Abstract:
A phase locked loop (PLL) circuit including a phase comparator for comparing a phase of a reference signal with a phase of a feedback signal, an oscillator for outputting an output signal at a frequency in accordance with an output of the phase comparator, a feedback loop for returning the output signal of the oscillator and supplying the output signal as the feedback signal, and a delay circuit for delaying the phase of the output signal output from the oscillator to a load circuit, wherein the delay circuit is provided outside the feedback loop.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Divisional of U.S. patent application Ser. No. 11/987,003 filed on Nov. 26, 2007, which claims the benefit of priority of Japanese Patent Application No. 2006-318638, filed on Nov. 27, 2006, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Disclosure 
         [0003]    The embodiments relate to a phase locked loop (PLL). 
         [0004]    2. Description of the Related Art 
         [0005]    In the field of semiconductor integrated circuit (IC) devices, technological development is in progress to achieve further enhancement of operation speeds and further reduction of power consumption. In a PLL circuit in a semiconductor IC device, reduction in noise occurring along the time axis, which is called “jitter,” is desired. In particular, in a semiconductor IC device including both analog and digital circuits, noise attributed to a digital circuit causes a PLL circuit to generate jitter. 
         [0006]      FIG. 1  illustrates an example of a conventional PLL circuit. In the conventional PLL circuit, a frequency of a reference signal ref-CLK generated from an oscillator circuit (not shown), such as a quartz oscillator, is divided into a 1/N frequency component by a first frequency divider  1 , and is then input into a phase comparator  2 . An output signal of a second frequency divider  3  having a division ratio of 1/M, for example, is input as a feedback signal into the phase comparator  2 . The phase comparator  2  detects a phase difference between the output signals of the first frequency divider  1  and the output signal of the second frequency divider  3 , and outputs a pulse signal in accordance with the phase difference to a charge pump  4 . 
         [0007]    As shown in  FIG. 1 , the charge pump  4  outputs an output voltage to a low pass filter  5  (LPF), in accordance with the pulse signal output from the phase comparator  2 . The output voltage of the charge pump  4  has a DC component and a pulse component from the pulse signal output from the phase comparator  2 . The DC component varies in accordance with frequency variation of the pulse signal, and the pulse component varies in accordance with the pulse signal corresponding to the phase difference. 
         [0008]    As further shown in  FIG. 1 , the low pass filter  5  smoothes the output voltage of the charge pump  4 , and outputs an output signal with a removed high frequency component to a voltage/current converter (WI converter)  6 . The V/I converter  6  converts the output voltage of the low pass filter  5  to a current, and outputs the current to a current control oscillator (ICO)  7 . 
         [0009]    As further shown in  FIG. 1 , the ICO  7  outputs an output signal out-CLK, having a frequency based on the output current of the V/I converter  6 , to a digital circuit  8  and to the second frequency divider  3 . The digital circuit  8  performs required operations in accordance with the output signal out-CLK. 
         [0010]    The PLL circuit of  FIG. 1  operates so as to conform the frequency and phase of the output signal of the second frequency divider  3  to the frequency and phase of the output signal of the first frequency divider  1 . 
         [0011]      FIG. 2  illustrates an operation in the case where the division ratio of each of the respective first and second frequency dividers  1  and  3  of  FIG. 1  is  1 . In accordance with the loop from the phase comparator  2  of  FIG. 1  to the ICO  7  of  FIG. 1 , the frequency and phase of the output signal out-CLK converge, so that the reference signal ref-CLK and the output signal out-CLK are in-phase with each other. 
         [0012]    As shown in  FIG. 2 , upon output of the output signal out-CLK from the ICO  7  of  FIG. 1 , the digital circuit  8  of  FIG. 1  operates a required switching operation, in accordance with a timing of a rise and fall of the output signal out-CLK. Therefore, in accordance with the operation of the digital circuit  8 , power-supply noise N, synchronous with the output signal out-CLK, is generated in a power supply Vcc potential and a ground GND potential that are supplied to the respective circuits, which are coupled between the phase comparator  2  and the ICO  7  through power supply lines. 
         [0013]    As further shown in  FIG. 2 , since a comparison operation is performed in the phase comparator  2  of  FIG. 1  in accordance with a timing of a rise or fall of the reference signal ref-CLK, the output signal is destabilized, due to the power-supply noise N. The output signal of the charge pump  4  of  FIG. 1 , operating on the common power supply with the digital circuit  8 , is also destabilized. 
