Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to Phased Locked Loops (PLLs) and, more particularly, to charge correction for a PLL filter.  
         [0003]     2. Description of the Related Art  
         [0004]     Phased Locked Loops (PLLs) are common components utilized in a variety of applications. For example, Frequency Modulation (FM) and Amplitude Modulation (AM) modulators utilize PLLs. PLLs operate by locking onto a phase and frequency of an input signal through continual adjustment of an oscillator. The PLL oscillator can be current or voltage driven. Typically, though, the PLL oscillator is a Voltage Controlled Oscillator (VCO).  
         [0005]     Referring to  FIG. 1  of the drawings, the reference numeral  100  generally designates a conventional PLL. A conventional PLL comprises a Phase-Frequency Detector (PFD)  102 , a charge pump  104 , a Low Pass Filter (LPF)  106 , a VCO  108 , and a frequency divider  110 .  
         [0006]     The illustration of the components of the PLL, though, do not necessarily lend to a complete explanation. The LPF  106  further comprises a capacitor  116  and a resistor  118  which operated on the principle of capacitive impedance where impedance of a capacitor is inversely proportional to the signal frequency. Also, the charge pump  104  further comprises a first current source  105 , a second current source  107 , a first switch  112 , and a second switch  114 .  
         [0007]     The PLL operates by maintaining charge on the first capacitor  116  of the LPF  106 . A reference signal or input signal is input into the PFD  102  through a first node  122  along with feedback from the frequency divider  110  through a second node  132 . Based on the comparison between the inputted signals, the PFD  102  either activates the first switch  112  of the charge pump  104  through a third node  124  or activates the second switch  114  of the charge pump  104  through a fourth node  126 . By activating the first switch  112 , the charge is added to the capacitor  116  of the LPF  106  through a fifth node  128 . By activating the second switch  114 , charge is removed from the capacitor  116  of the LPF  106  through the fifth node  128 .  
         [0008]     The active pulling down and pulling up the charge of the capacitor effectively changes the voltage of the LPF  106 . The voltage of the LPF  106  is then used to control the voltage of the frequency and phase of the VCO  108 . The voltage of the LPF  106  is maintained at the fifth node  128 , which is input into the VCO  108 . The VCO  108  then outputs an output signal through a sixth node  130  that has its phase and frequency synchronized with the input signal. The output signal from the VCO  108  is input into the frequency divider  110 . Also, the output signal of VCO  108  is used in a variety of circuits to perform a variety of tasks.  
         [0009]     With a conventional PLL  100  of  FIG. 1 , though, there are some disadvantages. Due to the advancement of Complimentary Metal-Oxide on a Semiconductor (CMOS) technology, the resulting thickness of the dielectric of the capacitor  116  of  FIG. 1  has become increasingly smaller. As a result of decreasing thickness of the dielectric, there has been an increase in the leakage current across the capacitor  116  of  FIG. 1 . The PLL, then cannot maintain, the proper voltage for the VCO  108  of  FIG. 1  resulting in drift of the locked in phase and frequency.  
         [0010]     Therefore, there is a need for a method and/or apparatus for correction of leakage voltage in a PLL that addresses at least some of the problems associated with conventional methods and apparatuses for PLLs.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention provides an apparatus for correcting charge leakage across an LPF. A voltage controlled Phased Locked Loop (PLL) is provided, wherein the PLL is at least configured to have a Low Pass Filter (LPF) and a Voltage Controlled Oscillator coupled at a first node. Also, a charge leakage correction circuit is provided that is at least coupled to the first node. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0013]      FIG. 1  is a block diagram depicting a conventional PLL;  
         [0014]      FIG. 2  is a block diagram depicting an improved PLL with a charge leakage correction circuit; and  
         [0015]      FIGS. 3   a  and  3   b  are graphs depicting the comparative operations of a PLL with and without current leakage correction. 
     
    
     DETAILED DESCRIPTION  
       [0016]     In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details concerning network communications, electromagnetic signaling techniques, and the like, have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the understanding of persons of ordinary skill in the relevant art.  
         [0017]     It is further noted that, unless indicated otherwise, all functions described herein may be performed in either hardware or software, or some combinations thereof. In a preferred embodiment, however, the functions are performed by a processor such as a computer or an electronic data processor in accordance with code such as computer program code, software, and/or integrated circuits that are coded to perform such functions, unless indicated otherwise.  
         [0018]     Referring to  FIG. 2  of the drawings, the reference numeral  200  generally designates an improved PLL with current leakage correction circuit. The improved PLL comprises a PFD  202 , a first charge pump  204 , an LPF  206 , a second charge pump  252 , a differentiator  250 , a VCO  208 , and a frequency divider  210 .  
         [0019]     The illustration of the most basic components of the improved PLL, though, do not necessarily lend to a complete explanation. The LPF  206  further comprises a capacitor  216  and a resistor  218  which operated on the principle of capacitive impedance where impedance of a capacitor is inversely proportional to the signal frequency. Also, the first charge pump  204  further comprises a first current source  205 , a second current source  207 , a first switch  212 , and a second switch  214 . The second charge pump  252  further comprises a third current source  253 , a fourth current source  256 , a third switch  254 , and a fourth switch  255 .  
