Abstract:
A digital FLL/PLL is provided which is capable of converging an oscillation frequency from a VCO to a desired frequency at a high speed even without setting a damping factor corresponding to each VCO gain. A digital FLL/PLL of the present invention includes: a comparator for comparing a channel signal to a loopback signal having an oscillation frequency to generate a signal error; a digital loop filter for generating a control voltage that determines the oscillation frequency, on the basis of the signal error; a VCO for controlling an oscillation frequency on the basis of the control voltage; a loopback path through which the oscillation frequency generated by the VCO is outputted as the loopback signal to the comparator; and a control section for monitoring the signal error, and controlling the digital loop filter such that the oscillation frequency of the VCO becomes a stationary state, when detecting that the signal error meets a predetermined condition after the channel signal is switched.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a digital frequency/phase locked loop (FLL: Frequency Locked Loop, PLL: Phase Locked Loop) used in a wireless communication device or the like, and, more specifically, relates to a digital FLL/PLL that converges an oscillation frequency to a desired frequency at a high speed on the basis of a signal error that is the difference between a channel signal and the oscillation frequency. 
       BACKGROUND ART 
       [0002]    In recent years, with technology of wireless LAN, third-generation mobile phone, digital broadcasting, and the like, digitalized communication/broadcasting has a purpose of switching the frequency of a channel signal. As a method for converging a frequency outputted from a wireless communication device or the like to the frequency of a channel signal when switching the frequency of the channel signal as described above, technology using a digital FLL/PLL is known. 
         [0003]      FIG. 12  is a diagram illustrating a digital FLL  900  in the conventional art. In  FIG. 12 , the digital FLL  900  includes a frequency comparator  910 , an FIR filter  920 , an IIR filter  930 , a digital-analogue convertor (DAC)  940 , a voltage-controlled oscillator (VCO)  950 , and a frequency-digital convertor  960 . 
         [0004]    The frequency comparator  910  compares a channel signal D_ref inputted to the digital FLL  900  to a loopback signal D_vco and outputs a frequency error signal D_error between the channel signal D_ref and the loopback signal D_vco. The FIR filter  920  and the IIR filter  930  output a control voltage signal D_vtune on the basis of the frequency error D_error outputted from the frequency comparator  910 . 
         [0005]    Here, the FIR filter  920  includes first to third delay blocks Z -1    921  to  923 , first and second adders  924  and  925 , and a multiplier  926  having a fixed multiplying factor of ⅓. The FIR filter  920  performs a moving average process on the frequency error D_error by using the third delay blocks Z -1    921  to  923 . In addition, the IIR filter  930  includes first and second multipliers  931  and  933 , first and second adders  932  and  934 , and a delay block Z -1    935 . An output of the FIR filter  920  is inputted to the first multiplier  931  and the first adder  932  of the IIR filter  930 . The first multiplier  931  multiplies the output of the FIR filter  920  by a weighting factor β. The first adder  932  adds an output of the second multiplier  933  to the output of the FIR filter  920 . The second multiplier  933  multiplies an output of the first adder  932  looped back via the delay block Z -1    935 , by a weighting factor α. The second adder  934  sums an output of the first multiplier  931  and an output of the first adder  932 , and outputs the summed output as the control voltage signal D_vtune to the DAC  940 . 
         [0006]    The control voltage signal D_vtune is analogue-converted by the DAC  940  and then inputted to the VCO  950 . The VCO  950  controls an oscillation frequency fout outputted from the VCO  950 , on the basis of the inputted control voltage signal. The oscillation frequency fout generated by the VCO  950  is digital-converted by the frequency-digital convertor  960  and returns as the loopback signal D_vco to the frequency comparator  910 . 
         [0007]    In this manner, the digital FLL  900  generates the control voltage signal D_vtune on the basis of the frequency error signal D_error between the channel signal D_ref and the loopback signal D_vco, and further controls the oscillation frequency fout outputted from the VCO  950 , on the basis of the control voltage signal D_vtune. 
         [0008]      FIG. 13  is a diagram illustrating a situation where the oscillation frequency fout from the VCO  950  of the digital FLL  900  in the conventional art converges to a desired frequency. In  FIG. 13 , between times t 0  and t 1 , the reference frequency of the channel signal D_ref and the oscillation frequency fout from the VCO  950  are in a stationary state at the same frequency f 1 . 
         [0009]    When the frequency of the channel signal D_ref is switched from f 1  to f 2  at time t 1 , the oscillation frequency fout from the VCO  950  does not instantly come into a stationary state at the frequency f 2 . The oscillation frequency fout from the VCO  950  converges to the desired frequency f 2  with repeated vibrations, and substantially comes into a stationary state at time t 3 . 
         [0010]    The reason why the oscillation frequency fout from the VCO  950  converges to the desired frequency f 2  with repeated vibrations as described above is that due to group delays of the FIR filter  920  and the IIR filter  930 , the frequency error signal D_error is not instantly transferred. 
         [0011]      FIG. 14A  is a diagram illustrating the frequency error signal D_error that is an output from the frequency comparator  910 , D_FIR that is an output from the FIR filter  920 , and D_IIR_B that is an output from the first multiplier  931  of the IIR filter  930 .  FIG. 14B  is a diagram illustrating D_IIR_A that is an output from the first adder  932  of the IIR filter  930 , and D_IIR_C that is an output from the second multiplier  933  of the IIR filter  930 . Hereinafter, timings of operations of the digital FLL  900  will be described with reference to  FIGS. 14A and 14B . 
         [0012]    Between times t 0  and t 1  between which the oscillation frequency fout from the VCO  950  is in a stationary state at the frequency f 1 , the frequencies of the frequency error signal D_error, D_FIR, and D_IIR_B are in a stationary state at 0 in  FIG. 14A , and the frequencies of D_IIR_A and D_IIR_C are in a stationary state at f 1  in  FIG. 14B . 
