Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/781,270 filed Feb. 17, 2004 entitled “LOW WANDER TIMING GENERATION AND RECOVERY” by Benedict A. Itri, now U.S. Pat. No. 7,415,092 issued on Aug. 19, 2008, which claims the benefit of U.S. Provisional Patent Application No. 60/530,055 filed Dec. 15, 2003, entitled, “LOW WANDER TIMING GENERATION AND RECOVERY,” by Benedict A. Itri, and which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates to communications and more particularly to circuits and methods for providing low wander timing generation and recovery. 
     2. Description of Related Art 
     Synchronizing modulation and demodulation frequencies is made difficult by the physical separation of communication devices where each device is conceivably driven by its own local clock. Take for example, current asymmetrical digital subscriber line (ADSL) systems. Current ADSL systems operate according to Discrete Multitone (DMT) frequency multiplexing where generally the central office (CO) modem generates this master clock signal. All corresponding client modems must recover the master clock signal from downstream data for processing such as sampling, demodulation, and transmission of upstream data. While modern clocks can be manufactured with considerable accuracy, difficulties in locking remote clocks and minute differences in manufacture and calibration has to date prevented the manufacture of a high-speed modem of the highest possible performance. 
     Prior Art  FIG. 1  is a block diagram of a communications system  10  including a transmitter  20 , a receiver  22 , a network  24  bi-directionally coupling the transmitter  20  and the receiver  22 , and a network link clock  26 . The communications system  10  illustrates one possible solution to the synchronization problem described above. In the communications system  10 , both the transmitter  20  and the receiver  22  attempt to synchronize on one clock, an analog front end (AFE) or network link clock  26  having a clock signal R 1 . Although theoretically sound, those skilled in the art will recognize that the network link clock is simply not a viable solution for a high speed modem as jitter and wander still degrade the clock signal preventing accurate synchronization. 
     It is good practice to separate the network link clock that governs the transmission of data between the transmitter and the receiver from the master clock that governs the data generation source. In this way the quality of transmission within the network link  24  can be designed independently of the quality of the master clock. The difficulty inherent in this separation of clock domains is that the master clock has to be faithfully reconstructed at the receiver side to provide correctly timed, synchronous output data. 
     Timing recovery is therefore required at the receiver side. In timing recovery, a receiver synchronizes a local clock with a master clock present at the transmitter via phase information contained directly or indirectly in the transmitted data stream. The data modulation and demodulation process that carries the data information over the link  24  may also necessitate the two sides to use a common network link clock if the link  24  is a synchronous communication link. Receicer synchronization to the network link clock is well understood to the skilled in the art and is not going to be described here. 
       FIG. 2  is a block diagram of a communications system  50  implementing timing recovery of the master clock according to prior art. As will be described below, the communications system  50  recovers timing directly from the received data. The communication system  50  includes a master clock  58 , a transmitter  60 , a receiver  62 , a network  64  bi-directionally coupling the transmitter  60  and the receiver  62 , a First In First Out (FIFO) buffer  66 , a control circuit  68 , and a receiver clock  70 . The receiver clock  70  is selected having a rate R 2  known to be substantially equivalent to but slightly greater than R 1 , for reasons explained below. 
     The transmitter  60  transmits data via the network  64  at a rate R 1  provided by the master clock  58  resident at the transmitter, or transported through the transmitter sourced by other network master transmitters. Data received at the receiver  62  is provided to and synchronized out through the FIFO buffer  66  at a recovered rate R 1 ′, which is an estimate of the transmitter rate R 1 . The rate R 1 ′ is obtained by the communications system  10  as follows. The FIFO buffer  66 , synchronizing the data through the buffer clocked by the signal R 1 ′, generates a signal indicative of the available buffer capacity. Available buffer capacity is indicative of the phase error between R 1  and R 1 ′ in that the FIFO buffer  66  filling up indicates that R 1 ′ is slower than R 1 , and vice versa. The control circuit  68  operates on its inputs R 2  and the FIFO buffer  66  error signal to generate the recovered signal R 1 ′. 