         [0014]    As a consequence, as shown further in  FIG. 2 , a jitter Z frequently occurs during the rise and fall of the output signal out-CLK. 
         [0015]    As one example of a related conventional technique, Laid-Open Japanese Patent Application (JP-A) No. 2005-79835 discloses a PLL circuit that includes a phase shifter interposed between a voltage control oscillator and a frequency divider to reduce jitter occurrence. However, in such a configuration, which includes the phase shifter in a feedback loop that feeds back the output signal of the voltage control oscillator to the phase comparator, the loop affects the lock-up speed until the frequency of the output signal of the voltage control oscillator is stabilized. Since the delay time caused as a result of the phase shifter has to be adjusted optimally, the phase shifter must include a delay length switching portion. Consequently, the circuit size of the phase shifter is increased. 
         [0016]    As another example, JP-A-2006-174243 discloses a PLL circuit, in which a delay value of a delay circuit in a voltage control oscillator is controlled to enable stepwise switching of an oscillation frequency characteristic. However, a jitter reduction feature is not disclosed. 
         [0017]    As still another example, JP-A-08-56157 discloses a PLL circuit that includes a variable delay circuit provided with a voltage control circuit. However, a jitter reduction feature is not disclosed. 
         [0018]    Conventional PLL circuits, such as described above, are problematic at least in that, when the timing of occurrence of power-supply noise N with the output signal out-CLK and the phase comparison timing in the phase comparator  2  overlap, the jitter Z increases in the output signal out-CLK. 
       SUMMARY 
       [0019]    Embodiments provide a PLL circuit that includes a phase comparator for comparing the phase of a reference signal with the phase of a feedback signal, an oscillator for outputting an output signal at a frequency in accordance with output of the phase comparator, a feedback loop for returning the output signal of the oscillator and supplying the output signal as the feedback signal, and a delay circuit for delaying the phase of the output signal output from the oscillator to a load circuit, wherein the delay circuit is provided outside the feedback loop. 
         [0020]    These aspects of the PLL circuit, together with other aspects and advantages, which will be subsequently apparent, reside in the details of construction and operation thereof, as more fully hereinafter described and claimed herein, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  illustrates a block diagram of a conventional PLL circuit; 
           [0022]      FIG. 2  illustrates a timing diagram of operation of the conventional PLL circuit of  FIG. 1 ; 
           [0023]      FIG. 3  illustrates a block diagram of a first exemplary PLL circuit, in accordance with an embodiment; 
           [0024]      FIG. 4  illustrates an ICO (current control oscillator) and a delay circuit for the PLL circuit of  FIG. 3 ; 
           [0025]      FIG. 5  illustrates a differential amplifier for the circuit of  FIG. 4 ; 
           [0026]      FIG. 6  illustrates an exemplary timing diagram of operation of the ICO for the circuit of  FIG. 4 ; 
           [0027]      FIG. 7  illustrates an exemplary timing diagram of operation of the PLL circuit of  FIG. 3 ; 
           [0028]      FIG. 8  illustrates an ICO of a second exemplary PLL circuit, in accordance with an embodiment; 
           [0029]      FIG. 9  illustrates an exemplary timing diagram of operation of the ICO for the PLL circuit of  FIG. 8 ; 
           [0030]      FIG. 10  illustrates an exemplary control voltage generator unit for the PLL circuit of  FIG. 8 ; 
           [0031]      FIG. 11  illustrates an exemplary delay circuit for the PLL circuit of  FIG. 8 ; 
           [0032]      FIG. 12  illustrates a block diagram of a third exemplary PLL circuit, in accordance with an embodiment; and 
           [0033]      FIG. 13  illustrates a block diagram of a fourth exemplary PLL circuit, in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0034]    In the following descriptions, many of the exemplary circuits are shown to include n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) in a variety of configurations. While MOSFET devices are used by example, the disclosed circuits may be implemented using any number of other transistor types, such as J-FETs, bipolar transistors, and so on. Additionally, while n-channel devices are used in the following examples, the same general approaches may also apply to circuits incorporating p-channel FETs or PNP bipolar transistors, for example. 
