Patent Publication Number: US-7216249-B2

Title: Clock generation system

Description:
FIELD OF THE INVENTION 
   The invention relates to a clock generation system for generating from a given frequency clock a first-reference frequency clock, a second-reference frequency clock, and a third-reference frequency clock respectively having frequencies having predetermined ratios to the frequency of the given clock. More particularly, the invention relates to a clock generation system suitable for generating a multiplicity of clocks having reference frequencies required by a DVD (Digital Versatile Disc) system. 
   BACKGROUND OF THE INVENTION 
   In the art of DVD system that utilizes, for example, a DVD player, a DVD-R, and a DVD-RW, it is necessary to provide a multiplicity of clocks of different reference frequencies (referred to as reference frequency clocks), including at least a 27 MHz clock series (first reference frequency series) for a video system, a 33.8688 MHz clock series (second reference frequency series) for a sound system (particularly CD) (the series including integral multiples of a sampling frequency of 44.1 kHZ), and a 36.864 kHz clock series (third reference frequency series) for another sound system (particularly DVD) (the series including integral multiples of 48 kHz and 32 kHz sampling frequencies). 
   These three reference frequency series can be obtained using individual oscillation modules. However, this approach is costly for a clock generation system because it requires one oscillation module for each of the required reference clock frequencies. Then, in order to reduce the cost, one might consider to generate two of the three reference frequency series from the remaining one, utilizing PLL (phase-locked loop) circuits, as shown in  FIG. 7 .  FIG. 7  shows a clock generation system conjectured by the inventor in the process of devising the present invention, which is shown as a reference, but not prior art. 
   In the system shown in  FIG. 7 , a first reference frequency clock Fr 1  (27 MHz) generated by an oscillation module is used as the basis for generating the remaining two reference frequency clocks, that is a second reference frequency clock Fr 2  (33.8688 MHz) and a third reference frequency clock Fr 3  (36.864 MHz). 
   In the first PLL circuit  70   a  as shown in  FIG. 7 , the first 27 MHz reference frequency clock Fr 1  is supplied to a first frequency divider  71   a,  which frequency-divides the input first clock by a factor of 625 and supplies it to one comparison input terminal P 1  of a phase comparator (PD)  73   a.  The second frequency divider  72   a  receives the output of the PLL circuit  70   a  and frequency-divides it by a factor of 3136, which is supplied to a PD  73   a  as another comparison input P 2 . The PD  73   a  compares the two inputs P 1  and P 2  and generates an output (referred to as comparison output) indicative of the phase difference between them. The comparison output is smoothed by a low-pass filter (LPF) before it is supplied to a voltage control oscillator (VCO)  75   a  as a control signal. The VCO  75   a  changes its oscillation frequency according to the control signal input so that the two inputs to the PD  73   a  coincide in frequency and in phase. The loop gain of this PLL circuit is large, so that remaining deviation is extremely small. Thus, the frequency of the output of the VCO  75   a  is converted to 135.4752 (=27×3136/625) MHz, in accordance with the frequency division ratio of the frequency dividers  71   a  and  72   a.    
   The output frequency of the VCO  75   a  is frequency-divided by a frequency divider  76   a  by a factor of 4, generating a second reference frequency clock Fr 2 . The output frequency of the VCO  75   a  is further frequency-divided by a 1/6 frequency divider  77   a,  a 1/8 frequency divider  78   a,  and a 1/12 frequency divider  79   a,  respectively, into 22.5792 MHz, 16.9344 MHz, and 11.2896 MHz. These frequencies have specific relationships with the second reference frequency clock Fr 2 . These clocks belonging to the second reference frequency series have integral multiple of the sampling frequency of 44.1 kHz for use with CD systems. 
   The second PLL circuit  70   b  also performs frequency division similar to that of the first PLL circuit  70   a,  except that the frequency division ratio of the first frequency divider  71   b  is 1/375, while that of the second frequency divider  72   b  is 1/2048. The output frequency of the VCO  75   b  is converted into 147.456(=27×2048/375) MHz in accordance with the division ratios of the frequency dividers  71   b  and  72   b.  Incidentally, reference numeral  73   b  indicates a PD, and  74   b  indicates an LPF. 
