Abstract:
An improved loop filter device for use in a phase lock loop that improves lock-in time and cycle-to-cycle jitter in the phase lock loop. The loop filter is used in a phase lock loop circuit having a phase frequency detector, a charge pump, a voltage controlled oscillator and a divider. The loop filter has a first capacitor with a first side and a second side, wherein the first side of the first capacitor is coupled to the output of the charge pump and the input of the voltage controlled oscillator. A CMOS switch is coupled to the second side of the first capacitor, wherein the CMOS switch is selectively operable between an open condition and a closed condition. At least one second capacitor is coupled in parallel with said switch, wherein the first capacitor is joined in series with said at least one second capacitor when the CMOS switch is in its open condition.

Description:
RELATED APPLICATIONS 
     This application is related in part to co-pending U.S. patent application Ser. No. 09/252,641, entitled, A Looped Circuit And Associated Method For Controlling The Relationship Between Current And Capacitance In CMOS And BICMOS Circuit Design, the disclosure of which is incorporated into this application by reference. 
    
    
     REFERENCE TO DOCUMENT DISCLOSURE 
     The matter in this application corresponds to the matter contained in Disclosure Document 446,511, filed Oct. 26, 1998, wherein this application assumes the priority date of that document. 
     BACKGROUND OF INVENTION 
     1) Field of the Invention 
     In general, the present invention relates to the design and manufacture of circuits containing phase lock loops. More particularly, the present invention relates to the circuit design of CMOS and BICMOS circuits that contain phase lock loops with on-chip loop filters. 
     2) Description of the Prior Art 
     In the prior art record, there are many phase lock loop circuits that are manufactured using CMOS and BICMOS manufacturing processes. The uses of CMOS and BICMOS manufacturing processes have many known advantages and disadvantages. Among the disadvantages is the inability to manufacture precision components with highly accurate absolute values. In many circuit designs, the circuit is engineered so that only the ratio of components needs to be accurate. In this manner, the importance of the accuracy of any one component is minimized. However, in design of the ‘Loop Filter’ in a phase lock loop, there is often a single resistor. It is highly desirable to manufacturer this resistor on-chip. By manufacturing the resistor on-chip, an external component is eliminated and the power can be reduced. The reduction in power results from the fact that currents can be lowered because the parasitic effects are much reduced with the resistor on-chip. However, if this resistor is manufactured using a CMOS or a BICMOS process, the variation in this resistor can be on the order of +/−50% or more over process and temperature. This is problematic because both the lock up time and the jitter of a phase lock loop are functions of the process and the temperature. This causes system performance to vary from manufactured lot to manufactured lot, and within each lot the system will vary as a function of temperature. 
     A common practice in CMOS and BICMOS processing is to laser trim the resistors used within a circuit to the values needed. This results in a precise resistor value within the circuit. Laser trimming is a time consuming and expensive process. In addition, the trimming process does not compensate for the change in resistor value that occurs when the temperature of the resistor changes. Changes in temperature can cause a CMOS resistor value to vary by +/−50%. 
     Most, if not all, of the existing techniques for incorporating an on-chip CMOS resistor into the loop filter of a phase lock loop, results in either process variation in the resistor or additional manufacturing using laser trimming. The temperature variation in the resistor is not addressed by laser trimming procedures. Therefore, even with laser trimmed circuits, the system performance of that circuit will vary widely with changes in temperature. This variation often forces the loop filter of a phase lock loop off the chip in order to meet design requirements. The off-chip design significantly increases the cost of the phase lock loop and the degree of power consumption. 
