BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a frequency synthesizer using a phase locked loop and, more particularly, to a frequency synthesizer in which a frequency gain variation of a voltage controlled oscillator is compensated.  
           [0003]    2. Background of the Related Art  
           [0004]    A frequency synthesizer refers to a circuit or a device that generates a signal of a specific frequency within a predetermined range to output the signal. Most frequency synthesizers employ a phase locked loop (PLL) scheme.  
           [0005]    [0005]FIG. 1 is a block diagram of a conventional phase locked loop frequency synthesizer. As shown in FIG. 1, the conventional phase locked loop frequency synthesizer includes a phase comparator  101 , a loop filter  103 , a voltage controlled oscillator  105  and a feedback divider  107 .  
           [0006]    The phase comparator  101  compares a reference signal Fin applied thereto with a signal outputted from the feedback divider  107  and generates a phase error signal when there is a phase difference between the two signals. This phase difference means a frequency difference between the reference signal Fin and the output signal of the feedback divider  107 . The loop filter  103  low-pass-filters the phase error signal outputted from the phase comparator  101  and stabilizes the signal. The voltage controlled oscillator  105  controls the frequency of its output oscillating signal Fout according to the phase error signal inputted from the loop filter  103 . The feedback divider  107  is connected between the voltage controlled oscillator  105  and phase comparator  101  to divide the signal Fout outputted from the voltage controlled oscillator  105  at a division rate N.  
           [0007]    In the phase locked loop frequency synthesizer shown in FIG. 1, loop gain is proportional to the multiplication result of phase gain of the phase comparator  101 , voltage gain of the loop filter  103  and frequency gain of the voltage controlled oscillator but inversely proportional to the division rate N of the feedback divider  107 . Accordingly, the phase locked loop frequency synthesizer is designed such that the gains and division rate N have predetermined values to satisfy a predetermined loop gain value.  
           [0008]    In the conventional phase locked loop frequency synthesizer, however, the voltage controlled oscillator  105  has a problem that its gain characteristics continuously vary with control voltage or oscillation frequency. In case of using integrated voltage controlled oscillator  105 , especially, its gain characteristics vary with a fabrication process, temperature and power voltage. A variation in the gain characteristics of the voltage controlled oscillator changes not only the gain characteristics of the frequency synthesizer but also the phase response characteristics of the frequency synthesizer. In other words, a variation in the gain characteristics of the voltage controlled oscillator  105  affects phase noise and stability of the entire system and deteriorates the performance of the phase locked loop frequency synthesizer.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, the present invention has been made to substantially obviate one or more problems due to limitations and disadvantages of the related art.  
           [0010]    An object of the present invention is to provide a control circuit and method for maintaining a uniform loop gain of a phase locked loop.  
           [0011]    Another object of the present invention is to provide a phase locked loop frequency synthesizer which can compensate a variation in frequency gain of a voltage controlled oscillator to uniformly maintain its gain characteristics.  
           [0012]    Still another object of the present invention is to a phase locked loop frequency synthesizer which can detect a gain variation of a voltage controlled oscillator and control frequency gain of the voltage controlled oscillator to obtain uniform frequency gain of the voltage controlled oscillator.  
           [0013]    To accomplish the above objects, according to one embodiment of the present invention, there is provided a phase locked loop frequency synthesizer, comprising a phase comparator for comparing phases of first and second signals applied thereto with each other and outputting a phase error signal when there is a phase difference between the two signals; a loop filter for filtering the phase error signal outputted from the phase comparator and stabilizing the filtered signal, to output a control signal; a voltage controlled oscillator for controlling frequency gain of a signal output in response to the control signal outputted from the loop filter; a divider for dividing the frequency of the output signal of the voltage controlled oscillator according to a division rate to apply it to the phase comparator as the second signal; a voltage detector for detecting control voltage from the control signal of the voltage controlled oscillator; and a controller for calculating a variation in gain characteristics of the voltage controlled oscillator using the control voltage outputted from the voltage detector and the division rate of the divider and adjusting gain of at least one of the phase comparator, the loop filter and the voltage controlled oscillator, to control gain of a loop composed of the phase comparator, the loop filter, the voltage controlled oscillator and the divider to be substantially uniform.  
           [0014]    Preferably, the division rate of the divider is set by the controller.  
           [0015]    Preferably, the phase comparator includes a charge pump circuit, and phase gain of the phase comparator is controlled by adjusting a current value of a driving bias current source included in the charge pump circuit.  