   The output frequency of the VCO  75   b  is frequency-divided by the frequency divider  76   b  by a factor of 4 to produce a third reference frequency clock Fr 3 . Additionally, the output frequency of the VCO  75   b  is frequency-divided by a 1/6 frequency divider  77   b,  a 1/8 frequency divider  78   b,  and a 1/12 frequency divider  79   b  to generate frequencies of 24.576 MHz, 18.432 MHz, and 12.288 MHz, respectively, which have specific frequency relationship with the third reference frequency clock Fr 3 . The frequencies of these clocks belonging to the third reference frequency series Fr 3 s are integral multiples of audio sampling frequencies 48 kHz and 32 kHz in DVD systems. 
   Clocks of a first reference frequency series Fr 1   s  are also generated. The series includes the first reference frequency clock Fr 1  (27 MHz) and a clock of 13.5 MHz obtained by frequency division of the first reference frequency clock Fr 1  by a 1/2 frequency divider  76   d.    
   Thus, one may choose necessary frequency clocks from the first through third reference frequency series Fr 1   s –Fr 3   s  for his use. 
   The S/N (signal-to-noise) ratios of the clocks generated by the clock generation system shown in  FIG. 7  can be obtained based on a known S/N theory as follows. As an example, S/N ratio of clocks of the second reference frequency series Fr 2   s  will be discussed. It will be understood that by the frequency division of the first reference frequency clock Fr 1  by a factor of 625, the S/N ratio is improved by 20 log 625 [dB]. Hence, theoretically, the S/N ratio of the output signal of the first frequency divider  71   a  equals (S/N ratio of the output signal+20 log 625) [dB]. Assuming that the S/N ratio of the first reference frequency clock is 80 [dB], it is 80+56=136 [dB]. Note that the S/N ratios are rounded to integers for simplicity. (It is also the case in the following discussion.) 
   It should be noted, however, that since a PLL circuit is in operation on the noise floor of a given IC (integrated circuit) on which the PLL circuit is formed, the S/N ratio of the PLL circuit is limited by the S/N ratio of the noise floor. The S/N ratio of the noise floor is governed by the fluctuations in the power supply potential, which is on the order of 90 [dB]. Hence, the S/N ratio of the PLL circuit is limited by the S/N ratio of the noise floor (90 [dB]). Hence, the S/N ratio of the output of the first frequency divider  71   a,  that is, the S/N ratio of one comparison input P 1  to the PD  73   a  is at most 90 [dB]. 
   Since the S/N ratios of the comparison inputs P 1  and P 2  to the PD  71   a  are the same, the S/N ratio of the comparison input P 2  is 90 [dB]. The S/N ratio of the comparison input to the second frequency divider  72   a  is lowered by 20log3136 [dB], since the input P 2  is stepped up by a factor of 3136. Therefore, the S/N ratio of the input to the second frequency divider  72   a  becomes (90 (for the comparison input P 2 ) −20log3136) [dB], or 20.3 [dB]. 
   Thus, S/N ratios of clocks of the second reference frequency series Fr 2   s  are 32.3 [dB] for the second reference frequency clock Fr 2 , 35.8 [dB] for the 22.5792 MHz clock, 38.3 [dB] for the 16.9344 MHz clock, and 41.8 [dB] for the 11.2896 MHz clock. 
   Similar calculations lead to S/N ratios of the clocks of the third reference frequency series Fr 3   s.  They are: 36.0 [dB] for the third reference frequency clock Fr 3 ; 39.5 [dB] for 24.576 MHz clock; 42.0 [dB] for 18.432 MHz clock; and 45.5 [dB] for 12.288 MHz clock. 
   In this way, using PLL circuits and frequency dividers as shown in  FIG. 7 , it is possible to generate clocks of a second reference frequency series Fr 2   s  which include a second reference frequency clock Fr 2  obtained by multiplying the frequency of the first reference frequency clock Fr 1  by a predetermined ratio, and clocks of a third reference frequency series Fr 3   s  which include a third reference frequency clock Fr 3  obtained by a similar multiplication. However, the S/N ratios of the clocks of the second and third reference frequency series are lowered to 30 [dB] −40 [dB]. This deterioration in S/N ratio is a problem that must be solved, since DVD systems, etc. in general requires a S/N ratio of at least 50 [dB], preferably more than 60 [dB]. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the invention to provide a clock generation system for generating a multiplicity of reference frequency clocks needed in DVD systems, which include clocks of a first reference frequency series (27 MHz series) for use in video system, a second reference frequency series (33.8588 MHz series) for use in audio system (particularly in CD), and a third reference frequency series (36.864 MHz series) for use in audio system (particularly in DVD), using a least number of oscillation modules and additional PLL circuits, the clock generation system capable of providing the first through the third clocks having sufficient S/N ratios in spite of the S/N ratio limitation by the noise floor. 