         [0013]    Here, when the frequency of the channel signal D_ref is switched from f 1  to f 2  at time t 1 , the frequency of D_error rapidly falls to near —(f 1 -f 2 ) in  FIG. 14A . This is because the frequency of the channel signal D_ref is switched to f 2  at time t 1  but the frequency of the oscillation frequency fout does not instantly become f 2 . The frequency difference between the channel signal D_ref and the loopback signal D_vco based on the oscillation frequency fout becomes about —(f 1 -f 2 ), and the frequency comparator  910  outputs a frequency error signal D_error having a frequency of —(f 1 -f 2 ). 
         [0014]    Then, the FIR filter  920  outputs D_FIR on the basis of the frequency error D_error outputted from the frequency comparator  910 . In  FIG. 14A , D_FIR delays from D_error. This is due to the delay properties of the FIR filter  920  (the third delay blocks Z -1    921  to  923  and the like). Further, D_FIR is multiplied by the weighting factor β by the first multiplier  931  of the IIR filter  930  and outputted as D_IIR_B. Here, the weighting factor β=0.3. 
         [0015]    Further, when the frequency of the channel signal D_ref is switched from f 1  to f 2  at time t 1 , the frequency of D_IIR_A falls from f 1  to f 2  slightly after time t 1  in  FIG. 14B . This is because D_IIR_A is obtained by adding the output of the second multiplier  933  to D_FIR, which is the output of the FIR filter  920 , and thus influenced by the above delay properties of the FIR filter  920 . Then, D_IIR_C is obtained by looping back the above D_IIR_A via the delay block Z -1    935  and multiplying D_IIR_A by the weighting factor α by the second multiplier  933  of the IIR filter  930 , and thus further delays from D_IIR_A. Here, the weighting factor α=1.0. 
         [0016]    As described above, according to the digital FLL  900 , when the frequency of the channel signal D_ref is switched from f 1  to f 2  at time t 1 , due to the group delays of the FIR filter  920  and the IIR filter  930 , the frequency error signal D_error is not instantly transferred, and the oscillation frequency fout from the VCO  950  converges to the desired frequency f 2  while repeatedly vibrating in a regular attenuation vibration cycle T (=1/ωn (ωn: natural frequency). In other words, in the digital FLL  900 , it takes a certain time (time t 3 −time t 1 ) until the oscillation frequency fout from the VCO  950  converges to the desired frequency f 2 . The above conventional art is disclosed, for example, in Non-Patent Literature 1. 
       Citation List  
     [Non Patent Literature] 
       [0017]    [NPL 1] Dean Banerjee, “PLL Performance, Simulation, and Design 4th Edition”, [online], [searched on Jan. 28, 2009], Internet &lt;http://www.national.com/appinfo/wireless/files/deansbook4.pdf&gt;. 
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention  
       [0018]    Here, when the natural frequency (on is increased by reducing the operation load of the digital filter (the FIR filter  920  and the IIR filter  930 ) only when switching the frequency of the channel signal D_ref, the attenuation vibration cycle T becomes low, and thus the oscillation frequency fout from the VCO  950  can be converged to the desired frequency at a high speed. 
         [0019]    However, when the VCO gain varies, it is necessary to adjust the natural frequency (on by setting a damping factor corresponding to each VCO gain. In other words, a desired natural frequency (on is not obtained unless the damping factor is corrected as appropriate by using the actual measured value of each VCO gain. Thus, when converging the oscillation frequency from the VCO to the desired frequency, the effect of speedup is not exerted at 100%. 
         [0020]    Therefore, an object of the present invention is to provide a digital FLL/PLL that is capable of converging an oscillation frequency from a VCO to a desired frequency at a high speed even without setting a damping factor corresponding to each VCO gain. 
       Solution to the Problems  
       [0021]    To achieve the above object, according to a first aspect of the present invention, a digital FLL/PLL or controlling an outputted oscillation frequency on the basis of a signal error that is a difference between an inputted channel signal and the oscillation frequency. The digital frequency/phase locked loop comprises: a comparator for comparing the channel signal to a loopback signal having the oscillation frequency to generate the signal error; a digital loop filter for generating a control voltage that determines the oscillation frequency, on the basis of the signal error; a VCO for controlling an oscillation frequency on the basis of the control voltage; a loopback path through which the oscillation frequency generated by the VCO is outputted as the loopback signal to the comparator; and a control section for monitoring the signal error generated by the comparator, and controlling the digital loop filter such that the oscillation frequency of the VCO comes into a stationary state, when detecting that the signal error is within a predetermined range based on 0 after the channel signal is switched. 
         [0022]    Further, the control section may monitor the signal error generated by the comparator, and may control the digital loop filter such that the oscillation frequency of the VCO comes into a stationary state, when detecting that the absolute value of the signal error is minimum after the channel signal is switched. 
         [0023]    Further, the control section may monitor a temporal average of the signal error generated by the comparator. Further, the control section may control the digital loop filter by using a temporal average of the control voltage generated by the digital loop filter. 
         [0024]    Further, the control section may have a function to correct a delay time occurring between an input and an output of the loopback path. 
         [0025]    Preferably, the digital loop filter includes an FIR filter and an IIR filter, and the control section sets 0 to a delay block of the FIR filter, and sets the control voltage generated by the digital loop filter to a delay block of the IIR filter. 
         [0026]    Further, preferably, the loopback path includes a frequency-digital convertor that performs analogue-digital conversion on the oscillation frequency generated by the VCO. 
