplicity, the number of samplings per bit in the absence of offset and jitter is preferably one. Then, a count is made of the occasions there are either two sampling times within a bit on no sampling times within a bit. It would also be possible to have more than one sampling time within a bit in the absence of offset and jitter, e.g. 2. Then the number of occasions of 3 or 1 sampling times in a bit would be counted. 
   The measurement period is preferably inversely proportional to the product of the bit rate and the difference between the original frequency and the offset frequency. Where the offset frequency is multiplied by an integer, the measurement period may be divided by that integer. 
   It is possible for the sampling to be at fixed intervals. However, where the offset clock signal is frequency multiplied by an integer factor, it is preferable that the sampling points are not regularly spaced by that integer factor, but are spaced by factors greater than or less than the integer factor. For example, if the integer is 4, then sampling may be at 3 and 5 intervals of the multiplied offset clock signal. 
   Thus a count is made of the occasions when there are more or less samplings, within the same bit than the predetermined number and the results of that count may be stored in a table whose size corresponds to the number of samples. The value stored in the table may increment and decrement depending whether the count is above or below the predetermined number. The value stored in the count thus increments and decrements depending on the jitter, with the increments and decrements occurring as the sampling point is close to the absolute transition point of the bits. It is then possible to use the difference between the maximum value counted and the minimum value counted, possibly with 1 subtracted, to be multiplied by the bit period to derived a coarse jitter value. Moreover, if the number of samples between the first occurrence of the maximum value and the last of the occurrence of the minimum value is determined, divided by the total number of samples, a fine jitter value may be determined. The jitter amplitude is then given by the sums of these two values. 
   It should be noted that where the offset clock is multiplied by an integer value, both of these values may need to be divided by that integer to obtain a jitter value which corresponds to the peak-to-peak value of the deviation of the phase function of the measured signal relative to time. It can also be noted that such a measurement is independent of bit rate, and independent of the shape of the binary signals being measured. 
   Thus, an aspect of the present invention may provide a system for measuring jitter in a digital signal having means for deriving a first clock signal from the digital signal, the first clock signal being offset by a predetermined frequency from the digital signal and being smoothed, means for sampling the digital signal using the first clock signal, such that, in the absence of jitter and said offset by a predetermined frequency, there are a predetermined number of sampling times in each bit of said digital signal, means for detecting occasions when the number of sampling times in any bit is different from the predetermined number, means for counting such occasions, and means for deriving a measurement of jitter from that count. 
   Another aspect of the invention relates to a method of measuring jitter using such a system. 
   The present invention, because it involves digital sampling and counting, can be embodied in a device which makes less use of analog circuits than known jitter measurement systems, which makes embodiments of the invention easier to produce. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  shows a schematic block diagram of a jitter measurement device being an embodiment of the present invention; 
       FIG. 2  is a flow-chart of the sampling sequence in the embodiment of  FIG. 1 ; 
       FIG. 3  is a block diagram of components of the jitter measurement device of  FIG. 1 ; 
       FIG. 4  shows in more detail a part (RXBERT) of the diagram of  FIG. 4 ; 
       FIG. 5  shows in more detail another part (RXJITTER) of the block diagram of  FIG. 3 ; 
       FIG. 6  shows in more detail yet another part (TXBERT) of the block diagram of  FIG. 3  and 
       FIG. 7  shows in more detail yet another part (TXJITTER) of the block diagram of  FIG. 3 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows schematically a jitter measurement device according to an embodiment of the present invention. In  FIG. 1 , a digital pulse train signal which may contain jitter is fed to an input  100 , and passed to a pattern clock converter  101 . The converter  101  performs a similar function to that in the known systems, in that it converts the digital pulse train received at input  100 , which may contain gaps in its pulse-train, into a continuous pulse-train with the same jitter as the original signal. That continuing pulse-train is then passed from the converter  101  to a clock frequency offset circuit  102 . The offset circuit  102  determines the frequency of the pulse-train received from the converter  101  using known clock recovery techniques, but then is offset by a frequency which is a small proportion of the frequency of the pulses received. 
   The offset clock pulses thus generated are passed to a phase locked loop (PLL)  103  with a long time constant. The loop has a phase comparator, a low pass filter and a voltage controlled oscillator, with the low pass filter having a very low cut off frequency it thus separates the relatively weak jitter component from the stronger modulation which is symmetric about the working frequency of the phase comparator. Therefore a slow-acting control voltage is produced which is used to regulate the oscillator to produce an average, constant phase. This generates a jitter-free pulse-train which can thus be used for a reference clock. 
   In this embodiment, the pulse-train thus generated is frequency multiplied by an integral factor. In the subsequent description, it will be assumed that integer factor is 4, but the embodiment is not limited to this. Thus, the output of the PLL  103  is a reference clock with a frequency multiplied by 4, and offset from the frequency of the digital signal received at the input  100  by a small frequency. 
   That reference clock is passed to a data sampler  104 , and is used to sample the pulse-train received at the input  100 . As can be seen from  FIG. 1 , the pulse-train input at input  100  is passed to the data sampler  104 , as well as to the convertor  101 . The action of that data sampler  104  will now be described with reference to the flow chart of  FIG. 2 . 
   As can be seen in  FIG. 2  a sampling step  110  is carried out, in which the pulse-train received at input  100  is sampled at a time determined by the reference clock signal from PLL  103 . The logical level of the sample is then compared with that of the previous sample. There are four possibilities. In two of them, shown at steps  111  and  112 , the sample is different from the previous sample, being either a change from logical zero to logical one (step  111 ) or a change from logical one to logical zero (step  112 ). In the other two alternatives, the sample is the same as the previous sample. In step  113 , both are at logical one, and in step  114  both are at logical zero. From step  111 , a three clock delay is imposed at step  115  and, assuming that the sampling operation has not yet been completed (step  116 ), processing returns to sampling step  110  for another sample. A similar procedure occurs at step  112 , except that a five clock delay is imposed at step  117 . 
