Abstract:
Method and circuitry for improving the accuracy and efficiency of a phase-locked loop. More specifically, the present invention relates to a method and device for monitoring the frequency discrepancy between two signals in conjunction with at least one data signal so as to improve the accuracy and efficiency of a phase-locked loop. In one embodiment of the present invention, two counters are used to check the frequency differential between a VCO signal and an external reference or input signal. An adjustable threshold is provided to determine whether the frequencies of the two signals are considered to be in a frequency-locked mode. A pair of flip-flops is used to minimize any erroneous detection of frequency discrepancy by validating two consecutive results of the frequency differential check. In addition, a data present signal is used to control the transition between the phase-locked mode and the frequency-locked mode to minimize the potential data loss.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   This application is a continuation of U.S. patent application Ser. No. 10/843,181, filed May 11, 2004 now U.S. Pat. No. 6,909,762, which is a continuation of U.S. application Ser. No. 09/632,665 now U.S. Pat. No. 6,760,394 filed Aug. 7, 2000 issued on Jul. 6, 2004, which claims the benefit of priority under 35 U.S.C. § 119 from U.S. Provisional Patent Application Ser. No. 60/148,415 filed on Aug. 11, 1999, the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to improving the accuracy and efficiency of a phase-locked loop. More specifically, the present invention relates to a method and device for monitoring the frequency discrepancy between two signals in conjunction with at least one data signal so as to improve the accuracy and efficiency of a phase-locked loop. 
   A phase-locked loop (PLL) is a circuit that is capable of synchronizing an output signal generated by an oscillator with a reference or input signal in frequency as well as in phase.  FIG. 1  shows a simplified block diagram of the functional elements of a conventional PLL. A conventional PLL generally includes a voltage-controlled oscillator (VCO)  10 , a phase detector  12 , and a loop filter  14 . A PLL uses feedback to maintain an output signal in a specific phase with a reference signal. The VCO  10  generally oscillates at an angular frequency which is determined by the output signal  20  of the loop filter  14  which, in turn, is controlled by the output signal  18  of the phase detector  12 . In turn, the output  22  of the VCO  10  and the external reference or input signal  16  dictate the output signal  18  of the phase detector  12 . Hence, if the phase error between the VCO output  22  and the external reference or input signal  16  is not zero or within a tolerable margin, the phase detector  12  will develop a nonzero output  18 , thereby via the loop filter  14  causing the VCO  10  to produce an output signal  22  that is synchronized or locked with the external reference or input signal  16  and reducing the phase error to an acceptable level. 
   The process of achieving a lock between the VCO output  22  and the external reference or input signal  16  involves two steps. First, the frequencies of the two signals  16 ,  22  have to be matched. When the two frequencies are matched, the two signals  16 ,  22  are sometimes referred to as being in a frequency-locked mode. Once the frequency-locked mode is achieved, the phases of the two signals  16 ,  22  are then matched thereby achieving a phase-locked mode. In other words, the frequency-locked mode is a prerequisite to achieving the phase-locked mode. Once the phase-locked mode is achieved, the PLL can then perform its intended functions. 
   PLLs are used in many applications including frequency synthesis, modulation, demodulation, and data and clock recovery. For example, in digital communications, it is frequently necessary to extract a coherent clock signal from an input data stream. A PLL is often used for this task by locking a VCO output to the input data stream. Once locked, the VCO output is essentially the clock signal of the input data stream and thus can then be used to extract the data bits from the input data stream. 
   Quite often, however, two signals for a variety of reasons may disengage from the phase-locked mode. This can happen when the two signals are no longer in frequency-locked mode. For example, when a data signal becomes jittery or disappears entirely, the frequencies of the data signal and the VCO signal can no longer match, thereby causing the two signals to disengage from the frequency-locked mode and subsequently from the phase-locked mode. Therefore, it would be desirable to provide a method and device that is capable of reliably detecting whether two signals are in frequency-locked mode thereby ensuring that the phase-locked mode is maintained. 
   In addition, different systems often require different degrees of precision  25  to achieve a frequency-locked mode depending on the purposes of the systems. Some systems may require two signals to be closely matched before a frequency-locked mode is considered achieved, while others may permit a wider margin of matching. Therefore, it would be desirable to provide a method and device that is capable of having an adjustable threshold for determining whether a frequency-locked mode is achieved. 
