Abstract:
A vernier phase error detection method is provided. The method comprises providing a first signal having a first cycle T 1,  wherein T 1 =1/N T; providing a second signal having a second cycle T 2,  wherein T 2 =1/M T; aligning a rising edge of the second signal with a rising edge of the first signal; when a second data sampled by the second signal is different from a first data sampled by the first signal at the Xth second cycle, a phase error Ø is evaluated by the following equation: Ø=(N/2−X)*T 1.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to an electronic device, and more particularly to an electronic device for phase error detection. 
         [0003]    2. Description of the Related Art 
         [0004]    Phase-locked loop (PLL) devices are applied in frequency generators, wireless receivers, communication devices and the like. A phase detector is an essential element in a PLL device as a stable, high accurate clock signal output is highly related to the accuracy of the phase error from the phase detector. Phase detectors range from very simple to complex in design. An XOR logic gate makes a passable phase detector. When the two compared signals are completely in phase, the two equal inputs to the XOR gate will output a constant level of zero. When a phase difference occurs, the XOR gate will output a “1” for the duration of the difference in phase between signals. Integration of the output signal results in an analog voltage proportional to the phase difference. A phase detector can also be made from an analog multiplier, sample and hold circuit, charge pump or a logic circuit consisting of flip-flops. These phase detectors have more desirable properties such as better accuracy at small phase differences or ability to phase lock to signals with large frequency mismatches. Although a complex phase detector generates a high accuracy phase error signal, the complex design causes unexpected errors, thus, a simple phase detector capable of performing high accuracy phase error detection method is desirable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The invention provides a vernier phase error detection method comprising providing a first signal having a first cycle T 1 , wherein 
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         [0000]    providing a second signal having a second cycle T 2 , wherein 
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         [0000]    aligning a rising edge of the second signal with a rising edge of the first signal; when a second data sampled by the second signal is different from a first data sampled by the first signal at Xth second cycle, a phase error Ø is evaluated by the following equation: 
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         [0006]    The invention provides a vernier phase detector comprising an alignment unit, a first sampler, a second sampler and a processing unit. The alignment unit aligns a rising edge of a first clock signal with a rising of a second clock signal. The first sampler is controlled by the first clock signal for sampling a data signal. The second sampler is controlled by the second clock signal for sampling the data signal. The processing unit determines a phase error signal between the first clock signal and the data signal, wherein when a first data sampled by the first sampler is different from a second data sampled by the second sampler, the processing unit determining the phase error signal based on the first clock signal and the second clock signal. 
         [0007]    The invention provides a PLL device, comprising a phase detector, a charge pump circuit, a loop filter, a voltage controlled oscillator and a feedback divider. The phase detector comprises an alignment unit, a first sampler, a second sampler and a processing unit. The alignment unit aligns a rising edge of a first clock signal with a rising of a second clock signal. The first sampler is controlled by the first clock signal for sampling a data signal. The second sampler is controlled by the second clock signal for sampling the data signal. The processing unit determines a phase error signal between the first clock signal and the data signal, wherein when a first data sampled by the first sampler is different from a second data sampled by the second sampler, the processing unit determining the phase error signal based on the first clock signal and the second clock signal. The charge pump circuit outputs a current based on the phase error signal, and the loop filter then transfers the current into a voltage. The voltage controlled oscillator outputs an output signal based on the voltage. The feedback divider receives the output signal to generate the first clock signal, wherein the output signal is multiple of the first clock signal. 
         [0008]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0010]      FIG. 1  is a schematic diagram of an embodiment of a phase detection method of the invention. 
           [0011]      FIG. 2  is a schematic diagram of another embodiment of a phase detection method of the invention. 
           [0012]      FIG. 3  is a block diagram of an embodiment of a vernier phase error detector of the invention. 