   If there was no offsetting of the reference clock from the PLL  103 , and the pulse-train received at input  100  had no jitter, then the effects of steps  111 ,  112 ,  115  and  117  would be for the sampling to switch across the logical transition of the pulse-train. If the sample was at logical level one, but had previously been a logical level zero, corresponding to step  111 , the three clock delay would move the sampling point back to logical level zero. Similarly, if the sampling was at logical level zero and the previous sampling at logical level one, the five clock delay  117  would move the sampling point back to logical level one. Thus, without offset and without jitter, the processing would pass alternately via steps  111  and  112 . 
   However, the offset circuit  102  output pulses to the PLL  103  which have an offset frequency relative to the pulse train received at input  100 . Thus, and still assuming that there is no jitter in the pulse-train received at input  100 , a sampling point which is initially spaced from the transition between logical levels would slowly move towards that transition, and would eventually reach it. As it crossed the transition, two sampling points would occur within the same pulse, and thus the step  113  would be triggered. From step  113 , a three clock delay again occurs at step  118 , but also a signal is passed to a counter step  119  which increments a counter (not shown in  FIG. 2 ) by one. From counter step  119 , processing again passes to the sampling step  110  via step  116 . After the sampling point had crossed the transition, it would again return to the options envisaged by steps  111  and  112 , the counter step  119  would not again be triggered. 
   Thus, in the absence of jitter and over a sampling period equal to the inverse of four times the clock offset times the reference clock, counter step  119  would be triggered only once. It can be observed from  FIG. 2  that if the movement of the sampling point was within a logical zero, indicated by step  114 , a five clock delaying step  120  would be triggered, and the counter step  119  activated to decrement the counter. Thus, in this case, the counter would count down once. 
   Now consider the effect of jitter in the pulse-train received by sample  100 . In the subsequent discussion, the position of the transitions in the pulse-train in the absence of jitter will be called the absolute transition point, to distinguish from the actual transition point. These two transition points differ due to jitter. Whilst the sampling point is remote from the absolute transition point, the processing envisaged by  FIG. 2  will pass alternately via steps  111  and  112 , assuming that the magnitude of jitter is less than the pulse width of the output of the PLL  103 . However, as the sampling point approaches the absolute transition point, due to the offset of the reference clock, there is a possibility that a sampling point will occur within the same pulse as the previous sampling point, due to jitter. At that time, either step  113  or step  114  is triggered, and the counter step  119  either increments or decrements the counter. 
   Thus, over a part of the total sampling period, the counter step  119  may be triggered several times, depending on the magnitude of the jitter. It is this variation in the counter triggered by counting step  119  which enables jitter to be measured, as will now be described. Due to the jitter, the values stored by the counter triggered by counter step  119  will count up and down as steps  113  and  114  are triggered, if it is possible that the steps  113  and  114  may not be triggered alternately so that the counter step  119  may be triggered by the increment of step  118  more than once, before the counter step  119  is triggered by decrement step  120 . It is also possible, of course, for the decrements at step  120  to be triggered more than once. As a result, over a measurement cycle, the counter may count up to a maximum value, and down to a minimum value. This is then used to determine the jitter as will now be described. 
   Referring again to  FIG. 1 , the counter step  119  triggers an accumulator  105 , which detects the counts and passes them to a store  106  to be stored in a table of a size corresponding to the measurement period. At the end of measurement period, triggered by end step  121 , the difference between the maximum counts stored and the minimum counts stored, is determined. If there were no jitter, the minimum count would be zero (or minus one) and the maximum count would be one (or zero). If there is jitter, however, either the maximum count or the minimum count may differ from that. Therefore, 1 is subtracted from the difference between the maximum count and the minimum count and multiplied by a quarter of the bit period of the input pulse-train received at input  100 . This one quarter multiple occurs because of the multiplication of the reference clock. This measurement gives a value known as “coarse jitter”. Secondly, the count table accumulator  105  is scanned to find the first occurrence at the maximum value count, and the last occurrence at the minimum value count. The difference in position is determined, divided by four and divided by the table size, which is equalled with a number of times the sampler  110  will be triggered during a measurement cycle. This gives a value known as the fine jitter. The sum of the course and fine jitter measurements are the peak-to-peak amplitude of the phase jitter of the input signals. 
   It can be noted that the term “jitter amplitude” designates the peak-to-peak value of the deviation of the phase function relative to time. The jitter amplitude is measured relative to the length of a clock period, so that it is independent of the shape of the binary signal of the pulse-train. Also, it is independent of bit rate, because it is relative to the clock period, making it a normalised parameter. It is thus possible to use this value to compare jitter amplitudes. 
   Moreover, and as shown in  FIG. 1 , the output of the table of store  106  may be passed to additional filter  107 , or a discrete Fourier transform carried out on the count values stored. This enables the frequency content of the phase jitter of the input pulse-train received at input  100  to be determined. 
   In the embodiment described above, the PLL  103  multiplies the offset clock frequency generated by offset circuit  102  by 4. Other factors are useful, but it should be noted that this factor then determines the delays in steps  115 ,  117 ,  118  and  120  in  FIG. 2 , and also the period of time of the measurement before end step  121  is reached. If, for example, a multiplier of 8 was used then steps  115  and  118  may have a seven clock delay, and steps  117  and  120  may have a nine clock delay. Moreover, the measurement period is then equal to the inverse of eight times the bit rate times the clock offset. Finally, when the fine jitter is measured, the subtraction of the table position of the first maximum value count from the table position of the last minimum count would then be divided by eight. 
     FIG. 3  is the top level functional block diagram for the entire jitter measurement device. It contains five main sections of circuitry, RX bit error rate testing (RX BERT)  10 , TX bit error rate testing (TX BERT)  11 , RX jitter  12 , TX jitter  13  and V40 interfacing circuitry  14 . The configuration can generate transmit jitter and also measure the incoming receive jitter while carrying out a bit error rate test at the same time. The V40 circuitry  14  controls operation of the configuration via V40 interface circuitry. 
   In the device of  FIG. 3 , the signals considered are shown in Table 1. 
   