   Further, as mentioned above, before a PLL can perform its intended functions, it must be engaged in a phase-locked mode first which, in turn, requires as a prerequisite a frequency-locked mode to be achieved. Conversely, a phase-locked mode is disengaged when the prerequisite frequency-locked mode is no longer present. Any unnecessary or mistaken disengagement of the frequency-locked mode thus disrupts the phase-locked mode and consequently prevents the PLL from performing its intended functions. Therefore, it would be desirable to provide a method and device that is capable of efficiently monitoring the activation of the frequency-locked mode so as to optimize the continued operation of a PLL. 
   Moreover, very often when a PLL is engaged in a phase-locked mode and no data is available for detection, the VCO signal tends to drift and eventually will no longer be considered to be in frequency-locked mode with the external reference or input signal. During this period when the frequency-locked mode is lost, a PLL is not capable of detecting incoming data and such data are thus lost. When this occurs, the phase-locked mode has to be disengaged so as to allow the frequency-locked mode to be re established so that as soon as data is available, the PLL can switch to the phase-locked mode to capture the data. Therefore, it would be desirable to provide a method and device that is capable of efficiently monitoring and controlling the transition between the phase-locked mode and the frequency-locked mode so as to minimize data loss. The present invention satisfies the above as well as other needs. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to efficiently control the transition between the phase-locked mode and the frequency-locked mode during the operation of a PLL. In one embodiment of the present invention, two counters are used to check the frequency differential between a VCO signal and an external reference or input signal. The external reference or input signal and the VCO signal are used to drive the two counters respectively. Both counters conduct count-downs in a cyclic manner. When the first counter arrives at an identifiable position in its count-down, the second counter is directed to begin its count-down from a predetermined position. When the first counter once again reaches the same identifiable position, the output of the second counter is checked to determine the differential between such output and the identifiable position of the first counter. Such differential can be selectively interpreted to provide an adjustable threshold to determine whether the frequencies of the two signals are considered to be in a frequency-locked mode. 
   The result of the frequency differential check is propagated through a pair of flip-flops. The pair of flip-flops are connected in series. Hence, the pair of flip-flops records the results of any two consecutive frequency differential checks. 
   The outputs of the two flip-flops are logically combined to a logic element which produces a low signal when both outputs of the flip-flops are high. In one exemplary implementation, a NAND logic function is provided to accept the output signals of the pair of flip-flops. The NAND logic function produces a low signal only when the results of both frequency differential checks are high, meaning that remedial action should be taken to rectify the frequency-locked mode. This provides the advantage that erroneous detection of a frequency discrepancy is minimized. This also provides the advantage that the PLL is given additional time to pull in the locked frequency when the PLL switches from the frequency-locked mode to the phase-locked mode. 
   In addition, a data present signal is logically combined with the output of the NAND logic function. An AND logic function, for example, produces a high signal only when both the data present signal and the output of the NAND logic function are high, meaning that data are present for detection and there is no frequency discrepancy. This, in turn, signifies that the phase-locked mode should be maintained. This minimizes unnecessary transition between the phase-locked mode and the frequency-locked mode thereby reducing the likelihood of data loss. 
   Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. 
   Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to accompanying drawings, like reference numbers indicate identical or functionally similar elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a simplified block diagram of the functional elements of a conventional PLL; 
       FIG. 2  shows a simplified functional block diagram of one embodiment of the present invention; 
       FIG. 3  is a simplified functional block diagram showing the functional components of one embodiment of the present invention; 
       FIG. 4  is a simplified schematic block diagram showing one embodiment of the present invention; 
       FIG. 5  is a logic circuit diagram illustrating one particular component of one embodiment of the present invention; and 
       FIG. 6  shows a simplified block diagram of the functional elements of a PLL in an exemplary embodiment according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will now be described.  FIG. 2  shows a simplified 10 functional block diagram of one embodiment of the present invention. The function of the lock detect element  24  is to produce a control signal  26  to indicate whether the frequency-locked mode or the phase-locked mode should be engaged. 