           [0013]      FIG. 4  is a block diagram of an embodiment of a PLL device of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0015]      FIG. 1  is a schematic diagram of an embodiment of a vernier phase detection method of the invention. T D  represents the period of the data signal. T 1  is the period of the first clock CLK 1 . T 2  is the period of the second clock CLK 2 . In this embodiment, T D  is equal to T 1 , and T 1  is slightly different from T 2 . If the data signal is sampled at the center of each high and low level when the first clock CLK 1  and the second clock CLK 2  are aligned with each other, the data signal is locked to the first clock CLK 1  or the second clock CLK 2 . If the data signal is not sampled at the center of each high and low level when the first clock CLK 1  and the second clock CLK 2  are aligned with each other, the data signal is not locked to the first clock CLK 1 . A phase error between the data signal and the first clock CKL 1  can be detected. 
         [0016]    With reference to  FIG. 1 , T 2  is slightly longer than T 1 , so after a clock cycle, the sampling edge of the second clock CLK 2  slightly lags that of the first clock CLK 1 . The phase deviation caused by the lag is represented by Δ. After another clock cycle, the second clock CLK 2  further lags the first clock CLK 1  by 2Δ, and so on. In this embodiment, when the second clock CLK 2  lags the first clock by 13Δ, the first clock CLK 1  and the second clock CLK 2  are realigned. It can be found that T 1 /T 2 =13/14. 
         [0017]    If the data signal is locked to the first clock CLK 1  as shown in  FIG. 1 , the sampling edge of the first clock CLK 1  is at the center of the high or low level when the alignment between the first clock CLK 1  and the second clock CLK 2  occurs. In this situation, the same data value (0 or 1) is sampled by the first clock CLK 1  and the second clock CLK 2  until the phase deviation reaches 7Δ. That is, a data value transition occurs when the phase deviation increases from 6Δ to 7Δ. This is because the phase deviation of 6.5Δ is about T D /2, the second clock CLK 2  starts to sample the next value of the data signal by crossing the transition. 
         [0018]      FIG. 2A ,  FIG. 2B , and  FIG. 2C  show the shifts of sampling edges for each clock cycle. With reference to  FIG. 2A , during the clock cycle  0 , the first clock CLK 1  and the second clock CLK 2  are aligned at the center of D 0 . This is a lock status. During the clock cycle  1 , the second clock CLK 2  deviates from the first clock CLK 1  by Δ. During the clock cycle  2 , the second clock CLK 2  deviates from the first clock CLK 1  by 2Δ, and so on. During the clock cycle  7 , the second clock CLK 2  has crossed the transition edge of D 0  and D 1 , so the second clock CLK 2  samples D 1  instead of D 0 . If D 0  has a different value from D 1  during the clock cycle  7 , the first clock CLK 1  and the second clock CLK 2  will have different sampled values. 
         [0019]    With reference to  FIG. 2B , during the clock cycle  0 , the first clock CLK 1  and the second clock CLK 2  are not aligned at the center of D 0 . This is not a lock status. During the clock cycle  1 , the second clock CLK 2  deviates from the first clock CLK 1  by Δ. During the clock cycle  2 , the second clock CLK 2  deviates from the first clock CLK 1  by 2Δ, and so on. During the clock cycle  3 , the second clock CLK 2  has crossed the transition edge of D 0  and D 1 , so the second clock CLK 2  samples D 1  instead of D 0 . If D 0  has a different value from D 1  during the clock cycle  3 , the first clock CLK 1  and the second clock CLK 2  will have different sampled values. 
         [0020]    It can be found from  FIG. 2A  and  FIG. 2B  that by detecting the number of clock cycles when the second clock CLK 2  crosses the data transition edge, one can know whether the data signal is in a lock status. If the data signal is in a lock status, the detected number of clock cycles is corresponding to T D /2. If the data signal is not in a lock status, the detected number of clock cycles is not corresponding to T D /2. The phases of the first clock CLK 1  and the second clock CLK 2  can be further adjusted to make the data signal locked by the first clock CLK 1  or the second clock CLK 2 . For example, in  FIG. 2B , if the first clock and the second clock are shifted left by 4Δ, then D 0  can be sampled at the center when the first clock CLK 1  and the second clock CLK 2  are aligned. 
