Abstract:
A phase locked loop (PLL) is used in a radio communication system such as a frequency mixer, a carrier frequency and the like. The phase locked loop (PLL) includes a phase/frequency detector for comparing a phase/frequency of a reference signal and a feedback signal. The phase/frequency detector includes: a NAND gate logic circuit for NANDing a first signal and a second signal to output a NANDed signal; a first latch unit for latching the NANDed signal and outputting the first signal in response to a reference frequency; and a second latch unit for latching the NANDed signal and outputting the second signal in response to a feedback frequency. The phase locked loop (PLL) further includes a filter controller for changing a bandwidth of a low pass filter in response to an output signal of the phase/frequency detector.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a semiconductor device; and, more particularly, to a phase locked loop (PLL) with a high-speed locking characteristic, which is capable of obtaining a fast locking time and a reduced jitter.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Generally, a phase locked loop (PLL) is widely used in a radio communication system, such as a frequency mixer, a carrier recovery circuit, a clock generator, a modulator/demodulator, and the like. In particular, systems employing a clock recovery circuit or a frequency hopping spread spectrum require a fast frequency/phase locking.  
           [0003]    [0003]FIG. 1 is a block diagram showing a conventional PLL.  
           [0004]    Referring to FIG. 1, a conventional PLL includes a phase/frequency detector (PFD)  10 , a charge pump unit  20 , a low-pass filter (LPF)  30 , a voltage-controlled oscillator (VCO)  40  and a frequency divider  50 .  
           [0005]    The PFD  10  compares a phase/frequency of a reference signal S R  having a predetermined frequency f R  with that of a feedback signal S F  having a feedback frequency f D , to thereby obtain a phase/frequency difference therebetween. Then, the PFD  10  produces a sequence of an up pulse UP and a down pulse DN according to the phase/frequency difference.  
           [0006]    The charge pump unit  20  converts the phase/frequency difference into a positive pump current signal and a negative pump current signal in response to the up pulse UP and the down pulse DN, respectively.  
           [0007]    The LPF  30  converts the positive pump current signal and the negative pump current signal into corresponding voltage signal.  
           [0008]    The VCO  40  receives the voltage signal outputted from the LPF  30  and generates an output signal S out  having a predetermined oscillation frequency f out  that is varied with the inputted voltage signal.  
           [0009]    The frequency divider  50  divides the oscillation frequency f OUT  to output a divided oscillation frequency f D .  
           [0010]    The PFD  10  again compares the reference signal S R  with a feedback signal S F  having the divided oscillation frequency f D  as the feedback frequency. Then, the frequency/phase of the reference signal S R  is synchronized with that of the feedback signal S F  after a predetermined time by repeatedly performing the above-described looping operation.  
           [0011]    In case where the reference signal is changed or a frequency division ratio of the frequency divider is changed, the PLL repeats the feedback loop procedures in order to obtain a new fixed phase. At this time, a locking time taken to reach a phase-locked state is determined by a characteristic function of the PLL.  
           [0012]    Two methods for reducing the locking time are disclosed in Yasuaki Sumi, “FAST SETTLING PLL FREQUENCY SYNTHESIZER UTILIZING THE FREQUENCY DETECTOR METHOD SPEEDUP CIRCUIT”, IEEE Transaction on Consumer Electronics, Vol. 43, No. 3, August 1997.  
           [0013]    One method is to employ a frequency detector method speedup circuit (FDMSC). The FDMSC includes a frequency detector for detecting a frequency difference and a charge controller. The charge controller is used to fix an input signal until a first frequency locking is completed when a frequency division ration is changed.  
           [0014]    The other method is to add an LPF, which has a changeable-bandwidth, to the FDMSC. In this method, a resistance ratio that determines a gain of an active LPF is adjusted according to a frequency difference. At this time, since the LPF has a smaller time constant only at a rising time, an input voltage signal of the VCO can reach fast a target voltage, thereby reducing the locking time.  