   The lock detect element  24  accepts as inputs an external reference clock  15  signal (Vref)  28  and a VCO signal  30 . In circuit applications, such as in high frequency fiber optic communication networks, the VCO signal  30  may be in the gigahertz range. In such applications, it is preferable to divide down the frequency of the VCO signal  30  by a certain predetermined factor, for example, 32 or 16, so as to make frequency matching with Vref  28  more manageable. The frequencies of Vref  28  and the VCO signal  30  are designated respectively as fref and fvco. 
   The lock detect element  24  then compares fref and fvco to determine if the frequency differential between them is within an adjustable predetermined threshold. Such adjustable predetermined threshold is dependent on the requirements of the system which uses the PLL. If the frequency differential is within the adjustable predetermined threshold, then the phase-locked mode is maintained; otherwise, certain mechanism may be triggered to engage the Vref  28  and the VCO signal  30  into the frequency-locked mode. Details with respect to detecting the difference between fref and fvco will be more fully described below. 
     FIG. 3  is a simplified functional block diagram showing the functional components of the lock detect element  24  in accordance with the present invention. The lock detect element  24  generally performs four basic functions  34 ,  36 ,  38  and  40 , namely, checking the frequency differential of the Vref  28  and the VCO signal  30 , validating the results of the frequency differential check, confirming whether data is present, and generating the appropriate control signal  26  to control the transition between the phase-locked mode and the frequency-locked mode. 
     FIG. 4  is a simplified schematic block diagram showing one exemplary embodiment of the present invention. This embodiment includes two digital counters  42 ,  44 , a step detector  46 , a bits checker  48 , two flip-flops  50 ,  52 , a NAND logic function  54 , and an AND logic function  56 . 
   A first counter  42  feeds the most significant bit (MSB)  58  of its output to a step detector  46 . The output  60  of the step detector  46  is connected to the second counter  44 . Selected output bits  62  of the second counter  44  are then fed into a bits checker  48 . 
   As will be explained below, the use of these selected output bits  62  provides the ability to control the amount of adjustable threshold to be used to determine whether Vref  28  and the VCO signal  30  are in a frequency-locked mode. The output  64  of the bits checker  48  is connected to a first flip-flop  50 . The output  66  of the first flip-flop  50  is entered into a second flip-flop  52 . The outputs  66 ,  68  of both flip-flops  50 ,  52  are accepted as inputs by a NAND logic function  54 . The result  70  of the NAND logic function  54  together with a data present signal  32  are then fed into an AND logic function  56  to produce a control signal  26 . The significance of the data present signal  32  will be more fully explained below. 
   The operation of the embodiment of the present invention as shown in  FIG. 4  will now be described. The signal Vref  28  is fed into the first counter  42  to initiate a count-down. The first counter  42  performs the count-down in a cyclic manner. In other words, the first counter  42  counts down to zero and then repeats the count-down from the largest number that it can handle. The count-down can start with an arbitrary number and the speed of the first counter  42  is controlled by the frequency of Vref  28  which is fref. 
   During the count-down, the output bits of the first counter  42  change with the clock  30  signal, i.e., Vref  28 , to generate the count. 
   As previously noted, the output MSB  58  of the first counter  42  is connected to the input of the step detector  46 . The function of the step detector  38  is to detect a rising step change in its input, i.e., the output MSB  58 . A rising step change means a change from a low (“0”) to a high (“1”). There is only one instance during the count-down when the output MSB  58  of the first counter  42  changes from a low to a high. That instance occurs when the first counter  42  reaches zero and restarts the count-down at the largest possible number. For example, for a 4-bit counter, the transition is from “0000” to “1111”. This is the point where the precise value of the first counter  42  is known when only the output MSB  58  is monitored. Monitoring other output bits of the first counter  42  individually does not allow the value of the first counter  42  to be determined precisely because these other bits can go from a low to a high in many instances during the count-down. By having the output MSB  58  of the first counter  42  connected to the input of the step detector  46 , the transition from the smallest number to the largest number during the count-down can be detected by the step detector. By being able to pinpoint this particular transition, a reliable starting point can be established to test the frequency differential between fref and fvco. Upon detecting the rising step change due to the output MSB  58  of the first counter  42 , the step detector  46  signals to a second counter  44  to initiate a count-down by the second counter  44 . 
