Abstract:
A clock filter circuit ( 20 ), which serves for filtering the clock of non-isochronous data signals having a selected one of at least two nominal data rates, has an auxiliary clock source ( 21 ) that generates an auxiliary clock signal ( 27 ) with a pulse repetition rate which is in the range between the at least two predetermined data rates, a delay line ( 22 ) connected to the auxiliary clock source ( 21 ) for creating a set of mutually delayed copies of the auxiliary clock signal and a multiplexer ( 23 ) that switches in a cyclic order between the delayed copies according to predetermined rules, which depend on the selected data rate to generate a filtered clock signal ( 28 ). A control circuit determines whether the rate of the filtered clock ( 28 ) signal must be increased or decreased as compared to said data signal and controls the multiplexer ( 23 ) to delay or advance the cyclical switching accordingly.

Description:
The invention is based on a priority application EP04 293 123.8 which is hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to the field of telecommunications and more particularly to a digital clock filter circuit for a gapped clock of a non-isochronous data signal and a method of filtering a clock signal of a non-isochronous data signal. 
   BACKGROUND OF THE INVENTION 
   Network elements in existing transport networks serve for establishing semi-permanent (“cross”)connections in the network. Such network elements include crossconnects and add/drop multiplexers. For crossconnecting high bitrate digital signals, it is advantageous to use a synchronous switch matrix. While today transport networks rely basically on the Synchronous Digital Hierarchy (SDH, ITU-T G.707), a new Optical Transport Hierarchy has been defined in ITU-T G.709, where the transport signals are no longer synchronous but asynchronous within a predetermined range of ±20 ppm from a nominal frequency. 
   Even when the transport signals are not synchronous, a synchronous switch matrix may nonetheless be used, if all payload channels are mapped internally into an common synchronous rate which is higher than the highest payload channel rate. Rate adaptation to the internal rate will be performed by bit stuffing. At the output of the network elements, however, the payload channels must then be de-mapped from the internal rate signal and the stuff bits removed. This will cause non-isochronous bit streams that represent the respective payload channels, since the bit clock of a bit stream has gaps from the removed stuff bits. Therefore, a narrow band phase lock loop (PLL) would be required at each payload channel for converting the non-isochronous bit stream back to an isochronous bit stream. Tight output jitter requirements would apply to this function. 
   Moreover, in certain applications, it would be advantageous if a network element would be able to process transport signals of different types. For instance, transmission equipment for payload bit rates in the Giga-bit range often provides as an option forward error correction (FEC) to the transport signals. The equipment needs therefore the ability to operate at different bit rates, i.e., with or without FEC, on the basis of configuration. On the other hand, it would be advantageous to provide the capability to process SDH and OTH type signals within the same equipment. For such applications, the bit rates must be accommodated to the actually used signal type, which might become very complex for narrow band PLLs at the outputs a network element. Clock circuits with a low Q factor could provide an automatic bit rate accommodation capability with in a range of ±10% with a single oscillator circuit only. High Q clock circuits, however, would require a separate crystal oscillator for each particular bit rate. A clock filter circuit for destuffed non-isochronous transport signals would hence require a particular voltage controlled crystal oscillator (VCXO) for every payload channel and for every bit rate. 
   It is therefore an object of the present invention, to provide a simplified clock circuit that can be configured to operate at least at two different bit rates. 
   SUMMARY OF THE INVENTION 
   These and other objects that appear below are achieved by a clock filter circuit for a gapped clock of a non-isochronous data signal having a selected one of at least two nominal data rates, which makes use of an all digital PLL for low bandwidth filtering of the gapped payload clock. 
   In particular, the clock filter circuit has an auxiliary clock source that generates an auxiliary clock signal with a pulse repetition rate which is in the range between the at least two predetermined data rates, a delay line connected to the auxiliary clock source for creating a set of mutually delayed copies of the auxiliary clock signal and a multiplexer that switches in a cyclic order between the delayed copies according to predetermined rules, which depend on the selected data rate, to generate a filtered clock signal. A control circuit determines whether the rate of the filtered clock signal must be increased or decreased as compared to said data signal and controls the multiplexer to delay or advance the cyclical switching accordingly. 