   In accordance with an aspect of the invention, there is provided a clock generation system for generating at least a clock having a first frequency (referred to as first-frequency clock), a clock having a second frequency (referred to as second-frequency clock), and a clock having a third frequency (referred to as third-frequency clock), comprising: 
   a first PLL circuit supplied with the first-frequency clock as a reference clock and adapted to generate a clock with an intermediate frequency (referred as intermediate-frequency clock) having a predetermined first ratio to the reference frequency; 
   a second PLL circuit supplied with the intermediate-frequency clock and adapted to generate the second-frequency clock, with the second frequency having a predetermined second ratio to the intermediate frequency; and 
   a third PLL circuit supplied with the intermediate-frequency clock and adapted to generate the third-frequency clock, with the third frequency having a predetermined third ratio to the intermediate frequency. 
   In accordance with another aspect of the invention, there is provided a clock generation system for generating at least a clock having a first frequency (referred to as first-frequency clock), a clock having a second frequency (referred to as second-frequency clock), and a clock having a third frequency (referred to as third-frequency clock), comprising: 
   a first PLL circuit supplied with a reference clock and adapted to generate an intermediate-frequency clock having an intermediate frequency which is a predetermined first ratio to the reference frequency; 
   a second PLL circuit supplied with the intermediate-frequency clock and adapted to generate the second-frequency clock, with the second frequency having a predetermined second ratio to the intermediate frequency; and 
   a third PLL circuit supplied with the intermediate-frequency clock and adapted to generate the third-frequency clock, with the third frequency having a predetermined third ratio to the intermediate frequency. 
   In accordance with a further aspect of the invention, there is provided clock generation system for generating at least a first-frequency clock having a first frequency, a second-frequency clock having a second frequency, a third-frequency clock having a third frequency, and a fourth-frequency clock having a frequency that is double of the first frequency, the clock generation system comprising: 
   a first PLL circuit supplied with the first-frequency clock as a reference clock and adapted to generate an intermediate-frequency clock having an intermediate frequency having a predetermined first ratio to the reference frequency; 
   a frequency divider for frequency dividing the first-frequency by a predetermined factor to generate the fourth frequency; 
   a second PLL circuit supplied with the intermediate-frequency clock and adapted to generate the second-frequency clock, with the second frequency having a predetermined second ratio to the intermediate frequency; and 
   a third PLL circuit supplied with the intermediate-frequency clock and adapted to generate the third-frequency clock, with the third frequency having a predetermined third ratio to the intermediate frequency. 
   In view of the fact that the S/N ratios of the PLL circuits are improved according to the frequency division ratio and lowered according to the multiplication ratio, and that it is limited by the S/N ratio of the noise floor, the invention generates a common intermediate-frequency clock in the first PLL circuit and the intermediate-frequency clock is supplied to the second and the third PLL circuits. This permits elimination of the limitation, or reduction of the influence, of the noise floor on the S/N ratio. Thus, in spite of the noise floor limitation on the S/N ratios, a second and a third-frequency clocks can be generated from a first-frequency clock with sufficient S/N ratios. 
   Furthermore, the inventive clock generation system can generate a first 27 MHz reference clock series for a video system, a second 33.8688 MHz reference clock series for a sound system (particularly for CD) (with the frequencies being integral multiples of a 44.1 kHz sampling frequency), and a third 36.864 kHz reference clock series for another sound system (particularly for DVD) (with the frequencies being integral multiples of 48 kHz and 32 kHz sampling frequencies), all with sufficient S/N ratios. 
   The use of a common reference clock adequate for the first through third-frequency clocks eliminates the limitation or minimizes the influence of the noise floor, thereby allowing generation of the first through the third-frequency clocks having sufficient S/N ratios. 
   The invention also permits generation of a fourth frequency clock whose clock frequency is double the frequency of the first-frequency clock (used as a reference frequency clock) from the clock outputted from the first PLL circuit as a common intermediate-frequency clock for the second and the third PLL circuits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram representation of a first embodiment of a clock generation system according to the invention. 
       FIG. 2  is a block diagram representation of a second embodiment of a clock generation system according to the invention. 
       FIG. 3  is a block diagram representation of a third embodiment of a clock generation system according to the invention. 
       FIG. 4  is a block diagram representation of a fourth embodiment of a clock generation system according to the invention. 
       FIG. 5  is a block diagram representation of a fifth embodiment of a clock generation system according to the invention. 