     A need therefore exists for a circuit design and associated method of manufacturing on-chip loop filters that are within exact performance parameters that remain stable across both process and temperature, thereby decreasing the lock-in time and improving the bit error rate in data recovery circuits used in many types of data storage and communication systems. This need is met by the present invention as described and claimed below. 
     SUMMARY OF THE INVENTION 
     An improved loop filter device for use in a phase lock loop that improves lock-in time and cycle-to-cycle jitter in the phase lock loop. The loop filter is used in a phase lock loop circuit having a phase frequency detector, a charge pump, a voltage controlled oscillator and a divider. The loop filter has a first capacitor with a first side and a second side, wherein the first side of the first capacitor is coupled to the output of the charge pump and the input of the voltage controlled oscillator. A CMOS switch is coupled to the second side of the first capacitor, wherein the CMOS switch is selectively operable between an open condition and a closed condition. At least one second capacitor is coupled in parallel with said switch, wherein the first capacitor is joined in series with said at least one second capacitor when the CMOS switch is in its open condition. The operation of the CMOS switch is controlled by the output of the phase frequency detector or some other dynamic loop parameter that is indicative of a phase of frequency error. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to the following description of exemplary embodiments thereof, considered in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic of a prior art second-order phase lock loop circuit; 
     FIG. 2 is a schematic of a second-order phase lock loop in accordance with the present invention; 
     FIG. 3 is a schematic of an alternate embodiment of a second-order phase lock loop in accordance with the present invention; and 
     FIG. 4 is a schematic of a time pulse generator for use in a second-order phase lock loop. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It is well know in the industry that the damping coefficient “ζ” Zeta, in a second-order phase lock loop, has a dominant effect on the lock-up time and jitter of that phase lock loop. 
     Referring to FIG. 1, a typical prior art second-order phase lock loop circuit  10  is shown. The phase lock loop circuit  10  contains a phase frequency detector  12 , wherein the phase frequency detector  12  has two inputs and two outputs. One of the inputs to phase frequency detector  12  is a reference clock or data signal. The other input of the phase frequency detector  12  is coupled to the output of a divider  14 . The input to divider  14  is, in turn, connected to the output of a voltage controlled oscillator (VCO)  16 . 
     The VCO  16  is connected to a charge pump  18 . The VCO  16  is also connected to a loop filter  20 . As is typical in prior art phase lock loop designs, the loop filter  20  contains a capacitor  22  joined in series with a resistor  24 . One side of the capacitor  22  connects to the VCO  16  and the charge pump  18 . The other side of the capacitor  22  is connected to the resistor  24 , wherein the resistor  24  is coupled to ground. 
     In operation, the phase and frequency of the signals on the inputs to the phase frequency detector  12  determine how the charge pump  18  charges the capacitor  22 . For the circuit in FIG. 1, it is well known that the damping coefficient (zeta) is given by:          Zeta     R   ,   C   ,   Kv   ,   N   ,   I       =         C   *   R     2     *     SQRT        (       Kv   *   I       2      π   *   N   *   C       )                                
     Where Zeta, defined from FIG. 1, is a function of the elements in the above equation. The quantity Kv is the gain of the VCO  16 . “I” is the output current of the charge pump  18 , and N is the value at which the divider  14  is set to divide. It is well known in the industry that the above equation is not constant across process and temperature and therefore causes the phase lock loops to be very difficult to manufacture entirely in a CMOS process. 
     The present invention sets forth a circuit design and associated method of manufacturing loop filters used in phase lock loops that allows precise control of the damping coefficient (Zeta) in phase lock loops. As such, the present invention improves the lock-up time and decreases the jitter in phase lock loops. Furthermore, the damping coefficient (Zeta) canube programmed to a new value to improve the bit error rate without losing lock. Whatever value of the damping coefficient (Zeta) is programmed, it will be held constant independent of process and temperature. The act of programming a new value of the damping coefficient (Zeta) does not cause jitter in the phase lock loop, except as the phase lock loop is affected with a new value for the damping coefficient (Zeta). 