     
       
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Signal Name 
               Description 
             
             
                 
             
           
           
             
               AD(0:7) 
               This signal is the V40&#39;s databus and the lower 8 bits of its 
             
             
                 
               address bus multiplexed together. Data travels backwards 
             
             
                 
               and forwards along this bus between the configuration and 
             
             
                 
               the V40. 
             
             
               AI(8:15) 
               This is the top 8 bits of the V40&#39;s address 
             
             
                 
               bus. It is an input to the configuration and 
             
             
                 
               indicates which address the V40 is accessing. 
             
             
               ASTB 
               This is the address/signal from the V40. It 
             
             
                 
               is high when the V40 is presenting its 
             
             
                 
               address on its external bus. 
             
             
               BEEPER 
               This signal oscillates at 2 megacycles per 
             
             
                 
               second and is divided in the smaller xilinx 
             
             
                 
               to form the beep signal. 
             
             
               CLKIN 
               This signal comes from the oscillator on the 
             
             
                 
               PAX A board and oscillates at 12.298 
             
             
                 
               megahertz. 
             
             
               CLKOUT 
               This signal is derived from signal CLKIN and 
             
             
                 
               oscillates at twice the frequency of CLKIN ie 
             
             
                 
               at 24.576 megahertz. 
             
             
               COMP 
               This is the comparison output to the phase 
             
             
                 
               lock loop. It is used in the generation of 
             
             
                 
               the received jitter clock SCLK. 
             
             
               COUNT 
               This signal indicates when a received jitter 
             
             
                 
               phase change is to be counted. It is high for 
             
             
                 
               phase changes of both plus a quarter of an 
             
             
                 
               interval and minus a quarter of an interval. 
             
             
                 
               The direction of the COUNT is controlled by 
             
             
                 
               the signal UP. 
             
             
               CRCERR 
               This signal pulses whenever received CRC 
             
             
                 
               error happens. 
             
             
               D(0:7) 
               This is the internal databus to the configuration. 
             
             
                 
               It carries all the data from the V40 to and from 
             
             
                 
               the configuration. It also carries the data which is 
             
             
                 
               stored in the V40&#39;s memory during DMA accesses. 
             
             
               DLTCLK 
               This signal oscillates at the same period as 
             
             
                 
               the transmit clock. It is fed to the Dallas 
             
             
                 
               chip to provide the transmit clock. It is 
             
             
                 
               also used to ensure the signals XTPOS and 
             
             
                 
               XTNEG have the right mark space ratio. 
             