           [0015]    In these methods, however, there are problems that a circuit configuration becomes complicated and a chip size is increased, thereby causing an increase in the power dissipation of the PLL.  
         SUMMARY OF THE INVENTION  
         [0016]    It is, therefore, an object of the present invention to provide a phase-locked loop (PLL) which is capable of obtaining a fast locking time and a reduced jitter.  
           [0017]    In accordance with an aspect of the present invention, there is provided a phase/frequency detector for comparing a phase/frequency of a reference signal having a reference frequency and that of a feedback signal having a feedback frequency in a phase locked loop (PLL), comprising: a NAND gate logic circuit for NANDing a first signal first signal and a second signal to output a NANDed signal; a first latch means for latching the NANDed signal and outputting the first signal in response to the reference signal; and a second latch means for latching the NANDed signal and outputting the second signal in response to the feedback signal.  
           [0018]    In accordance with another aspect of the present invention, there is provided a phase locked loop (PLL) comprising: a phase/frequency detection means for comparing a phase/frequency of a reference signal having a predetermined reference frequency with that of a feedback signal having a predetermined feedback frequency to generate a up pulse and a down pulse according to a phase/frequency difference, wherein the phase/frequency detection means includes two latch circuits and one gate logic circuit; a charge pump means for providing a positive pump current signal and a negative pump current signal in response to the up pulse and the down pulse;  
           [0019]    a filter means for converting the positive pump current signal and the negative pump current signal into corresponding voltage signal; and a voltage controlled oscillation means for receiving the voltage signal to generate an output signal having a predetermined oscillation frequency.  
           [0020]    Furthermore, the phase-locked loop (PLL) further comprises a filter control means for performing a switching operation in response to the up pulse and the down pulse, thereby changing a resistance of the filter means. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 is a block diagram showing a conventional PLL;  
         [0023]    [0023]FIG. 2 is a block diagram illustrating a PLL in accordance with the present invention;  
         [0024]    [0024]FIG. 3 is a circuit diagram illustrating a PFD shown in FIG. 2;  
         [0025]    [0025]FIG. 4 is a timing chart of a latch circuit in a PFD shown in FIG. 2; and  
         [0026]    [0026]FIG. 5 is a circuit diagram illustrating a filter control unit shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    [0027]FIG. 2 is a block diagram illustrating a PLL in accordance with the present invention.  
         [0028]    Referring to FIG. 2, the PLL in accordance with the present invention includes a phase/frequency detector (PFD)  200 , a charge pump unit  210 , a low-pass filter (LPF)  220 , a filter control unit  230 , a voltage-controlled oscillator (VCO)  240 , and a frequency divider  250 .  
         [0029]    The PFD  200  receives a reference signal S R′  having a predetermined frequency f R′  and a feedback signal S F′  having a predetermined feedback frequency f D′ . Then, the PFD  200  compares a phase/frequency of the reference signal S R′  with that of the feedback signal S F′ , to thereby obtain a phase/frequency difference therebetween. Then, the PFD  200  generates a up pulse UP and a down pulse DN according to the phase/frequency difference.  
         [0030]    The charge pump unit  210  generates a positive pump current signal and a negative pump current signal in response to the up pulse UP and the down pulse DN, respectively.  
         [0031]    The filter control unit  230  controls a bandwidth of the LPF  220  in response to the up pulse UP and the down pulse DN. That is, while the phase/frequency is unlocked, the filter control unit  230  performs a switching operation to change a resistance of the LPF  220 . As a result, the bandwidth of the LPF  220  is changed. Meanwhile, if the phase/frequency is locked, the filter control unit  230  is switched off, so that the LPF  220  has its own fixed bandwidth.  
         [0032]    The LPF  220  converts the pump current signal into corresponding voltage signal in response to the pump current signal. The LPF  220  implemented with a resistor and a capacitor has a predetermined bandwidth and its bandwidth is controlled by the filter control unit  230 .  