     Referring to FIG. 2, a schematic of a second-order phase lock loop circuit  30  is shown in accordance with the present invention. The phase lock loop circuit  30  has a phase frequency detector  32 , charge pump  34 , VCO  36  and divider  38 , as did the prior art phase lock loop of FIG.  1 . The phase frequency detector  32  has an up output and a down output, both of which are coupled to the charge pump  34 . The phase frequency detector  32  also has two inputs. One of the inputs to the phase frequency detector  32  is a timing clock or data signal. The other input of the phase frequency detector  32  is coupled to the output of the divider  38 . The input to the divider  38  is connected to the output of the VCO  36 . The input to the VCO  36  is, in turn, connected to a improved loop filter  40 . The improved loop filter  40  contains a first capacitor  42 , a second capacitor  44 , a CMOS switch  46 , a time pulse generator  48  and a reference voltage source  50 . 
     The first capacitor  42  is coupled to the output of the charge pump  34  and the input of the VCO  36 . The first capacitor  42  is connected to one side of the CMOS switch  46  and to one side of a second capacitor  44 . The other side of the CMOS switch  46  is connected to the reference voltage source  50  and to the opposite side of the second capacitor  44 . The reference voltage source  50  is connected to ground. 
     The gate of the CMOS switch  46  is connected to the time pulse generator  48 . The time pulse generator  48  produces an output time pulse “tw”. The input to the time pulse generator  48  is connected to the output of an OR gate  52 . One of the inputs to the OR gate  52  is connected to the up output of the phase frequency detector  32  and to the up input of the charge pump  34 . The other input to the OR gate  52  is connected to the down output of the phase frequency detector  32  and to the down input of the charge pump  34 . 
     As has been previously mentioned, the phase frequency detector  32  has an up output and a down output. If either of these outputs is high, the “OR” gate  52  will be high. When the “OR” gate  52  goes high, it triggers the time pulse generator  48  and causes the time pulse generator  48  to output a timing pulse of width “tw”. The timing pulse ‘tw’ will be described in detail later in this document. The output of the time pulse generator  48  is normally high. When the time pulse generator  48  outputs a timing pulse “tw”, its output goes low for a time equal to “tw”. When the output of the time pulse generator  48  is high, the CMOS switch  46  is in a low impedance state and the second capacitor  44  is effectively shorted out of the loop filter circuit  40 . Both sides of the second capacitor  44  are connected to the reference voltage source  50  through a low impedance DC path. When either output of the phase frequency detector  32  goes high, the time pulse generator  48  will initiate the output of a low timing pulse “tw”. When the output of the time pulse generator “tw” is low, the CMOS switch  46  is in a high impedance state. This puts the first capacitor  42  in series with the second capacitor  44 . The effective capacitance between the output of the charge pump  34  and the reference voltage source  50  is the series combination of the first capacitor  42  and the second capacitor  44 . 
     The output of the time pulse generator  48  goes low just before the charge pump  34  starts to output current to the series combination of the first capacitor  42  and the second capacitor  44 . The second capacitor  44  is typically on the order of {fraction (1/15)} the size of the first capacitor  42 , however other sized capacitors can also be used. When the charge pump  34  is supplying current, and the CMOS switch  46  is in a high impedance state, the voltage on the first capacitor  42  is increasing much more rapidly than when the CMOS switch  46  is in a low impedance state. The time period “tw” for which the CMOS switch  46  is in a high impedance state is given by:        tw   =     (     t   -     tp   2       )                            
     Where t is a fixed time and ‘tp’ is the time that the output of the “OR” gate  52  is high. The fixed time “t” should be less than the maximum period of either of the signals that are connected to the input of the phase frequency detector  32 . The value of the second capacitor  44  is then determined by: 
     
