Patent Publication Number: US-7218176-B2

Title: Digital phase-locked loop circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority of Taiwanese Application No. 092125806, filed on Sep. 18, 2003. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a digital phase-locked loop circuit, more particularly to a digital phase-locked loop circuit that requires a relatively small circuit layout. 
   2. Description of the Related Art 
   Equalizers are usually built in control chips for optical disk drives suitable for CD-ROM, CD-R, CD-RW and DVD-ROM applications, and serve to amplitude equalize radio frequency signals generated by an optical pickup heads when reading an optical disk. Therefore, it is important to stabilize a −3 dB frequency (f E ) of an equalizer filter so as not to affect post-processing by a digital signal processor. 
     FIG. 1  illustrates a control loop composed of a conventional phase-locked loop circuit  1  and a voltage controlled oscillator  2  for adequately adjusting a control voltage signal (vin) inputted to an equalizer filter  6  so as to stabilize a −3 dB frequency (f E ) of the equalizer filter  6 . The conventional phase-locked loop circuit  1  serves to generate the control voltage signal (vin) that is used to enable the voltage controlled oscillator  2  to generate a target frequency output (f B ). The conventional phase-locked loop circuit  1  includes a frequency divider  11  for frequency dividing an output frequency (f o ) outputted by the voltage controlled oscillator  2  in response to the control voltage signal (vin), a phase comparator  12  for phase comparing the output the frequency divider  11  with the target frequency output (f B ) and for outputting an error signal (Te) according to a phase difference therebetween, a charge pump  13  driven in response to the error signal (Te) and outputting a current output Io, and a low-pass filter  14 , which includes a resistor (R 0 ) and a capacitor (C 0 ), for integrating the current output from the charge pump  13  and for outputting the control voltage signal (vin). 
   It is assumed that the −3 dB frequency (f E ) of the equalizer filter  6  is represented by the following equation: f E =k 1 ×vin, where k 1  is a constant associated with electrical characteristics of electronic components in the equalizer filter  6  and changes with actual operating temperature conditions, and that the target frequency output (f B ) is represented by the following equation: f B =k 2 ×vin, where k 2  is a constant associated with electrical characteristics of electronic components in the control loop and also changes with actual operating temperature conditions. Since a ratio of k 1  and k 2  almost does not change even at different operating temperature conditions, the control voltage signal (vin) generated by the conventional phase-locked loop circuit  1  can stabilize the −3 dB frequency (f E ) of the equalizer filter  6 . 
   However, in view of the above configuration, the frequency divider  11 , the phase comparator  12  and the charge pump  13  require a relatively large layout area (about 280000 μm 2 ) for fabrication. 
   SUMMARY OF THE INVENTION 
   Therefore, the object of the present invention is to provide a digital phase-locked loop circuit that requires a relatively small layout area. 
   According to the present invention, there is provided a digital phase-locked loop circuit for generating a control voltage signal that is used to enable a voltage controlled oscillator to generate a target frequency output. The digital phase-locked loop circuit comprises: 
   a counter for outputting a count value corresponding to an output frequency outputted by the voltage controlled oscillator in response to the control voltage signal; 
   a comparator coupled to the counter and receiving the count value therefrom, the comparator comparing the count value with a target value associated with the target frequency output and outputting a comparison signal according to a comparison result therebetween; 
   a digital reference value generator coupled to the comparator and receiving the comparison signal therefrom, the digital reference value generator outputting a digital reference value according to the comparison signal, and including a register for storing the digital reference value therein; and 
   a digital-to-analog converter coupled to the digital reference value generator and receiving the digital reference value therefrom, the digital-to-analog converter generating the control voltage signal from the digital reference value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment with reference to the accompanying drawings, of which: 
       FIG. 1  is a schematic circuit block diagram showing a conventional phase-locked loop circuit; and 
       FIG. 2  is a schematic circuit block diagram showing the preferred embodiment of a digital phase-locked loop circuit according to this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2  illustrates the preferred embodiment of a digital phase-locked loop circuit  4  for generating a control voltage signal (vcon) that is used to enable a voltage controlled oscillator  5  to generate a target frequency output (f B ) according to the present invention. The relationship between the −3 dB frequency output (f E ) of an equalizer filter  6 , and the target frequency output (f B ) can be represented by the following equations:
   f   E   =k 1× vcon     f   B   =k 2× vcon   
where k 1  is a constant associated with electrical characteristics of electronic components in the equalizer filter  6 , and k 2  is a constant associated with electrical characteristics of electronic components in a loop composed of the voltage controlled oscillator and the digital phase-locked loop circuit  4 . Both k 1  and k 2  change with actual operating temperature conditions. However, a ratio of k 1  and k 2  almost does not change even at different operating temperature conditions. Therefore, the control voltage signal (vcon) can also be used to stabilize the −3 dB frequency output (f E ) of the equalizer filter  6 .
