Abstract:
Phase locked loop circuit ( 105 ) having a double entry VCO ( 158 ) and two independent charge pumps ( 171, 172 ), each connected with one of the entries of the VCO. Each of the VCO entries has a different gain coefficient, thereby allowing a better optimisation and control of the device bandwidth and a reduced phase noise. Can be employed in radio transmitters and/or receivers and allows simultaneous and precise FM modulation both inside and outside the PLL bandwidth.

Description:
REFERENCE DATA  
       [0001]     This application is a continuation in the USA of international patent application 2003WO-EP09900 (WO0525069), filed on Sep. 6, 2003, the contents whereof are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention concerns a phase locked loop circuit, and in particular but not exclusively, a PLL generator for application in a frequency synthesizer to be used in a transceiver for a wireless digital communication network.  
       DESCRIPTION OF RELATED ART  
       [0003]     It is known to employ a PLL to generate signals which are in a precise predetermined phase and frequency relation with an input signal, in general with a stabilized reference oscillator. PLL frequency synthesizers are often employed in transceivers used in digital wireless networks, which must be able to generate a variety of closely-spaced channel or local oscillator frequencies with very short channel-switch times. In these very demanding applications the PLL must provide excellent frequency stability, a low level of spurious tones and phase noise and fast locking and switching times.  
         [0004]      FIG. 1   a  represents a typical PLL of type  11 . This circuit  3  comprises a phase and frequency comparator  31  for measuring the phase and frequency difference between an input signal REF and the frequency divided output signal  42 . The phase and frequency comparator  31  generates the two digital control signals U and D for a charge pump  35  to respectively charge and discharge the loop filter cell  34 . The amount of charge generated by the charge pump  35  is for example proportional to the width of the U and D pulses according to a proportionality constant K φ . The voltage across the filter cell  34  determines the output frequency of the signal OUT at the output of the VCO  36 , according to a second proportionality constant K VCO . The output signal is fed back to the second input of the comparator  31  via the frequency divider  39 , which establishes the ratio between the output frequency and the input reference frequency, to which the output frequency is locked.  
         [0005]     U.S. Pat. No. 6,329,882 proposes a self-biased PLL for a timing of a computer system comprising two independent charge pumps, driven by the same phase detector. The signal of the second charge pump is summed to the signal coming from the bias generator for reducing the jitter of the output signal. This document describes a VCO having a single differential input.  
         [0006]     U.S. Pat. No. 5,870,003 describes another PLL for a clock of a processor. In this case the VCO has a second, current-sensitive differential input  
         [0007]     The object of U.S. Pat. No. 6,329,882 and U.S. Pat. No. 5,870,003 is to provide a stable clock for a computer system. The bandwidth of the PLL is not a very critical parameter in this application, and is usually rather low.  
         [0008]     Patent application EP780985 describes a circuit comprising a PLL having a phase detector and a double-input VCO one terminal thereof is used for biasing purposes. The bias voltage is provided by a second auxiliary PLL.  
         [0009]     U.S. Pat. No. 5,870,003 describes a high-frequency PLL for clocking a computer system having two charge pumps piloting a Delay-Interpolating VCO.  
         [0010]     A limitation of the above techniques lies in the large spread and variability of the K VCO  parameter which, in the case of a wide tuning range VCO can have a variability up to a factor  5 , depending on the VCO input voltage, which induces a variability of the same order in the PLL bandwidth.  FIGS. 7   c  and  7   d  represent a typical VCO characteristic in which K VCO  depends strongly on the control voltage V CTRL .  
         [0011]     This limitation is further exemplified by the Bode plots of  FIGS. 1   b  and  1   c . The plot  1   b  represents the spread in the open-loop gain in a sample of wide tuning range PLLS. In this case the PLL are equipped with frequency and phase comparators, which induces a second-order type behaviour, but with a considerable gain spread between extreme cases.  
         [0012]     The upper limit of the tuning range is approximately given by the open loop unity gain frequency. Plot  1   c  represents the closed-loop gain for the same sample of PLL. It can be seen that the gain spread directly translates in a large spread in the loop bandwidth.  
         [0013]     In order to obviate this limitation it is known to add an A/D converter for sensing the loop filter voltage, which is also the VCO control voltage, in order to adapt the K φ , gain and maintain a constant K φ ×K VCO  product in the full PLL dynamic, and therefore a constant bandwidth. A drawback of this solution is the need of A/D and D/A converters, and also a reduced precision in the case of an imperfect K VCO  calibration, an increased circuit complexity and compromise between charge-pump noise and current range. This solution, in which K VCO  is measured during an auto-calibration sequence, allows a precise bandwidth control, but does not lend itself well to very low-power applications, in which the PLL may be switched repeatedly on and off. Each power cycle implies in fact a new calibration, which is both time- and power-consuming.  