       
         
           C=t/R 
         
       
     
     As the second capacitor  44  is made larger, the dynamic range required by the charge pump  34  is reduced. The damping coefficient (Zeta) in this invention is given by:          Z     tw   ,   C2   ,   C3   ,   Kv   ,   N   ,   I       =       C1   C2     *     tw   2     *     sqrt   (     Kv     2      π                 N   *   C1       )                              
     The coefficient before the square root in the last equation no longer depends on resistance (R) and the error in the first capacitor  42  has been transformed into the error in the ratio of the first capacitor  42  divided by the second capacitor  44 . The ratio of similar capacitors can be held to very tight tolerances. When the phase lock loop  30  is in lock, the damping coefficient (Zeta) depends linearly on the fixed time “t”, because “tp/2” is very small. This allows the damping coefficient (Zeta) to be adjusted linearly by changing the fixed time “t”, as will be later described. 
     An alternate method of changing the damping coefficient (Zeta) is to replace the second capacitor  44  with parallel capacitors. A simple four bit parallel combination of capacitors can give a 16 to 1 range change in the damping coefficient (Zeta). 
     Referring to FIG. 3, an alternate embodiment of a second-order phase lock loop circuit  60  is shown. The phase lock loop circuit  60  is identical to the circuit shown in FIG. 2, except a bias voltage (Vbias) is supplied to both the charge pump  34  and time pulse generator  48 . Since the circuit embodiments of FIG.  2  and FIG. 3 share many similar components, the reference numbers that were used in describing the circuit of FIG. 2 will again be used to describe like components in the circuit of FIG.  3 . 
     The bias voltage (Vbias) received by the charge pump  34  and the time pulse generator  48 , is generated by an external circuit. The specifics of that external circuit are described and claimed in co-pending U.S. patent application Ser. No. 09/252,641, entitled, A Looped Circuit And Associated Method For Controlling The Relationship Between Current And Capacitance In CMOS and BiCMOS Circuit Design, which has already been incorporated into this specification by reference. 
     The value of the bias voltage (Vbias) is used to hold the ratio of the charge pump current to capacitance constant across process and temperature. In FIG. 3, the bias voltage (Vbias) is an input to the time pulse generator  48 . The bias voltage (Vbias) is also an input to the charge pump  34  where it is used to bias the current mirrors that are a scaled version of the current mirrors used in the circuit that produces the bias voltage. Accordingly, by using common scaling techniques for CMOS transistors, current sources and capacitors, the damping coefficient (Zeta) depends only on the ratio of capacitors, a reference frequency, and a reference voltage. 
     In FIG. 3, the bias voltage (Vbias) is received by the time pulse generator  48 . Referring now to FIG. 4, an embodiment of the time pulse generator  48  is shown. The time pulse generator  48  uses the bias voltage (Vbias) to generate precise time width pulses “tw” in typical CMOS processing, independent of process variations and temperature. The time pulse generator  48  includes a D-flip-flop  62 , two current sources  64 ,  66 , a non-overlapping pulse generator  68 , a comparator  70  and three CMOS switches  72 ,  74 ,  76 . 
     The D-flip-flop  62  has its “D” input connected to a high signal “HI”. The D-flip-flop  62  also has its clock input “CLK” connected to a trigger signal  71  and to the input of the non-overlapping pulse generator  68 . The non-overlapping pulse generator  68  generates a Q output. The Q output is connected to the gate of a first CMOS switch  72 . 
     The positive clear input “CLR” of the D-flop-flop  62  is connected to the output of the comparator  70 . The Q bar output “˜Q” of the D-flop-flop  62  is connected to the gate of a second CMOS switch  74  and to the output pulse “tw”. The bias voltage (Vbias) is connected to the control voltage of both the first current source  64  and the second current source  66 . The first current source  64  has its source connected to VDD. The drain of the first current source  64  is connected to the drain of the second CMOS switch  74 . The drain of the first current source  64  also leads to one side of a capacitor  78 , the drain of the third CMOS switch  76  and to the positive input of the comparator  70 . 
     The second current source  66  also has its source connected to VDD. The drain of the second current source  66  is connected to the drain of the first CMOS switch  72 . The drain of the second current source  66  also is connected to one side of the third CMOS switch  76 . 
     A programmable digital to analog converter  80  is also provided. The voltage output (Vref) of the programmable digital to analog converter  80  is connected to the negative input of the comparator  70 . The Qbar output of the non-overlapping pulse generator  68  is connected to the gate of the first CMOS switch  72 . The first CMOS switch  72  has its source connected to the drain of the second current source  66  and to one side of the third CMOS switch  76 . The time pulse generator  48  generates a time pulse width that approximates “tw” as described above by the equation:        tw   =     (     t   -     tp   2       )                            
     The time pulse “tw” is initiated from a rising edge on the trigger pulse  71  in FIG. 4, which causes the Qbar output of the D-flip-flop  62  to transition to the low state. This puts the second CMOS switch  74  into a high impedance state and allows the current from the first current source  64  to start charging the capacitor  78 . This causes the voltage of the capacitor  78  to start ramping. 
     The trigger pulse  71  goes into the non-overlapping pulse generator  68 . The rising edge of the trigger pulse  71  initiates the non-overlapping pulse generator  68 , as is well known in the industry. The non-overlapping pulse generator  68  has two outputs. The shorter pulse of the non-overlapping pulse generator  68  is applied to the gate of the third CMOS switch  76 . The longer pulse width is applied to the gate of the first CMOS switch  72 . This causes the first CMOS switch  72  to go into a high impedance state slightly before the third CMOS switch  76  goes into a low impedance state. When the trigger pulse  71  drops back to a low state, the third CMOS switch  76  goes into a high impedance state slightly before the first CMOS switch  72  goes into a low impedance state. This process directs the current from the second current source  66 , through the third CMOS switch  76 , to the capacitor  78 . The current from the second current source  66  adds charge to the capacitor  78  to reduce the fixed time “t” it takes to charge the capacitor  78  from ground to the reference voltage (Vref) by approximately tp/2. 
     As has been previously explained, the time “t” in the above equation is readily given by: 
     
