Abstract:
A phase locked loop (PLL) circuit adjusts a voltage controlled differential oscillator to generate an output frequency signal, which is a selected multiple of an input reference signal. The PLL circuit includes an oscillator control circuit for increasing and decreasing the PLL output frequency signal, a frequency detector for detecting a phase shift between the reference signal and the PLL output signal and produces an error signal, and a fast lock circuit for detecting when the output frequency signal passes the selected multiple of the reference signal. This circuit design provides improved jitter performance, tolerates process variation, and extends the PLL operating frequency range.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to phase locked loop (PLL) circuits. More particularly, this invention relates to a PLL circuit which is based on a voltage controlled differential oscillator and an advanced common biasing technique, which tolerates process variations and calibrates current ranges for operational request frequency that provides frequency stability with temperature change without the use of a bandgap reference bias circuit.  
           [0002]    Phase-locked loops are often used in the I/O interfaces of digital integrated circuits in order to hide clock distribution delays and to improve overall system timing. The maintenance of the timing throughout a circuit is important. Timing becomes particularly critical for applications requiring high-speed processing of information, such as with video processors.  
           [0003]    The timing throughout a circuit deviates from the system clock when noise is introduced by various system components and as a result of capacitive effects due to system interconnections. In recent years, the demand has risen for devices capable of high-speed processing. As a result, the demand for PLL circuits that quickly compensate for electronic noise and capacitive delays has also risen. The problem is that the amount of phase shift produced as a result of the supply, substrate noise and capacitor load is directly related to how quickly the PLL can correct the output frequency.  
           [0004]    One type of design used by those skilled in the art to eliminate the noise present in the circuit at the required speed is a self-bias signal technique. Referring to FIG. 1, this prior art PLL circuit is a self-biasing configuration which is composed of a phase comparator, charge pump, loop filter, bias generator and a voltage-controlled oscillator (VCO). This PLL circuit also uses an additional charge pump current to the bias generator V bp  output. For a typical PLL, the charge pump current and the loop filter resistance are constant. These conditions give rise to a constant damping factor and a constant loop bandwidth. A constant loop bandwidth can constrain the achievement of a wide operating frequency range and low input tracking jitter. If the frequency is disturbed, the phase error that results from each cycle of the disturbance will accumulate for many cycles until the loop can compensate for the frequency error. The error will be accumulated for a number of cycles, which is proportional to the operating frequency divided by the loop bandwidth. Thus the loop bandwidth would have to be positioned as close as possible to the reference frequency bandwidth to minimize the total phase error. The result is that the frequency bandwidth must be conservatively set for stability at the lowest operating frequency with worst case process variations, rather than set for optimized jitter performance. The self-biased PLL also exhibits much faster locking times only when locking from similar or higher operating frequencies. If, however, the self-biased PLL is started at a very low operating frequency, it will exhibit very slow locking times.  
           [0005]    Accordingly, there is a need for a PLL circuit which provides a fast lock-up, improved jitter performance, tolerates process variations, and extends the PLL operating frequency range.  
         SUMMARY  
         [0006]    The present invention is a phase locked loop (PLL) which is based on a common bias technique, comprising a voltage controlled differential oscillator, fast lock-up circuit, self-calibration current range setting circuit and a high-speed phase frequency detector. This design provides improved speed in locking to the frequency of an incoming signal, extends the PLL operating frequency range, improves PLL jitter performance, and provides greater immunity to environmental noise, which results in improved power supply rejection ratio (PSRR).  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a typical PLL block diagram;  
         [0008]    [0008]FIG. 2 is a block diagram of the circuit in accordance with the preferred embodiment of the present invention;  
         [0009]    [0009]FIG. 3A is a logic gate diagram of the speed-up circuit in accordance with the preferred embodiment of the present invention;  
         [0010]    [0010]FIG. 3B is a state diagram of the speed-up circuit inputs in accordance with the preferred embodiment of the present invention;  
         [0011]    [0011]FIG. 3C is a logic table for the speed-up circuit in accordance with the preferred embodiment of the present invention;  
         [0012]    [0012]FIG. 4A is a diagram of the bias generator;  
         [0013]    [0013]FIG. 4B is a diagram of a delay element of the VCO in accordance with the preferred embodiment of the present invention (only one is shown though there are a plurality of delay cells  20   a . . .  20   n  used);  
         [0014]    [0014]FIG. 5 is an example of a graph of four I-V curves in accordance with the preferred embodiment of the present invention;  
         [0015]    [0015]FIG. 6 is a flow diagram in accordance with the preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    The first embodiment of the present invention will be described with reference to the drawing figures wherein like numerals represent like elements throughout.  