           [0016]    Also, preferably, the loop filter includes a variable gain amplifier, and voltage gain of the loop filter is controlled by adjusting a gain value of the variable gain amplifier.  
           [0017]    The voltage detector is preferably composed of an analog-digital converter.  
           [0018]    Preferably, the voltage controlled oscillator includes at least two voltage controlled oscillators, and one of the voltage controlled oscillators is activated according to a control signal provided by the controller.  
           [0019]    Also, preferably, the voltage controlled oscillator includes at least one inductor and capacitor that determine a frequency band, and frequency gain of the voltage controlled oscillator is varied by controlling an impedance value of the inductor or capacitor.  
           [0020]    In accordance with one embodiment of the present invention, there is also provided a method for detecting frequency gain of a voltage controlled oscillator of a phase locked loop frequency synthesizer including a phase comparator for comparing phases of first and second signals applied thereto with each other and outputting a phase error signal when there is a phase difference between the two signals, a loop filter for filtering the phase error signal outputted from the phase comparator and stabilizing the filtered signal, to output a control signal, a voltage controlled oscillator for controlling frequency gain of a signal output in response to the control signal outputted from the loop filter, and a divider for dividing the frequency of the output signal of the voltage controlled oscillator according to a division rate to apply it to the phase comparator as the second signal, the method comprising the steps of: a first step of setting the division rate of the divider to a predetermined first division rate, and detecting control voltage from the control signal; a second step of setting the division rate of the divider to a predetermined second division rate, and detecting control voltage from the control signal; and a third step of calculating the frequency gain of the voltage controlled oscillator using the frequency of the first signal, the control voltages detected at the first and second steps, the first and second division rates.  
           [0021]    Preferably, the frequency gain of the voltage controlled oscillator corresponds to Fin×(N 1 -N 2 )/(V 1 -V 2 ) where Fin is the first signal, N 1  and N 2  denote the first and second division rates, respectively, V 1  and V 2  represent the control voltages detected at the first and second steps, respectively.  
           [0022]    In accordance with another embodiment of the present invention, there is provided a method for detecting frequency gain of a voltage controlled oscillator of a phase locked loop frequency synthesizer including a phase comparator for comparing phases of first and second signals applied thereto with each other and outputting a phase error signal when there is a phase difference between the two signals, a loop filter for filtering the phase error signal outputted from the phase comparator and stabilizing the filtered signal, to output a control signal, a voltage controlled oscillator for controlling frequency gain of a signal output in response to the control signal outputted from the loop filter, and a divider for dividing the frequency of the output signal of the voltage controlled oscillator according to a division rate to apply it to the phase comparator as the second signal, the method comprising the steps of: a first step of setting the frequency of the output signal of the voltage controlled oscillator to a predetermined first frequency; a second step of detecting control voltage from the control signal; a third step of controlling the division rate of the divider to vary the frequency of the output signal of the voltage controlled oscillator by a predetermined frequency value, and detecting control voltage from the control signal; a fourth step of calculating the frequency gain of the voltage controlled oscillator using the control voltages detected at the second and third steps and the predetermined frequency value; and a fifth step of comparing the frequency of the output signal of the voltage controlled oscillator with a predetermined second frequency and repeatedly performing the second and fourth steps until the frequency of the output signal has a value identical to the second frequency value.  
           [0023]    Preferably, the frequency gain of the voltage controlled oscillator corresponds to Fstep/(V 1 -V 2 ) where Fstep is the predetermined frequency, V 1  denotes the control voltage detected at the second step, and V 2  represents the control voltage detected at the third step.  
           [0024]    In accordance with another embodiment of the present invention, there is provided a method for detecting frequency gain of a voltage controlled oscillator of a phase locked loop frequency synthesizer including a phase comparator for comparing phases of first and second signals applied thereto with each other and outputting a phase error signal when there is a phase difference between the two signals, a loop filter for filtering the phase error signal outputted from the phase comparator and stabilizing the filtered signal, to output a control signal, a voltage controlled oscillator for controlling frequency gain of a signal output in response to the control signal outputted from the loop filter, and a divider for dividing the frequency of the output signal of the voltage controlled oscillator according to a division rate to apply it to the phase comparator as the second signal, the method comprising the steps of: a first step of detecting a control voltage value from the control signal at a predetermined reference frequency; a second step of varying the frequency of the output signal of the voltage controlled oscillator from the reference frequency by a predetermined specific frequency and detecting control voltage from the control signal; a third step of varying the frequency of the output signal of the voltage controlled oscillator from the reference frequency by the specific frequency and detecting control voltage from the control signal; and a fourth step of calculating the frequency gain of the voltage controlled oscillator using the control voltages respectively detected at the second and third steps and the frequency of the output signal.  
