Abstract:
A phase locked loop circuit comprises a voltage controlled oscillator with a control input to which a variable control voltage is applied and a phase-frequency discriminator with an output connected to a loop filter to produce the control voltage. To provide the phase locked loop circuit with a low loop gain and a wide frequency pulling range, a plurality of discrete capacitors is associated with the voltage controlled oscillator. A switch array selectively activates and deactivates each of the capacitors in the voltage controlled oscillator. Each number of currently activated capacitors determines one out of a plurality of partial ranges of frequencies through which the oscillator can be tuned by a variation of the control voltage in a range between predetermined upper and lower control voltage limits. A total frequency range through which the oscillator can be tuned is divided thus into a plurality of partial frequency ranges each defined by a different number of activated capacitors.

Description:
[0001]     The present invention relates to a phase locked loop circuit with a voltage controlled oscillator that has a control input to which a variable control voltage is applied.  
       BACKGROUND  
       [0002]     When a phase locked loop (PLL) operating as a variable frequency oscillator is intended to have a wide frequency pulling range and the frequency variation is achieved with a variable capacitance element in an analog control loop, the control loop must have a high gain. A high loop gain may cause high phase and/or frequency jitter. These problems limit the frequency pulling range that can be achieved with an analog control loop.  
       SUMMARY  
       [0003]     The present invention provides a phase locked loop circuit with a low loop gain and a wide frequency pulling range.  
         [0004]     In a described embodiment, the phase locked loop circuit comprises a voltage controlled oscillator that has a control input to which a variable control voltage is applied and a phase-frequency discriminator with an output connected to a loop filter to produce the control voltage. A plurality of discrete frequency tuning elements, e.g. capacitors, is associated with the voltage controlled oscillator. A switch array selectively activates and deactivates each of the frequency tuning elements in the voltage controlled oscillator. Each number of currently activated frequency tuning elements determines one out of a plurality of partial ranges of frequencies through which the oscillator can be tuned by a variation of the control voltage in a range between predetermined upper and lower control voltage limits. A total frequency range through which the oscillator can be tuned is divided into a plurality of partial frequency ranges each defined by a different number of activated frequency tuning elements. Since each partial frequency range can be relatively narrow, the control loop needs only a low loop gain. The total frequency range through which the oscillator can be tuned mainly depends on the number of capacitors that can be selectively activated. Thus, the inventive phase locked loop circuit combines analog with digital control of the loop to achieve low analog gain and a wide frequency pulling range.  
         [0005]     In one arrangement, the partial frequency ranges overlap each other so that the control loop may always operate in a linear range.  
         [0006]     To control the selective activation and deactivation of the frequency tuning elements, a threshold comparator monitors the control voltage and controls the switching array to activate or deactivate one of the frequency tuning elements when either of the upper and lower control voltage limits is reached or exceeded. However, since the oscillator responds immediately to a change of the capacitance that determines its frequency and the control voltage needs much more time to react to the change of frequency, preferred embodiments of the invention have an inhibiting circuit which is set whenever one of the capacitors is activated or deactivated to inhibit an activation or deactivation of further frequency tuning elements. The inhibiting circuit is reset only after the control voltage has evolved away from the upper or lower control voltage limit by a predefined amount. Preferably, the control voltage is forced away from the control voltage limit during the set condition of the inhibiting circuit. Since the loop filter normally includes a filter capacitance charged to the control voltage, the phase-frequency discriminator is functionally disconnected from the loop filter during the set condition of the inhibiting circuit, and the capacitance is selectively connected to a charge sink or to a charge source to reduce or increase the control voltage applied to the control input of the voltage controlled oscillator.  