         [0017]    Referring to FIG. 2, a PLL circuit  10  made in accordance with the present invention is shown. The PLL circuit  10  includes a phase frequency detector (PFD)  11 , a speed-up circuit  12 , a charge pump  13 , a charge pump control circuit  14 , a loop filter  15 , a bias generator  16 , a current range control circuit  17 , a voltage-controlled oscillator (VCO)  18 , and a frequency divider  19 . The PLL circuit  10  receives a reference frequency Fref from an outside source. Along with the reference frequency F ref  , the phase frequency detector (PFD)  11  receives a reset signal PLL reset  and the divided PLL  10  output frequency signal F O /N. Coupled to the phase frequency detector  11  are the speed-up circuit  12  and the charge pump  13 . As those skilled in the art should know, the PFD  11  determines the phase and frequency difference between the reference frequency F ref  and the divided PLL  10  output signal F O /N. If the PFD  11  detects a difference between the two input signals F ref , F O /N, the phase error signals U, D are sent to the speed-up circuit  12  and the charge pump  13 . The duration of the output signals U, D pulse widths depend on the amount of phase and frequency error that is detected by the PFD  11 .  
         [0018]    The speed-up circuit  12 , shown in FIG. 3A, is controlled by the phase error signals U, D output from the PFD  11 . Coupled to the PFD  11  and the charge pump control switch  14 , the speed-up circuit  12  receives the phase error signals U, D and a reset signal PLL reset . The purpose of the speed-up circuit  12  is to monitor the crossing of the PLL  10  output frequency F O /N and the reference frequency F ref , as will be disclosed hereinafter. The speed-up circuit  12  comprises a plurality of invertors  3 , a plurality of AND gates  5 , two S-R flip flops  7 , a NAND gate  8 , and a delay stage  9 . Initially, when the PLL circuit  10  receives a reset signal PLL reset , the speed-up circuit  12  signals the charge pump control switch  14  to adjust the charge pump  13  current to the maximum. When the speed-up circuit  12  detects the crossing of the two frequencies F O , F ref , the speed-up circuit  12  signals the charge pump control switch  14  to reduce the charge pump  13  current. The output of the speed-up circuit  12 , as the PLL circuit  10  attempts to match the output frequency F O /N with the reference frequency F ref , is illustrated in the state diagram of FIG. 3B and the logic table of FIG. 3C.  
         [0019]    Referring to FIGS. 3B and 3C, when the speed-up circuit  12  receives the reset signal PLL reset  and the reset PFD  11  outputs U, D, the speed-up circuit  12  signals the charge pump  13  to output its maximum current, thus discharging the loop filter capacitor C 1  to 0V. This triggers the VCO  18  to output its maximum frequency F O . The PLL  10  output frequency F O /N will decrease as the capacitor C 1  charges and will eventually be equal to the reference frequency F ref . This condition is identified by U, D changing from 00,01,11 to 00,10,11. As the PLL  10  comes out of reset, the signals U, D are in the state 0,0. Since the PLL  10  output frequency F O /N is faster than the reference frequency F ref , the next state of U,D is 0, 1. This indicates to the charge pump  13  to charge the capacitor C 1 , reducing the PLL  10  output frequency F O /N. On the next rising edge of the reference frequency F ref , the signals U, D go to 1, 1. This state causes the PFD  11  to reset and return U, D to its neutral state 0, 0. This process continues until the PLL  10  output frequency Fo/N is slower than the reference frequency F ref . This condition is indicated by the U, D signals going to a state 1, 0. This condition signals the speed-up circuit  12  to output a logical zero (0) Q 3  to the charge pump control circuit  14 , indicating that the charge pump  13  should reduce its current to the level prescribed by the charge control signal CC signal  to be disclosed hereinafter.  