           [0025]    Preferably, the frequency gain of the voltage controlled oscillator corresponds to (F 1 -F 2 )/(V 1 -V 2 ) where F 1  is the frequency of the output signal at the second step, F 2  is the frequency of the output signal at the third step, V 1  denotes the control voltage detected at the second step, and V 2  represents the control voltage detected at the third step.  
           [0026]    In accordance with one embodiment of the present invention, there is provided a method for uniformly controlling a loop gain of a voltage controlled oscillator of a phase locked loop frequency synthesizer including a phase comparator for comparing phases of first and second signals applied thereto with each other and outputting a phase error signal when there is a phase difference between the two signals, a loop filter for filtering the phase error signal outputted from the phase comparator and stabilizing the filtered signal, to output a control signal, a voltage controlled oscillator for controlling frequency gain of a signal output in response to the control signal outputted from the loop filter, and a divider for dividing the frequency of the output signal of the voltage controlled oscillator according to a division rate to apply it to the phase comparator as the second signal, the method comprising the steps of: a first step of setting the frequency of the output signal of the voltage controlled oscillator to a predetermined first frequency; a second step of detecting control voltage from the control signal; a third step of controlling the division rate of the divider to vary the frequency of the output signal of the voltage controlled oscillator by a predetermined frequency value, and detecting control voltage from the control signal; a fourth step of calculating the frequency gain of the voltage controlled oscillator using the control voltages detected at the second and third steps and the predetermined frequency value; and a fifth step of comparing the frequency of the output signal of the voltage controlled oscillator with a predetermined second frequency and repeatedly performing the second and fourth steps until the frequency of the output signal has a value identical to the second frequency value; and a sixth step of setting a desired output signal frequency of the voltage controlled oscillator, grasping the frequency gain of the voltage controlled oscillator at the corresponding frequency as a value calculated through the first to fifth steps, and controlling gains of the phase comparator and loop filter.  
           [0027]    Preferably, frequency gain of the voltage controlled oscillator corresponds to Fstep/(V 1 -V 2 ) where Fstep is the predetermined frequency, V 1  denotes the control voltage detected at the second step, and V 2  represents the control voltage detected at the third step.  
           [0028]    In accordance with another embodiment of the present invention, there is provided a method for controlling loop gain of a voltage controlled oscillator of a phase locked loop frequency synthesizer including a phase comparator for comparing phases of first and second signals applied thereto with each other and outputting a phase error signal when there is a phase difference between the two signals, a loop filter for filtering the phase error signal outputted from the phase comparator and stabilizing the filtered signal, to output a control signal, a voltage controlled oscillator for controlling frequency gain of a signal output in response to the control signal outputted from the loop filter, and a divider for dividing the frequency of the output signal of the voltage controlled oscillator according to a division rate to apply it to the phase comparator as the second signal, the method comprising the steps of: a first step of detecting a control voltage value from the control signal at a predetermined reference frequency; a second step of varying the frequency of the output signal of the voltage controlled oscillator from the reference frequency by a predetermined specific frequency and detecting control voltage from the control signal; a third step of varying the frequency of the output signal of the voltage controlled oscillator from the reference frequency by the specific frequency and detecting control voltage from the control signal; a fourth step of calculating the frequency gain of the voltage controlled oscillator using the control voltages respectively detected at the second and third steps and the frequency of the output signal; and a fifth step of controlling gain of the phase comparator or gain of the loop filter, to control the loop gain to be substantially uniform.  
           [0029]    Preferably, the frequency gain of the voltage controlled oscillator corresponds to (F 1 -F 2 )/(V 1 -V 2 ) where F 1  is the frequency of the output signal at the second step, F 2  is the frequency of the output signal at the third step, V 1  denotes the control voltage detected at the second step, and V 2  represents the control voltage detected at the third step.  
