Abstract:
A clock conditioning circuit including a phase detector circuit configured to provide an analog tuning signal indicative of a phase relationship between a reference clock to be conditioned and a generated clock. The controlled oscillator is configured to produce the generated clock, with the generated clock having an output frequency adjustable in response to an analog tuning signal applied to a control signal input of the controlled oscillator. Converter circuitry is provided to produce a digital representation of the analog tuning signal when the mode control circuitry is in a tracking mode. In the event the reference clock is lost, the mode control circuitry switches to a holdover mode so as to provide an analog holdover signal to the control signal input based upon the digital representations produced just prior to the loss of the reference clock.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to clock generating circuitry and, in particular, to circuitry for maintaining the frequency of an output clock when the input clock is lost. 
         [0003]    2. Description of Related Art 
         [0004]    For wired and wireless network applications, the recovered clock from a serial communications link is typically noisy and needs to be cleaned up by a clock conditioner or clock jitter cleaner. The outputs of a clock conditioner are used to provide low noise clock sources for other system function blocks such as analog to digital converters (ADC), digital to analog converters (DAC), serializer/deserializer devices (SERDES) and the like. In such applications, the input clock to the clock conditioner may be lost due to failures in the communication link such as a broken wire, failure of the SERDES device and the like. When the input clock is lost, it is typically required by the communication system that the clock conditioner maintain precise output frequencies for long periods of time such as several days or even longer. 
         [0005]      FIG. 1  shows a typical prior art clock conditioning circuit that is essentially a phase locked loop (PLL). A clock CLKR to be conditioned, sometimes referred as a reference clock, is fed to one input of a phase frequency detector  16 . Clock CLKR may be divided down in frequency by a divider  18 . A conditioned clock CLKO is fed to a second input of phase detector  16 , where an optional frequency divider  20  can also be used. Frequency dividers  18  and  20  allow the frequencies of the two clocks CLKR and CLKO to differ but still be maintained in phase with one another. Phase detector  16  is typically comprised of a pair of D type flip flops with one flip-flop being set by the rising edge of CLKR (or the divided version of CLKR) and the other being set by the rising edge of CLKO (or the divided version of CLKO). Once both flip-flops are set, both are reset at the same time following a small delay. Thus, if the two clocks are in phase, the rising edges will be concurrent so that simultaneous narrow pulses will be produced at the two detector outputs. If the rising edge of CLKR precedes that of CLKO, the pulse width of output UP will be greater than the narrow pulse width of output DN with the difference in duration relating to the time difference of the rising edges. Conversely, if the rising edge of CLKR follows that of CLKO, then the pulse width of output DN will be greater than that of output UP, with the difference in width again being a function of the rising edge time difference. 
         [0006]    The two outputs UP and DN are coupled to the respective inputs of a charge pump circuit  22 , with circuit  22  including a high side pump component  22 A controlled by signal UP and a low side pump component  22 B controlled by signal DN. Details of the charge pump circuit  22  are shown in  FIG. 2 . The high side component  22 A includes a current source  34 A which can be switched to a pump output  24  by way of a switch  36 A controlled by signal UP. The low side component  22 B includes a current source  34 B which can be switched to the pump output  24  by way a switch  36 B controlled by signal DN. The two current sources  36 A and  36 B are of equal magnitude, with current source  34 A sourcing current to output  24  and current source  34 B sinking current from the output. As will be described, the output  24  of the charge pump  22  is connected to a low pass filter which operates to essentially integrate the current pulses provided by the charge pump components  22 A and  22 B. 
         [0007]    Returning to  FIG. 1 , as previously noted, the output of the charge pump circuit is filtered by a low pass filter  26 . Filter  26  is typically in the form of a single capacitor connected between output  24  and the circuit common connected in parallel with a series-connected capacitor and resistor combination. Thus, the output of the filter on line  30  is directly connected to the filter input on line  24 . The filter output on line  30  is an error or tuning signal which is provided to the control input of the crystal based voltage controlled oscillator (VCXO)  28 . As is well known, a voltage controlled oscillator provides an output signal having a frequency which can be varied in response to changes in a control input (tuning signal), with instantaneous changes in frequency corresponding to a change in phase. Oscillator  28  is configured to provide a clock CLKO which it in phase with the reference clock CLKR based upon the magnitude of the input tuning signal. Note that phase noise beyond the PLL loop bandwidth present on clock CLKR is substantially cleaned from clock CLKO. 