             
               DMAACK 
               This signal comes from the V40 and indicates that a DMA 
             
             
                 
               cycle is occurring. 
             
             
               DMARQ 
               This signal is generated by the configuration 
             
             
                 
               and is used to indicate to the V40 that a DMA 
             
             
                 
               request is pending. 
             
             
               DOJIT 
               This signal goes high whenever a twelfth of 
             
             
                 
               a unit interval jitter hit is to be inserted 
             
             
                 
               into the transmit jitter. The transmit jitter 
             
             
                 
               is comprised of a twelfth of a unit interval 
             
             
                 
               hits. 
             
             
               E1CLK 
               This signal goes high once per received bit 
             
             
                 
               in the RX jitter circuitry. Pulses on the 
             
             
                 
               E1CLK are counted and after every 8 counts a 
             
             
                 
               jitter result is DMA&#39;d into the V40&#39;s memory. 
             
             
               FASERR 
               This signal pulses whenever the receiver detects a FAS 
             
             
                 
               error. 
             
             
               HLDRQ 
               This signal is passed to the V40 and is held 
             
             
                 
               permanently low in this configuration. 
             
             
               INJERR 
               The V40 controls the signal and can pulse it 
             
             
                 
               in order to inject a bit error into the 
             
             
                 
               transmit Bert pattern. 
             
             
               IOEN 
               This signal is used whenever the V40 carries out a IO 
             
             
                 
               operation. 
             
             
               IORD 
               This signal goes low whenever the V40 is carrying out a IO 
             
             
                 
               read instruction. 
             
             
               IOWR 
               This signal goes low whenever the V40 carries out an IO 
             
             
                 
               rate instruction. 
             
             
               JCLKI 
               This signal is sourced from the jitter 
             
             
                 
               attenuator chip. It oscillates at the same 
             
             
                 
               frequency as the receive clock less 
             
             
                 
               1/(3 × 2 18 ) (approximately 1.27 parts per 
             
             
                 
               million). This signal is quadruple in 
             
             
                 
               frequency to form signal SCLK which is used 
             
             
                 
               to sample the received jitter. 
             
             
               JITAMP 
               This signal goes high whenever the V40 is 
             
             
                 
               writing to the jitter amplitude register on 
             
             
                 
               the transmit jitter circuitry. 
             
             
               JMODI 
               The transmit jitter waveform. It indicates 
             
             
                 
               whether the jitter waveform is varying in 
             
             
                 
               phase or otherwise. 
             
             
               JQ(0:2) 
               These signals are high whenever the V40 is 
             
             
                 
               writing to the transmit jitter frequency 
             
             
                 
               registers. 
             
             
               MNADDR 
               This signal is high wherever the received 
             
             
                 
               jitter circuitry has taken a jitter sample 
             
             
                 
               which is less than or equal to the previous 
             
             
                 
               minimum jitter sample. It causes the 
             
             
                 
               configuration to latch the DMA address of 
             
             
                 
               the next DMA cycle. At the end of the 
             
             
                 
               received jitter measurement the V40 reads 
             
             
                 
               this address to determine the received 
             
             
                 
               jitter. 
             
             
               MRD 
               This signal goes low whenever the V40 executes a memory 
             
             
                 
               read instruction. 
             
             
               MWRD 
               This signal goes low whenever the V40 executes a memory 
             
             
                 
               write instruction. 
             
             
               MWRI 
               This signal goes low whenever the V40 executes a memory 
             
             
                 
               write instruction. 
             
             
               MXCADDR 
               This signal goes high whenever the received 
             
             
                 
               jitter measurements is higher than any of 
             
             
                 
               the previous received jitter measurements. 
             
             
                 
               This signal is used to latch an address 
             
             
                 
               which is later used by the V40 to determine 
             
             
                 
               the received jitter. 
             
             
               OFFCLK 
               This signal is the received clock offset by 
             
             
                 
               −1/(3 × 2 18 ) (approximately −1.27 parts per 
             
             
                 
               million). This signal has quarter of a unit 
             
             
                 
               interval hits on it and is dejittered using 
             
             
                 
               the jitter attenuator chip. 
             
             
               RSERI 
               This is similar to RSER. 
             
             
               RSTS 
               This signal from the Dallas chip goes high 
             
             
                 
               during time slot 16 of the E1 frame and is 
             
             
                 
               decoded to indicate phase or CRC errors. 
             
             
               RXCKEN 
               This is the received clock enable signal 
             
             
                 
               for the RX Bert circuitry. It goes high for 
             
             
                 
               one CLKOUT period each received bit. 
             
             
               RXER 
               This signal is the data signal to the WG gate array. 
             
             
               RFER 
               This signal from the Dallas chip is de-coded to indicate 
             
             
                 
               FAS or CRC errors. 
             
             
               RFSYNC 
               This signal is used to synchronise the 
             
             
                 
               received time slot selection circuitry and 
             
             
                 
               also de-coded to indicate phase or CRC 
             
             
                 
               errors. 
             