           [0030]    In accordance with another embodiment of the present invention, there is provided a method for controlling frequency gain of a voltage controlled oscillator of a phase locked loop frequency synthesizer to be substantially uniform, the frequency synthesizer including a phase comparator for comparing phases of first and second signals applied thereto with each other and outputting a phase error signal when there is a phase difference between the two signals, a loop filter for filtering the phase error signal outputted from the phase comparator and stabilizing the filtered signal, to output a control signal, a voltage controlled oscillator for controlling frequency gain of a signal output in response to the control signal outputted from the loop filter, and a divider for dividing the frequency of the output signal of the voltage controlled oscillator according to a division rate to apply it to the phase comparator as the second signal, the method comprising the steps of: a first step of detecting a control voltage value from the control signal at a predetermined reference frequency; a second step of varying the frequency of the output signal of the voltage controlled oscillator from the reference frequency by a predetermined specific frequency and detecting control voltage from the control signal; a third step of varying the frequency of the output signal of the voltage controlled oscillator from the reference frequency by the specific frequency and detecting control voltage from the control signal; a fourth step of calculating the frequency gain of the voltage controlled oscillator using the control voltages respectively detected at the second and third steps and the frequency of the output signal; and a fifth step of comparing the calculated frequency gain with a predetermined reference gain and controlling the frequency gain of the voltage controlled oscillator to be substantially uniform.  
           [0031]    Preferably, the frequency gain of the voltage controlled oscillator corresponds to (F 1 -F 2 )/(V 1 -V 2 ) where F 1  is the frequency of the output signal at the second step, F 2  is the frequency of the output signal at the third step, V 1  denotes the control voltage detected at the second step, and V 2  represents the control voltage detected at the third step.  
           [0032]    It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings;  
         [0034]    [0034]FIG. 1 is a block diagram of a conventional phase locked loop frequency synthesizer;  
         [0035]    [0035]FIG. 2 is a block diagram of a phase locked loop frequency synthesizer according to an embodiment of the present invention;  
         [0036]    [0036]FIG. 3 is a flow chart for showing a procedure of detecting and compensating a frequency gain variation according to control voltage of the voltage controlled oscillator shown in FIG. 2 according to an embodiment of the present invention;  
         [0037]    [0037]FIG. 4 is a flow chart for showing a procedure of detecting and compensating a frequency gain variation according to control voltage of the voltage controlled oscillator shown in FIG. 2 according to another embodiment of the present invention;  
         [0038]    [0038]FIG. 5 is a flow chart for showing a procedure of detecting and compensating a frequency gain variation according to control voltage of the voltage controlled oscillator shown in FIG. 2 according to another embodiment of the present invention;  
         [0039]    [0039]FIG. 6 is a block diagram showing the phase locked loop frequency synthesizer in which a gain variation of the voltage controlled oscillator is compensated according to an embodiment of the present invention in more detail;  
         [0040]    [0040]FIG. 7 is a block diagram showing a phase locked loop frequency synthesizer in which a gain variation of a voltage controlled oscillator is compensated according to another embodiment of the present invention in more detail; and  
         [0041]    [0041]FIG. 8 is a block diagram of a gain controllable voltage controlled oscillator according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0043]    Basic Composition Of a Phase Locked Loop Frequency Synthesizer According to an Embodiment Of the Present Invention  
         [0044]    [0044]FIG. 2 is a block diagram of a phase locked loop frequency synthesizer according to an embodiment of the present invention. Referring to FIG. 2, the phase locked loop frequency synthesizer according to an embodiment of the invention includes a phase comparator  201 , a loop filter  203 , a voltage controlled oscillator  205 , a feedback divider  207 , a voltage detector  209  and a controller  211 .  
         [0045]    The phase comparator  201  compares the phase of a reference signal Fin with the phase of a signal outputted from the feedback divider  207  and generates a phase error signal when there is a phase difference between the two signals. Phase gain of the phase comparator  201  has a predetermined value in the initial state and it is controlled by a first control signal Vc 1  applied from the controller  211 .  
         [0046]    The loop filter  203  filters the phase error signal generated by the phase comparator  201  and stabilizes the signal. The phase error signal outputted from the loop filter  203  is applied to the voltage controlled oscillator  205  as its control voltage. As well-known in the art, a low pass filter is generally used as the loop filter  203 . Voltage gain of the loop filter  203  is set to have a predetermined value in the initial state and it is controlled by a second control signal Vc 2  applied from the controller  211 .  
         [0047]    The voltage controlled oscillator  205  controls the frequency of its output signal Fout according to the control voltage V 1 pf applied thereto. Frequency gain of the voltage controlled oscillator  205  is determined by a value obtained by dividing a variation in the frequency of the output signal Fout by a variation in the control voltage V 1 pf. The frequency gain is controlled by a third control signal Vc 3  outputted from the controller  211 .  