         [0008]    As previously noted, in the event the reference clock signal CLKR is missing for some reason, many systems require that the clock conditioning circuitry maintain the frequency at the correct value for extended periods of time. One prior art approach is to force the output  24  to a high impedance.  FIG. 3  depicts an alternative prior art charge pump circuit  44  which includes an upper component  44 A and a lower component  44 B. The construction of charge pump  44  is similar to that of charge pump  22  and includes upper and lower current sources  34 A and  34 B and associated switches  36 A and  36 B. The alternative charge pump  44  further includes isolation switches  38 A and  38 B that are closed during normal charge pump operation. In the event of the loss of the reference clock CKLR, a loss of clock detector (not depicted) will generate a hold signal VHO 1  which will operate to open switches  38 A and  38 B thereby isolating the outputs of the charge pump  44  from output line  24 / 30 . The tuning voltage on line  24 , at the time of the loss of CKLR will tend to be maintained by the low pass filter which is essentially a capacitance connected between line  24  and ground. Thus, the tuning voltage to the control input to the VCXO on line  24 / 30  will be held in place so that the frequency output of VCXO  28  is maintained. However, the tuning voltage will change primarily due to leakage currents represented by element  46 . These leakage currents may be attributable to the leakage current in the charge pump output, or the VCXO input or leakage through the capacitors of low pass filter  26  and can be on the order of 1 nA or larger. Assuming that the tuning voltage is about 1.65 V (VDD/2 or 3.3V/2) and assuming that the effective capacitance between line  24 / 30  and ground is about 10 μF, then two hours after the loss of CLKR the tuning voltage will drop by 0.93 volts. Assuming that the frequency gain Kvcxo of VXCO is 100 ppm/V, the output frequency will drift 72 ppm in just two hours. 
         [0009]    In order to further reduce the change in frequency after a loss of a reference clock, another prior art approach is to first isolate the output of the charge pump circuit as previously described by opening switches  38 A and  38 B in response to signal VHO 1 . In addition, after line  24 / 30  has been isolated from the charge pump circuit  44  a fixed voltage VDD/2 is applied to line  24 / 30 . Voltage VDD/2, which typically is the nominal VCXO tuning voltage, is provided by a buffer circuit  42  when switch  40  is closed in response to a signal VHO 2  which is produced along with signal VHO 1  when a loss of the reference clock CLKR is detected. Unfortunately, this approach is not capable of holding an accurate output frequency over changes in supply voltage, ambient temperature and VCXO tuning characteristic variations. For example, lab testing has confirmed that, even when changes in the VCXO tuning characteristics are not considered, the output frequency can vary between −10 ppm and +30 ppm for a typical commercial VCXO when its tuning voltage is held at VDD/2 and the supply voltage is varied between +3.15V and +3.45V and the ambient temperature is varied between −40 C and +85 C. 
         [0010]    There is a need for a clock conditioning system which includes a phase locked loop which is capable of accurately maintaining an output clock frequency over extended periods of time after the input or reference clock has been lost. As will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings, the present invention provides this improved capability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a block diagram of a prior art clock conditioning circuit utilizing a phase locked loop. 
           [0012]      FIG. 2  is a diagram of a prior art charge pump circuit used in the  FIG. 1  clock conditioning circuit. 
           [0013]      FIG. 3  is a diagram of another prior art charge pump circuit illustrating various attempts to maintain a clock output frequency after a loss of the input clock. 
           [0014]      FIG. 4  is a diagram of a clock conditioning system which utilizes one embodiment of the present invention. 
           [0015]      FIG. 5  is a diagram of the part of the  FIG. 4  clock conditioning system, including the DAC used in the conditioner holding mode, with the DAC further acting as part of the ADC used in the conditioner tracking mode. 
           [0016]      FIG. 6  shows some of the details of the DAC of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Referring again to the drawings,  FIG. 4  shows a clock conditioning system including a PLL which utilizes one embodiment of the present invention. The system includes circuitry for two reference clocks CLKIN 1  and CLKIN 0  which are coupled to the inputs of a multiplexer  48 , with one clock acting as the backup for the other. One of the two clocks is selected by a signal originating from a Holdover Control block  50  in response to signals CLKIN_SEL 0  and CLKIN_SEL 1 . Each of the two clocks has an associated loss of signal detector  52 A and  52 B, with the outputs of the two detectors being sent to the Holdover Control block  50 . One of the two reference clocks is selected for normal operation and is connected to the input of a phase locked loop (PLL) as will be described. The selected reference clock is provided to a frequency divider  54  with the divided clock being provided to one input of a phase detector/charge pump circuit  16 / 44 , with the charge pump circuit being similar to circuit  44  of  FIG. 3 . The divided clock output is also fed to the input of a digital lock detector (DLD)  58  used for detecting the PLL lock status. The analog output on line  24 / 30  of phase detector/charge pump circuit  16 / 44  acts as a tuning voltage for VCXO  28 . Note that VCXO  28  can also, by way of further example, be implemented by a discrete, voltage-tunable crystal oscillator or by a non-crystal based VCO. 