             
               RSER 
               This is the E1 data from the Dallas chip. 
             
             
                 
               It is passed to the WG gate ray to measure bit errors. 
             
             
               RSTS 
               This signal from the Dallas chip goes high 
             
             
                 
               during time slot 16 of the E1 frame and is 
             
             
               RECONEN 
               This signal is used to reconfigure the 
             
             
                 
               xilinx when the jitter test is complete. 
             
             
               RCHCLK 
               This signal from the Dallas chip is the 
             
             
                 
               channel clock for the E1 receive frame. It 
             
             
                 
               is de-coded to indicate FAS or CRC errors. 
             
             
               RDLCLK 
               This is the receive clock which is passed 
             
             
                 
               to the Dallas chip. It is similar to 
             
             
                 
               signal RXCKEN but is extended by one clock 
             
             
                 
               period to meet the Dallas chip specifications. 
             
             
               SCLK 
               This is the master clock used by the RX 
             
             
                 
               jitter circuitry. It oscillates at normally 
             
             
                 
               8.192 megahertz, minus 1/(3 × 2 18 ) 
             
             
                 
               (approximately 1.27 parts per million). It 
             
             
                 
               is used to sample in incoming received data 
             
             
                 
               to detect jitter. 
             
             
               SIGIN 
               This is the signal input to the 4046 phase 
             
             
                 
               up loop. It is used to quadruple the signal 
             
             
                 
               JCLKI to form signal SCLK. 
             
             
               SMP(0:7) 
               This signal is the raw sample jitter from the received jitter 
             
             
                 
               circuitry. 
             
             
               STOPPED 
               This signal is controlled by the V40 and is 
             
             
                 
               driven high when the received jitter 
             
             
                 
               measurement is stopped. 
             
             
               PDLCLK 
               This signal is the 2 megabit transmit clock 
             
             
                 
               generated from the transmit BERT circuitry. 
             
             
                 
               transmit jitter circuitry to insert a 12th 
             
             
                 
               of a unit interval jitter hit into the 
             
             
                 
               transmit clock. This signal prevents jitter 
             
             
                 
               hits from being inserted while the transmit 
             
             
                 
               bit is marking. This makes sure that the 
             
             
                 
               transmitted bits meet the pulse mask. 
             
             
               TMO 
               This signal originates in the Dallas chip 
             
             
                 
               and indicates the start of the transmit 
             
             
                 
               multiframe. It is used to synchronise the 
             
             
                 
               transmit time slot select circuitry. 
             
             
               TNEG 
               This signal originates in the Dallas chip 
             
             
                 
               and together with signal TPOS forms the 
             
             
                 
               transmit E1 stream. 
             
             
               TPOS 
               This signal originates in the Dallas chip 
             
             
                 
               and is used to generate the E1 stream. 
             
             
               TWO 
               This signal goes high when ever the 
             
             
                 
               received jitter is too much for the 
             
             
                 
               received jitter circuitry to cope with. The 
             
             
                 
               V40 can read whether this line as ever been 
             
             
                 
               high. If this is the case then the jitter 
             
             
                 
               measurement is discarded. 
             
             
               TXBERT 
               This signal goes high during time slots 
             
             
                 
               where bit error rate test signals are being 
             
             
                 
               transmitted. 
             
             
               TXBRTS 
               This signal goes high whenever a transmitted PRBS bit is to 
             
             
                 
               be sent. 
             
             
               TXCKEN 
               This signal goes high for one CLKOUT period each 
             
             
                 
               transmit bit. 
             
             
               TXCLK 
               This is the signal pass to the counter timer chip to indicate 
             
             
                 
               the transmit bit rate. 
             
             
               TSPDAT 
               This is the transmitted PRBS signal which is injected into 
             
             
                 
               the transmit data stream. 
             
             
               UP 
               This signal indicates the polarity of a 
             
             
                 
               receive jitter phase change and is used in 
             
             
                 
               conjunction with signal COUNT to accumulate 
             
             
                 
               the received jitter. 
             
             
               V24RX 
               This signal is the received V24 data which is passed to the 
             
             
                 
               V40. 
             
             
               V24RXD 
               This signal is the same as signal V24RX. 
             
             
               V24TX 
               This is the V24 data from the V40 transmitted out of the 
             
             
                 
               V24 port. 
             
             
               V24TXD 
               This signal is the same as V24TX. 
             
             
               VCO 
               This signal comes from the 4046 phase lock 
             
             
                 
               loop. It is used in the process whereby 
             
             
                 
               signal JCLKI is quadruple in frequency to 
             
             
                 
               form signal SCLK. 
             
             
               WGCLK 
               This signal is used to clock data into the 
             
             
                 
               WG gate array during bit error tests. The 
             
             
                 
               WG gate array then measures bit errors. 
             