         [0048]    The feedback divider  207  divides the frequency of the signal Fout outputted from the voltage controlled oscillator  205 . The division rate N of the feedback divider  207  is decided on the basis of a fourth control signal Vc 4  outputted from the controller  211 .  
         [0049]    The voltage detector  209  detects the control voltage V 1 pf, outputted from the loop filter  203  and applied to the voltage controlled oscillator  205 , and outputs it to the controller  211 .  
         [0050]    The controller  211  detects a frequency gain variation of the voltage controlled oscillator  205  and controls phase gain of the phase comparator  201  or voltage gain of the loop filter  203 , to compensate a frequency gain variation of the voltage controlled oscillator  205 . Furthermore, the controller  211  detects a variation in the frequency gain characteristics of the voltage controlled oscillator  205  and applies the third control signal Vc 3  capable of compensating the detected variation to the voltage controlled oscillator  205  so as to maintain uniform gain characteristics of the voltage controlled oscillator  205 .  
         [0051]    Although FIG. 2 shows that the controller  211  outputs all of the first, second and third control signals, a variation in the gain characteristics of the voltage controlled oscillator  205  can be compensated even if only one of the first, second and third control signals is applied to the corresponding element. In other words, the gain characteristics variation of the voltage controlled oscillator  205  can be compensated by controlling only the phase gain of the phase comparator  201  without controlling the voltage gain of the loop filter  203  and the frequency gain of the voltage controlled oscillator  205 , and the loop gain can be maintained uniform by controlling only the voltage gain of the loop filter  203  or frequency gain characteristics of the voltage controlled oscillator  205 .  
         [0052]    Method of Detecting and Compensating Gain Characteristics of the Voltage Controlled Oscillator According to an Embodiment of the Present Invention  
         [0053]    A method of detecting the gain characteristics of the voltage controlled oscillator  205  and compensating a variation in the gain characteristics according to an embodiment of the present invention is explained below.  
         [0054]    In the phase locked loop frequency synthesizer, the output signal frequency Fout is represented by the value obtained by multiplying the input signal frequency Fin by the division rate N of the feedback divider  207 . That is, the output signal frequency Fout corresponds to Fin×N in the case that the division rate is N.  
         [0055]    The frequency gain of the voltage controlled oscillator  205  can be represented by the value obtained by dividing a variation of the output signal frequency Fout by a variation of the control voltage V 1 pf. That is, output signal frequency Fout 1  when the division rate is N 1  is N 1 ×Fin and output signal frequency Fout 2  when the division rate is N 2  becomes N 2 ×Fin. In the case that control voltages applied to the voltage controlled oscillator  205  are V 1 pf 1  and V 1 pf 2 , frequency gain of the voltage controlled oscillator  205  becomes Fin×(N 1 -N 2 )/(V 1 pf 1 -V 1 pf 2 ).  
         [0056]    Through this method, desired frequency gain of the output frequency Fout can be obtained. Loop gain of the frequency synthesizer can be maintained uniform by controlling the phase gain of the phase comparator  201  or voltage gain of the loop filter  203 . More specifically, phase gain of the phase comparator or voltage gain of the loop filter  203  is reduced when frequency gain of the voltage controlled oscillator  205  becomes higher than a predetermined reference gain but the phase gain of the phase comparator  201  or voltage gain of the loop filter  203  is increased in the case that the frequency gain of the voltage controlled oscillator  205  becomes lower than the reference gain so that the loop gain of the frequency synthesizer can maintain a uniform value irrespective of a variation in the frequency gain of the voltage controller oscillator  205 .  
         [0057]    In accordance with another embodiment of the present invention, the measured frequency gain characteristics of the voltage controlled oscillator  205  can be negatively fed back and controlled to converge the frequency gain of the voltage controlled oscillator  205  on a desired value. Accordingly, the frequency gain of the voltage controlled oscillator  205  can be maintained uniform irrespective of the control voltage or oscillating frequency.  
         [0058]    [0058]FIG. 3 is a flow chart of a procedure that detects frequency gain characteristics KVCO according to control voltage VLPF of the voltage controlled oscillator  205  and controls phase gain Kpd of the phase comparator  201  and voltage gain K 1 pf of the loop filter  203  so as to compensate the gain characteristics KVCO of the voltage controlled oscillator  205  according to an embodiment of the present invention.  