         [0020]    The speed-up circuit  12  operates to quickly detect the matching of the reference and output frequencies F ref , F O /N, and then output a control signal Q 3  to the charge pump control circuit  14  to reduce the current of the charge pump  13  in order to find the optimal bias current range l bias , as will be disclosed hereinafter, to be output to the VCO  18 . This speed-up circuit  12  provides a fast frequency lock by signaling the initialization of the charge pump  13  current output to its maximum and signaling for the reduction of this current when the frequencies F ref  and F O /N are equal.  
         [0021]    Referring back to FIG. 2, the control signal Q 3  output from the speed-up circuit  12  is a logical one or a logical zero and is received by the charge pump control circuit  14 , which is coupled to the speed-up circuit  12  and the charge pump  13 . The charge pump control circuit  14 , using a variation of switches, converts the signal from the speed-up circuit  12  and the charge control signal CC signal  to logic signals S 1 , S 2 , S 3 , which will adjust the amount of current the charge pump  13  outputs. Initially, when the speed-up circuit  12  indicates that the charge pump  13  should output its maximum current, by outputting a logical one (1) signal Q 3 , the logic signals S 1 , S 2 , S 3  will be equivalent to 1, 1, 1, respectively. When the speed-up circuit  12  outputs a logical zero (0) signal Q 3 , indicating that the charge pump  13  should reduce its current, the logic signals S 1 , S 2 , S 3  output from the charge pump control circuit  14  will be equivalent to 1, 1, 0 or 1, 0, 1, respectively, for example. This output S 1 , S 2 , S 3  is set by the charge control signal CC signal , an outside input signal whose value depends on the operation for which the PLL  10  output is to be used. The charge pump control circuit  14  converts the charge control signal CC signal  to the logic signals S 1 , S 2 , S 3 . This instructs the charge pump  13  to switch out an internal current source (not shown) when the logic signal S 1 , S 2 , S 3  associated with the current source is zero (0), thereby dividing the current of the charge pump  13  by a number m (e.g., 3, 6, or 9).  
         [0022]    The charge pump control circuit  14  provides the PLL  10  with the ability to slew the output frequency F O  toward lock at the fastest rate possible, instead of at a constant rate, using the maximum charge pump  13  output current. As should be well known to those having skill in the art, the faster damping of the PLL  10  output frequency F O  is a result of the high frequency response to the error signals U, D from the PFD  11 . Once the PLL circuit  10  outputs the desired frequency, the charge pump control circuit  14  reduces the charge pump  13  output current, which reduces the output frequency response of the PLL  10  and improves jitter performance.  
         [0023]    The charge pump  13  outputs a current that charges or discharges the capacitors C 1 , C 2  of the loop filter  15  to a voltage level VLPF. The charge pump  13  receives the error signals U, D, a voltage input V bp  from the common bias generator  16 , and the logic signals S 1 , S 2 , S 3  from the charge pump control circuit  14 . V bp  is a reference bias voltage which controls the charge pump output current. It is well known to those skilled in the art that the charging and discharging of the loop filter  15  capacitors C 1 , C 2  create a voltage change V LPF  across the loop filter  15 . As V LPF  increases, the charge pump  13  output current decreases. This voltage change V LPF  is a reference for the common bias generator  16  and the current control circuit  17  that generates the reference signals for controlling the amount of delay created by each delay element  20   a . . . 20   n  of the VCO  18 .  
         [0024]    The loop filter  15  comprises a first capacitor C 1  and a second capacitor C 2  with a variable resistor  22 . The variable resistor  22  comprises a symmetric load. The loop filter  15  is coupled to the common bias generator  16  as well as the charge pump  13 . This loop filter  15  receives a current output generated by the charge pump  13  using the bias voltage V bp , and an input voltage V dd . As the current from the charge pump  13  shifts to adjust the frequency of the delay cells  20   a . . .  20   n , the variable resistor  22  is also adjusted. As those skilled in the art should know, the use of the variable resistor  22  in the loop filter  15  allows the bias generator  16  to maintain stability over a wide bandwidth.  