     FIG. 5  shows a logic circuit which can be used to implement the step detector  46 . This logic circuit can be used in conjunction with the first counter  42  which performs a count-down function. In this embodiment, the step detector  46  includes a pair of flip-flops  76 ,  78 , an inverter  80 , and a NOR logic function  82 . The two flip-flops  76 ,  78  are connected in series. The input to the flip-flop  76  is the output MSB  58  of the first counter  42 . One output of the flip-flop  76  is coupled to the inverter  80 . The output of the inverter  80  and the output of the flip-flop  78  are then combined as inputs to the NOR logic function  82 . The output of the NOR logic function  82  is the output  60  of the step detector  46 . The operation of the step detector  46  as implemented by this logic circuit is straightforward. It can be easily seen that the only instance when the output of the NOR logic function  82  is high is when the output of the flip-flop  76  is high and the output of the flip-flop  78  is a low, meaning that the output MSB  58  of the first counter  42  has transitioned from a “0” to a “1”. 
   Prior to receiving the count-down initiation signal  60  from the step detector  46 , the second counter  44  is held in reset. When the count-down initiation signal  60  is received from the step detector  46 , the second counter  44  similarly begins a count down sequence beginning from a predetermined number set by the system designer. This predetermined number is related to the adjustable threshold and its significance will be explained below. The speed of the second counter  44  is driven by the VCO signal  30  whose frequency is fvco. 
   At the moment that the second counter  44  is instructed to start counting, the value of the first counter  42  is known. As mentioned above, in order to start the second counter  44 , the step detector  46  must have detected a rising step change from the output MSB  58  of the first counter  42  which, in turn, means that the first counter  42  is about to turn over its count-down sequence and begin from the largest possible number again. By starting the second counter  44  at that point, the frequency differential between fref and fvco can be determined because the precise starting points of the two counters  42 ,  44  are known. 
   After the second counter  44  initiates its count-down, the first counter  42  continues its own count-down. When the first counter  42  completes one count-down cycle (i.e., when the step detector  46  detects another rising step change from the output MSB  58  of the first counter  42 ), the output bits  62  of the second counter  44  are checked by the bits checker  48  to determine where the second counter  44  is in its count-down cycle. At that moment, the first counter  42  is known to be at its smallest possible number, i.e., “0”, so the output bits  62  of the second counter  44  are checked to see if they represent a “0” as well. The bits checker  48  can be implemented using, for example, an OR logic function to perform this check. Hence, by examining the second counter  44  at that point, it can be determined how far apart the two frequencies, fref and fvco, are. 
   It should be noted that the check performed by the bits checker  48  is affected by the initial predetermined number from which the count-down of the second counter  44  begins. The result  64  of the bits checker  48  has to be adjusted accordingly based on the difference between the initial predetermined number of the second counter  44  and the initial count-down number of the first counter  42  (which is usually “0”) when the count-down of the second counter  44  begins. 
   It should be further understood that the frequency differential check can be similarly performed using counters which count up in a cyclic manner and a step detector which detects a transition from a “1” to a “0”. In fact, any combination of logic elements can be used as long as an identifiable position can be repeatedly ascertained during count cycles. 
   The number of output bits  62  of the second counter  44  that need to be examined depends on the desired amount of threshold within which the frequency differential of fref and fvco must fall if fref and fvco are to be considered in a frequency-locked mode. If a very precise threshold is required, more output bits, or perhaps the entire output of the second counter, may need to be checked. If a more lenient threshold is permitted, then only the more significant output bits may need to be examined and the less significant output bits can be ignored. Hence, by selectively examining the number of output bits  62  of the second counter  44 , an adjustable threshold can be implemented depending on the desired precision of the frequency-locked mode. 
   The result  64  produced by the bits checker  48  is then recorded by the first flip-flop  50 . For example, if all the examined output bits  62  of the second counter  44  are “0&#39;s”, then a result of “0” is produced by the bits checker  48 . This indicates that the two frequencies, fref and fvco, are within the allowable threshold and thus in frequency-locked mode. Otherwise, a “1” is produced indicating a frequency discrepancy. On the next rising step change of the output MSB  58  of the first counter  42 , the result  66  of the first flip-flop  50  is passed onto the second flip-flop  52  and a new result  64  from the bits checker  48  is passed into the first flip-flop  50 . The outputs  66 ,  68  of the two flip-flops  50 ,  52  are fed into the NAND logic function  54 . 