         [0059]    In the initializing block, an output signal frequency Fo of the voltage controlled oscillator  205  is set to minimum frequency Fmin, at step  301 . As described above, since the output signal frequency Fo can be represented by the value obtained by multiplying the input signal frequency Fin by the division rate N of the feedback divider  207 , the output signal frequency Fo can be set to the minimum frequency Fmin when the controller  211  sets the division rate N of the feedback divider  207  to a minimum value.  
         [0060]    At step  303 , a variable VLPF 1  is set as the control voltage VLPF. The control voltage VLPF of the voltage controlled oscillator  205  is detected through the voltage detector  209  and applied to the controller  211 . Then, the division rate N of the feedback divider  207  is controlled to increase the output signal frequency Fo of the voltage controlled oscillator  205  by a predetermined frequency Fstep and to detect control voltage VLPF in each frequency band, at step  305 .  
         [0061]    As described above, the frequency gain of the voltage controlled oscillator  205  corresponds to the value obtained by dividing a variation in the output signal frequency Fo by a variation in the control voltage VLPF so that the frequency gain of the voltage controlled oscillator  205  can be found out through the steps  301  and  303 . Specifically, the frequency gain of the voltage controlled oscillator  205  corresponds to KVCO [Fo]=Fstep/(VLPF-VLPF 1 ) when the output signal frequency is Fo (step  307 ).  
         [0062]    The steps  303  and  305  are repeated until the output signal frequency Fo of the voltage controlled oscillator  205  becomes maximum frequency Fmax, to detect the gain of the voltage controlled oscillator  205  in each frequency band, at step  309 .  
         [0063]    Upon completion of the initializing block, a desired output signal frequency Fo is set at step  311 . Then, at step  313 , phase gain Kpd of the phase comparator  201  or voltage gain K 1 pf of the loop filter  203 , which can satisfy a desired loop gain, can be calculated by setting gain KVCO of the voltage controlled oscillator  205  in case of the output signal frequency Fo to KVCO [Fo] calculated in the initializing block. Accordingly, the controller  211  can control the phase gain Kpd of the phase comparator  201  or voltage gain K 1 pf of the loop filter  203  through the first or second control signal Vc 1  or Vc 2  to maintain the entire gain of the phase locked loop frequency synthesizer uniform.  
         [0064]    [0064]FIG. 4 shows a method for sequentially increasing the output signal frequency of the voltage controlled oscillator  205  from minimum frequency Fmin up to maximum frequency Fmax. In accordance with another embodiment, it is possible to detect the control voltage and calculate frequency gain of the voltage controlled oscillator  205  while decreasing the output signal frequency of the voltage controlled oscillator from maximum frequency Fmax to minimum frequency Fmin.  
         [0065]    [0065]FIG. 4 is a flow chart of a procedure that detects the frequency gain characteristics according to the control voltage of the voltage controlled oscillator  205  and controls gain Kpd of the phase comparator  201  or gain K 1 pf of the loop filter  203  in order to maintain a desired loop gain according to another embodiment of the present invention.  
         [0066]    Referring to FIG. 4, a desired output signal frequency Fo of the voltage controlled oscillator  205  is set, and a variable Fset is set to the output frequency Fo, at step  401 . The voltage detector  207  detects control voltage VLPF of the voltage controlled oscillator  205  and sets variable VLPF 1  to the detected voltage VLPF, at step  403 . Then, the division rate N of the divider  207  is controlled such that the output signal frequency Fo of the voltage controlled oscillator  205  is reduced by a predetermined frequency Fstep, and control voltage VLPF of the voltage controlled oscillator  205  in this state is detected, at step  405 . Step  407  judges whether or not the detected voltage VLPF is identical to the voltage value of variable VLPF 1 . The step  405  is executed when the two voltage values identical to each other and step  409  is executed when they are not. That is, the output frequency Fo is reduced until control voltage VLPF in the case that the output frequency Fo of the voltage controlled oscillator has been decreased by the predetermined frequency Fstep has a value different from the control voltage VLPF 1  at the initially set frequency Fset. At step  409 , variables Fo 1  and VLPF 2  are respectively set to Fo and VLPF that are outputted at step  405 , variable Fo is set to the initially set Fset. Accordingly, variable Fo 1  is set as a frequency (referred to as ‘first frequency’ hereinafter) that is lower than the initially set output frequency Fset by a predetermined frequency (Fstep or multiples of Fstep) and variable VLPF 2  is set as a control voltage value (referred to as ‘first voltage’ hereinafter) of the voltage controlled oscillator  205  at the first frequency.  