       FIG. 6  is a table showing the clocks of the respective frequency series along with the S/N ratios involved. 
       FIG. 7  is a block diagram of an exemplary clock generation system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The invention will now be described in detail by way of examples with reference to the accompanying drawings. 
   Referring to  FIG. 1 , there is shown in block diagram a first embodiment of a clock generation system according to the invention. This clock generation system is designed to receive as a reference clock a first reference frequency clock of 27 MHz for use in a video system to generate clocks of a first frequency series (or 27 MHz series) for video system, a second reference frequency series (or 33.8688 MHz series) for audio system (particularly for CD), and a third reference frequency series (or 36.864 MHz series) for audio system (particularly for DVD). 
   In view of the fact that the S/N ratio of a PLL circuit gets improved by the frequency division ratio thereof, lowered by the multiplication ratio, and limited by the S/N ratio of the noise floor, a first-frequency clock Fr 1  is supplied as the reference clock to a first PLL circuit  10   a,  which generates an intermediate-frequency clock Fim 1  having a frequency as determined by the first reference frequency clock Fr 1  and a first predetermined ratio. 
   The intermediate-frequency clock Fim 1  is entered in a second PLL circuit  10   b,  which generates a second-base clock) for generating a second reference clock having a frequency as determined by the intermediate-frequency clock Fim 1  and a second predetermined ratio. The second-base clock is frequency-divided to generate clocks of a second reference frequency series Fr 2   s  that includes the frequency of the second reference frequency clock Fr 2 . 
   The intermediate-frequency clock Fim 1  is also supplied to a third PLL circuit  10   c,  which generates a third-base clock) for generating a third reference clock having a frequency as determined by the intermediate-frequency clock Fim 1  and a third predetermined ratio. The frequency of the third-base is frequency-divided to generate a third reference frequency series Fr 3   s  that includes the frequency of the third reference clock Fr 3 . 
   In this way, a multiplicity of PLL circuits are connected in series, wherein a common intermediate-frequency clock Fim 1  is generated by the first stage PLL circuit  10   a,  which is supplied to the second and third PLL circuits to distribute frequency division ratios among the PLL circuits. This helps to circumvent the limitation or suppress the influence of the noise floor on the S/N ratios of the clocks generated. 
   The clock system of  FIG. 1  is formed on an IC. The first reference frequency clock Fr 1  for use as the reference clock may be generated by an internal oscillation module provided in the IC, or may be supplied from an external IC. 
   The first PLL circuit  10   a  is supplied with the first reference frequency clock Fr 1  (27 MHz). Here, it is assumed that the S/N ratio of this clock Fr 1  is 80 [dB], and S/N ratio of the noise floor is 90 [dB], as in the example shown in  FIG. 7 . 
   In the first PLL circuit  10   a,  a first frequency divider  11   a  frequency-divides the clock Fr 1  by 25, and feed the divided clock to one comparison input P 1  of a PD  13   a.  The S/N ratio of this comparison input P 1  is theoretically 108 [dB] (=80+20log25), but on account of the limitation of the noise floor, the S/N ratio drops to 90 [dB]. A second frequency divider  12   a  frequency-divides the output of the first PLL circuit  10   a  by 128, and supplies the resultant clock to another comparison input P 2  of the PD  13   a.  The S/N ratio of the comparison input P 2  turns out to be 90 [dB]. 
   PD  13   a,  LPF  14   a,  and VCO  15   a  of the first PLL circuit  10   a  only differ from corresponding circuits of PLL circuits shown in  FIG. 7  in that some of their parameters are different, and these PLL circuits operate in the same manner. The frequency of the output of the VCO  15   a  is converted to 138.24 (=27×128/25) MHz in accord with the frequency division ratio of frequency dividers  11   a  and  12   a.  The S/N ratio of the output of the VCO  15   a  is 48 [dB] (=90−20 log 128). The output of the VCO  15   a  is further frequency-divided by a frequency divider  16   a  by the ratio of 1/2, resulting in a first intermediate-frequency clock Fim 1  (69.120 MHz). As a consequence of the frequency division (by 2) by the frequency divider  16   a,  the S/N ratio of this clock Fim 1  becomes 54 [dB]. 
   This clock Fim 1  (69.120 MHz, 54 [dB]) are utilized as a common input clock to the second PLL circuit  10   b  and the third PLL circuit  10   c.    
   The second PLL circuit  10   b  is composed of a first frequency divider  11   b  (division ratio of 1/50), a second frequency divider  12   b  (division ratio 1/98), a PD  13   b,  an LPF  14   b,  and a VCO  15   b.  Although the division ratios are different, the second PLL circuit operates in the same way as the first PLL circuit  10   a.    