         [0007]     Although the analog loop gain remains constant within each partial frequency range of the oscillator, it changes from one partial range to another. Since a change in loop gain affects the bandwidth of the loop and a change of bandwidth is detrimental to the performance of the loop, the described implementation provides a compensation mechanism. Specifically, a charge pump associated with the loop filter is supplied with a charge current that is varied in accordance with the number of activated capacitors or, the particular partial frequency range in which the oscillator operates, in a manner to keep the bandwidth of the loop substantially constant throughout the total pulling range of the oscillator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     Further advantages and features of the invention will be apparent from the description of example embodiments, with reference to the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a block diagram of a phase locked loop circuit embodiment illustrating principles of the invention;  
         [0010]      FIG. 2  is a schematic diagram illustrating a variable capacitance in a voltage controlled oscillator;  
         [0011]      FIG. 3  is a chart showing overlapping partial frequency ranges of the oscillator;  
         [0012]      FIG. 4  is a diagram that illustrates the evolution of a control voltage in operation of the oscillator;  
         [0013]      FIG. 5  is a schematic diagram of an alternative frequency tuning circuit; and  
         [0014]      FIG. 6  is a schematic diagram of a further alternative frequency tuning circuit. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0015]     The phase locked loop (PLL) circuit in  FIG. 1  is a variable frequency oscillator. In a generally conventional manner, the analog control loop includes a voltage controlled oscillator (VCO)  10 , a frequency divider  12  connected to the output of the VCO  10 , a phase-frequency discriminator (PFD)  14  having a first input connected to the output of the frequency divider  12  and a second input which receives a reference frequency fin, a charge pump (CP)  16  connected to the “up” and “down” outputs of the PFD  14 , and a loop filter (LF)  18  having an input connected to the output of the LF  16  and an output connected to a control input of the VCO  10  and providing a frequency control signal V CRTL . The frequency divider  12  divides by “n” the output of the VCO  10  at frequency f out  to provide a system frequency f sys .  
         [0016]     The VCO  10  includes a variable capacitor C and a number of fixed capacitors C 1  to C 5 , the number of which is of course exemplary only. Each of the fixed capacitors C 1  to C 5  can be selectively switched functionally in parallel with variable capacitor C by a switching array contained within VCO  10 . The capacitance of variable capacitor  10  is controlled by the control voltage V CRTL . The effective capacitance that determines the operating frequency of the oscillator is the sum of the capacitances of capacitor C and any of the fixed capacitors C 1  to C 5  connected in parallel by the switching array within VCO  10 . It should be understood that in an actual circuit design in CMOS technology, the capacitors C and C 1  to C 5  would be distributed between the stages of a ring oscillator that would typically form the variable frequency oscillator.  
         [0017]     The VCO  10  has one control input for each of the fixed capacitors C 1  to C 5 . The control inputs “1” to “5” are connected to corresponding outputs of a mode select circuit  20 . The mode select circuit  20  has a clock input and a control input, both connected to corresponding outputs of a control voltage sensor  22 . The output of the mode select circuit  20  are “thermometer coded”, i.e. the more outputs have a logic “high” value, the more of the fixed capacitors C 1  to C 5  will be activated (switched in parallel with capacitor C) by the switching array within VCO  10 .  
         [0018]     The control voltage sensor  22  has an input connected to the output of LF  18  to receive the control voltage V CRTL . The control voltage sensor  22  compares the control voltage V CRTL  to predetermined upper and lower reference voltage levels. Whenever the control voltage V CRTL  reaches one of these voltage levels, the control voltage sensor supplies a clock pulse to the mode select circuit  20 . At the same time, the control voltage sensor  22  supplies an “higher/lower” control signal indicative of whether the upper or lower reference level is reached and whether a fixed capacitor is to be activated or deactivated so that a corresponding output of the mode select circuit  20  is changed from one logic state to the opposite logic state.  
         [0019]     The “upper/lower” control signal is also fed to a control input of a logic circuit  24 . A first output of the logic circuit  24  is connected to the output of the LF  18 . A second output of the logic circuit  24  is connected to a control input of a switch  26  connected between the outputs of the PFD  14  and the inputs of the CP  16 . The logic circuit  24  further has inputs “Set” and “Reset” connected to corresponding outputs of the control voltage sensor  22 .  
         [0020]     A controllable load current source  28  is associated with the charge pump CP  16  to supply a variable charge current, and has control inputs connected to the outputs “1” to “5” of the mode select circuit  20 .  
         [0021]     In  FIG. 2 , an inverting buffer  30  has variable capacitor C and fixed capacitors C 1  to C 5  connected to its output. Buffer  30  may be one out of e.g. five stages of a ring oscillator in the VCO  10 . Each of the fixed capacitors C 1  to C 5  has one terminal connectable to ground Gnd through a corresponding switch SWI to SW 5  of the switching array within VCO  10 . Each switch SW 1  to SW  5  is controlled by a corresponding one of the control inputs “1” to “5”. By selectively controlling the switches SW 1  to SW 5 , the corresponding capacitors C 1  to C 5  are connected in parallel with capacitor C.  