         [0067]    At step  411 , the output frequency Fo of the voltage controlled oscillator  205  is increased from the output frequency Fset initially set by a predetermined frequency Fstep, and control voltage VLPF in this state is detected. At step  413 , the detected voltage VLPF is compared with the voltage value of variable VLPF 1 , and the output frequency of the voltage controlled oscillator  205  is increased by Fstep until the two voltage values become different from each other. When the two voltages have different values, step  415  is executed. Through steps  411  and  413 , variable Fo is set to a frequency value (referred to as ‘second frequency’ hereinafter) that is increased by a predetermined frequency (Fstep or multiples of Fstep) from the initially set output frequency Fset, and a control voltage value (referred to as ‘second voltage’ hereinafter) of the voltage controlled oscillator  205  is stored as variable VLPF.  
         [0068]    As described above, gain KVCO of the voltage controlled oscillator  205  corresponds to the value obtained by dividing a variation of the output frequency by a variation of the control voltage. Variables Fo 1  and VLPF 2  are respectively set as the first frequency and first voltage and variables Fo and VLPF are respectively set as the second frequency and second voltage through steps  401  to  413  so that gain KVCO of the voltage controlled oscillator can be calculated as (Fo-F 1 )/(VLPF 2 -VLPF) at step  415 .  
         [0069]    Then, variable Fo is set to the initially set frequency Fset at step  417 . At step  419 , a variable KVCO [Fo] is set as the frequency gain value KVCO of the voltage controlled oscillator  205 , calculated at step  415 , and gain of the phase comparator  201  or gain of the loop filter  203  for obtaining a desired loop gain is calculated. Accordingly, the controller  211  can output the first or second control signal Vc 1  or Vc 2  to maintain uniform loop gain irrespective of a variation in the gain of the voltage controlled oscillator  205 .  
         [0070]    [0070]FIG. 5 is a flow chart of a procedure that detects the frequency gain characteristics according to the control voltage of the voltage controlled oscillator shown in FIG. 2 and controls frequency gain of the voltage controlled oscillator  205  through negative feedback according to an embodiment of the present invention. The procedure of controlling frequency gain of the voltage controlled oscillator  205  according to an embodiment of the invention is explained below with reference to FIG. 5. Steps  501  to  509  for detecting the first output frequency and the first voltage, steps  511 ,  512  and  513  for detecting the second output frequency and the second voltage and step  515  for calculating gain of the voltage controlled oscillator are identical to those explained in FIG. 4 so that explanations therefore are omitted.  
         [0071]    Upon calculation of gain KVCO(mea) of the voltage controlled oscillator  205  at the specific frequency Fset, the gain KVCO(mea) is compared with a desired gain KVCO(target) at step  517 . When the two gain values are identical to each other, the procedure of controlling gain of the voltage controlled oscillator  205  is finished. In the case that the calculated gain KVCO(mea) of the voltage controlled oscillator  205  is different from the desired gain KVCO(target), the controller  211  outputs the third control signal Vc 3  to control the gain of the voltage controlled oscillator. That is, the controller reduces the gain KVCO of the voltage controller oscillator when the calculated gain KVCO(mea) of the voltage controller oscillator is larger than the desired gain KVCO(target) but increases the gain KVCO when it is smaller than the desired gain. Steps  503  to  517  are repeated until desired gain characteristics are obtained.  
         [0072]    Concrete Embodiments in which the Method of Compensating the Gain Characteristics of the Voltage Controlled Oscillator According to an Embodiment of the Present Invention is Applied to a Phase Locked Loop Frequency Synthesizer According to an Embodiment of the Present Invention  
         [0073]    [0073]FIG. 6 is a block diagram of a phase locked loop frequency synthesizer in which gain of a phase comparator is controlled using a charge pump to compensate a gain variation of a voltage controlled oscillator according to an embodiment of the present invention. Referring to FIG. 6, the phase locked loop frequency synthesizer according to an embodiment of the present invention includes a phase comparator  601 , a loop filter  603 , a voltage controlled oscillator  605 , a feedback divider  607 , a voltage detector  609 , and a controller  611 .  
         [0074]    The phase comparator  601  includes a phase detection unit  601   a  and a charge pump circuit  601   b . The gain of the phase comparator  601  can be controlled by adjusting a driving bias variable current source of the charge pump circuit  601   b.    