     Unfortunately, the communications system  50  of  FIG. 2  suffers in that a wander error will only be corrected once sufficient information is extracted from the data, which means the receiver clock can wander quite sometime before being corrected. Jitter may simply not be corrected. 
     What is needed is a timing generation and recovery scheme with sufficient precision to support a high-speed modem communication system. 
     SUMMARY OF THE INVENTION 
     The present invention teaches a variety of timing generation and recovery schemes for providing high precision clock synchronization in cascaded communications systems where each point of communication has a unique clock. To accomplish the high precision, one embodiment of the present invention teaches quantizing information related to phase relation between a master clock at the transmitter and a network link clock. This quantized phase information can be transmitted with very little bandwidth, recovered and the receiver and used to recover the timing information with high precision. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, features and characteristics of the present invention will become more apparent to those skilled in the art from a study of the following detailed description in conjunction with the appended claims and drawings, all of which form a part of this specification. In the drawings: 
         FIG. 1  illustrates a communication system of the prior art; 
         FIG. 2  illustrates another communication system of the prior art; 
         FIG. 3  is a flow chart of a timing generation and recovery method in accordance with one embodiment of the present invention; 
         FIG. 4  is a block diagram of a communication system in accordance with one embodiment of the present invention; 
         FIG. 5  is a block diagram of a timing generation circuit in accordance with another embodiment of the present invention; 
         FIG. 6  is a block diagram of another timing generation circuit in accordance yet another embodiment of the present invention; 
         FIG. 7  is a block diagram of a timing recovery circuit according to one aspect of the present invention; and 
         FIG. 8  is a block diagram of a higher precision timing generation circuit in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention teaches a variety of timing generation and recovery schemes for providing high precision clock synchronization in cascaded communications systems where each point of communication has a unique clock. To accomplish the high precision, one embodiment of the present invention teaches quantizing information related to phase relation between a master clock at the transmitter and a network link clock. This quantized phase information can be transmitted with very little bandwidth, recovered at the receiver and used to recover the timing information with high precision. 
     A first embodiment of the present invention will now be described with reference to  FIG. 3  and  FIG. 4 .  FIG. 3  is a flow chart of a timing generation and recovery method  80  in accordance with one embodiment of the present invention.  FIG. 4  illustrates a block diagram of a communications system  100  in accordance with another embodiment of the present invention. The communications system  100  includes a network link clock  102  generating a network link clock signal RN, a transmitter  104 , a receiver  106 , a network  108  coupling the transmitter  104  and the receiver  106 , and a master clock  110  generating a master clock signal RT. The network  108  may be a cable system, or any other suitable network. 
     In  FIG. 3 , the method  80  begins at a master modem such as that present at a central office (CO) etc. where data, a master clock signal RT and a network link clock signal RN are provided to a transmitter. In a step  82 , the transmitter  104  calculates a phase relation as a function f(RT, RN) between the master clock signal RT and the network link clock signal RN. The phase relation provides condensed information regarding the phase error between the two clock signals RT and RN. In preferred embodiments the function f(RT, RN) quantizes the phase information. 
     In a step  84 , the transmitter  104  sends downstream data at a rate specified by the master clock signal RT, as well as transmitting the quantized phase signal. In certain embodiments, the quantized phase signal is transmitted via an overhead channel and takes minimal bandwidth relative to the data. In a step  86 , a receiver  106  receives the downstream data together with the phase signal, as well as the network link clock signal RN. In a step  88 , the receiver  106  recovers an estimate RT′ of the master clock signal RT from the network link clock signal RN and the received phase signal. 
     As will be appreciated, the embodiment described above with reference to  FIGS. 3 and 4  presents a scheme at a relatively high level of abstraction. To further explain the present invention, the next several FIGS. provide some specific examples that are well suited for use in a wireline or wireless modem system. 
       FIG. 5  illustrates a block diagram of one suitable circuit for implementing a timing generation circuit  140  suitable within the transceiver  104  described above with reference to  FIGS. 3-4 . As will be appreciated, the timing generation circuit  140  can be useful in a variety of applications. 