             
               WGDATA 
               This is the data passed to the WG gate 
             
             
                 
               array from the receive BERT circuitry. It 
             
             
                 
               is used to perform bit error rate tests on. 
             
             
               WGERR 
               This signal originates in the WG gate array 
             
             
                 
               and indicates when a received bit error has 
             
             
                 
               occurred. It is passed to a counter timer 
             
             
                 
               chip where bit errors are measured. 
             
             
               XRNEG 
               This is the re-timed received E1 data which is passed to the 
             
             
                 
               Dallas chip. 
             
             
               XPNEGI 
               This is the raw E1 data from the B board. 
             
             
               XRBLS 
               This is the re-timed received E1 data which is passed to the 
             
             
                 
               Dallas chip. 
             
             
               XRPOSI 
               This signal is the raw received E1 data from the B board. 
             
             
               XSM 
               This signal is XRNEGI re-timed to the clock 
             
             
                 
               CLKOUT. The received clock is recovered 
             
             
                 
               from this signal. 
             
             
               XSP 
               This is the signal XRPOSI re-timed to the 
             
             
                 
               clock CLKOUT. Along with signal XSM this 
             
             
                 
               signal is used to generate the received 
             
             
                 
               clock. 
             
             
               XSPU 
               This is the unbuffered received E1 data 
             
             
                 
               which is passed to the jitter detection 
             
             
                 
               circuitry. Jitter is detected on this 
             
             
                 
               signal. 
             
             
               XTNEG 
               This signal is passed to the B board and is used to generate 
             
             
                 
               the transmit E1 string. 
             
             
               XTPOS 
               This signal is passed to the B board and is used to generate 
             
             
                 
               the transmit E1 string. 
             
             
                 
             
           
        
       
     
   
   The various components of the system of  FIG. 3  will now be considered in more detail. Starting with the RX bit error rate testing circuitry (RX BERT)  10 , the detailed structure of this circuitry is shown in more detail in  FIG. 4 . As can be seen, there are several circuit elements. The first is CLOCK GEN component  20  is used to double the frequency of the signal CLKIN. This forms a higher frequency clock CLKOUT which has a frequency of about 25½ meahertz. The logic for this clock doubling is placed in a CLB map at position AA. This ensures that the logic is very close on the LCA to the global clock buffer GCLK. The circuit works by forming a signal CLKBUF which is identical to the signal CLKIN except delayed by a small amount of time. The clock CLKOUT is passed to a GETCLOCK component  21 . 
   This GETCLOCK component  21  recovers the clock from the received E1 data to be used in the TX Bert circuitry. The raw incoming E1 data is sampled by the system clock CLKOUT and then the positive and negative streams are gated together to form signal RESET. This signal resets a four bit divided by twelve counter. This counter is then used to generate received blocks during times when there are no marks on the received data. CLB map in this drawing is used to try and squash as much logic as possible into the system. Thus, the GETCLOCK component  21  corresponds to the pattern clock converter  101  in  FIG. 1 . 
   The signals shown in  FIG. 4  are then listed in Table 2. 
   
     
       
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Signal Name 
               Description 
             
             
                 
             
           
           
             
               CLKOUT 
               This is the 24½ megahertz system clock. 
             
             
               CNT0 through to 
               These tour signals form a divide by twelve counter. It 
             
             
               CNT3 
               is divide by twelve as the received bit rate is a twelfth 
             
             
                 
               of the system clock. This counter is reset by the signal 
             
             
                 
               RESET. This occurs whenever a mark is received on 
             
             
                 
               the incoming data. 
             
             
                 
               During strings of 0&#39;s where there is no timing 
             
             
                 
               information on the received E1 data then this counter 
             
             
                 
               is used to 1 generate the signal RXCKEN which is the 
             
             
                 
               received clock enable. 
             
             
               RDLCLK 
               This signal is generated for the Dallas chip. The signal 
             
             
                 
               RXCKEN is only one CLKOUT clock period wide. 
             
             
                 
               This is not a wide enough pulse to clock the Dallas 
             
             
                 
               chip so the extra signal RDLCLK is generated which is 
             
             
                 
               twice as long to clock the Dallas chip. 
             
             
               RESET 
               This signal pulse is high whenever a mark is received 
             
             
                 
               on the incoming E1 data and is used to synchronise 
             
             
                 
               the received counter. 
             
             
               RNEG 
               This signal is fed to the Dallas chip and is the received 
             
             
                 
               negative E1 data. 
             
             
               RNEG0 
               This is the same signal as RNEG. 
             
             
               RPOS 
               This is the received E1 positive pulses which are fed to 
             
             
                 
               the Dallas chip. 
             
             
               RPOS0 
               This is the same signal as RPOS. 
             
             
               RXCKEN 
               This signal is generated in his block and is the received 
             
             
                 
               clock enable. This signal goes high for one CLKOUT 
             
             
                 
               period every single received bit. 
             