         [0014]     PLL bandwidth also affects directly the dynamic behaviour and output phase noise of the PLL. A bandwidth adapted to the application must be high enough to ensure that the PLL can follow the designed frequency variation, but also not too high in order to provide a “flywheel” action for smoothing over noise and jumps in the input signal. Closed loop bandwidth must therefore be strictly controlled especially in the demanding applications of high-speed wireless digital communication.  
       BRIEF SUMMARY OF THE INVENTION  
       [0015]     It is an aim of the present invention to provide a PLL circuit having an improved bandwidth control.  
         [0016]     It is likewise an object of the present invention to propose a PLL circuit which is free from the drawback of the related art.  
         [0017]     These objects of the present invention are obtained by a device according to the appended independent claim, the dependent claims describing various optional features of the invention. In particular these objects are provided by a phase locking loop circuit, comprising: 
        a phase and frequency comparator for receiving an input reference signal and a feedback signal;     a voltage controlled oscillator having a first analogue voltage input and a second analogue voltage input, independent from said first analogue voltage input, for controlling a frequency produced from said controlled oscillator;     a first charge pump receiving a signal from said frequency and phase detector and delivering a first control signal to said first voltage input of said voltage controlled oscillator;     a second charge pump receiving a signal from said frequency and phase detector and delivering a second control signal to said second voltage input of said controlled oscillator.     a feedback path, for feeding a correction signal back to said phase and frequency comparator.        
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The invention will be better comprised with reference to the accompanying claims and detailed description, illustrated by the figures wherein:  
         [0024]      FIG. 1   a  represents a PLL of conventional type;  
         [0025]      FIGS. 1   b  and  1   c  represent the bandwidth of the PLL of  FIG. 1   a  as Bode plots;  
         [0026]      FIG. 2   a  represents a PLL according to an aspect of the present invention;  
         [0027]      FIGS. 2   b  and  2   c  represent the bandwidth of the PLL of  FIG. 2   a  as Bode plots;  
         [0028]      FIG. 3   a  represents a frequency and phase detector;  
         [0029]      FIG. 3   b  represents a timing diagram of the detector of  FIG. 3   a.    
         [0030]      FIG. 4  represents a charge pump;  
         [0031]      FIG. 5  represents a VCO according to the present invention;  
         [0032]      FIG. 6  represents a Frequency synthesizer according to an aspect of the present invention;  
         [0033]      FIGS. 7   a - 7   d  shows, in diagrammatical form, the voltage-frequency VCO characteristics of the VCO used in the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]      FIG. 2  represents a simplified schematic of a PLL  5  according to a first aspect of the present invention. In the circuit of  FIG. 2   a  the input signal REF is fed to a frequency and phase detector  51  which produces two signals U and D, according to the relative phase and frequency differences of the signal present at its two entries. For example the frequency and phase detector  51  could be a known edge-sensitive lead-lag type detector, as illustrated by  FIGS. 3   a  and  3   b  or any other frequency and phase detector.  
         [0035]      FIG. 3   a  shows a simplified schematic of an edge sensitive frequency and phase detector  51  suitable for the present invention. The frequency and phase detector  51  comprises in this case two flip-flops  103  and  104  and a logic AND gate  106  arranged in such a way that a voltage pulse is generated at the U output whenever a pulse at the ref input leads ahead a corresponding pulse at the div input. Conversely, whenever the ref input lags behind the div input, pulses are generated at the D output, as it is schematized on  FIG. 3   b.    
         [0036]     The signals U and D are then split and fed to the charge pumps  71  and  72 , which are now described with reference to  FIG. 4  representing a simplified schematic of a charge pump suitable for the present invention. The charge pumps  71  and  72  generate charge pulses whose polarity is positive or negative according to whether a pulse is present at the inputs U or D. Each of the charge pumps  71  and  72  is characterised by a gain coefficient, respectively K φ1  and K φ2 .  
         [0037]     The detector represented on  FIG. 3   a  is used in this possible mode of realization of the present invention. However the skilled person will appreciate that many other types of phase and frequency detectors exist, not limited to two output control signals, which could be replaced to the detector  51  without leaving the scope of the present invention.  
