Abstract:
There is provided a Clock recovery apparatus comprising: an early/late voter for deciding whether a current sampling point needs to be advanced or retarded, wherein said early/late voter passes an Up/Down signal to an interpolator for maintaining a clock signal; a frequency accumulator and rate multiplier 30 for generating further signals which are summed with those of the Up/Down signal of the early/late voter to provide an improved control signal to the phase interpolator. The accumulator is responsive to frequency changes in the input signal, and said interpolator acts on said Up/Down signals to adjust the clock signal by stepping it forward or backward according to control need, so that said sampling point can be advanced or retarded.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to clock recovery as may be necessary for example to extract data from a datastream. Where no separate clock is available at the receiver, a clock to sample the data must be generated from the datastream itself.  
       SUMMARY OF THE INVENTION  
       [0002]     This and other objects and features are provided, in accordance with one aspect of the invention by a clock recovery apparatus comprising an early/late voter for deciding whether a current sampling point needs to be advanced or retarded, wherein the early/late voter passes an Up/Down signal to an interpolator for maintaining a clock signal. A frequency accumulator and rate multiplier generates further signals which are summed with those of the Up/Down signal of the early/late voter to provide an improved control signal to the phase interpolator. The accumulator is responsive to frequency changes in the input signal, and the interpolator acts on said Up/Down signals to adjust the clock signal by stepping it forward or backward according to control need, so that said sampling point can be advanced or retarded.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0003]      FIG. 1   a  is a diagram showing the idealised form for a datastream input to a receiver;  
         [0004]      FIG. 1   b  is a diagram showing an example of the actual form of the datastream input to a receiver, suffering form the effect of noise and distortion;  
         [0005]      FIG. 2  is a schematic diagram showing an arrangement for performing eye scan;  
         [0006]      FIG. 3  is a schematic diagram showing a modified arrangement for performing eye scan where both frequency and phase control is provided;  
         [0007]      FIG. 4  is a schematic diagram of an exemplary rate multiplier according to a preferred embodiment of the present invention; and  
         [0008]      FIG. 5  presents example performance figures for a particular embodiment of this invention which used a 10-bit rate multiplier together with an interpolator with a resolution of 64 phase steps per unit interval (UI) and whose phase could be updated at a maximum rate of one phase step every 2UI.  
     
    
     DETAILED DESCRIPTION  
       [0009]     In  FIG. 1  there is shown a datastream made up of differential signals  1  and  2  (R×P and R×N) which, in this case, show a stream of alternate 1&#39;s and 0&#39;s. The data may be recovered successfully if a clock can be generated to trigger sampling at points S in the datastream, such as for example, a sampling at time of 5 will recover the first 1.  
         [0010]     In practice, the datastream received will not be of the idealised form shown in  FIG. 1 ( a ), but more likely be subject to noise and distortion, as shown in  FIG. 1 ( b ). In order to determine the sampling instant S, an attempt is made to identify the transitions or edges of the signals  3  and  4 . It may then be deduced that samples should be taken substantially midway between the detected edges. Since the edges are subject to noise and jitter they must be tracked and the actual instantaneous sampling point moved accordingly.  
         [0011]     An arrangement for achieving this is shown in  FIG. 2 . Input signals  11  are received at receiver  12  and processed by equaliser  13 . The data is sampled by data sampler  14  to produce the data output  15 . The data sampler is controlled by a clock signal  16  such that if clock signal  16  transitions at the desired sampling instant S, the data will be correctly recovered. Hence signal  16  constitutes a clock recovered at the receiver.  
         [0012]     Recovered clock signal  16  is maintained by a stepped phase interpolator  19 , which may be controlled to alter the precise sampling instant, such that the changes in the incoming signal may be tracked. Since the data rate of the incoming signal may be very high (equivalent for example to a clock data rate of 2 GHz or more) it is not practicable to control the position of each and every sampling point. Accordingly, the interpolator is used to maintain a clock signal and is adjusted from time to time, for example at one quarter of the recovered clock rate. This is achieved by detecting the edges of the received signal in edge sampler  17  and then using early/late voting  18  to decide whether the current sampling point needs to be advanced or retarded. Accordingly an Up/Down signal is fed to step the phase interpolator. The interpolator is then stepped forward or backward according to the control need.  
         [0013]     It will be appreciated that the arrangement of  FIG. 2  provides the control to the phase interpolator, based on the phase difference between the current and the desired sampling points.  