   The combination of the two flip-flops  50 ,  52  and the NAND logic function  54  provides protection against erroneous detection of frequency discrepancy and thus prevent the unnecessary initiation of measures to put the two frequencies, fref and fvco, back into frequency-locked mode. For example, erroneous detection of frequency discrepancy can occur when a noise burst corrupts the result  64  produced by the bits checker  48 . 
   By connecting the two flip-flops  50 ,  52  in series, two consecutive results of the frequency differential checks performed by the bits checker  48  are maintained. The NAND logic function  54  interprets these two consecutive results  66 ,  68  and produces a signal  70  to initiate steps to rectify the frequency-locked mode only when both results  66 ,  68  indicate that there is a frequency discrepancy. For example, if one of the two consecutive results  66 ,  68  from the bits checker  48  is a “0” indicating that there is a frequency match, then the NAND logic function  54  by its nature of operation will produce a high signal, i.e., a “1”. The only time the NAND logic function  54  produces a low signal or a “0” is when both its inputs  66 ,  68  are high or “1&#39;s”. Hence, in order for the NAND logic function  54  to generate a low signal, both consecutive results  66 ,  68  from the bits checker  48  have to be high indicating a frequency discrepancy in both instances. The foregoing provides an advantage in preventing the system from engaging in any premature corrective measures when only one instance of frequency discrepancy exists. It should be further understood that to provide for increased accuracy in detecting frequency discrepancy, additional flip-flops may be used to record the history of the frequency differential check. 
   The output  70  of the NAND logic function  54  and the data present signal  32  are then fed into an AND logic function  56  to produce either a high signal indicating that the phase-locked mode should be maintained or a low signal indicating that the frequency-locked mode should be engaged. In one embodiment, the AND logic function  56  is implemented by using a NAND logic function  72  connected in series with an inverter  74 . The operation of the AND logic function  56  produces a high control signal  26  only when both inputs  32 ,  70  are also high; in all other instances, the control signal  26  is low. Hence, the AND logic function  56  produces the high control signal  26  only when both the data present signal  32  is high indicating the presence of data and the output  70  of the NAND logic function  54  is high indicating that there is no frequency discrepancy. In all other cases, such as when the data present signal  32  is low indicating that no data is available, or the output  70  of the NAND logic function  54  is low indicating that there is a frequency discrepancy, or both, the control signal  26  produced by the AND logic function  56  remains low. A low control signal  26  generated by the AND logic function  56  signifies that the frequency-locked mode should be engaged. 
   As shown in  FIG. 6 , a PLL according to an exemplary embodiment of the present invention includes a VCO  110 , a phase detector  112 , a loop filter  114  and a lock detect element  124 . The lock detect element  124  can be substantially the same as the lock detect element  24  of  FIGS. 2 and 3 . 
   The use of the data present signal  32  provides an additional measure of protection against undertaking premature actions to engage the phase-locked mode. As mentioned above, a PLL is often used to ascertain the proper clock signal frequency in order to retrieve data from an input data stream. When data is not present, the frequency locked mode should be maintained and kept ready for retrieval of data. On the other hand, when data is present and there is no frequency discrepancy, then the phase-locked mode should be maintained to continue to retrieve data. By using the data present signal  32  as one of the inputs to the AND logic function  56 , the phase-locked mode is not automatically engaged upon the determination that there is no frequency discrepancy between fref and fvco. This is significant because when the phase-locked mode is engaged with no data present, fvco will drift and eventually will no longer match fref. fref and fvco then have to be restored to the frequency-locked mode. Consequently, any data that comes in during this restoring period will be undetected and lost. In contrast to the situation where the frequency-locked mode is maintained and kept ready, when data becomes available, the phase-locked mode can be promptly engaged to capture the data. Hence, with the use of the data present signal  32 , unnecessary transition between the phase-locked mode and the frequency-locked mode is minimized thereby reducing the likelihood of losing data. 
   It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference for all purposes in their entirety.