         [0033]    The VCO  240  receives the voltage signal from the LPF  220  to generate an output signal S OUT′  having a predetermined oscillation frequency F OUT′ .  
         [0034]    The frequency divider  250  divides the oscillation frequency f OUT′  to output a divided oscillation frequency f D′ .  
         [0035]    The PFD  200  again compares the reference signal S R′  having the frequency f R′  with the feedback signal S F′  having the divided oscillation frequency f D′  as the feedback frequency. After repeating the above-described looping operation, the frequency/phase of the reference signal S R′  is locked with that of the feedback signal S F′ .  
         [0036]    [0036]FIG. 3 is a circuit diagram illustrating the PFD  200  in accordance with the present invention.  
         [0037]    Referring to FIG. 3, the PFD  200  includes a first latch circuit  300 , a NAND gate logic circuit  320 , and a second latch circuit  330 .  
         [0038]    First, the NAND gate logic circuit  320  NANDs a first output Q 1  of the first latch circuit  300  and a second output Q 2  of the second latch circuit  310  to output a NANDed signal D 1 .  
         [0039]    The first latch circuit  300  receives and latches the NANDed signal D 1  and generates the first output Q 1  as the up pulse UP in response to the reference signal S R′ .  
         [0040]    The second latch circuit  310  receives and latches the NANDed signal and generates the second output Q 2  as the down pulse DN in response to the feedback signal S F′ .  
         [0041]    The first latch circuit  300  includes: a PMOS transistor  301  having a source coupled to a power terminal VDD and a gate receiving the NANDed signal; an NMOS transistor  302  having a drain coupled to a drain of the PMOS transistor  301 , a gate receiving the reference signal S R′  and a source coupled to a ground terminal GND; a PMOS transistor  303  having a source coupled to the power terminal VDD and a gate receiving the reference signal S R′ ; and an NMOS transistor  304  having a gate coupled to the drain of the NMOS transistor  302 , a drain coupled to a drain of the PMOS transistor and a source coupled to the ground terminal GND. At this time, the up pulse UP is outputted from the drain of the PMOS transistor  303 .  
         [0042]    The NAND gate logic circuit  320  includes: a PMOS transistor  321 , coupled between the power terminal VDD and a node N 1 , whose gate receives the first output Q 1 ; a PMOS transistor  322 , coupled between the power terminal VDD and the node N 1 , whose gate receives the second output Q 2 ; an NMOS transistor  323  having a drain coupled to the node N 1  and a gate receiving the first output Q 1 ; and an NMOS transistor  324  having a drain coupled to a source of the NMOS transistor  323 , a gate receiving the second output Q 2  and a source coupled to the ground terminal GND. At this time, the node N 1  is an output terminal of the NAND gate logic circuit  320 .  
         [0043]    The second latch circuit  310  includes: a PMOS transistor  311  having a source coupled to the power terminal VDD and a gate receiving the NANDed signal; an NMOS transistor  312  having a drain coupled to a drain of the PMOS transistor  311 , a gate receiving the feedback signal S F′  and a source coupled to the ground terminal GND; an NMOS transistor  314  having a gate coupled to the drain of the NMOS transistor  312  and a source coupled to the ground terminal GND; and a PMOS transistor  313  having a source coupled to the power terminal VDD, a gate receiving the feedback signal S F′  and a drain coupled to the drain of the NMOS transistor  314 . At this time, the down pulse DN is outputted from the drain of the PMOS transistor  313 .  
         [0044]    Hereinafter, an operation of the latch circuit  300  contained in the PFD  200  will be described with reference to FIGS. 3 and 4.  
         [0045]    In case where the output signal Dl of the NAND gate logic circuit  320  is high and the reference frequency f R′  is falling, the first output Q 1  is always high. Additionally, in case where the output signal D 1  of the NAND gate logic circuit  320  is low, the first output Q 1  is always low regardless of the reference frequency f R′ .  
