1. Field of the Invention
The present invention relates to a signal receiving circuit utilizing a timing recovery parameter generating circuit, and particularly relates to a signal receiving circuit utilizing a timing recovery parameter generating circuit that utilizes a Mueller & Muller algorithm.
2. Description of the Prior Art
In general, signal processing circuits include a timing recovery circuit for amending sample phases of a sampler to obtain correct signals. FIG. 1 illustrates a prior art signal receiving circuit 100. The signal receiving circuit 100 includes a sampler 101, an analog digital converter (ADC) 103, a digital signal processor 105 and a timing recovery circuit 107. The digital signal processor 105 includes an equalizer 109 and a quantizer 111, and the timing recovery circuit 107 includes a timing recovery parameter generating circuit 113, a loop filter 115 and a voltage controlled oscillator (VCO) 117. The sampler 101 is used for sampling an analog signal AS to generate a sampled signal SS, and the ADC 103 is used for transferring the sampled signal SS to a digital signal DS. The digital signal DS is processed by the equalizer 109 and the quantizer 111 to form a processed digital signal PDS. The timing recovery parameter generating circuit 113 generates a timing recovery parameter TP according to an equalized digital signal EDS and the processed digital signal PDS, then the loop filter 115 and the VCO 117 adjust the sampling clock signal SCLK according to the timing recovery parameter TP.
In this system, the signal at the receiving terminal can be shown as
            x      ⁡              (        t        )              =                            ∑          k                ⁢                              a            k                    ⁢                      h            ⁡                          (                              t                -                kT                            )                                          +              n        ⁡                  (          t          )                      ,wherein n(t) is White Gaussian Noise, and T is the period. If the sample timing of a mth symbol is supposed to be τ+MT, than the sampled symbol can be shown as
            x      ⁡              (                  τ          +          mT                )              =                  h        ⁡                  (          τ          )                    ⁡              [                              a            m                    +                                    1                              h                ⁡                                  (                  τ                  )                                                      ⁢                                          ∑                                  i                  =                                      -                    ∞                                                  ∞                            ⁢                                                          ⁢                                                a                                      m                    -                    i                                                  ⁢                                  h                  ⁡                                      (                                          τ                      +                      iT                                        )                                                                                +                                    n              ⁡                              (                                  τ                  +                  mT                                )                                                    h              ⁡                              (                τ                )                                                    ]              ,wherein
      1          h      ⁡              (        τ        )              ⁢            ∑              i        =                  -          ∞                    ∞        ⁢                  ⁢                  a                  m          -          i                    ⁢              h        ⁡                  (                      τ            +            iT                    )                    indicates noise, and the timing recovery circuit 107 is used for enabling the sampler 101 to sample at a suitable phase for making the SNR ratio as high as possible.
FIG. 2 illustrates an impulse response with ISI situation. FIG. 3 is a schematic diagram illustrating how the prior art utilizes impulse response to find sampling points. The impulse response is indicated as h(t), and the impulse response at the receiver is the sum of the filter at the transmitting terminal, and the filter and channel at a receiving terminal. As shown in FIG. 2, the symbol h0 is the impulse response of a current signal, and h1, h−1 are, respectively, impulse responses of signals of a previous period and a next period. Conventionally, there will be ISI (Inter-Symbol Interference) between h0,h1, and h−1, and the impulse response shown in FIG. 2 includes serious ISI. ISI is an important reference for the timing recovery circuit 107, however. Normally, a timing function
      f    ⁡          (      τ      )        =                    1        2            ⁢              (                              h            ⁡                          (                              τ                +                T                            )                                -                      h            ⁡                          (                              τ                -                T                            )                                      )              =                  1        2            ⁢              (                              h            1                    -                      h                          -              1                                      )            is utilized for computing a best sampling point, as shown in FIG. 3. In FIG. 3, a zero-crossing point x is a middle point of a current symbol h0, and is theoretically a best sampling point. Also, the timing function can be computed from the Mueller & Muller algorithm.
FIG. 4 is a circuit diagram of a prior art Mueller and Muller algorithm. As shown in FIG. 4, the timing recovery parameter generating circuit 113 is a circuit utilizing a Mueller and Muller algorithm, and generates a timing adjusting parameter TP for following processing devices. Relevant details of the Mueller and Muller algorithm are disclosed in K. H. Mueller and M. Muller, “Timing Recovery in Digital Synchronous Data Receivers,” IEEE Trans. Communications, vol. Com-24, pp. 516-531, May 1976.
As described above, the Mueller & Muller algorithm can be utilized to get correct sampling points via ISI. However, the equalizer 109 shown in FIG. 1 may eliminate ISI, such that the determination of the sampling points may fail. As shown in FIG. 5, there will be no ISI between h0, h1, and h−1 after processing of the equalizer 109. Such symbol will have a region Y at a location at which the zero crossing point is supposed to exist after being processed by the timing function. In this case, a new sampling point may locate at any point in the Y region and the zero crossing point may shift, such that the sampling point will be incorrectly selected and the timing recovery circuit 107 may break. Moreover, in this structure, the closed loop includes an equalizer, which may diverge.
Additionally, since the impulse response of channels is asymmetric, the asymmetry will get more serious if the transmission line for transmitting signals is increased. FIG. 6 is a schematic diagram illustrating how to utilize the impulse response of FIG. 5 to find sampling points. As shown in FIG. 6, the symbol h is not as perfect as h0, h1, h−1 shown in FIGS. 3 and 5 but has an extended region Z, which will disturb the selecting of correct sampling points. The longer the line, the more accurate the zero point, i.e. the sampling point will reach the next signal. Since the resistance of the equalizer for the previous signal interference is larger than that for the next signal interference, such a situation is best avoided.
Therefore, a new invention is needed to solve the above-mentioned problems.