   The clock filter circuit according to the invention requires less circuit board area, lower component costs and shows a lower power consumption than existing solutions. It allows full integration into AISCs and provides a simple configuration of the clock rate by simply adapting the PLL algorithm (i.e., the rules according to which the switching is performed). Moreover, it is less prone to crosstalk from neighboring channels, which in existing solutions could cause a false lock problem. 
   The invention requires only a single auxiliary clock source as auxiliary clock for a number of payload channels. While other digital PLLs generate intrinsic jitter that needs to be removed by a subsequent analogue PLL, the proposed solution requires no analogue filtering (—but which does not exclude that an analogue PLL is nevertheless used to improve the clock quality!). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described below with reference to the accompanying drawings, in which 
       FIG. 1  shows a block diagram of a configurable circuit that generates a clock signal from a fixed auxiliary clock, 
       FIG. 2  shows in a block diagram a digital clock filter circuit making use of the circuit of  FIG. 1 , 
       FIG. 3  shows an alternative circuit for generating a fixed auxiliary clock, 
       FIG. 4  shows a controlled delay line for use in the circuit of  FIG. 2 , and 
       FIGS. 5   a  and  5   b  show in two diagrams the principle of generating a clock signal by cyclically switching between delayed copies of an auxiliary clock. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Clock filter circuits are used on the I/O boards of a network element for a transport network. It serves to filter the data clock of payload channels contained in the transport signals to be sent. In the preferred embodiment, each I/O board receives 16 payload channels from the switch matrix of the network element. These channels are then de-mapped from an internal transport frame. Due to the removal of the stuff bits, the de-mapped payload channels are provided at a gapped clock. The clock filter circuit smoothes these gaps and generates a transmit line signal with low intrinsic jitter to meet the requirements defined in the applicable standards. Each payload channel has its own individual clock signal. 
   On the I/O board, each payload channel coming from the matrix is (after de-mapping from the internal frame) written to a buffer memory at its gapped payload bit clock and read back from the buffer using a filtered read clock. The clock filter circuit that generates the filtered read clock is what the present invention relates to. The recovered payload signals can then be multiplexed to form an outgoing line signal or can be outputted as individual tributaries, depending on what purpose the network element serves for. 
   According to the invention, each clock filter circuit contains a digitally controlled oscillator (DCO). In the preferred embodiment, the DCO is designed as a configurable reference frequency generator for either STM-16 or OTU1 bit rates and generates a clock signal at a fourth of the target frequency, i.e., 622 MHz for STM-16 or 666.5 MHz for OTU1. The clock output shall comply to the jitter requirements of the target bit rates, i.e.,
         &lt;800 ps broadband jitter for SDH   &lt;450 ps broadband jitter for OTH   &lt;160 ps high frequency band jitter for SDH   &lt;150 ps high frequency band jitter for OTH,
 
in the frequency bands between 5 kHz and 20 kHz for broadband and 1 MHz to 20 MHz for the high frequency band.
       

   The mean frequency between the SDH and the OTU1 rate is 644.297.143 Hz. This frequency is 29/28 times the SDH rate and 29/30 times the OTU1 rate. The SDH rate could be obtained by “stealing” every 29 th  cycle from this mean frequency to generate the number of 622.080.000 cycles per second, which is one fourth of the STM-16 rate. Conversely, the OTU1 rate could be obtained by adding one cycle every 29 cycles to generate the number of 666.514.285.7 cycles per second, which is one fourth of the OTU1 rate. 
   A circuit that generates these rates out of the mean rate is shown in  FIG. 1 . It uses a crystal oscillator  11  as auxiliary clock source to generate the auxiliary clock signal  17  at the mean rate of 644.297.143 Hz. A basic idea of the invention is to switch in a cyclic order between delayed copies of this auxiliary clock. The phase shifts are preferably small enough to meet the jitter requirements. In  FIG. 1 , a delay line  12  provides 28 phase shifted copies of the auxiliary clock  17 . The phase shift between two neighbored clock copies is therefore 55.4 ns. This ensures that even the high frequency jitter is far below the limit when switching between neighbored clock copies. 