         [0027]    Further, preferably, the digital frequency/phase locked loop further comprises: a subband selection circuit for controlling selection of a subband in which the VCO oscillates at a desired frequency; and a switch, provided between the digital loop filter and the VCO, for switching between inputs of the control voltage generated by the digital loop filter and a control voltage from the subband selection circuit. The subband selection circuit fixes a control voltage inputted to the VCO, during the selection of the subband, and changes the control voltage inputted to the VCO, after the selection of the subband. The switch switches to connect the subband selection circuit to the VCO, at start of the selection of the subband, and switches to connect the digital loop filter to the VCO, when the oscillation frequency of the VCO comes into a stationary state. 
         [0028]    Further, the digital frequency/phase locked loop may further comprise a DAC for performing digital-analogue conversion on the control voltage generated by the digital loop filter. 
         [0029]    To achieve the above object, a second aspect of the present invention applies the digital FLL/PLL described above by incorporating the digital FLL/PLL into a wireless communication device or the like. 
       ADVANTAGEOUS EFFECTS OF THE INVENTION 
       [0030]    As described above, according to the present invention, the digital loop filter is controlled into a stationary state on the basis of the signal error, thereby implementing a digital FLL/PLL that is capable of converging the oscillation frequency from the VCO to a desired frequency at a high speed. In other words, the present invention does not converge the oscillation frequency from the VCO to a desired frequency at a high speed by adjusting the natural frequency ωn to decrease the attenuation vibration cycle T. Thus, even when the VCO gain varies, the present invention can converge the oscillation frequency from the VCO to a desired frequency at a high speed without making correction to a damping factor corresponding to each VCO gain. It should be noted that when a stationary state is provided in a short time after the frequency of the channel signal is switched, each device can be set in a sleep mode and thus reduction of current consumption can be achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    [ FIG. 1 ]  FIG. 1  is a diagram illustrating a digital FLL  100  according to a first embodiment of the present invention. 
           [0032]    [ FIG. 2 ]  FIG. 2  is a diagram illustrating a situation where an oscillation frequency fout from a VCO  150  of the digital FLL  100  according to the first embodiment of the present invention converges to a desired frequency. 
           [0033]    [ FIG. 3 ]  FIG. 3  is a flowchart illustrating an operation of a control section  170  of the digital FLL  100  according to the first embodiment of the present invention. 
           [0034]    [ FIG. 4A ]  FIG. 4A  is a diagram illustrating a frequency error signal D_error that is an output from a frequency comparator  110 , D_FIR that is an output from an FIR filter  120 , and D_IIR_B that is an output from a first multiplier  131  of an IIR filter  130 . 
           [0035]    [ FIG. 4B ]  FIG. 4B  is a diagram illustrating D_IIR_A that is an output from a first adder  132  of the IIR filter  130 , and D_IIR_C that is an output from a second multiplier  133  of the IIR filter  130 . 
           [0036]    [ FIG. 4C ]  FIG. 4C  is a diagram representing the absolute value of the frequency error signal D_error illustrated in  FIG. 4A . 
           [0037]    [ FIG. 4D ]  FIG. 4D  is a diagram illustrating a digital FLL  100   b  according to the first embodiment of the present invention. 
           [0038]    [ FIG. 4E ]  FIG. 4E  is a diagram illustrating a digital FLL  100   c  according to the first embodiment of the present invention. 
           [0039]    [ FIG. 5 ]  FIG. 5  is a diagram illustrating a digital FLL  200  according to a second embodiment of the present invention. 
           [0040]    [ FIG. 6 ]  FIG. 6  is a diagram illustrating a situation where an oscillation frequency fout from a VCO  150  of the digital FLL  200  according to the second embodiment of the present invention converges to a desired frequency. 
           [0041]    [ FIG. 7 ]  FIG. 7  is a flowchart illustrating an operation of the digital FLL  200  according to the second embodiment of the present invention. 
           [0042]    [ FIG. 8A ]  FIG. 8A  is a diagram illustrating relationships between a control voltage inputted to the VCO  150  and an oscillation frequency when subbands (N−1) to (N+2) are selected. 
           [0043]    [ FIG. 8B ]  FIG. 8B  is a diagram illustrating a digital FLL  200   b  according to the second embodiment of the present invention. 
           [0044]    [ FIG. 8C ]  FIG. 8C  is a diagram illustrating a digital FLL  200   c  according to the second embodiment of the present invention. 
           [0045]    [ FIG. 9A ]  FIG. 9A  is a diagram illustrating a digital PLL  300  according to a third embodiment of the present invention. 
           [0046]    [ FIG. 9B ]  FIG. 9B  is a diagram illustrating a digital PLL  300   b  according to the third embodiment of the present invention. 
           [0047]    [ FIG. 9C ]  FIG. 9C  is a diagram illustrating a digital PLL  300   c  according to the third embodiment of the present invention. 
           [0048]    [ FIG. 10 ]  FIG. 10  is a diagram illustrating a polar modulation circuit  400  according to a fourth embodiment of the present invention. 
           [0049]    [ FIG. 11 ]  FIG. 11  is a diagram illustrating a wireless communication device  500  according to a fifth embodiment of the present invention. 
           [0050]    [ FIG. 12 ]  FIG. 12  is a diagram illustrating a digital FLL  900  in the conventional art. 
           [0051]    [ FIG. 13 ]  FIG. 13  is a diagram illustrating a situation where an oscillation frequency fout from a VCO  950  of the digital FLL  900  in the conventional art converges to a desired frequency. 
           [0052]    [ FIG. 14A ]  FIG. 14A  is a diagram illustrating a frequency error signal D_error that is an output from a frequency comparator  910 , D_FIR that is an output from an FIR filter  920 , and D_IIR_B that is an output from a first multiplier  931  of an IIR filter  930 . 