     In communication system of  FIG. 5 , the network link clock signal RN and the master clock signal RT are well defined with respect to each other. The transmitter  104  includes a variable divider circuit  150 , a variable divider circuit  152 , a detector circuit  154 , a quantizer circuit  156 , and a modulus control circuit  158 . The variable divider  150  and the variable divider  152  are each controllable to divide the frequency of their input signal by an integer adjustable by arbitrary integer offsets +/−N and +/−M, respectively. This division process enables each divider to generate a reference signal with a common nominal rate. Those dividers are necessary whenever the two clock frequencies RN and RT are not equal but are rationally related. Additionally, the phase difference between the two signals controls the frequency dividers  150  and  152  via the modulus control block  158 . The detector  154  receives and measures a phase relation between the two reference signals RN and RT. The measured phase relation is fed into the quantizer circuit  156  that in turn generates the output signal f(RT, RN). The quantizer circuit  156  also provides an output signal for driving the modulus control circuit  158 . The modulus control circuit  158  provides output signals controlling the quantities +/−N, +/−M variation for the divider circuits  150 ,  152 , respectively. 
       FIG. 6  shows a specific embodiment of a timing generation circuit  190 . The timing generation circuit  190  is for a communications system using a master clock signal RT having a frequency rate of 44.736 MHz and a network link clock signal RN having a frequency rate of 35.328 MHz. As will be appreciated, these are arbitrary but familiar and common frequencies for circuitry. For example, 44.736 MHz is the transmission rate of DS3 systems used in telephony as part of the Synchronous Digital Hierarchy (SDH) system. The timing generation circuit  190  includes two variable modulus counters  200  and  202  used as variable dividers, two divider circuits  204  and  206 , two D-flip-flops  208  and  210  operating as the detector, a register  212 , and a divider circuit  214 . In this specific embodiment, the variable divider offsets +/−M and +/−N are both equal to +/−1. 
     Operation of the timing generation circuit  190  is as follows. The master clock signal RT is divided by the nominal value  233  at the variable modulus counter  200  to generate a 192 kHz reference. Similarly, the network link clock signal RN is divided by the nominal value  184  at the variable modulus counter  202  to generate a 192 kHz reference. Both reference signals are further divided by 24 to a nominal rate of 8 kHz. Both dividers are able to change their nominal dividing value by +/−1. 
     The D-flip-flops are used to measure the phase relationship between the master clock signal RT and the network link clock signal, brought down to a nominal rate of 8 kHz. If the D-flip-flop output is a “1” then “phase” is deleted by varying the modulus of the counter  202  to  183  and the modulus of the counter  200  to  232  simultaneously for one detector reference clock at 8 kHz. Changing the phase simultaneously results in a phase change relative to the master clock RT of:
 
((1/184)−(1/133))*233=0.261
 
unit intervals (UI) of the RT clock. This phase change of approximately 0.25 UI is four times better than if one simply changed only the master clock signal RT modulus. By performing this phase adjustment every 8 kHz, the maximum parts per million (PPM) that can be tracked is:
 
2*(0.261*8000*1,000)/(44.736*1,000,000)+/−46.6738 PPM.
 
     The phase comparisons are made every 8 kHz. The quantized phase relation is transmitted through an overhead channel every 4 kHz (once per frame) and requires a minimum of 2-bits per frame with no redundancy. 
     Having explained the operation of the transmitter according to the teachings of the invention, we now proceed to explain the operation of the receiver. 
       FIG. 7  illustrates a block diagram of one specific embodiment of a phase locked looped timing recovery circuit  240 . The timing recovery circuit is well suited for use in a communications system using a master clock signal RT having a frequency rate of 44.736 MHz and a network link clock signal RN having a frequency rate of 35.328 MHz. As will be appreciated, both the timing generation circuit  190  and the timing recovery circuit  240  are required in the present invention. 