 
   The digital phase-locked loop circuit  4  includes a frequency divider  44 , a counter  45 , a comparator  46 , a digital reference value generator  41 , a digital-to-analog converter  42 , a low-pass filter  43 , and a control circuit  47 . 
   The frequency divider  44  is adapted to be coupled to the voltage controlled oscillator  5  and is adapted for frequency dividing an output frequency (vconclk) outputted by the voltage controlled oscillator  5  in response to the control voltage signal (vcon). 
   The counter  45  is coupled to the frequency divider  44 , and receives a divided output frequency therefrom. The counter  45  outputs a count value (vconclk_cnt) from the divided output frequency corresponding to the output frequency (vconclk) at a constant clock frequency. 
   The comparator  46  is coupled to the counter  45 , and receives the count value (vconclk_cnt) therefrom. The comparator  46  compares the count value (vconclk_cnt) with a target value (target_cnt) associated with the target frequency output (f E ), and outputs a comparison signal (vcondac) according to a comparison result therebetween. In this embodiment, the comparator  46  further outputs a measured difference value (cnt_diff) that is a difference between the target value (target_cnt) and the count value (vconclk_cnt). 
   The digital reference value generator  41  is coupled to the comparator  46 , and receives the comparison signal (vcondac) therefrom. The digital reference value generator  41  outputs a digital reference value (vconref) according to the comparison signal (vcondac), and includes a register  411  for storing the digital reference value (vconref) therein. In this embodiment, the digital reference value (vconref) in the register  411  is updated in increments when the count value (vconclk_cnt) is less than the target value (target_cnt), and in decrements when the count value (vconclk_cnt) is greater than the target value (target_cnt). The updating procedure of the digital reference value (vconref) will be described in greater detail in the succeeding paragraphs 
   The digital-to-analog converter  42  is coupled to the digital reference value generator  41 , and receives the digital reference value (vconref) therefrom. The digital-to-analog converter  42  generates the control voltage signal (vcon) from the digital reference value (vconref) in a conventional manner. 
   The low-pass filter  43  is coupled to the digital-to-analog converter  42  for processing the control voltage signal (vcon) therefrom. In this embodiment, the low-pass filter  43  includes a resistor (R) that has one end (r 1 ) coupled to the digital-to-analog converter  42 , and the other end (r 2 ) adapted to be coupled to the voltage controlled oscillator  5  and the equalizer filter  6  for outputting the control voltage signal (vcon) thereto, a capacitor (C) that has a first end (c 1 ) and a second grounded end (c 2 ), and a control switch  431  coupled between the first end (c 1 ) of the capacitor (C) and the other end (r 2 ) of the resistor (R). 
   The control circuit  47  is coupled to the comparator  46  and the control switch  431  of the low-pass filter  43 . The control circuit  47  receives the measured difference value (cnt_diff) from the comparator  46 , compares the measured difference value (cnt_diff) with a standard difference value (conclk_diff), and selectively enables and disables the low-pass filter  43  according to a comparison result between the measured difference value (cnt_diff) and the standard difference value (conclk_diff.). In this embodiment, since the low-pass filter  43  has a longer response time in an enabled mode than that in a disabled mode, the control circuit  47  enables the low-pass filter  43  (i.e., the control switch  431  is turned on by the control circuit  47 ) when the measured difference value (cnt_diff) is less than the standard difference value (conclk_diff), and disable the low-pass filter  43  (i.e., the control switch  431  is turned off by the control circuit  47 ) when the measured difference value (cnt_diff) is greater than the standard difference value (conclk_diff) so that the count value (vconclk_cnt) can be kept close to the target value (target_cnt) as fast as possible. 
   The control circuit  47  is further coupled to the digital reference value generator  41 , and enables the digital reference value generator  41  to update the digital reference value (vconref) in the register  411  at one of a lower update frequency and a higher update frequency according to the comparison result between the measured difference value (cnt_diff) and the standard difference value (conclk_diff). In this embodiment, the digital reference value (vconref) in the register  411  is updated at the lower update frequency when the measured difference value (cnt_diff) is less than the standard difference value (conclk_diff), and at the higher update frequency when the measured difference value (cnt_diff) is greater than the standard difference value (conclk_diff). Preferably, the lower update frequency cannot be greater than a −3 dB frequency band of the low-pass filter  43 . 
   In view of the above configuration, the present invention which includes the counter  45 , the comparator  46 , the digital reference value generator  41 , the digital-to-analog converter  42 , etc., can be fabricated in a circuit layout having a layout area of about 16320 μm 2  which is much less than that of the aforesaid conventional phase-locked loop circuit (about 280000 μm 2 ). Moreover, the capacitor (C) of the low-pass filter  43  has a capacitance smaller than that of the aforesaid conventional phase-locked loop circuit. The object of the invention is thus met. 
   While the present invention has been described in connection with what is considered the most practical and preferred embodiment, it is understood that this invention is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.