         [0046]    Basically, if the reference frequency f R′  is low, the first latch circuit  300  performs a data input operation, and if the reference frequency f R′  is high, the first latch circuit  300  performs a data latching operation. That is, if the reference frequency f R′  is low and the output signal D 1  is falling, the first output Q 1  is also falling immediately.  
         [0047]    Meanwhile, the conventional PFD detects a negative edge of the reference frequency and generates a reset signal if a negative edge of another frequency is detected. However, as shown in a circle portion  400  of FIG. 4, the PDF in accordance with the present invention does not generate a reset signal if a state of the reference frequency and the feedback frequency is changed. At this time, since there exists a delay time due to the latch circuit and the NAND gate logic circuit, the clock CLK and the input data D 1  cannot be falling at the same time.  
         [0048]    An operation of the second latch circuit  320  is the same as that of the first latch circuit  300 .  
         [0049]    [0049]FIG. 5 is a circuit diagram illustrating the filter control unit  230  shown in FIG. 2.  
         [0050]    Referring to FIG. 5, the filter control unit  230  includes an Exclusive-OR (XOR) gate  500  and a bandwidth control circuit  504 . The XOR gate  500  XORs the up pulse UP and the down pulse DN to output a XORed signal as a control signal. The bandwidth control circuit  504  changes the resistance of the LPF  220  I response to the XORed signal by a switching operation.  
         [0051]    The XOR gate  500  in accordance with the present invention includes a first inverter  501 , a second inverter  502  and a transmission gate  503 .  
         [0052]    The first inverter  501  includes: a PMOS transistor  505  having a source coupled to a power terminal VDD and a gate receiving the up pulse UP; and an NMOS transistor  506  having a drain coupled to a drain of the PMOS transistor  505 , a gate receiving the up pulse UP and a source coupled to a ground terminal GND.  
         [0053]    The second inverter  502  includes: a PMOS transistor  507  having a source coupled to the up pulse UP and a gate receiving the down pulse DN; and an NMOS transistor  508  having a drain coupled to a drain of the PMOS transistor  507 , a gate receiving the down pulse DN and a source coupled to an output of the first inverter  501 . At this time, the XORed signal is outputted from the drain of the PMOS transistor  507 .  
         [0054]    The pass gate  503  includes: a PMOS transistor  509  having a source coupled to an output of the second inverter  502 , a gate receiving the up pulse UP and a drain coupled to the down pulse DN; and an NMOS transistor  510  having a drain coupled to the source of the PMOS transistor  509 , a gate receiving the output of the first inverter  501  and a drain coupled to the down pulse DN.  
         [0055]    The bandwidth control circuit  504  includes: an NMOS transistor  511  having a gate receiving the XORed signal and a source coupled to the ground terminal GND; a resistor  512  coupled to a drain of the NMOS transistor  511 ; and a capacitor  513  coupled between the gate of the NMOS transistor  511  and the ground terminal GND. The capacitor  513  is used to stabilize the PLL with respect to a small phase difference.  
         [0056]    At this time, when either the up pulse UP or the down pulse DN is high, the XOR gate logic circuit  500  generates the XORed signal of a high level signal. Then, the NMOS transistor  511  contained in the bandwidth control circuit  504  is turned on in response to the XORed signal. As a result, the resistor  512  is electrically coupled in parallel to a resistor R contained in the LPF  220 .  
         [0057]    Functionally explaining, during the unlocked period, the bandwidth of the LPF  220  is widened by electrically coupling the resistor  511  to the resistor R. Meanwhile, if the locking is completed, the NMOS transistor  511  is turned off in response to the XORed signal so that the LPF  220  has its own fixed resistance. That is, there is no change in the bandwidth of the LPF  220 .  
         [0058]    While the conventional PDF is implemented with D-flip flops, the PDF in accordance with the present invention is implemented with the dynamic latch circuits so that the PLL obtains a high-speed operation. Furthermore, since a dead zone is very small, a jitter and a phase noise can be remarkably reduced and a locking time can be also shortened.  
         [0059]    Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.