           [0053]    [ FIG. 14B ]  FIG. 14B  is a diagram illustrating D_IIR_A that is an output from a first adder  932  of the IIR filter  930 , and D_IIR_C that is an output from a second multiplier  933  of the IIR filter  930 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0054]    Hereinafter, each embodiment of the present invention will be described with reference to the drawings. 
       First Embodiment 
       [0055]      FIG. 1  is a diagram illustrating a digital FLL  100  according to a first embodiment of the present invention. In  FIG. 1 , the digital FLL  100  includes a frequency comparator  110 , an FIR filter  120 , an IIR filter  130 , a VCO  150 , a frequency-digital convertor  160 , and a control section  170 . The digital FLL  100  according to the first embodiment of the present invention is typically applied to a frequency synthesizer. 
         [0056]    The frequency comparator  110  compares a channel signal D_ref inputted to the digital FLL  100  to a loopback signal D_vco and outputs a frequency error signal D_error between the channel signal D_ref and the loopback signal D_vco. The FIR filter  120  and the IIR filter  130  output a control voltage signal D_vtune on the basis of the frequency error D_error outputted from the frequency comparator  110 . 
         [0057]    Here, the FIR filter  120  includes first to third delay blocks Z -1    121  to  123 , first and second adders  124  and  125 , and a multiplier  126  having a fixed multiplying factor of ⅓. The FIR filter  120  performs a moving average process on the frequency error D_error by using the first to third delay blocks Z -1    121  to  123 . Further, the IIR filter  130  includes first and second multipliers  131  and  133 , first and second adders  132  and  134 , and a delay block Z -1    135 . 
         [0058]    An output of the FIR filter  120  is inputted to the first multiplier  131  and the first adder  132  of the IIR filter  130 . The first multiplier  131  multiplies the output of the FIR filter  120  by a weighting factor β. The first adder  132  adds an output of the second multiplier  133  to the output of the FIR filter  120 . The second multiplier  133  multiplies an output of the first adder  132  looped back via the delay block Z -1    135 , by a weighting factor α. The second adder  134  sums an output of the first multiplier  131  and an output of the first adder  132 , and outputs the summed output as the control voltage signal D_vtune. 
         [0059]    The control voltage signal D_vtune is inputted to the VCO  150 . The VCO  150  controls an oscillation frequency fout outputted from the VCO  150 , on the basis of the inputted control voltage signal. Here, a loopback path through which the oscillation frequency fout generated by the VCO  150  is looped back to the frequency comparator  110  includes the frequency-digital convertor  160 . The oscillation frequency fout generated by the VCO  150  is digital-converted by the frequency-digital convertor  160  and returns as the loopback signal D_vco to the frequency comparator  110 . 
         [0060]    In this manner, the digital FLL  100  generates the control voltage signal D_vtune on the basis of the frequency error signal D_error between the channel signal D_ref and the loopback signal D_vco, and further controls the oscillation frequency fout outputted from the VCO  150 , on the basis of the control voltage signal D_vtune. 
         [0061]    The configuration and the operation of the digital FLL  100  described so far are the same as the configuration and the operation of the digital FLL  900  in the conventional art. The digital FLL  100  according to the first embodiment of the present invention further includes the control section  170 . Hereinafter, the difference between the digital FLL  100  according to the first embodiment of the present invention and the digital FLL  900  in the conventional art will be described in detail with a description concerning an operation of the control section  170 . 
         [0062]      FIG. 2  is a diagram illustrating a situation where the oscillation frequency fout from the VCO  150  of the digital FLL  100  according to the first embodiment of the present invention converges to a desired frequency. In  FIG. 2 , between times t 0  and t 1 , the reference frequency of the channel signal D_ref and the oscillation frequency fout from the VCO  150  are in a stationary state at the same frequency f 1 . 
         [0063]    When the frequency of the channel signal D_ref is switched from f 1  to f 2  at time t 1 , the oscillation frequency fout from the VCO  150  does not instantly come into a stationary state at the frequency f 2 . However, the oscillation frequency fout from the VCO  150  does not converge to the desired frequency f 2  with repeated vibrations, as illustrated in  FIG. 13 , between times t 2  and t 3 , but substantially comes into a stationary state at the desired frequency f 2  at time t 2 . This is because at time t 2 , the control section  170  controls the FIR filter  120  and the IIR filter  130  on the basis of the frequency error signal D_error. 
         [0064]      FIG. 3  is a flowchart illustrating the operation of the control section  170  of the digital FLL  100  according to the first embodiment of the present invention. Further,  FIG. 4A  is a diagram illustrating the frequency error signal D_error that is an output from the frequency comparator  110 , D_FIR that is an output from the FIR filter  120 , and D_IIR_B that is an output from the first multiplier  131  of the IIR filter  130 .  FIG. 4B  is a diagram illustrating D_IIR_A that is an output from the first adder  132  of the IIR filter  130 , and D_IIR_C that is an output from the second multiplier  133  of the IIR filter  130 . Hereinafter, timings of operations of the digital FLL  100  will be described with reference to  FIGS. 3 ,  4 A, and  4 B. 
         [0065]    Between times t 0  and t 1  between which the oscillation frequency fout from the VCO  150  is in a stationary state at the frequency f 1 , the frequencies of the frequency error signal D_error, D_FIR, and D_IIR_B are in a stationary state at 0 in  FIG. 4A , and the frequencies of D_IIR_A and D_IIR_C are in a stationary state at f 1  in  FIG. 4B . This is the same as the stationary state illustrated in  FIGS. 14A and 14B . 
         [0066]    In  FIG. 3 , when the frequency of the channel signal D_ref is switched from f 1  to f 2  (at time t 1  in  FIG. 2 ), the control section  170  starts a process for converging the oscillation frequency fout from the VCO  150 , to a desired frequency at a high speed. Then, the control section  170  executes steps S 101  to S 104  in order. 