         [0025]    The current range control circuit  17 , coupled to the loop filter  15  and the bias generator  16 , comprises two voltage comparators (not shown) and provides two (2) single bit outputs R 0 , R 1 . The current range control circuit  17  receives the loop filter output voltage V LPF  and adjusts the level of the bias current I bias  generated in the common bias generator  16  through the output leads R 0 , R 1 . The voltage comparators in the current range control circuit  17  monitor the voltage V LPF  against an internal reference voltage V ref  which is indicative of the point where a small change in the voltage V LPF  causes a large change in the bias current I bias , as disclosed hereinafter. The reference voltage V ref , which can be fixed or variable, is illustrated in the FIG. 5 I bias  I-V curves  60 - 66 . As those skilled in the art should know, each of these curves  60 - 66  includes an approximate narrow linear region, illustrated by the curves up to the voltage level V ref  in FIG. 5, and a nonlinear region. The nonlinear region, where a small increase in the voltage creates a large decrease in the current, produces a large change in the delay elements  20   a . . .  20   n  of the VCO  18 , causing increased jitter within the PLL circuit  10 . If the PLL circuit  10  is operated past a certain voltage point V ref  on the curves, into the nonlinear region, the amount of jitter will increase and the overall performance of the PLL circuit  10  will decrease. Therefore, when the voltage V LPF  is greater than the reference voltage V ref , PFD  11 , speed-up circuit  12 , and the charge pump control circuit  14  receive a local reset signal L reset  from the current range control circuit  17  which resets the voltage V LPF  to zero. The current range control circuit  17  outputs R 0 , R 1  switch, adjusting the bias current I bias  to a lower level, as will be described hereinafter.  
         [0026]    Initially, when the PLL circuit  10  is reset, the current range control circuit  17  signals the bias generator  16  to switch on all four currents sources I 1 , I 2 , I 3 , I 4 , which is indicated by the output 0, 0 for the two outputs R 0 , R 1 , respectively (shown in FIG. 4A). When the speed-up circuit  12  detects the crossing of the reference and output frequencies F ref , F O /N, the current control circuit  17  is adjusted to find the optimal bias current I bias  curve to be utilized by the bias generator  16  for output to the VCO  18 . As disclosed above, when the voltage comparators in the current range control circuit  17  detect that V LPF  is too high, the current range outputs R 0 , R 1  switch by one (1). For example, after the resetting of the PLL circuit  10 , the current range outputs R 0 , R 1  are equivalent to 0, 0. When the comparator detects the high voltage V LPF , the current range outputs R 0 , R 1  switch to 0, 1, respectively, which indicates that the common bias generator  16  should switch off current source I 4 . The current range control circuit  17  also outputs an internal reset signal L reset  to the PFD  11 , speed-up circuit  12 , and the charge pump control circuit  14 , which resets the loop filter  15  voltage V LPF  to zero (0). Each time the voltage comparators of the loop filter  15  detect this condition, the current range outputs R 0 , R 1  increase by one (1), and another current source is switched off. This process continues until R 0 , R 1  is equivalent to 1, 1, where the only remaining current source is I 1 . At this point, the current range control circuit  17  signals to the common bias generator  16  to remain at the lowest current level I 1 .  
         [0027]    If the reference voltage V ref  is greater than the loop filter voltage V LPF , the current range control circuit  17  indicates to the bias generator  16  to remain at the present current level, which is considered the optimal operating point for the process utilizing the output frequency signal F O . As those skilled in the art should know, even though the current range control circuit  17  is illustrated utilizing two (2) single bit outputs R 0 , R 1 , a single two-bit output may also be utilized, or any signaling scheme which provides a selective control output.  