         [0035]    Still further, while terms “drain” and “source” are used for ease of explanation and to adhere to traditional engineering usage, it should -be recognized that a drain and source of a FET transistor may be considered interchangeable, and for the following descriptions merely thought of as a first end and a second end of a semiconductor channel unless otherwise stated or apparent to one of ordinary skill in the art. 
         [0036]      FIG. 3  illustrates a first exemplary PLL circuit in accordance with an embodiment. In the drawings and description hereinafter, like reference characters refer to the similar components/portions as those of the conventional example. A phase comparator  2  of  FIG. 3  performs a comparison between a reference signal output from a first frequency divider  1  and a feedback signal output from a second frequency divider  3 , and outputs a phase difference signal to a charge pump  4 . 
         [0037]    As shown in  FIG. 3 , an output signal out-CLK of an ICO  7   a  is output to a digital circuit  8  (load circuit) via a delay circuit  11   a.    
         [0038]      FIG. 4  illustrates more specific exemplary configurations of the ICO  7   a  of  FIG. 3  and the delay circuit  11   a  of  FIG. 3 . In the ICO  7   a  of  FIG. 4 , differential amplifiers  12   a  to  12   c  are coupled to one another in a ring shape, in which an output current lout of a V/I converter  6  of  FIG. 3  is supplied to the respective differential amplifiers  12   a  to  12   c.    
         [0039]    As shown in  FIG. 5 , the differential amplifiers  12   a  to  12   c  of  FIG. 4  are each configured such that a bias current IB thereof varies with variations in the output current Iout of the V/I converter  6  of  FIG. 3 . According to a configuration shown in  FIG. 5 , in the ICO  7   a  of  FIG. 4 , the frequency of the output signal out-CLK is varied due to variations in the output current lout of the V/I converter  6  of  FIG. 3 . 
         [0040]      FIG. 6  illustrates output signal waveforms of the ICO  7   a  of  FIG. 4 , configured as a function of the three differential amplifiers  12   a  to  12   c.  As shown in  FIG. 6 , output signals n 1 , bar-n 1  to n 3 , and bar-n 3 , respectively, have waveforms shifted by a predetermined interval. 
         [0041]    As shown in  FIG. 4 , in the delay circuit  11   a,  phase inverted signals are generated by inverter circuits  13   a  to  13   c  from the output signal out-CLK of the ICO  7   a,  and the phase inverted signals are input into a differential amplifier  12   d.  The differential amplifier  12   d  has a similar configuration to each of the respective differential amplifiers  12   a  to  12   c.  Similarly, as in the ICO  7   a,  the output current Tout of the V/I converter  6  of  FIG. 3  is input as a bias current into each of the differential amplifiers  12   a  to  12   c.  Consequently, in the delay circuit  11   a , a delay time is generated for the output signal out-CLK of the ICO  7   a  at a constant rate at all times. 
         [0042]    As shown in  FIG. 7 , in the configuration in which the ICO  7   a  of  FIG. 4  is configured as a function of the three differential amplifiers  12   a  to  12   c,  an output signal out-DL of the delay circuit  11   a  of  FIG. 4  is formed to have a waveform that is ⅙ cycle delayed relative to the output signal out-CLK of the ICO  7   a.  The output signal of the differential amplifier  12   d  is output to the digital circuit  8  via an inverter circuit  13   d  of  FIG. 4 . 
         [0043]    As further shown in  FIG. 7 , when the division ratio of each of the first and second frequency dividers  1  and  3  is assumed to be about 1, the frequency and phase of the output signal out-CLK converge to be in-phase with the reference signal ref-CLK in accordance with the loop in the range from the phase comparator  2  to the ICO  7   a  of  FIG. 3 . 
         [0044]    As further shown in  FIG. 7 , an output signal out-DL, which is obtained by delaying the output signal out-CLK of the ICO  7   a  by the ⅙ cycle, is generated by the delay circuit  11   a  of  FIG. 4 , and the output signal out-DL is output to the digital circuit  8 . 
         [0045]    As further shown in  FIG. 7 , in the digital circuit  8  of  FIG. 3 , a necessary switching operation is performed in accordance with the rising and falling of the output signal out-DL of the delay circuit  11   a . With operation of the digital circuit  8 , noise N, synchronous with the output signal out-DL, is generated in a power supply Vcc potential and a ground GND potential supplied to the respective circuits through the loop in the range from the phase comparator  2  to the ICO  7   a.  In this case, however, the power-supply noise N is not synchronous with the rising and falling of the reference signal ref-CLK, that is, comparison timing in the phase comparator  2 . 
