Abstract:
In a network where frequency synchronous lower bit rate signals are combined to form a high bit rate signal by using hierarchies of multiplexers, i.e., concatenated multiplexers of different bit rate levels, an apparatus is provided for adjusting the phase deviations that occur between data input signals and a common clock signal. In one embodiment, the apparatus includes delay lines for delaying the input signals of the multiplexers of the lower hierarchies.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of European Patent Application No. 98306713.3, which was filed on Aug. 21, 1998. 
     TECHNICAL FIELD 
     This invention relates generally to the field of phase correlation and, more particularly, to correlating the phases of a clock signal and data signals. 
     BACKGROUND OF THE INVENTION 
     In high bit rate networks frequency synchronous lower bit rate data signals are combined by multiplexers to form the high bit rate signal. Often hierarchies of multiplexers are used to form the high bit rate signal, i.e., multiplexers of different bit rate levels are concatenated. In a first hierarchy from low bit rate data signals medium bit rate data signals are formed which then are multiplexed to a high bit rate data signal by a multiplexer of a second hierarchy. Usually all data signals of a common hierarchy are frequency synchronous. After forming the medium bit rate data signals, multiplexing them to the high bit rate data signal causes a problem, because the medium bit rate data signals often show a phase deviation from the common frequency used for multiplexing, i.e., a phase deviation from a common clock signal. 
     The phase deviation can be adjusted by using a phase correlator and a delay line for phase adjusting the medium bit rate data signals to the common clock signal of the medium bit rate data signals. But adjusting the phase deviation for the medium bit rate data signals causes problems, because an apparatus for adjusting the phase deviations of the medium bit rate data signals to the common clock signal has to operate at relatively high frequencies. 
     SUMMARY OF THE INVENTION 
     According to the principles of the invention, an apparatus is provided for correlating phases of a clock signal and data signals to adjust for phase deviations in a network having multiplexers of a first hierarchy concatenated with multiplexers of at least one higher hierarchy. More specifically, the apparatus according to the principles of the invention includes means for correlating phases of the clock signal and data signals of the higher hierarchy, to generate control signals indicative of a phase deviation of the data signals to the clock signal of the higher hierarchy, delay means controlled by the control signals to adjust for the phase deviation of the data signals to the clock signal of the higher hierarchy, whereat the delay means at least delay one input signal of each multiplexer of a lower hierarchy. 
     An advantage of the present invention is that it allows correction of the phase deviation of each data input of the multiplexer of the highest hierarchy separately. Another advantage of the present invention is that the phase adjustment is effected at a location in the multiplexing hierarchy which is at least one hierarchical level below where the phase deviation occurs. As the adjustment of the phase deviation thus is effected for a lower bit rate signal, less timing critical circuitry can be used to achieve the phase adjustment, as the clock phase margin at re-timing D-flip-flops usually used in the multiplexers is more relaxed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A more complete understanding of the present invention may be obtained from consideration of the following detailed description of the invention in conjunction with the drawing, in which: 
     FIG. 1 shows an exemplary embodiment of an apparatus for combining low bit rate data signals to form a high bit rate data signal according to the principles of the invention; and 
     FIG. 2 shows an exemplary embodiment of a means for generating a control signal to adjust a phase deviation according to the principles of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Depicted in FIG. 1 is an apparatus for combining low bit rate data signals to form a high bit rate data signal. The apparatus has a common first clock signal C which is coupled to adjustable delay means, e.g., delay lines, T 1  to Tm. The output of the delay lines T 1  to Tm are coupled to inputs of multiplexers M 1  to Mm respectively, which form a first multiplexing hierarchy. The multiplexers M 1  to Mm each have n inputs, for example, for inputting n low bit rate data signals all being frequency synchronous. The n inputs of the multiplexers M 1  to Mm and the low bit rate data signals are not shown for reason of simplicity, as they are not relevant for the understanding of the present invention. At outputs of the multiplexers M 1  to Mm medium bit rate data signals D 1  to Dm respectively are available. The multiplexing of the n low bit rate data signals by each of the multiplexers M 1  to Mm is clocked by the common first clock signal C via the delay lines T 1  to Tm respectively. The medium bit rate data signals D 1  to Dm are coupled to m inputs of a multiplexer M which combines the m medium bit rate data signals to a high bit rate data signal D, forming a second multiplexing hierarchy. The multiplexing of the m medium bit rate data signals is clocked by a second clock signal Cxn which is generated from the first clock signal C by a frequency multiplier xn. To avoid phase deviations between the medium bit rate data signals D 1  to Dm to the second clock signal Cxn, control signals R 1  to Rm are generated by the multiplexer M to control the delay time of the delay lines T 1  to Tm respectively. 
     Depicted in FIG. 2 is a means for generating a control signal for adjusting phase deviations of the input signals of the multiplexer M, as explained above. There are m means as depicted in FIG. 2 present in multiplexer M, i.e., for each medium bit rate data signal D 1  to Dm one such means is provided to form the control signals R 1  to Rm. The means for generating the control signal has two inputs, one for the medium bit rate data signal Dx and a second one for the second clock signal Cxn. The input signals are fed to a data acquisition means FF, e.g., a D-flip-flop. The medium bit rate data signal Dx is coupled to a data input of the flip-flop FF, the clock signal Cxn  15  is coupled to a clock input of the flip-flop FF, to allow the data acquisition or transfer of data stemming from the medium bit rate data signal Dx to an output Qx of the flipflop FF. The output signal Qx is one of the m signals fed to the multiplexer M to be multiplexed to form the high bit rate data signal D. The flip-flop FF itself is already present, as it is part of the multiplexer M. 
     The input signal Dx and the output signal Qx of the flip-flop are coupled to a phase correlator K, e.g., an EXOR gate. To compensate for the propagation time of the flipflop FF a delay circuit τ, e.g. a delay line or a buffer, can be provided to couple the input signal Dx to the EXOR gate K. As the output signal Qx of the flip-flop FF is subject to the clock signal Cxn, the output signal of the EXOR gate K is a measure for the phase deviation between the data signal Dx and the clock signal Cxn. An output of the EXOR gate K is coupled to a loop filter I, e.g., an integrating filter, to form a control signal Rx, which is fed to one of the delay lines T 1  to Tm, as shown in FIG.  1 . 
     By adjusting phase deviations in the way explained above, it is possible to correct the phase deviation of each data input D 1  to Dm of the multiplexer M to the clock signal Cxn separately. The phase adjustment is effected at a location in the multiplexing hierarchy which is one hierarchical level below where the phase deviation occurs. As the adjustment of the phase deviation for the medium bit rate data signals D 1  to Dm is effected at the side of the low bit rate data signals for the clock signal C, controlling the multiplexers M 1  to Mm, which form the medium bit rate signals D 1  to Dm, the phase adjustment is shifted to a less critical location. 
     The foregoing is merely illustrative of the principles of the invention. Those skilled in the art will be able to devise numerous arrangements, which, although not explicitly shown or described herein, nevertheless embody principles that are within the spirit and scope of the invention. For example, more than two multiplexing hierarchies can be present. Moreover, phase deviations can be adjusted by controlling delay lines that are coupled to multiplexers not being direct predecessors of the multiplexer of the highest multiplexing hierarchy. Accordingly, the scope of the invention is limited only by the claims that follow.