         [0038]     The signals generated by the charge pumps  71  and  72  are finally connected to the dual input VCO (Voltage Controlled Oscillator)  58 , which is schematically represented on  FIG. 5 . The VCO has two analogue control voltage inputs  81  and  82 , each of which acts on an array of varactor diodes, modifying the total capacity C seen across the inductor L. The output frequency of such a circuit is given by F OUT =½π√{square root over (L·(C 1 +C 2 ))}. Since the input control voltage  82  is applied to N rows of varactors and the input control voltage  81  is applied to one row only, it follows that, all diodes being identical, the gain K VCO2  of the input  82  is N times the gain K VCO1  of the input  81 , that is: K VCO2 =N×K VCO1 . It is also possible, within the frame of the present invention, to employ varactor diodes of different characteristics in the different sections of the VCO  58  for providing two variable capacitors having different voltage coefficients.  
         [0039]     Even if this particular embodiment of the invention involves an LC oscillator, the invention is not limited to this class of circuit. An equivalent double-input VCO could in fact be obtained by other types of controlled oscillator, for example by a ring oscillator.  
         [0040]     By referring now again to  FIG. 2   a , one can appreciate that the proportional and integral part of the loop filter are split in the device of the invention. The first VCO input  81 , which has a lower K VCO1 , sees the voltage across the resistance  96  and takes care of the proportional part of the control loop, and determines the bandwidth of the PLL. The voltage at the first PLL input  81  is substantially fixed, and, as a result the variations of K VCO1  and the spread in PLL bandwidth are greatly reduced.  
         [0041]      FIGS. 7   a  and  7   b  represent the FN characteristic of the VCO  58  in function of the first control voltage  81 . During normal circuit functioning the variations of the control voltage  81  are contained within a relatively narrow range  107 . As a consequence variations in K VCO1  and in the PLL bandwidth are minimized.  
         [0042]     The second VCO input  82  sees the voltage across the filtering capacitor  97 , and is used to implement the integral part of the control loop. The K VCO2  coefficient varies largely according with the input voltage. Preferably this control voltage at the second VCO input  82  can swing from rail-to-rail. The induced K VCO2  variation does not however affect directly the bandwidth of the PLL, which is mainly dependent from K VCO1 .  
         [0043]      FIGS. 7   c  and  7   d  represent the FN characteristic of the VCO  58  in function of the second control voltage  82 .  
         [0044]     This aspect of the invention will be better comprised when comparing the open-loop and closed-loop bandwidth of this circuit with those of conventional PLL of  FIGS. 1   a ,  1   b  and  1   c .  FIG. 2   b  represents the open loop gain of a circuit like the one of  FIG. 2   a . One can see from the plot of  FIG. 2   b  that the spread in K VCO2  translates in a variation of the zero position in the open-loop transfer function. Above this frequency the open-loop gain is dominated by the relatively stable contribution of K φ1  and K VCO1 .  
         [0045]     The unity-gain frequency varies therefore very little. The closed-loop bandwidth is thus remarkably stable, as illustrated by the plot of  FIG. 2   c.    
         [0046]     This architecture has also other advantages. In particular the design constraints of the two charge pumps  71  and  72  are rather different, and each of them can be independently optimized. Charge pump  71  needs to have good noise performance, but only a limited output swing. The second charge pump  72 , on the contrary should preferably provide rail-to-rail swing, for maximal PLL frequency range, but the noise specification can be somewhat relaxed thanks to the filtering action of capacitor  97 .  
         [0047]     The contribution of thermal noise of resistor  96  to the output is also reduced, thanks to the low value of the K VCO1  factor.  
         [0048]      FIG. 6  represents a variant embodiment of the present invention. The circuit of  FIG. 6  is a frequency synthesizer and modulator which can be employed in a digital radiofrequency transmitter, and in particular in a low-power transmitter for a wireless telecommunication network, like a Bluetooth network, an 802.11 network, a GSM network, a network based on a DECT protocol or the like. The PLL  105  comprises a multiple divider  159  for obtaining a series of output frequencies, each of which is an integer or non-integer multiple of the reference frequency Ref. The multiple divider can foresee a time-switched double ratio divider, a randomized multiple ratio divider, a delta-sigma modulator or any other fractional ratio frequency synthesis technique.  
         [0049]     For use in a FM transmitter the circuit includes a variable source  183 , which is used for the modulation of the output signal. The skilled person will appreciate that other disposition of the source or modulation schemes are likewise possible within the scope of the present invention. The variable source  183  and the divider  159  are controlled by a digital controlling circuit not represented, allowing a precise FM modulation outside of the PLL bandwidth thanks to the controlled K VCO1  (modulation inside PLL bandwidth being assured by digital control).  
         [0050]     The same circuit, here illustrated in connection with a transmitter, could also serve as local oscillator in a direct conversion, low IF or heterodyne receiver.