         [0046]    Among other things, the PLL circuit of  FIG. 3  enables the following effects and advantages to be achieved: 
         [0047]    (1) Since the output signal out-CLK of the ICO  7   a  is output to the digital circuit  8  via the delay circuit  11   a , the occurrence of power-supply noise N attributed to the operation of the digital circuit  8  and the occurrence of comparison timing by the phase comparator  2  may be controlled so as not to overlap. Consequently, the occurrence of jitter in the output signal out-CLK of the ICO  7   a  may be suppressed. 
         [0048]    (2) The delay circuit Ha has a similar configuration to each of the respective differential amplifiers  12   a  to  12   c  constituting the ICO  7   a.  The delay circuit  11   a  is also supplied with the output current lout of the V/I converter  6 , as the bias current commonly supplied to the differential amplifiers  12   a  to  12   c  to configure the circuit for automatically adjusting the delay time. Consequently, the delay time in the delay circuit  11   a  is generated at times at a constant rate relative to the cycle of the output signal out-CLK of the ICO  7   a.  The delay time of the delay circuit  11   a  may be automatically adjusted as a function of variations in the frequency of the output signal out-CLK of the ICO  7   a.    
         [0049]    (3) The delay circuit  11   a  may be configured to include the single differential amplifier  12   d,  which has the same configuration as each of the differential amplifiers  12   a  to  12   c . Consequently, the delay circuit  11   a  may be formed into a simplified configuration. 
         [0050]      FIGS. 8 to 11  illustrate a second exemplary PLL circuit, in accordance with embodiments, which includes an ICO  7   b  and a delay circuit  11   b,  respectively, in place of the ICO  7   a  and the delay circuit  11   a  of  FIG. 3 . The ICO  7   b  includes an odd number of inverter circuits, and the delay circuit  11   b  includes an inverter circuit having the same configuration as the respective inverters of the ICO  7   b.  Other portions of the configuration are similar to corresponding portions of the first PLL circuit of  FIG. 3 . 
         [0051]    As shown in  FIG. 8 , in the ICO  7   b,  an odd number of inverter circuits ( 14   a  to  14   c ) are coupled to one another in a ring shape via respective transfer gates  15   a  to  15   c.  The transfer gates  15   a  to  15   c  are each configured in such a manner that a P-channel MOS transistor (PMOS) and an N-channel MOS transistor (NMOS) are coupled together in parallel. A common control voltage V 1  is input to PMOS-side gate terminals of the respective transfer gates  15   a  to  15   c,  and a common control voltage V 2  is input to NMOS gate terminals Of the respective transfer gates  15   a  to  15   c.    
         [0052]    As further shown in  FIG. 8 , when the control voltage V 1  decreases, the control voltage V 2  increases, and the transfer currents of the transfer gates  15   a  to  15   c  are increased thereby, the frequency of the output signal out-CLK of the ICO  7   b  is increased. In contrast, when the control voltage V 1  increases, the control voltage V 2  decreases, and the transfer current of the transfer gates  15   a  to  15   c  is decreased thereby, the frequency of the output signal out-CLK of the ICO  7   b  is decreased. 
         [0053]    As further shown in  FIG. 9 , the waveforms of output signals n 1  to n 3  of the respective inverter circuits  14   a  to  14   c  of  FIG. 8  are shifted by a predetermined interval. 
         [0054]    As shown in  FIG. 10 , a control voltage generator unit  16  generates the control voltages V 1  and V 2  of  FIG. 8 . More specifically, the output current lout of the V/I converter  6  of  FIG. 3  is supplied to the drains of NMOS transistors T 1  and T 2  of  FIG. 10  and the drain of the NMOS transistor T 1 . The sources of the transistors T 1  and T 2  are coupled to a ground GND, thereby producing a current mirror circuit. A drain voltage of the transistor T 1  is supplied as the control voltage V 2 . 
         [0055]    As further shown in  FIG. 10 , the drain of the transistor T 2  is coupled to the drain and gate of a PMOS transistor T 3 , and the source of the transistor T 3  is coupled to a power supply Vcc. The control voltage V 1  is output from the drain of the transistor T 3 . 