         [0067]    At step S 101 , the control section  170  monitors the frequency error signal D_error, which is from the frequency comparator  110 . When the frequency error signal D_error does not meet a predetermined condition, the control section  170  continues to monitor the frequency error signal D_error (No at step S 102 ). It should be noted that between times t 1  and t 2 , in  FIG. 4A , D_error, D_FIR, and D_IIR_B exhibit the same characteristics as those in  FIG. 14A , and in  FIG. 4B , D_IIR_A and D_IIR_C exhibit the same characteristics as those in  FIG. 14B . 
         [0068]    When the frequency error signal D_error meets the predetermined condition after the frequency of the channel signal D_ref is switched from f 1  to f 2 , namely, when the control section  170  detects that the frequency error signal D_error meets the predetermined condition after starting the monitoring of the frequency error signal D_error, the control section  170  proceeds to a process at step S 103  (Yes at step S 102 ). At step S 103 , the control section  170  obtains the control voltage signal D_vtune, which is the output from the IIR filter  130 , and proceeds to a process at step S 104 . 
         [0069]    Here, the control section  170  can determine whether or not the frequency error signal D_error meets the predetermined condition, on the basis of whether or not the frequency error signal D_error is 0. In other words, when the frequency error signal D_error is not 0 such as between times t 1  and t 2  in  FIG. 2 , the control section  170  continues to monitor the frequency error signal D_error (No at step S 102 ). When the control section  170  detects that the frequency error signal D_error is 0 after starting the monitoring of the frequency error signal D_error (e.g., at time t 2  in  FIG. 4A ), the control section  170  proceeds to the process at step S 103  (Yes at step S 102 ). 
         [0070]    Alternatively, the control section  170  may detect whether or not the frequency error signal D_error meets the predetermined condition, on the basis of whether or not the frequency error signal D_error is within a predetermined range based on 0. It should be noted that the predetermined range based on 0 is preferably close to 0. In this case, when the control section  170  detects that the frequency error signal D_error is within the predetermined range based on 0 after starting the monitoring of the frequency error signal D_error, the control section  170  advances the processing to step S 103  (Yes at step S 102 ). This is for assuredly advancing the operation to steps subsequent to the step S 103  even when the frequency error signal D_error does not completely become 0 due to reasons of digital signal processing. 
         [0071]    Alternatively, the control section  170  may detect whether or not the frequency error signal D_error meets the predetermined condition, on the basis of whether or not the absolute value of the frequency error signal D_error is minimum.  FIG. 4C  is a diagram representing the absolute value of the frequency error signal D_error illustrated in  FIG. 4A .  FIG. 4C  illustrates the case where the absolute value of the frequency error signal D_error reaches minimum at time t 2 . When the control section  170  detects that the absolute value of the frequency error signal D_error is minimum after starting the monitoring of the frequency error signal D_error (e.g., at time t 2  in  FIG. 4C ), the control section  170  advances the processing to step S 103  (Yes at step S 102 ). 
         [0072]    At step S 104 , the control section  170  sets 0 to the first to third delay blocks Z -1    121  to  123  of the FIR filter  120 , and sets the control voltage signal D_vtune obtained at step S 103  to the delay block Z -1    135  of the IIR filter  130 . By so doing, at time t 2 , the frequencies of D_FIR and D_IIR_B become 0 in  FIG. 4A , and the frequencies of D_IIR_A and D_IIR_C become f 2  in  FIG. 4A . 
         [0073]    Here, the FIR filter  120  and the IIR filter  130  of the digital FLL  100  according to the first embodiment of the present invention will be compared to the FIR filter  920  and the IIR filter  930  of the digital FLL  900  in the conventional art. At step S 104 , the control section  170  sets 0 to the first to third delay blocks Z -1    121  to  123  of the FIR filter  120 , and sets the control voltage signal D_vtune obtained at step S 103  to the delay block Z -1    135  of the IIR filter  130 , whereby at time t 2 , the FIR filter  120  and the IIR filter  130  of the digital FLL  100  according to the first embodiment of the present invention come into a state that is the same as that at time t 3  of the FIR filter  920  and the IIR filter  930  of the digital FLL  900  in the conventional art (see  FIG. 13 ). 
         [0074]    Therefore, the oscillation frequency fout from the VCO  150  of the digital FLL  100  according to the first embodiment of the present invention does not converge to the desired frequency f 2  with repeated vibrations as illustrated in  FIG. 13 , between times t 2  and t 3 , but substantially comes into a stationary state at the desired frequency f 2  at time t 2 . 
         [0075]    As described above, according to the digital FLL  100  according to the first embodiment of the present invention, at time t 2  when the control section  170  detects that the frequency error signal D_error meets the predetermined condition, the control section  170  controls the digital loop filter into a stationary state (a state at time t 3  in  FIG. 13 ), whereby the oscillation frequency fout from the VCO  150  can be converged to the desired frequency at a high speed. 
         [0076]    Further, according to the digital FLL  100  according to the first embodiment of the present invention, a stationary state is provided in a short time after the frequency of the channel signal is switched, and thus each device can be set in a sleep mode and reduction of current consumption can be achieved. 
         [0077]    Further, in order to maximally exert the effects of the present invention, the digital FLL  100  may operate, for example, as follows. At step S 101  in  FIG. 3 , the control section  170  monitors the frequency error signal D_error, which is from the frequency comparator  110 . At that time, the control section  170  may use a temporal average of the frequency error signal D_error for monitoring the frequency error signal D_error. By so doing, the control section  170  can reduce the influence of a noise component included in the frequency error signal D_error, when monitoring the frequency error signal D_error. 