             
               RXP 
               This signal is used in combination with signal 
             
             
                 
               RXCKEN to generate the signal RDLCLK which is 
             
             
                 
               used to clock the Dallas chip. 
             
             
                 
             
           
        
       
     
   
   The component  22  is used to generate the enables for the RX BERT circuitry. A patched signal USERTA goes high whenever the received data is to be passed to the WG gate array for PRBS testing. Two other CLB maps are used simply to compress the logic into the smallest space as possible. The block consists of an 8 bit counter which is formed by signals CNT 0  through to CNT 7 . This counter is reset to 0 by the signal RFSYNC from a Dallas chip  23 . This counter is then de-coded to form the time slot select for the received PRBS data. Note that the high ordered 5 bits of the counter from signal CNT 3  through to CNT 7  are reset by the signal RFSYD. Again this technique is used to try and conserve space. The signal USERTS which is patched is then gated with the received clock enable to form the clock to the WG gate array which is signal WGCLK. 
   As mentioned above, the TSSEL component  22  receives the signal RFSYNC from the Dallas ship  23 . That signal is then passed to a G703ERRS component  24 . This component  24  is used to generate the CRC and FAS error signals. These signals are generated from gated signals from the Dallas chip  23 . The signal CRC error goes low whenever the signals RF since and RFER are high simultaneously, likewise the signal FASERR goes low whenever the signals RCHCLK and RFER are high while the signal RSTS is low. 
   Next the RX jitter circuit  12  will be considered in more detail. Its internal structure is shown in  FIG. 5 . Again, it has several circuit elements. The first is a CLOCKOFF component  30 . The component  3 Q offsets the incoming received E1 clock by minus 1/(3×2 18 ). (approximately 1.27 parts per million) before passing this clock to a Dallas jitter attenuator  31 . It has a function which is used to divide the receive clock by 65,536. It also contains test functions and SLPYREG which are used to offset the clock by adding single periods of the clock CLKOUT every 65,536 received bits. Thus, the CLOCKOFF component  30  corresponds to the offset circuit  102  in  FIG. 1 . 
   The CLIPYCNT function uses a four bit counter which performs a divide by twelve operation. Bits zero and one divide by three, and bits two and three divide by four, given a total of divide by twelve. The counter clock enabled by signal which goes high for one CLKOUT clock period every 65,536 received bits. The output of the counter is used to determine where in the twelve bit shift register in function SLIPYREG the received clock is inserted. In this way twelfth of a unit interval phase changes are introduced into the received clock in order to offset it by minus 1.27 parts per million. The SLIPYREG function uses a twelve bit shift register. It is used to inject slowly increasing twelfth of a unit interval jitter phase hits into received clock. Every 65,536 the point at which the received clock is injected into the shift register is moved closer to the beginning of the shift register. The output of the shift register ie the offset clock is at the last twelfth tap. When finally the RX clock has been injected into the first bit of the shift register and it is time to access another twelfth of a unit interval phase shift. This received clock is discarded and then the received clock is then injected into the end of the shift register. In this way the clock is offset. The MISSCNT function uses a linear feedback shift register counter. It consists of a sixteen bit shift register, of which four taps are fed back to the input. Other gates are used to detect when the shift register counter has reached its terminal count. This forms signal HIGHNR which is the output. 
     FIG. 5  shows that the output JCLKI of the Dallas jitter attenuator  31  passes to a PLLSTUFF component  32 . This PLLSTUFF component  32  is used to multiply the signal JCLKI by four to form the jitter sample block SCLK. It does this by doubling the frequency using the phase lock loop and then doubling the frequency from the phase up loop by two using an edge detection method. The Dallas jitter attenuator jitter  31  acts a phase lock loop which acts to remove the jitter component from the OFFCLK signal derived from the CLKOFF component  30 . This function of the Dallas jitter attenuator  31 , together with the PLLSTUFF component  32  thus form the PLL  103  of  FIG. 1  which, as previously described, produces a jitter-free pulse-train, and then multiplies that pulse-train by the integer factor of 4. 
   A JITDET component  33  samples the incoming E1 data and from this measures the received jitter. It also recovers an E1 receive clock from the incoming E1 data stream. Thus, the JITDET component  33  forms the data sampler  104  in  FIG. 1 . It receives the offset and multiplied clock signal from PLLSTUFF component  32 , and also the incoming signal which is being sampled for jitter. 
   A JITCOUNT component  34  generates the 8 bit jitter sample data. It consists of an 8 bit up/down counter which is enabled by the signal COUNT and the direct of the count is controlled by signal UP. The counter is set to value 80 HEX while the signal STOPPED is high. Notice that signal CNT 7  is inverted before emerging from this component  34 . Thus the JITCOUNT component  34  forms the accumulator  105 . 