         [0018]    Once again, a low pass filter  26  is provided which integrates the output of the charge pump circuit. The tuning voltage controls the frequency of the output clock CLKO on line  32  which is fed back to the other input of the phase detector input by way of divider circuit  60 . The feedback clock is also coupled to another input of the digital lock detector (DLD)  58 . As is well known, the tuning voltage adjusts the instantaneous frequency of clock CLKO so that when the loop is locked, the phase difference between the selected reference clock CLKIN 1 /CLKINO and the output clock CLKO is reduced to some minimum value. Further, phase noise of the reference clocks beyond the PLL loop bandwidth is also substantially reduced. 
         [0019]    As will be explained in greater detail, the clock conditioning system of  FIG. 4  has two basic operating modes. Normally, when the selected reference clock (CLKIN 1  or CLKINO) is operational, the system is placed in a tracking mode of operation. In that mode, the PLL is fully operational, including the charge pump circuit. The tuning voltage on line  24 / 30  is monitored using an ADC so as to produce a digital representation of the tuning voltage. The digital representation tracks the analog tuning voltage throughout the tracking mode. In the event the selected reference clock fails, the clock conditioning system is switched from the tracking mode to a holdover mode. More details on the switching between the tracking mode and the holdover mode are discussed later. In the holdover mode, the digital representation of the tuning voltage produced just prior to the loss of the reference clock is held. The held digital representation is used in conjunction with a DAC to produce a substitute fixed tuning signal so that the VCXO produces an output clock CLKO of the same frequency of the clock just prior to the loss of the reference clock. The output of the charge pump circuit is isolated from the DAC output charge pump circuit so that the charge pump output does not interfere with the substitute tuning signal produced by the DAC. By way of example, this isolation can be achieved by opening switches  38 A and  38 B of the  FIG. 3  charge pump circuit. 
         [0020]    When in the tracking mode, the PLL including the charge pump, is fully operational, with a signal CP_Tri produced by control block  50  for isolating the charge pump output, being in a de-asserted state. The output of a DAC  66  of  FIGS. 4 ,  5  and  6  is isolated from tuning voltage line  24 / 30  by an open switch  74 . As is known in the prior art, a DAC can be used in combination with an up/down counter and comparator to create an ADC circuit. In the tracking mode, DAC  66  forms part of an ADC circuit as shown in  FIG. 5 . A comparator  70  compares the magnitude of the tuning voltage on line  24 / 30  to the buffered output of DAC  66 . In the event the analog output of ten bit DAC  66  is less than the tuning voltage, comparator  70  switches an Up/Down counter  76  to the count up mode. The counter, which is continuously clocked by a ten bit programmable ripple counter  76 , will count up to a new value which will cause the output of DAC  66  to increase. Eventually, the increase value of the DAC  66  output will cause comparator  70  to switch states so that the up/down counter  76  will count down. Thus, the ten bit DAC  66  digital input provided by counter  76  will track the analog tuning voltage. Note that the digital signal will continuously change by one LSB even when the tuning voltage is constant. DAC  66  update rate is at a programmable clock rate equal to the N 1  divider  60  ( FIG. 4 ) output divided by a programmable value using ripple counter  78 . The speed of DAC  66  only needs to be fast enough to track the VCXO tuning voltage when the tuning voltage has settled. 
         [0021]    Multiple approaches can be used for switching the clock conditioner system from the tracking mode to the holdover mode. One approach is to switch is response to an externally generated signal “To Holdover”. For example, an FPGA or a Micro-controller can assert the “To Holdover” signal to force the PLL into the holdover mode when it determines that the input clock to the PLL has failed. A second approach is to switch when the PLL transitions from a lock status to an unlocked status as determined by DLD  58  or when detectors  52 A and  52 B indicate that the selected reference clock has failed. Signal CP_Tri is asserted so as to isolate the output of the charge pump circuit from the tuning voltage line  24 / 30 . The digital output of counter  76  is held during this mode so that the analog output remains fixed. In addition, signal Vtr is asserted by block  50  so that switch  74  connects the analog DAC  66  output to the tuning voltage line  24 / 30  so that VCXO  28  will produce a clock CLKO of the same frequency of the clock just prior to the loss of the reference clock. 