         [0075]    That is, the controller  611  detects gain characteristics of the voltage controlled oscillator  605  and applies a first control signal Vc 1  for compensating the detected gain characteristics of the voltage controller oscillator  605  to the bias variable current source, to maintain the entire gain characteristics of the phase locked loop frequency synthesizer uniform.  
         [0076]    Furthermore, according to another embodiment of the present invention, the voltage detector  609  can be composed of an analog/digital converter to provide control voltage V 1 pf of the voltage controller oscillator  609  as a digital signal to the controller  611 , as shown in FIG. 6.  
         [0077]    [0077]FIG. 7 is a block diagram of a phase locked loop frequency synthesizer in which gain of a loop filter is controlled to compensate gain characteristics of a voltage controlled oscillator according to another embodiment of the present invention. Referring to FIG. 7, the phase locked loop frequency synthesizer according to another embodiment of the invention includes a phase comparator  701 , a loop filter  703 , a voltage controlled oscillator  705 , a feedback divider  707 , a voltage detector  709  and a controller  711 .  
         [0078]    In the phase locked loop frequency synthesizer according to another embodiment of the invention, the loop filter  703  includes a filtering unit  703   a  and a variable gain amplifier  703   b . A variation in the gain characteristics of the voltage controller oscillator  705  can be compensated by controlling gain of the variable gain amplifier  703   b.    
         [0079]    Specifically, the controller  711  detects the gain characteristics of the voltage controller oscillator  705  and applies a control signal Vc 2  for compensating the detected gain characteristics of voltage controller oscillator  705  to the variable gain amplifier  703   b  of the loop filter  703 , to maintain the entire gain of the phase locked loop frequency synthesizer uniform.  
         [0080]    [0080]FIG. 8 is a block diagram showing an embodiment of detecting gain characteristics of the voltage controller oscillator and feeding back the gain of the voltage controller oscillator to control it. Referring to FIG. 8, the voltage controller oscillator  205  included in the phase locked loop frequency synthesizer shown in FIG. 2 is composed of a plurality of voltage controlled oscillators  205   a ,  205   b  and  205   c  and switches SW 1 , SW 2  and SW 3 , to control frequency gain of the voltage controller oscillator  205 .  
         [0081]    Frequency gains of the voltage controlled oscillators  205   a ,  205   b  and  205   c  have different characteristics according to control voltage, and an appropriate voltage controller oscillator is selected according to a third control signal Vc 3  applied to the switches SW 1 , SW 2  and SW 3 .  
         [0082]    Specifically, in the case that the frequency gain of the voltage controller oscillator is larger than a desired value, a voltage controller oscillator having smaller frequency gain is selected in order to decrease the frequency gain. On the contrary, a voltage controller oscillator having larger frequency gain is selected when the frequency gain is smaller than the desired value.  
         [0083]    Although FIG. 8 shows independent multiple voltage controller oscillators, frequency gain can be changed using a single voltage controller oscillator in such a manner that capacitance or inductance of the oscillation node of an LC-tank voltage controlled oscillator, for example, is varied through a switch. Moreover, while three voltage controlled oscillators having different frequency gain characteristics are shown in FIG. 8, it is well-known in the art that the number of voltage controlled oscillators can be increased or decreased.  
       INDUSTRIAL APPLICABILITY  
       [0084]    According to the present invention, gain characteristics of the phase locked loop frequency synthesizer can be maintained uniform by detecting a variation in frequency gain of the voltage controlled oscillator and compensating the variation. Furthermore, uniform gain characteristics of the voltage controller oscillator can be maintained by detecting a gain variation of the voltage controlled oscillator and feeding back it to control frequency gain of the voltage controlled oscillator.  
         [0085]    The forgoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.
A phase locked loop frequency synthesizer in which frequency gain of a voltage controlled oscillator is compensated is disclosed. The phase locked loop frequency synthesizer measures a frequency gain variation of the voltage controlled oscillator and compensates the variation by controlling phase gain of a phase comparator or voltage gain of a loop filter. Gain characteristics of the voltage controlled oscillator are detected and fed back to control frequency gain of the voltage controlled oscillator, so as to allow the voltage controlled oscillator to have uniform frequency gain. Accordingly, the phase locked loop frequency synthesizer can obtain uniform loop gain irrespective of a frequency gain variation of the voltage controlled oscillator and provide optimum phase noise characteristics and stability.