         [0078]    Further, at step S 103  in  FIG. 3 , similarly, the control section  170  may use a temporal average of the control voltage signal D_vtune for obtaining the control voltage signal D_vtune. By so doing, the control section  170  can reduce the influence of the noise component included in the control voltage signal D_vtune, when obtaining the control voltage signal D_vtune. 
         [0079]    By using the temporal average of at least either one of the frequency error signal D_error or the control voltage signal D_vtune as described above, the control section  170  can set a value that reduces the influence of the noise component, at step S 104 . 
         [0080]    Further, in the example described above, the control section  170  calculates the temporal averages of the frequency error signal D_error and the control voltage signal D_vtune. However, a component other than the control section  170  may calculate them. In this case, the digital FLL circuit  100  may be configured to further include, for example, at least either one of an averaging section  180  or an averaging section  190  as in a digital FLL  100   b  illustrated in  FIG. 4D . The averaging section  180  calculates a temporal average of the frequency error signal D_error outputted from the frequency comparator  110 , and outputs the temporal average to the control section  170 . The averaging section  190  calculate a temporal average of the control voltage signal D_vtune, and outputs the temporal average to the control section  170 . 
         [0081]    It should be noted that since the influence of the noise component is reduced as described above, it is effective to calculate the temporal averages of the frequency error signal D_error and the control voltage signal D_vtune. However, if the timing of determining at step S 102  whether or not the predetermined condition is met and the timing of obtaining the control voltage signal D_vtune at step S 103  are out of synchronization with each other, the effects of the present invention are reduced. Thus, the frequency error signal D_error and the control voltage signal D_vtune are desirably temporally averaged at the same level. 
         [0082]    Further, for the timing of obtaining the control voltage signal D_vtune at step S 103 , it is desirable to take into consideration a delay time occurring at the frequency-digital convertor  160 . In other words, the control section  170  desirably has a function to correct a delay time occurring between an input and an output of the loopback path. 
         [0083]    Further, the digital FLL  100  according to the first embodiment may be configured to further include a DAC  140  as in a digital FLL  100   c  illustrated in  FIG. 4E . The DAC  140  performs digital-analogue conversion on the control voltage signal D_vtune generated by the IIR filter  130 , and outputs the resultant signal to the VCO  150 . 
         [0084]    Further, other than the frequency synthesizer, the digital FLL  100  according to the first embodiment of the present invention may be applied to a frequency modulation circuit. The frequency modulation circuit performs frequency modulation on an inputted modulation signal, and outputs the resultant signal as a frequency modulation signal. 
       Second Embodiment 
       [0085]      FIG. 5  is a diagram illustrating a digital FLL  200  according to a second embodiment of the present invention. In  FIG. 5 , the digital FLL  200  includes a frequency comparator  110 , an FIR filter  120 , an IIR filter  130 , a VCO  150 , a frequency-digital convertor  160 , a control section  170 , a switch  210 , and a subband selection circuit  220 . The digital FLL  200  according to the second embodiment of the present invention differs from the digital FLL  100  according to the first embodiment of the present invention in including the switch  210  between the IIR filter  130  and the VCO  150  and in including the subband selection circuit  220  for selecting a subband. In  FIG. 5 , the same components as those in  FIG. 1  are designated by the same reference characters, and the detailed description thereof is omitted. In the present embodiment, the difference from the digital FLL  100  according to the first embodiment of the present invention will be described in detail. 
         [0086]      FIG. 9  is a diagram illustrating a situation where an oscillation frequency fout from the VCO  150  of the digital FLL  200  according to the second embodiment of the present invention converges to a desired frequency. In  FIG. 6 , between times t 0  and t 1 , the reference frequency of a channel signal D_ref and the oscillation frequency fout from the VCO  150  are in a stationary state at the same frequency f 1 . 
         [0087]    When the frequency of the channel signal D_ref is switched from f 1  to f 2  at time t 1 , the oscillation frequency fout from the VCO  150  does not instantly come into a stationary state at the frequency f 2 , and substantially come into a stationary state at the desired frequency f 2  at time t 2   a.  The digital FLL  200  performs subband selection between times t 1  and t 1   a  and changes a control voltage to the VCO  150  between times t 1   a  and t 2   a,  thereby causing the frequency error signal D_error to approach 0. 
         [0088]      FIG. 7  is a flowchart illustrating an operation of the digital FLL  200  according to the second embodiment of the present invention. In  FIG. 7 , when the frequency of the channel signal D_ref is switched from f 1  to f 2  (at time t 1  in  FIG. 6 ), the digital FLL  200  starts a process for converging the oscillation frequency fout from the VCO  150 , to a desired frequency at a high speed. Then, the digital FLL  200  executes steps S 201  to S 210  in order. 
         [0089]    At step S 201 , the digital FLL  200  switches an input terminal of the switch  210  to the terminal A side to connect the subband selection circuit  220  to the VCO  150 . 
         [0090]    At step S 202 , a lower bit outputted from the subband selection circuit  220  is fixed. 
         [0091]    By steps S 201  and S 202 , the lower bit outputted from the subband selection circuit  220  is inputted as a control voltage signal to the VCO  150  via the switch  210 . Since the lower bit outputted from the subband selection circuit  220  is fixed at step S 202 , the control voltage signal inputted to the VCO  150  is also fixed. 
         [0092]    At step S 203 , an upper bit outputted from the subband selection circuit  220  is changed, whereby subband selection is performed while a subband setting is changed. Here, the subband selection will be described.  FIG. 8A  is a diagram illustrating relationships between the control voltage inputted to the VCO  150  and an oscillation frequency when subbands (N−1) to (N+2) are selected. In the present embodiment, by fixing the lower bit outputted from the subband selection circuit  220 , the control voltage inputted to the VCO  150  is fixed, and the subband selection is performed. In  FIG. 8A , for example, by fixing the control voltage inputted to the VCO  150  at Vo and changing the upper bit outputted from the subband selection circuit  220 , a subband in which the oscillation frequency is the desired frequency f 2  is searched for while the subband setting is changed. Examples of the method of searching for a subband include binary search. 