         [0020]     In a conventional PLL as depicted in  FIG. 1 , though, maintaining a constant “locked” voltage can be difficult because of technological changes. Due to better and better CMOS technology, the thickness of the capacitor dielectric (not shown) has decreased. As a result, current leakage across the dielectric (not shown) becomes problematic because the voltage across the capacitor  116  of  FIG. 1  fluctuates. These fluctuations translate into severe short-term jitter in the output characteristic of the VCO  108 . The addition of correction circuitry (the second charge pump  252  of  FIG. 2  and a differentiator  250  of  FIG. 2 ) reduces the fluctuations resulting in a clean signal.  
         [0021]     The improved PLL operates by maintaining charge on the capacitor  216  of the LPF  206 . A reference signal or input signal is input into the PFD  202  through a first node  222  along with feedback from the frequency divider  210  through a second node  232 . Based on the comparison between the inputted signals, the PFD  202  either activates the first switch  212  of the first charge pump  204  through a third node  224  or activates the second switch  214  of the first charge pump  204  through a fourth node  226 . By activating the first switch  212 , the charge is added to the capacitor  216  of the LPF  206  through a fifth node  228 . By activating the second switch  214 , charge is removed from the capacitor  216  of the LPF  206  through the fifth node  228 .  
         [0022]     The active pulling down and pulling up the charge of the capacitor effectively changes the voltage of the LPF  206 . The voltage of the LPF  206  is then used to control the voltage of the frequency and phase of the VCO  208 . The voltage of the LPF  206  is maintained at the fifth node  228  which is input into the VCO  208 . The VCO  208  then outputs an output signal through a sixth node  230  that has a phase and frequency that is synchronized with the input signal. The output signal from the VCO  208  is input into the frequency divider  210 . Also, the output signal of VCO  208  is used in a variety of circuits to perform a variety of tasks.  
         [0023]     However, also attached to the fifth node  228 , is a second charge pump  252  and differentiator  250 . While the PFD  202 , first charge pump  204 , and LPF  206  are in the process of achieving phase and frequency lock, the differentiator  250  remains off. Thus, initially, the second charge pump  252  and the differentiator  250  are inactive. A lock detector  260  monitors the voltages of the first node  222  and the second node  232  to determine if phase and frequency lock have been achieved. Once lock is achieved, the differentiator  250  is enabled through the lock detection node  251 . The differentiator  250  then monitors the voltage at the fifth node  228 .  
         [0024]     In the process of monitoring the voltage at the fifth node  228 , the differentiator can determine the rate of change of the voltage at the fifth node  228  with respect to time or effectively determine the derivative of the voltage (dV/dt). The derivative of the voltage (dV/dt) is proportional to the leakage current through the capacitor  216  of the LPF  206 . If the rate of change of the voltage is greater than zero (dV/dt&gt;0), then the voltage on the fifth node  228  is too high, and the fourth switch  255  of the second charge pump  252  is engaged. When the fourth switch  255  is engaged, the fourth current source  256  draws current from the fifth node  228  to lower the voltage to the proper level. If the rate of change of the voltage is less than zero (dV/dt&lt;0), then the voltage on the fifth node  228  is too low, and the third switch  254  of the second charge pump  252  is engaged. When the third switch  254  is engaged, the third current source  253  supplies current to the fifth node  228  to increase the voltage to the proper level. Also, when the rate of change of the voltage is zero (dV/dt=0), then the third switch  254  and the fourth switch  255  are disengaged.  
         [0025]     Referring to  FIGS. 3   a  and  3   b  of the drawings, the reference numeral  300  generally designates graphs depicting the comparative operations of a PLL with and without current leakage correction. Both  FIGS. 3   a  and  3   b  voltages versus time graphs at node  228  of  FIG. 2 .  
         [0026]     In section  1  of  FIGS. 3   a  and  3   b,  the first charge pump  204  of  FIG. 2  is on and the second charge pump  252  of  FIG. 2  is off. During this phase of operation, the PFD  202  and the first charge pump  204  of  FIG. 2  are actively seeking phase and frequency lock. The PFD  202  of  FIG. 2  actively engages the first switch  212  and second switch  214  of the first charge pump  204  of  FIG. 2  to achieve the proper voltage at the capacitor  216  of the LPF  206  of  FIG. 2 .  
         [0027]     In section  2  of  FIG. 3   a,  when lock is achieved the first charge pump  204  of  FIG. 2  is off. Also, the second charge pump  252  of  FIG. 2  is off for the purposes of illustration. After phase and frequency lock have been achieved, the voltage, in section  2  of  FIG. 3   a,  is not constant. This is due to the leakage current associated with the capacitor  216  of  FIG. 2 .  
         [0028]     In section  2  of  FIG. 3   b,  when phase and frequency lock are achieved, the first charge pump  204  of  FIG. 2  is off and the second charge pump  252  of  FIG. 2  is on. The second charge pump  252  of  FIG. 2  actively corrects voltage fluctuations across the LPF  206  of  FIG. 2  to maintain a constant voltage. Therefore, after phase and frequency lock have been achieved, the voltage, in section  2  of  FIG. 3   b,  is constant.  
         [0029]     It will further be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. This description is intended for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.

Technology Category: h