   A multiplexer  13  switches between the delayed copies of the auxiliary clock in a cyclical order. The auxiliary clock signal is also provided to a counter  16  and a gate circuit  15 . The counter  16  counts from 1 up to 29 and provides a control signal every 29 th  clock cycle, which causes the gate circuit  15  (i.e., an XOR circuit) to inhibit one clock pulse from the auxiliary clock signal  17 . The output of the gate circuit  15  is fed to a second counter  14 , which generates a 5 bit control signal for the multiplexer  13 . This control signal designates the clock copy to which the multiplexer  13  has to switch over. The second counter  14  can be configured to count either up or down from 1 to 28 or from 28 to 1, respectively. 
   The operation of the clock circuit  10  is as follows: For the SDH clock of 622 MHz, the counter  14  counts downwards and multiplexer  13  switches to the more delayed clock copy with every cycle of the auxiliary clock for 28 consecutive cycles and as a 29 th  step, to keep the selected clock copy for two cycle. By shifting 28 times, one clock cycle in the output clock  18  is lost as compared to the auxiliary clock. In other words, the multiplexer  13  outputs only 27 cycles while the auxiliary clock generates 28 cycles and in the 29 th  cycle, the selected clock is kept for one further cycle, which produces the 28 th  cycle in the output clock  18 . 
   The last step of keeping the selected copy for two rather than for only one clock cycle provides the potential for further reduction of the output clock  18  and hence for adapting the output clock  18  to changed signal condition by controlling the 28 th  and 29 th  steps as will be explained below. If the selected clock copy is kept also in the 28 th  step, the phase of the output clock  18  decreases by 55.4 ps and if the in the 29 th  step the next copy is selected rather than keeping the selected copy for a second cycle, the output clock phase increases by 55.4 ps. This allows a modification of the output clock  18  in the range of ±1189 ppm, which is by far enough for the required ±20 ppm. 
   For generating the OTU1 rate, counter  14  counts upwards and multiplexer  13  switches to the less delayed clock in order to gain phase. After shifting 28 times, the output clock  18  has gained one complete clock cycle, i.e. outputs 29 cycles while the auxiliary clock  17  generates 28 cycles, only. In the 29th step, the selected clock copy is kept, which provides a further cycle that adds to the 29 cycles. Thus, the output clock  18  has 30 cycles while the auxiliary clock  17  generates 29 cycles, only. 
   The clock circuit shown in  FIG. 1  is not jet controllable. In order to make it controllable the gate circuit must be made controllable for the steps  28  and  29 . A controllable clock filter circuit  20  is shown in  FIG. 2 . Same or equal components are designated by reference numerals increased by 10 as compared to  FIG. 1 . 
   To allow identifying steps  28  and  29 , counter  26  is connected to the gate circuit  25  by a 2 bit output. These two bits indicate the 28 th  and 29 th  step, respectively. Moreover, the gate circuit  25  has a control input  29 . Control input  29  will be connected to a comparator (not shown) on the I/O board, which compares the output clock signal  28  with the data signal to be recovered. If the output clock signal  28  is ahead of the data signal, the control signal  29  indicates to delay the output clock  28 . Conversely, if the output clock signal  28  lags behind the data signal, the control signal  29  indicates to advance the output clock  28 . The frequency control works in a similar way for the OTU1 frequency reference as for the STM-16 frequency reference explained before. 
   The comparator can be implemented with simple counters that count the gapped bit clock of the data signal to be recovered and the output clock  28 , increments the counts over a predefined time interval, and compares these counts. Preferably, the output clock is adjusted in synchronism with the frame clock of the internal frames or an integer multiple thereof. This will reduce waiting time jitter caused by the removed stuff bits. 
   As an alternative, control input  29  can be connected to a monitor that detects any destuff operations (i.e., gaps) in the payload channel and determines therefrom the control signal that advances or delay switching in multiplexer  23 . 
   The circuits shown in  FIGS. 1 and 2  need a crystal oscillator as auxiliary clock source. It should be clear that several clock filter circuits can be supplied by a single crystal oscillator. However, in certain applications, a reference frequency might be already available from another source. This is the case for instance in an SDH network element, where the network element is supplied with an SDH frequency, anyway. In this case, it might be advantageous to derive the auxiliary clock required by the invention from this already available SDH frequency. An alternative auxiliary clock source  31  that derives the auxiliary clock signal from an SDH frequency and that therefore does not require a free-running crystal oscillator is shown in  FIG. 3 . 