   The output from this JICOUNT component  34  is signal SMP(0:7). That output SMP(0:7) passes to a JITOUT component  35 . This component  35  is used to transfer measured jitter into the V40&#39;s memory. This memory forms the sampler  106  of  FIG. 1 . It is also used to detect the amplitude of the received jitter. It does this by storing the addresses of the first time that a maximum valued jitter sample was stored and also the address of where the last minimum value jitter sample was stored. The difference between these addresses ratio to the size of the whole DMA buffer gives an indication of the jitter amplitude. The current value of the sample jitter is stored in a shift register, along with the maximum value recorded up until now and the minimum value recorded up until now. These are compared in a block called compare which indicates when bigger or smaller samples are received. These signals are processed to generate latches for addresses. 
   Next the TXBERT circuit  11  will be considered in more detail with reference to  FIG. 6 . The TXBERT circuit  11  has a TXTSSEL component  40 . The component  40  is used to generate the transmit enables for the transmit Bert data. It consists of an 8 bit counter formed by the signals CNT 0  through to CNT 7 . This counter is reset by the signal TMO which indicates the start of the transmit multiframe. The signal TMO comes from the Dallas chip  23 . It is latched and gated to form signal TS since which directly resets the counter. The output of this counter is then decoded to form a signal TXBERT. This signal goes high during which data is to be transmitted. In unframed mode this signal is patched permanently high. The signal is patched in the CLBTX time slot select. Note that signal TFSYNC directly resets the high five bits of the eight bit counter whereas the low three bits of the counter are set to the value 001 by this signal. This ensures everything lines up with the timing of the TMO signal. 
   The output TXBERTS of the TXTSSEL component  41  passes to a TXPRBS component  41 . This component  41  is used to generate the transmit PRBS Bert pattern. It consists of a 15 bit shift register formed by signals TAP 0  through to TAP 14 . Various outputs from this shift register are then gated together and fed back to the input of shift register to generate a PRBS pattern. The CLBTXPRBS select is patched to select which taps are enabled. The CLB map TX polarity select is patched to determine the polarity of the transmitted PRBS data. Signal INJER is controlled by the V40  14 . When this signal toggles high during the transmission of Bert data a bit error is injected into the transmitted data stream. This bit error signal is decoded to signal BERR which inverts the output of the PRBS shift register. Note the output of the shift register occurs from the eighth tap signal TAP 7  although it could have come from any of the other taps if desired. 
   The TXJITTER circuit  13  will now be described with reference to  FIG. 7 . It has a TXCKEN component  50 . This component  50  is used to generate the transmit clock. The transmit clock can be jittered under the influence of signals DOJIT and JMOD 1 . When signal DOJIT is high a twelfth of a unit interval phase hit is introduced into the transmit click if a polarity depending the state of signal JMOD 1 . These phase hit insertion happen during the time when the line is not marking except in high jitter situations. 
     FIG. 7  also shows a TXHDB3 component  42 . This component  42  is used to encode the transmit data in a HDB3 format. Note it can be patched so that the transmit data is AMI. The configuration must do this encoding as the Dallas chip  23  can only encode for HDB3 during unframed transmission when the HDB3 coding is needed. For this reason the Dallas transmitter is always used to transmit AMI data. The CLB maps TX line code and TX framing are patched to enable AMI mode. In this mode, no extra violations are inserted into the transmit data. 
     FIG. 7  also shows a GRADREGO component  51 . This component  51  contains the circuitry which is used to set the frequency of the transmitted jitter. It consists of a nineteen bit counter which is formed by signals JCNT(0:19) together with registers which are used to compare against this count value. The output INCAMP indicates when it is time to inject a twelfth of a unit jitter hit into the transmitted jitter waveform. The block EXTRACLK also enables fine tuning of the jitter frequency. The output INCAMP of the component  51  passes to an AMPREG component  52 . 
   The component  51  is used to set the amplitude of the transmitted jitter. It consists of an eight bit latch which the V40  14  can write to and an eight bit counter which is compared to the contents of this latch to indicate when the required jitter amplitude has been reached. 
   The INCAMP signal also passes to a JITGEN component  57 . This component is used to control the generation of transmit jitter in the TX jitter generation circuitry. 
   It can be seen from the above discussion of  FIGS. 3 to 7  that the embodiment of  FIG. 1  makes use primarily of digital components. This makes embodiments of the present invention easier and cheaper to produce. In the embodiment of  FIG. 1 , the PLL circuit  103  needs to be an analog circuit, but the fact that the PLL circuit  103  has a low time constant means that it is easy to produce and is thus inexpensive. 
   In the above discussion, it is assumed that the pulse-train received at input  100  is a co-directional digital data signal, in which the clock information and data are included together in one signal. The present invention may also be applied to clock signals which are not included with data, clocks still being recovered in the same way as discussed above. Moreover, the present invention may be used to investigate the jitter of an analog signal, by converting that to a digital signal before being input to input  100 .