         [0093]    As described above, by changing the upper bit outputted from the subband selection circuit  220 , the subband setting is repeatedly changed (No at step S 204 ), a subband N that meets that FN≦f2&lt;F(N+1) is selected as illustrated in  FIG. 8A  (Yes at step S 204 , time t 1   a  in  FIG. 6 ). 
         [0094]    At step S 205 , after the subband selection is completed (Yes at step S 204 ), the upper bit outputted from the subband selection circuit  220  is fixed. 
         [0095]    At step S 206 , the lower bit outputted from the subband selection circuit  220  is changed to change the control voltage inputted to the VCO  150 . 
         [0096]    The VCO  150  controls the oscillation frequency fout outputted from the VCO  150 , on the basis of the inputted control voltage signal. The oscillation frequency fout outputted from the VCO  150  is inputted as a loopback signal D_vco to the frequency comparator  110  via the frequency-digital convertor  160 . The frequency comparator  110  compares the channel signal D_ref to the loopback signal D_vco and outputs a frequency error signal D_error between the channel signal D_ref and the loopback signal D_vco. 
         [0097]    In this manner, the lower bit outputted from the subband selection circuit  220  is changed to change the control voltage inputted to the VCO  150  and further to change the loopback signal D_vco. Thus, the frequency error signal D_error outputted from the frequency comparator  110  is also changed. 
         [0098]    Similarly as described in the first embodiment of the present invention, the control section  170  monitors the frequency error signal D_error. When the frequency error signal D_error does not meet a predetermined condition, the lower bit outputted from the subband selection circuit  220  is changed such that the frequency error signal D_error approaches 0 (No at step S 207 ). 
         [0099]    When the frequency error signal D_error meets the predetermined condition, namely, when the control section  170  detects that the frequency error signal D_error meets the predetermined condition, the control section  170  proceeds to a process at step S 208  (Yes at step S 207 ). 
         [0100]    The control section  170  can determine whether or not the frequency error signal D_error meets the predetermined condition, similarly as in the first embodiment. For example, when the frequency error signal D_error is not 0 (between times t 1   a  and t 2   a  in  FIG. 6 ), the control section  170  changes the lower bit outputted from the subband selection circuit  220  such that the frequency error signal D_error approaches 0 (No at step S 207 ). When the frequency error signal D_error is 0 (at time t 2   a  in  FIG. 6 ), namely, when the control section  170  detects that the frequency error signal D_error is 0, the control section  170  proceeds to the process at step S 208  (Yes at step S 207 ). Alternatively, the control section  170  may detect whether or not the frequency error signal D_error meets the predetermined condition, on the basis of whether or not the frequency error signal D_error is within a predetermined range based on 0 or whether or not the absolute value of the frequency error signal D_error is minimum. 
         [0101]    At step S 208 , the control section  170  obtains the control voltage signal D_vtune that is an output from the IIR filter  130 , and proceeds to a process at step S 209 . 
         [0102]    At step S 209 , the control section  170  sets 0 to first to third delay blocks Z -1    121  to  123  of the FIR filter  120 , and sets the control voltage signal D_vtune obtained at step S 103  to a delay block Z -1    135  of the IIR filter  130 . 
         [0103]    At step S 210 , the digital FLL  200  switches the input terminal of the switch  210  to the terminal B side to connect the IIR filter  130  to the VCO  150 . 
         [0104]    As described above, according to the digital FLL  200  according to the second embodiment of the present invention, after the subband selection is performed, the control voltage inputted to the VCO  150  is changed in order to cause the frequency error signal D_error to approach 0. At time t 2   a  when the control section  170  detects that the frequency error signal D error meets the predetermined condition, the control section  170  controls the digital loop filter into a stationary state (the state at time t 3  in  FIG. 13 ), whereby the oscillation frequency fout from the VCO  150  can be converged to the desired frequency at a high speed. 
         [0105]    Further, according to the digital FLL  200  according to the second embodiment of the present invention, a stationary state is provided in a short time after the frequency of the channel signal is switched, and thus each device can be set in a sleep mode and reduction of current consumption can be achieved. 
         [0106]    Similarly as in the first embodiment, the digital FLL circuit  200  according to the second embodiment may be configured to further include at least either one of an averaging section  180  or an averaging section  190  as in a digital FLL  200   b  illustrated in  FIG. 8B . The averaging section  180  calculates a temporal average of the frequency error signal D_error outputted from the frequency comparator  110 , and outputs the temporal average to the control section  170 . The averaging section  190  calculates a temporal average of the control voltage signal D_vtune, and outputs the temporal average to the control section  170 . 
         [0107]    Further, the digital FLL  200  according to the second embodiment may be configured to further include a DAC  140  between the switch  210  and the VCO  150  as in a digital FLL  200   c  illustrated in  FIG. 8C . Hereinafter, an operation different from that in  FIG. 7  when the digital FLL  200   b  includes the DAC  140  will be described. At step S 201 , the digital FLL  200   b  switches the input terminal of the switch  210  to the terminal A side to connect the subband selection circuit  220  to the DAC  140 . By steps S 201  and S 202 , the lower bit outputted from the subband selection circuit  220  is inputted to the DAC  140  via the switch  210 . The signal inputted to the DAC  140  is analogue-converted by the DAC  140  and then inputted as a control voltage signal to the VCO  150 . 