         [0028]    Referring to FIG. 4A and 4B, the present invention utilizes a single common bias generator  16  comprising a bias current generator  56 , a bias voltage generator  58  and a differential amplifier  54 . The differential amplifier  54  reduces the noise from the power supply, which improves the power supply rejection ratio performance of the PLL circuit  10 . The bias current generator  56  includes four symmetric loads  70 ,  72 ,  74 ,  76  and a switching circuit  57 . Each of the symmetric loads  70 ,  72 ,  74 ,  76  (which may or may not have the same device widths), has as its voltage source V dd  and the low pass filter  15  output signal V LPF  as its gate voltage. The four symmetric loads  70 ,  72 ,  74 ,  76  act as current sources I 1 , I 2 , I 3 , I 4 . These current sources I 1 , I 2 , I 3 , I 4  are switched on and off by the switching circuit  57 . The current range control circuit  17  outputs R 0 , R 1  dictate to the switching circuit  57  which of the current sources I 1 , I 2 , I 3 , I 4  should be on and which should be off. As should be known to those skilled in the art, there are numerous switching circuits that are responsive to a digital input and can be used as described herein. In this manner, the current range control circuit  17  provides four (4) discrete levels of bias current I bias  to control the delay elements  20   a . . .  20   n  within the VCO  18 . This allows for better control of the PLL circuit  10  by providing the flexibility of varying the current slope without having to vary the delay cell itself. The bias current I bias  is output from the bias current generator  56  to the bias voltage generator  58 .  
         [0029]    The bias voltage generator  58  comprises two n channel transistors  51 ,  52  and one p channel transistor  53 . The gate voltage to n channel transistors  51 ,  52  is connected to a differential amplifier  54 . This differential amplifier  54  eliminates the noise generated by power supply voltage V dd . The n channel transistors  51 ,  52  are configured such that the bias current I bias  through transistor  51  is mirrored onto transistor  52  and reflected up to transistor  53 . When the bias current I bias  is mirrored in this way, two reference voltages are created. The two reference voltages are the bias voltages V bn  and V bp  which determine the amount of delay for each delay element  20   a . . .  20   n  within the VCO  18 . Using this common bias generator  16  and a selectable number of delay cells will provide better linearity in the overall delay.  
         [0030]    The voltage controlled oscillator (VCO)  18  is coupled to the common bias generator  16  and a frequency divider  19 . Although only one delay element  20   a  is shown for clarity, it should be understood that the VCO  18  includes a plurality of differential delay elements  20   a . . .  20   n . The delay elements  20   a . . .  20   n  are configured in such a way that the voltage inputs V + , V − of the delay elements come from the voltage outputs V O   + , V O   − of the preceding delay elements. The voltage outputs V O   + , V O   − from the last delay element  20   n  are feedback to the voltage inputs V + , V 31  of the first delay element  20   a . This configuration generates the desired output frequency F O .  
         [0031]    Each delay cell  20   a . . .  20   n  contains two p channel transistors  21  and  22  and three n channel transistors  23 ,  24 ,  25 . Transistors  23 ,  24  act as switches in the delay cell  20   a  and determine the actual delay for each element based on the bias current I bias  via the voltages V bn  and V bp . The p channel transistors  21 ,  22  act as current sources for the transistors  23 ,  24 . Transistor  25  acts as a current source as well. The p channel transistors  21 ,  22  are biased by the voltage V bp . Since the amount of bias current I bias  determines the voltage level of V bp , the delay element delay time changes with the V bp . Transistors  23  and  24  receive a voltage input V + and V − . The current supplied by transistor  22  does not pass through transistor  24  when transistor  24  is “off” (or not conducting). Likewise, when transistor  23  is not conducting, the current provided by transistor  21  does not pass through transistor  23 . There are parasitic capacitances at the inputs of transistors  23  and  24  that charge and discharge to affect the voltages V + and V − , which rise and fall. When transistors  23  and  24  are on and off, respectively, the charge on the parasitic capacitors at the input of transistors  23  and  24  on the subsequent delay cell will be affected. When transistor  23  is turned on, it discharges the parasitic capacitances of the next delay cell and V O− changes from (V dd −Vds  21 ) to (0V+V ds   25 +V ds   23 ). Likewise, when transistor  24  is off, transistor  22  charges the capacitance of the following delay cell and V O   + changes from (0V+V ds    25 +V ds    24 ) to (V dd− Vds  22 ), Vds  22  at saturation. As is well known to those skilled in the art, the delay provided by the delay cell is equivalent to the duration between turning on transistor  23  and turning off transistor  24 , and when the voltages V + and V − are equal. When this point is reached, the transistors in the next delay cell are activated. V O   + and V O   − are the output voltages of each delay cell that provide the input voltages V + , V − to the next delay cell.  