         [0022]     In the exemplary embodiment illustrated in  FIG. 3 , a total frequency pulling range extending from 250 MHz to 1.25 GHz is divided into six partial ranges referred to as “modes” 0 to 5. Mode 0 corresponds to all capacitors C 1  to C 5  being deactivated and is thus the highest partial frequency range. Mode 5 corresponds to all capacitors C 1  to C 5  being activated and is thus the lowest partial frequency range. The partial frequency ranges overlap each other at the upper and lower ends. In the example shown, each mode is tuned through in a time period of 1 μs by varying the control voltage V CRTL  through the available range of substantially linear loop operation.  
         [0023]      FIG. 3  illustrates a process where the input frequency fin to PFD increases and the control V CRTL  increases accordingly. At point “2” in the diagram, the control voltage reaches a predetermined upper reference voltage level designated “⅘”, which is a ⅘ fraction of a fixed reference voltage. The control voltage sensor  22  now delivers to mode select circuit  20  a clock pulse and a control signal “higher” so that the mode select circuits forces one of its outputs from logic “high” to logic “low”, thereby deactivating one of the capacitors C 1  to C 5  in VCO  10 . Simultaneously, the control voltage sensor  22  delivers to the logic circuit  24  a “Set” signal and the “higher” signal. The logic circuit now inhibits the PFD  14  by opening switch  26  and disconnecting the PFD  14  from the charge pump. In addition, the logic circuit  24  connects the output of LF  18  to a current sink to discharge the capacitance contained therein, thereby reducing the control voltage V CRTL  until point “3” in the diagram of  FIG. 4  is reached. In a practical implementation, the capacitance in LF  18  would be discharged to ground through a pull-down resistor. At point “3” in  FIG. 4 , the control voltage sensor  22  detects a control voltage of another fraction of the fixed reference voltage, ¾ in this example. The control sensor  22  now delivers a “Reset” signal to logic circuit  24 , which causes switch  26  to close and disconnects the output of LF  18  from the current sink. If the oscillator frequency is still too low, the control voltage V CRTL  increases again from point “3” and the mode switching process is repeated at point “4”. At point “5”, the control voltage V CRTL  reaches the ¾ fraction of the reference voltage, and the logic circuit  24  returns to its non-inhibiting condition.  
         [0024]     In  FIG. 4 , it is assumed that the frequency is too high after the second mode switching step so that the control loop reduces the control voltage V CRTL  after point “5” until the control loop locks in at point “6”.  
         [0025]     The inhibiting and pull-down functions of the logic circuit allow the control loop sufficient time to adapt the control voltage after each mode switching step. A similar process as the one illustrated in  FIG. 4  occurs when the frequency fin of the input signal is reduced. In this case, a lower predetermined reference voltage is a ⅕ fraction of the fixed reference voltage and a ¼ fraction is the level at which the control voltage sensor  22  delivers a “Reset” signal to logic circuit  24 . To raise the control voltage after a mode switching step wherein a fixed capacitor is activated, the logic circuit  24  would connect the output of LF  18  to a current source through a pull-up resistor.  
         [0026]     To maintain substantially constant the bandwidth of the loop over the entire frequency pulling range, the current source  28  supplies a variable current to the charge pump CP  16  in accordance with the current operating mode of the oscillator.  
         [0027]     In the  FIG. 5  embodiment, an inverting buffer  30   a  has a variable capacitor Ca connected to its output. Buffer  30   a  is supplied from a plurality of current sources Io, I 1 , I 2 , I 3 , . . . that can be connected in parallel by means of an array of switches Sw 1 , Sw 2 , Sw 3 , . . . . Like in  FIG. 2 , the arrangement shown is but one stage of the ring oscillator that forms the VCO. The more of the current sources are connected in parallel, the faster the capacitor Ca will be charged and the higher will be the frequency of the oscillator.  
         [0028]     In the  FIG. 6  embodiment, buffer stages  30   0 ,  30   1 ,  30   2 ,  30   3 , . . . are connected in parallel to variable capacitor Cb and can be selectively enabled by application of an appropriate control signal to an enable input. Each of the enable inputs E of buffer stages  30   1 ,  30   2 ,  30   3 , . . . is selectively connected to a control voltage source through one of the switches Sw 1 , Sw 2 , Sw 3 , . . . in a switching array.  
         [0029]     Those skilled in the art to which the invention relates will appreciate that variations, additions, deletions and substitutions can be made in the described example embodiments, without departing from the spirit and scope of the invention, as defined by the claims.