         [0014]     This system works well, but may be incapable of tracking, for example, large changes in the frequency of the incoming signal as may be encountered for example, in a frequency swept spread spectrum transmission.  
         [0015]     The above problem is overcome by the arrangement of  FIG. 3 , in which the addition of a frequency accumulator and rate multiplier  30  is shown to generate further signals  32 , which are summed with those of the phase response of the early/late voter to provide an improved control signal to the phase interpolator.  
         [0016]     The purpose of the accumulator  30  is to be responsive to frequency changes in the input signal and may for example be configured to detect trends in the results of the early/late voter, rather than simply react to the error signal. In this way the range of frequencies for which this system is able to recover the clock may be greatly enhanced.  
         [0017]     In  FIG. 4  there is shown an exemplary rate multiplier. A binary counter  41  produces for example, four bits of counter output, which are logically combined in gates  42 ,  43  and  44  to provide a series of different pulse trains. Dependent upon a control word  45  applied to and gates  46 ,  47 ,  48  and  49 , these pulse trains are selectively combined to provide an output pulse train, containing a number of pulses in a period defined by the control word.  
         [0018]     Using exemplary embodiments, the improvement provided by the arrangement of  FIG. 3  has been calculated and is shown in the table of  FIG. 5 .  
         [0019]     The table in  FIG. 5  presents example performance figures for a particular embodiment of this invention which used a 10-bit rate multiplier together with an interpolator with a resolution of 64 phase steps per unit interval (UI) and whose phase could be updated at a maximum rate of one phase step every 2UI. The table gives values of the 1st order loop offset frequency tracking range (f 1  in ppm), the maximum 2nd order loop offset frequency tracking range (f 2   max  in ppm), the 2nd order loop frequency step size (Δf 2  in ppm) and the maximum 2nd order loop frequency tracking rate (∂2/∂ max  in ppm/UI) as a function of the voting period and loop bandwidth setting (LBW[1:0]).  
         [0020]     The 1st order loop tracking frequency is determined by the fact that the interpolator can at most be adjusted by 1/64 UI every voting period, thus giving: f 1 =1/64/VP (where VP is the voting period in UI).  
         [0021]     The 2nd order loop could theoretically step by one phase step (1/64UI) every 2 UI which would allow it to track offset frequencies of 1/64/2=7813 ppm. However, in this embodiment the frequency accumulator was limited to 3/4 of its maximum range (768 rather than 1024) giving a maximum value of 5860 ppm.  
         [0022]     For the 10-bit rate multiplier used, each increment of the frequency register results in one extra phase step every 1024 phase update cycles (2048 UI), equivalent to a frequency adjustment of 1/64/2048=7.6 ppm. This represents the maximum frequency resolution/accuracy of the 2nd order loop. The loop bandwidth setting (LBW[1:0]) is a 2-bit control value which determines the step size that is applied to the 2nd order loop frequency accumulator as a result of each vote: for LBW=00 the 2nd order loop is disabled and the frequency register forced to zero. For LBW=01, 10 and 11, the frequency accumulator is incremented in step sizes of 1, 2 and 4 respectively, giving values for Δf 2  of 7.6, 15.3 and 30.5 ppm respectively.  
         [0023]     The maximum 2nd order frequency tracking rate is simply obtained from the frequency step size divided by the voting period. This means that there is naturally a trade off between the frequency resolution of the 2nd order loop and the rate at which it can track changes in data freqeuncy.  
         [0024]     From  FIG. 5  it may be seen that the first order system ( FIG. 2 ) is capable of capturing over a frequency range f 1 . Adding the second order loop ( FIG. 3 ) super imposes this capture range on a much larger capture range f 2 , which extends the overall capture range to the extent that the arrangement is useful with frequency stepped spread spectrum signals.  
         [0025]     A number of other improvements are also derived. For example, if the frequency is increasing and the accumulator  30  may be directly loaded with a code which defines a frequency, which is higher than that to which the interpolated could simply be stepped. Hence, any increased frequency can be captured much more quickly. For example, if a sequence of early/late votes all show that the present clock speed is late, the maximum correction may be immediately applied.  
         [0026]     System testing is also facilitated, for example, suppose a known frequency offset were deliberately introduced to the transmitted signal. For receiver left to a free run, that same offset should be present as a control word in the accumulator of the receiver. Hence a text vector value is immediately present.  
         [0027]     While the invention has been shown and described with reference to preferred embodiments thereof, it is well understood by those skilled in the art that various changes and modifications can be made in the invention without departing from the spirit and scope of the invention as defined by the appended claims.