         [0056]    As further shown in  FIG. 10 , in the control voltage generating portion  16 , when the output current lout of the V/I converter  6  of  FIG. 3  increases, the control voltage V 2  is increased and the drain voltage of the transistor T 2  is increased, whereby the control voltage V 1  is decreased. In contrast, when the output current lout of the V/I converter  6  of  FIG. 3  decreases, the control voltage V 2  is decreased, and the drain current of the transistor T 2  is decreased, whereby the control voltage V 1  is increased. 
         [0057]      FIG. 11  illustrates the delay circuit  11   b.  The delay circuit  11   b  includes a transfer gate  15   d  and an inverter circuit  14   d.  The control voltage V 1  is input to a PMOS-side gate of the transfer gate  15   d,  and the control voltage V 2  is input to an NMOS-side gate of the transfer gate  15   d.  The output signal out-CLK of the ICO  7   b  of  FIG. 8  is output as the output signal out-DL to the digital circuit  8  of  FIG. 3  via the transfer gate  15   d  and the inverter circuit  14   d.    
         [0058]    As shown in  FIG. 11 , in the delay circuit  11   b,  when the control voltage V 1  is decreased, the control voltage V 2  is increased, and the transfer current of the transfer gate  15   d  is increased thereby, the delay time of the delay circuit  11   b  is decreased. In contrast, when the control voltage V 1  is increased, the control voltage V 2  is decreased, and the transfer current of the transfer gate  15   d  is decreased thereby, the delay time of the delay circuit  11   b  is increased. 
         [0059]    When the frequency of the output signal out-CLK of the ICO  7   b  of  FIG. 8  is increased, the delay time of the delay circuit  11   b  of  FIG. 11  is decreased. In contrast, when the frequency of the output signal out-CLK of the ICO  7   b  of  FIG. 8  is decreased, the delay time of the delay circuit  11   b  of  FIG. 11  is increased. Consequently, a delay time in the delay circuit  11   a  is generated at all times at a constant rate relative to the cycle of the output signal out-CLK of the ICO  7   b.    
         [0060]    Accordingly, in the PLL circuit that includes the ICO  7   b  of  FIG. 8  and the delay circuit.  11   b  of  FIG. 11 , effects and advantages similar to those of the first PLL circuit of  FIG. 3  may be obtained. 
         [0061]      FIG. 12  illustrates a third exemplary PLL circuit, in accordance with an embodiment of As shown in  FIG. 12 , in the configuration in which, for example, an ICO  7   b  is similar to the ICO  7   b  of the first PLL circuit of  FIG. 8 , the delay circuit  11   b  is interposed between the first frequency divider  1  and the phase comparator  2 . 
         [0062]    As further shown in  FIG. 12 , in addition to the output current lout of the V/I converter  6 , an idling current lid is supplied to the delay circuit  11   b.  Even in a state where the output current lout of the V/I converter  6  is not supplied at the time of activation of the PLL circuit of  FIG. 12 , the reference signal ref-CLK is supplied to the phase comparator  2  when the idling current lid is supplied to the delay circuit  11   b.    
         [0063]    As further shown in  FIG. 12 , in the configuration that includes a delay circuit  11   b  interposed between the first frequency divider  1  and the phase comparator  2 , the configuration enters into a state in which the comparison timing in the phase comparator  2  does not overlap the timing of the rising and falling of the output signal out-CLK of the ICO  7   b.  Consequently, occurrence of jitter in the output signal out-CLK of the ICO  7   b  may be suppressed. 
         [0064]      FIG. 13  illustrates a fourth exemplary PLL circuit, in accordance with an embodiment. As shown in  FIG. 13 , a delay circuit  11   b  similar to the delay circuit  11   b  of  FIG. 12  is interposed before the first frequency divider  1 . The PLL circuit of  FIG. 13  enables similar effects and advantages to be obtained to those of the PLL circuit of  FIG. 12 . 
         [0065]    The ICO  7   a  and delay circuit  11   a  of the first PLL circuit of  FIG. 3  may be used instead of the respective ICO  7   b  and delay circuit  11   b  of the respective PLL circuits of  FIGS. 12 and 13 . 
         [0066]    According to any one of the embodiments described above, the PLL circuit capable of suppressing jitter occurrence in the output signal may be provided without insertion of the delay circuit in the feedback loop and without introducing circuit size enlargement. 
         [0067]    The many features and advantages of the embodiments are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the embodiments that fall within the true spirit and scope thereof. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the inventive embodiments to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope thereof.