         [0032]    The frequency output from the VCO  18  is then input to a frequency divider  19 . Since the output frequency F O , is a multiplied version of the reference frequency F ref , by a factor of N times, the frequency divider  19  eliminates this N factor for comparison to the reference frequency F ref .  
         [0033]    The flow diagram in accordance with the present invention is illustrated in FIG. 6. The PLL  10  receives a reset signal PLL reset  from an external source and resets all PLL circuit  10  components (step  700 ). Upon receipt of this reset signal PLL reset , the PFD  11  resets the up and down output signals U, D (step  701 ). The speed-up circuit  12  receives the reset signal PLL reset  as well as the output signals of the PFD  11  and sends a signal to the charge pump control circuit  14  to boost the current to maximum (step  702 ). The charge pump control circuit  14 , upon receipt of the signal from the speed-up circuit  12 , outputs logic signals SI, S 2 , S 3  to the charge pump  13  to adjust the current in the charge pump  13  to its maximum (step  703 ). The VCO  18  is set to the maximum output frequency (step  704 ). The maximum output frequency F O  is then forwarded to the frequency divider  19  where it outputs F O /N to the phase frequency detector  11  for comparison to the reference frequency F ref  (step  709 ). If F O /N is equal to F ref , the current range control circuit  17  locks at the present current level (step  710 ) and the charge pump control circuit  14  reduces the charge pump  13  current output (step  711 ). Once the current output of the charge pump  13  is reduced, the PLL circuit  10  is in the lock range position (step  712 ). If the two frequencies F O /N, F ref  are not equal, the PFD  11  outputs signals U, D whose duration depend on the amount of phase and frequency error that is detected between the two frequency signals F ref , F O /N (step  705 ). The charge pump  13  receives the control signals U, D, S 1 , S 2 , S 3  from both the PFD  11  and the charge pump control circuit  14  and outputs a current, which sources or sinks the loop filter capacitors C 1 , C 2  (step  706 ). As a result of the charging or discharging of the loop filter capacitors C 1 , C 2 , the control voltage V LPF  is generated (step  707 ). If the loop filter voltage V LPF  is greater than V ref  and the current range control circuit  17  is not at the minimum current level, the current control circuit  17  outputs a local reset signal L reset  to the loop filter  15 , which resets the voltage V LPF  to zero (0) (step  707   a ). If the current range control circuit  17  is at the minimum current level, the common bias generator  16  outputs the bias current I bias  and bias voltages V bp , V bn  to the VCO  18  (step  708 ), which generates the output frequency F O  (step  709 ). Since the current range control circuit  17  is at its minimum current level I 1 , the PLL circuit is in the lock range position (step  712 ). If the current range control circuit  17  is not at its minimum current level I 1 , the current range control outputs R 0 , R 1  switch to the next lower level (step  707   b ). Because of the local reset signal of the current range control circuit  17 , the VCO is set to its maximum frequency output (step  704 ). The described process continues until F ref  is equal to F O /N.  
         [0034]    This design of the PLL circuit  10 , in accordance with the preferred embodiment, will achieve a wide operating frequency range with a fast lock up circuit and good jitter performance over a wide power supply voltage range and short lock in time. The differential VCO  18 , operating in the biasing current mode, provides a much wider operating frequency range with high common-mode noise immunity. The common biasing technique provides the necessary bias with less sensitivity to temperature and process variations. It also provides better power supply rejection ratio and current range calibration regulation when the power supply droops or when process variations change.  
         [0035]    While a specific embodiment of the present invention has been shown and described, many modifications and variations can be made by one skilled in the art without departing from the spirit and scope of the invention. The above description serves to illustrate and not limit the particular form in any way.