       Third Embodiment 
       [0108]    The digital FLLs  100  and  200  described in the first and second embodiments of the present invention can be applied as a digital PLL used in a wireless communication device or the like. 
         [0109]      FIG. 9A  is a diagram illustrating a digital PLL  300  according to a third embodiment of the present invention. In  FIG. 9A , the digital PLL  300  includes a phase comparator  310 , an FIR filter  120 , an IIR filter  130 , a VCO  150 , and a control section  170 . The digital PLL  300  according to the third embodiment of the present invention differs from the digital FLL  100  according to the first embodiment of the present invention illustrated in  FIG. 1 , in including the phase comparator  310  instead of the frequency comparator  110  and in not including the frequency-digital convertor  160 . 
         [0110]    In the digital PLL  300 , an oscillation frequency outputted from the VCO  150  is inputted as a loopback signal to the phase comparator  310  without any changes. The phase comparator  310  compares a channel signal D_ref to the loopback signal and outputs a phase error signal D_error between the channel signal D_ref and the loopback signal. In addition, a loopback path through which the oscillation frequency fout generated by the VCO  150  is looped back to the phase comparator  310  typically includes a DAC. The other process is the same as that of the digital FLL  100  according to the first embodiment of the present invention illustrated in  FIG. 1 , and needless to say, the same effects are obtained. 
         [0111]    Similarly as in the first embodiment, the digital PLL circuit  300  according to the third embodiment may be configured to further include at least either one of an averaging section  180  or an averaging section  190  as in a digital FLL  300   b  illustrated in  FIG. 9B . Further, the digital PLL  300  according to the third embodiment may be configured to further include a DAC  140  between the switch  210  and the VCO  150  as in a digital FLL  300   c  illustrated in  FIG. 9C . 
         [0112]    Needless to say, the digital FLL  200  described in the second embodiment can similarly be applied as a digital PLL. 
       Fourth Embodiment 
       [0113]      FIG. 10  is a diagram illustrating a polar modulation circuit  400  according to a fourth embodiment of the present invention. In  FIG. 40 , the polar modulation circuit  400  includes a signal generation section  410 , a phase modulator  420 , a regulator  430 , and a power amplifier  440 . 
         [0114]    In the polar modulation circuit  400 , the signal generation section  410  generates an amplitude signal and a phase signal. The amplitude signal is inputted to the regulator  430 . In addition, a direct-current voltage is supplied from a power supply terminal to the regulator  430 . The regulator  430  supplies a voltage Vcc controlled in accordance with the inputted amplitude signal, to the power amplifier  440 . Typically, the regulator  430  supplies a voltage Vcc proportional to the magnitude of the inputted amplitude signal, to the power amplifier  440 . The phase signal generated by the signal generation section  410  is inputted to the phase modulator  420 . The phase modulator  420  performs phase modulation on the phase signal and outputs a phase modulation signal. The power amplifier  440  amplifies the phase modulation signal with the voltage Vcc supplied from the regulator  430 . A signal Vout resulting from the amplification by the power amplifier  440  is outputted as a transmission signal from an output terminal. 
         [0115]    The digital FLL/PLL of the present invention can be incorporated as a modulator used in the phase modulator  420  of the polar modulation circuit  400 . 
       Fifth Embodiment 
       [0116]      FIG. 11  is a diagram illustrating a wireless communication device  500  according to a fifth embodiment of the present invention. In  FIG. 11 , the wireless communication device  500  includes an antenna  510 , a power amplifier  520 , a modulator  530 , a switch  540 , a low noise amplifier  550 , a demodulator  506 , and a digital FLL/PLL  570  of the present invention. 
         [0117]    When transmitting a wireless signal, the modulator  530  modulates a desired high frequency signal outputted from the digital FLL/PLL  570 , with a baseband modulation signal, and outputs the resultant signal. The high frequency modulation signal outputted from the modulator  530  is amplified by the power amplifier  520 , and radiated from the antenna  510  via the switch  540 . 
         [0118]    When receiving a wireless signal, a high frequency modulation signal received by the antenna  510  is inputted into the low noise amplifier  550  via the switch  540 , amplified, and inputted into the demodulator  506 . The demodulator  506  demodulates the inputted high frequency modulation signal into a baseband modulation signal with the high frequency signal outputted from the digital FLL/PLL  570 . A plurality of the digital FLL/PLLs  570  may be used on the transmission side and the reception side. Furthermore, the digital FLL/PLL  570  may also serve as a modulator. 
       INDUSTRIAL APPLICABILITY 
       [0119]    The present invention can be used in a wireless communication device or the like, and is useful particularly for the case where it is desired to converge the oscillation frequency of a VCO to a desired frequency at a high speed, or the like. 
       DESCRIPTION OF THE REFERENCE CHARACTERS 
       [0000]    
       
           100 ,  200 ,  900  digital FLL 
           110 ,  910  frequency comparator 
           120 ,  920  FIR filter 
           130 ,  930  IIR filter 
           140 ,  940  DAC 
           150 ,  950  VCO 
           160 ,  960  frequency-digital convertor 
           170  control section 
           180 ,  190  averaging section 
           121  to  123 ,  135 ,  921  to  923 ,  935  delay block Z -1    
           124 ,  125 ,  132 ,  134 ,  924 ,  925 ,  932 ,  934  adder 
           126 ,  131 ,  133 ,  926 ,  931 ,  933  multiplier 
           210 ,  540  switch 
           220  subband selection circuit 
           300  digital PLL 
           310  phase comparator 
           400  polar modulation circuit 
           410  signal generation section 
           420  phase modulator 
           430  regulator 
           440 ,  520  power amplifier 
           500  wireless communication device 
           510  antenna 
           530  modulator 
           550  low noise amplifier 
           560  demodulator 
           570  digital FLL/PLL