         [0021]    However, T D  need not be the same as T 1 . With reference to  FIG. 2C , during the clock cycle  0 , the first clock CLK 1  and the second clock CLK 2  are aligned at the center of D 0 . This is a lock status. It is noted that the period of first clock CLK 1  or the second clock CLK 2  is different from that of the data signal, so that during the clock cycle  1 , both the first clock CLK 1  and the second clock CLK 2  are not at the center of D 0 , and the second clock CLK 2  deviates from the first clock CLK 1  by Δ. During the clock cycle  2 , the second clock CLK 2  deviates from the first clock CLK 1  by 2Δ, and so on. During the clock cycle M, the second clock CLK 2  has crossed the transition edge of D 0  and D 1 , so the second clock CLK 2  samples D 1  instead of D 0 . If D 0  has a different value from D 1  during the clock cycle M, the first clock CLK 1  and the second clock CLK 2  will have different sampled values. In this embodiment, during the clock cycle N, the first clock CLK 1  and the second clock CLK 2  are realigned at the center of D 1 . 
         [0022]      FIG. 5  is a diagram showing the relationship between the data signal, the first clock CLK 1  and the second clock CLK 2 . For each clock cycle, the first clock CLK 1  gets a further phase shift T D /P with respect to the rising edge of the data signal, where P is a natural number. Similarly, for each clock cycle, the second clock CLK 2  gets a further phase shift T D /Q with respect to the rising edge of the data signal, where Q is a natural number. That is, for each clock cycle, the second clock CLK 2  gets a further phase shift T D /Q−T D /P with respect to the first clock CLK 1 . Every P first clock cycles (or every Q second clock cycles), the first clock CLK 1  and the second clock CLK 2  are realigned. If the realignment occurs at the center of the high level or low level of the data signal, it is a lock status. It is preferred that P and Q are relatively prime. One convenient embodiment is Q=P+1 or P−1. However, even if P and Q are not relatively prime, the first clock CLK 1  and the second clock CLK 2  will eventually be realigned. The realignments can be detected to determine whether a lock status is reached. 
         [0023]      FIG. 3  is a block diagram of an embodiment of a vernier phase error detector. The vernier phase error detector  30  comprises a first sampler  31 , a second sampler  32 , an aligning unit  33 , and a processing unit  36 . The aligning unit  33  outputs the first clock CLK 1  and the second clock CLK 2  to sample the data signal. The aligning unit  33  also determines the time when the first clock CLK 1  and the second clock CLK 2  are aligned. The processing unit  36  determines a phase error according to the first sampled value (sampled by the first clock CLK 1 ), the second sampled value (sampled by the second clock CLK 2 ), and the alignments of the first clock CLK 1  and the second clock CLK 2 . The vernier phase error detector  30  can further comprise a first buffer for storing the first sampled value and a second buffer for storing the second sampled value. 
         [0024]      FIG. 4  is a block diagram of an embodiment of a PLL device of the invention. The PLL comprises a phase error detector  41 , a charge pump circuit  42 , a loop filter  43 , a voltage-controlled oscillator  44  and a feedback divider  45 . The phase error detector  41  is described in  FIG. 3 , for brevity, description of like structures are omitted. The charge pump circuit  42  converts a phase error signal from the phase error detector  41  into a charge current for charging or discharging the loop filter  43 . The loop filter  43  limits the rate of charge of a capacitor therein to generate a voltage corresponding to the phase error signal, and the voltage-controlled oscillator  44  then generates an output signal based on the voltage. The feedback divider  45  receives the output signal to generate a first clock signal CLK 1 , wherein the frequency of the output signal is multiple of the frequency of the first clock signal. In this embodiment, the frequency of the output signal is also a multiple of the frequency of the second clock. In another embodiment, the second clock signal is also generated by the feedback divider  45 . 
         [0025]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.