     The timing recovery circuit  240  includes a variable modulus counter  250 , a detector circuit  252 , a digital loop filter  254 , a digital-to-analog converter (DAC)  256 , a voltage controlled oscillator  258 , a variable modulus counter  260 , and a modulus control circuit  262 . The network link clock signal RN is divided by 184+/−1 at the counter  250  to generate a 192 kHz reference. The estimate of RT, RR, is divided by 233+/−1 at the counter  260 , to generate a 192 kHz reference. The phase relation is recovered by the receiver modem using the overhead channel information via circuitry not illustrated and provided to the modulus control circuit  262 . The modulus control circuit  262  controls both variable modulus counters  250  and  262  according to the phase difference provided via the overhead channel. The detector  252  measures the phase relationship between RR and RN. The digital loop filter  254  is a lowpass filter, and the oscillator  258  generates RR according to a voltage provided by the DAC  256 . 
     The circuit of  FIG. 7  recovers the network link clock RN through the following process: After an initial acquisition period, the timing loop will reach a steady state where the output clock frequency RR matches the transmitter frequency RT. Then the modulus circuitry comprising of  250 ,  252 ,  260  and  262  will re-create the phase variations of RN around the reconstructed clock RT by way of repeating the process followed at the transmitter ( FIG. 5 ). 
     Next we present a further enhancement of the current invention that allows even finer phase granularity in the clock tracking system. 
       FIG. 8  illustrates a block diagram of a timing generation circuit  300  capable of providing finer precision than the timing generation circuit  190  of  FIG. 6 . The timing generation circuit  300  includes a variable modulus counter  302 , a detector  304  (a D-flip-flop), a phase accumulator circuit  306 , a FIFO  308 , a delta-sigma modulator circuit  310 , a variable modulus counter  312 , a divider circuit  314 , a gate  316 , and a modulus control circuit  318 . The variable modulus counter  302  and the variable modulus counter  312  are each operable to vary by +/−N and +/−M from their nominal divide value respectively for each reference period. By properly selecting the integers M and N, the phase relationship between the references can be adjusted with fine precision. 
     A D-flip-flop acting as the detector  304  continually measures the phase between the master clock and the network link clock references. Any positive phase output, i.e. logic “1” from the detector  304 , results in “phase” being deleted for the next reference period by changing the network link clock modulus by −M and the master clock modulus by −N. Any negative phase output, i.e. logic “0” from the detector  304 , results in “phase” being added to the next reference period by changing the network link clock modulus by +M and the master clock modulus by +N. 
     Each time a phase adjustment is made the amount of phase that is added or deleted relative to the network link clock can be calculated as Phase Adjustment=(233*M−184*N)/233=Phase Resolution/233. 
     Thus when the sum of the phase accumulator  306  register  320  reaches a count of +/−233, then a single network link clock is added or deleted. The inverted output of the detector  304  is used to multiply the phase resolution value since for positive detector outputs phase is deleted. The logic “0” output is arithmetically interpreted as −1. 
     The output of the phase accumulator  306  is examined at the frame rate 4 kHz or once per frame. Prior to transmission, the phase accumulator  306  is processed by the first order delta-sigma modulator  310 . It will be appreciated that higher order modulator schemes may be used. The modulator  310  helps reduce low frequency wander by pushing the low frequency wander components into the higher frequency bands, which could then be filtered by the receiving clock tracking circuitry. The modulator  310  operates at the frame rate 4 kHz. When the 1-bit output is high, a value  233  is added to the phase accumulator register  320  for a single reference period. This occurs since the logic ‘1’ output indicates that a single network link clock has been deleted. The average value of the 1 bit modulator  310  output represents the amount of phase added or deleted over a 4 kHz frame. 
     The block diagram of  FIG. 8  explains the required operations on the transmitter side in this enhanced embodiment of the invention. The operations on the receiver side are similar to the ones explained before based on the block diagram of  FIG. 7 . The difference in this embodiment is that the RN divider  260  is fixed to its nominal value  184 , while only the divider  260  is allowed to vary around its nominal value of 233 by +/−1. 
     Allowing for the addition/deletion of a single network link clock every 4 kHz results in the ability of handling phase precision of +/−80 PPM. Recommended values are M=4, N=5, and phase resolution=12. 
     In addition to the above mentioned examples, various other modifications and alterations of the invention may be made without departing from the invention. Accordingly, the above disclosure is not to be considered as limiting and the appended claims are to be interpreted as encompassing the true spirit and the entire scope of the invention.

Technology Category: 5