       
         t=I*Vref/C 1   
       
     
     Where C 1  is a scaled version of the first capacitor  42  (FIG. 3) used in the phase lock loop, and the current “I” is generated from the first current source  64 . The first current source  64  is a scaled version of the current source used to generate “I” in the external circuit that produces the bias voltage (Vbias). 
     The voltage on the capacitor  78  is initially at ground but it starts charging when the rising edge of a trigger pulse  71  is received. The capacitor  78  continues to charge until the voltage on capacitor  78  reaches the reference voltage (Vref). At this point, the output of the comparator goes high and clears the D-flip-flop, which forces “tw” high. The output of the programmable digital to analog converter  80  is the reference voltage, which supplies the voltage for the negative input to the comparator  70 . The voltage from the programmable digital to analog converter  80  typically is generated from an on-chip band gap reference. The total current charging the capacitor  78  is in general the sum of two currents. The first current is from the first current source  64  and is used to generate the fixed time “t”. The second current is from the second current source  66 . The second current source  66  is designed to approximately reduce the pulse width “tw” by tp/2. 
     There are many techniques for reducing the width of a timing pulse. An approximation that is satisfactory is to scale the current from the second current source  66  so that it would charge the capacitor  78  to the reference voltage (Vref) in the nominal period of the input clock. This value of current for the second current source  66  is given by: 
     
       
           I=C*Vref/ (NOMINAL PERIOD) 
       
     
     If the fixed time “t” was chosen to be equal to the nominal period then the first and second current sources will be identical. The current from the second current source  66  is summed for the time that the input line trigger pulse  71  is high. The additional current summed in the capacitor  78  causes the time the output “tw” is low to be reduced by approximately tp/2. 
     The circuit in FIG. 4 can be simplified if desired by removing the second current source  66 , the first CMOS switch  72 , the second CMOS switch  74 , and the non-overlapping pulse generator  68 . This will result in a pulse width ‘tw’ that is equal to a fixed time t. This time as shown by a previous equation depends on the first current source  64 , the programmable digital to analog converter output and the capacitor  78 . The damping coefficient (Zeta) can be easily programmed, by programming either the first current source  64  or the reference voltage. This reprogramming can be done while the phase lock loop is in lock, to minimize the bit error rate. 
     The capacitors and current sources may be replaced with programmable capacitors and current sources if greater flexibility is desired. The invention disclosed here is very flexible. There is great range of fixed and variable times that may be used to control the second CMOS switch  74 . There are many ways to make the basic invention presented here programmable. There are many digital circuits that could be implemented to generate the timing pulse tw that would perform well. It will be understood that the specifics of the embodiments of the present invention described above illustrates only a few exemplary embodiments of the present invention. A person skilled in the art can therefore make numerous alterations and modifications to the shown embodiment utilizing functionally equivalent components and circuit layouts to those shown and described. All such modifications are intended to be included within the scope of the present invention as defined by the appended claims.