   The circuit  31  shown in  FIG. 3  is suitable to generate the auxiliary clock  37  out of an SDH frequency clock signal  37 ′ that is readily available on the I/O board. A delay line  32  generates 28 delayed copies of the SDH reference clock signal  37 ′. A counter  34  counts the clock pulses of clock signal  37 ′ and delivers the count values as a 5 bit control signal to a multiplexer  33 , which switches in a cyclic order between these copies. Similar to circuit  10  shown in  FIG. 1 , circuit  31  generates an output clock signal  37  having 29 clock cycles while input clock signal  37 ′ had only 28 cycles. The output signal can thus be used as the auxiliary clock signal  17  or  27  in  FIG. 1  or  2 , respectively. However, this alternative method of generating the auxiliary clock adds some more intrinsic jitter to the output of the DCO. 
   Similar as in  FIG. 3 , the circuit shown in  FIG. 1  can be used to generate a free-running OTU1 clock. It is therefore possible to design a STM-12/OTU1 I/O board without a single crystal oscillator. 
     FIG. 4  shows an implementation of the delay lines  12 ,  22 , and  32  used in  FIGS. 1 ,  2 , and  3 , respectively. The delay line  42  is implemented by a series of buffers  44 , where each buffer adds more delay to the output of the preceding buffer. The delay of the individual buffers is controlled in such a way to drive the overall delay of the delay line  42  to equal one clock cycle. 
   This is achieved by phase comparator  45 , which compares the phase of the input signal  47  with the phase of the most delayed output signal  48  of the delay line  42  and adjusts the delay values of the buffers accordingly. A digital low pass filter  46  is provided to integrate any variations over time so that no short term phase hits may occur. The delay control is advantageous to compensate for the process specific parameter outcome and for temperature and supply voltage variations. 
   As an alternative, the delay line can be implemented by loaded delay lines, i.e. by a series of adjustable LC elements. For example, the delay line can be implemented using adjustable varactor diodes. 
   Another improvement of the invention concerns the multiplexers  13 ,  23 , and  33  in  FIGS. 1 ,  2 , and  3 , respectively. In order to avoid that the multiplexer switches the clock copies in the vicinity of a clock slope, individual delay elements can be provided for the particular switches within the multiplexer. The delay buffers for the multiplexer control can be controlled by the same control signal that controls the phase shift delay line in  FIG. 4 . 
   Moreover, the multiplexer control input from counter  14 ,  24 , or  34 , respectively, can be 28 bits parallel instead of 5 bits encoding in order to avoid any delay caused by the decoding of the control signal. 
     FIGS. 5   a  and  5   b  show by way of example the switching principle of the invention:  FIG. 5   a  shows an auxiliary clock signal  57  and four delayed copies  57   a - 57   d , which are delayed in steps of 90° (i.e., π/2 or a fourth of a clock cycle). By switching in a cyclic order from signal  57  to signal  57   a , to signal  57   b , to signal  57   c , to signal  57   d , back to signal  57   a  and so forth, output clock signal  58  is obtained. As can be seen from the figure, the output clock  58  has four clock cycles while the auxiliary clock  57  has five, i.e., the output clock rate is ⅘ from the auxiliary clock rate. 
     FIG. 5   b  shows the opposite case, where the output clock signal  58  is obtained by switching in a reverse order from signal  57   d  to signal  57   c , to signal  57   b , to signal  57   a , to signal  57 , back to signal  57   c  and so forth. In this case, switching is from the more delayed copies to the less delayed copies of the auxiliary clock signal  57 . As can be seen from  FIG. 5   b , the output clock signal  58  has now 5 clock cycles while the auxiliary clock  57  had only four. The output clock rate is hence 5/4 from the auxiliary clock rate. 
   The clock filter circuit described above is not only useful for network elements employing synchronous switch matrices but also for other systems using cell based or packet based switch matrices, since such matrices inherently produce delay variations, which need to be smoothed by low bandwidth filtering.