         [0022]    Similarly, multiple approaches can be used for switching the clock conditioner system from the holdover mode back to the tracking mode. One approach is to switch back in response to an externally generated signal “To Holdover”. For example, a FPGA or a Micro-controller can de-assert signal “To Holdover” to force the PLL back to the tracking mode when it determines the input clock to the PLL is valid. A second approach is to switch back when DLD  58  determines that the difference between the input clock frequency and the holdover clock frequency is small enough or the selected reference clock is valid. Should the conditions for switching the clock conditioner system from the holdover mode back to the tracking mode be met, the conditioning system will revert back to the tracking mode where normal PLL operation is resumed. 
         [0023]    Note that for many applications, a second PLL  84  following the first PLL is preferably used to multiply the VCXO CLKO frequency to a higher frequency signal. That signal can then be divided by divider circuitry  86  to generate multiple clock outputs at desired frequencies. The second PLL can be constructed by those skilled in the art of PLL design. 
         [0024]    In one exemplary implementation, the components disposed within block  56  of  FIG. 4  are implemented in a common integrated circuit. In that case, the PLL filter  26  and VCXO  28  are external to the integrated circuit. 
         [0025]    Many existing low power, high linearity DAC architectures can be used for tracking and holding the VCO tuning voltage and can be designed by those skilled in the art. In one embodiment, DAC  66  is a sub-ranging DAC as shown in  FIG. 6 . which is based on the classical resistive divider architecture. DAC  66  includes a coarse stage  60 A and a fine stage  60 B. Each stage includes an array of 32 resistors and associated switches (not designated), with the state of the switches being controlled by a digital control block  88 . 
         [0000]    The control signals for the coarse stage  60 A are derived by the control block  88  from the 5 MSBs of the 10 bit DAC control word. The control signals for the fine stage  60 V are derived by control block  88  from the 5 LSBs of the control word. The coarse stage  60 A receives a reference voltage Vref from a voltage reference source (not depicted). The coarse stage  60 A divides reference voltage Vref into 32 subranges, with the control signals derived from the 5 LSBs determining which of the 32 subranges is applied to the upper and lower terminals  90 A and  90 B of the fine stage  60 B resistive divider. The DAC output voltage is then chosen by closing the appropriate switch in the fine stage  60 B, thereby connecting the desired tap on the resistive divider to the DAC output buffer  68 . 
         [0026]    The buffer  68 , switch  74  and comparator  70  of  FIG. 5  are also depicted in  FIG. 6 , with some additional components being shown. By way of example, a low pass filter  92  is disposed intermediate the DAC output  91  and buffer  68 . The buffer  68  includes a resistance  94  to improve the capacitive driving capability of the buffer. In addition, the non-inverting input of comparator  70  is also connected to a low pass filter  96 . These various components operate to reduce switching transients and to lower the noise bandwidth of the circuit. 
         [0027]    In order to ensure accurate tracking, it is important that the DAC  66  be monotonic and have a low differential non-linearity (DNL) error value. Further, to achieve low phase noise on the output clock CLKO in the holdover mode, DAC  66  and its buffer  68  should be optimized for reduced low-frequency noise. The holdover clock CLKO frequency accuracy is related to the VCXO tuning sensitivity, the DAC  66  resolution, the DAC  66  accuracy, supply and ambient temperature variation and the variation of the VCXO characteristics over temperature, etc. 
         [0028]    Assuming that the ambient temperature does not change significantly during the holdover mode, the measured holdover frequency accuracy using the approach of  FIG. 4  is within +/−3 ppm for three different test parts evaluated over supply and temperature variations using a commercial 153.6 MHz VCXO (tuning gain Kvcxo of 100 ppm/V) from Epson Toyocom. This holdover accuracy is about ten times better that that of the prior art method of  FIG. 3  where the tuning voltage is held at VDD/2 when in the holdover mode. 
         [0029]    Thus, an improved clock conditioning system has been disclosed. Although an exemplary embodiment of the system has been described in some detail, it is to be understood that various changes can be made by those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.