   The second PLL circuit  10   b  is supplied with the clock Fim 1 , generating a second-base clock of 135.4752 MHz (=69.120×98/50) in accord with the division ratios of the frequency dividers  11   b  and  12   b.    
   The S/N ratio of the output of the first frequency divider  11   b,  i.e. the first input P 1  to the PD  13   b, i s not limited by the noise floor, and becomes 88 [dB] (=54+20log50). The S/N ratio of the input to the second frequency divider  12   b,  i.e. the S/N ratio of the second-base clock, becomes 48.2 [dB] (=88−20 log 98). 
   This second-base clock (135.4752 MHz, 48.2 [dB]) is again frequency-divided by 4 by a frequency divider  16   b,  resulting in a second reference frequency clock Fr 2  (33.8688 MHz, 60.2 [dB]). This second reference clock is further frequency-divided by a 1/6 frequency divider  17   b,  a 1/8 frequency divider  18   b,  and a 1/12 frequency divider  19   b,  resulting in clocks of 22.5792 MHz (63.7 [dB]), 16.9344 MHz (66.2 [dB]), and 11.2896 MHz (69.7 [dB]), respectively, which are associated with the second-frequency clock Fr 2 . These clocks constitute a second reference frequency series Fr 2 . 
   The third PLL circuit  10   c  is composed of a first frequency divider  11   c  (division ratio 1/45), a second frequency divider  12   c  (division ratio 1/96), a PD  13   c,  an LPF  14   c,  and a VCO  15   c.  Although the division ratios are different, this PLL circuit  10   c  has the same function as the first PLL circuit  10   a.    
   Like the second PLL circuit  10   b,  the third PLL circuit  10   c  is also supplied with the clock Fim 1  and generates a 147.456 MHz (=69.120×96/45) third-base clock for generating the third reference clock in accord with the division ratios of frequency dividers  11   c  and  12   c.    
   The output of the first frequency divider  11   c,  or the first input P 1  to the PD  13   c,  is 87 [dB] (=54+20 log 45), which is not limited by the noise floor. The S/N ratio of the signal input to the second frequency divider  12   c,  or the S/N ratio of the third-base clock, is 47.4 [dB] (=87−20 log 96). 
   The third-base clock (147.456 MHz, 47.4 [dB]) is frequency-divided by 4 by the frequency divider  16   c,  which results in the third reference frequency clock Fr 3  (36.864 MHz, 59.4 [dB]). The resultant third reference clock Fr 3  is further frequency-divided by a 1/6 frequency divider  17   c,  a 1/8 frequency divider  18   c,  and a 1/12 frequency divider  19   c  to generate clocks of a third reference frequency series Fr 3   s  associated with the third reference frequency clock Fr 3 . The third reference frequency series includes 24.576 MHz (63.0 [dB]), 18.432 MHz (65.4 [dB]), and 12.288 MHz (69.0 [dB]). 
   In addition, the first reference frequency clock Fr 1  (27 MHz, 80 [dB]), which is supplied to the first PLL circuit  10   a  as a reference clock, and the clock that results from the reference clock by the frequency division by a 1/2 frequency divider  16   d  (13.5 MHz, 86 [dB]) are also output as clocks of the first reference frequency series Fr 1   s.    
   The clocks of the first reference frequency series Fr 1   s  including the first reference frequency clock Fr 1 , the clocks of the second reference frequency series Fr 2   s  including the second reference frequency clock Fr 2 , and clocks of the third reference frequency series Fr 3   s  including the third reference frequency clock Fr 3  have sufficiently high S/N ratios as compared with the clock of the cited reference frequency, so that they can be selectively utilized. 
   It should be appreciated that in the first embodiment shown herein although the first PLL circuit  10   a  of the first stage has an S/N ratio a little limited by the noise floor, the subsequent PLL circuits, i.e. second PLL circuit  10   b  and third PLL circuit  10   c,  are not influenced by the noise floor. That is, the division ratios of the subsequent PLL circuits  10   a,    10   b,  and  10   c  are set so that the S/N ratios of these PLL circuits are not determined by the S/N ratio of the noise floor. Only the S/N ratio of the first PLL circuit  10   a  can be limited by the noise floor, since the S/N ratio is dependent on the S/N ratio of the first reference clock Fr 1  input to the first PLL circuit  10   a.  Thus, the S/N ratios of the clock generation system can be most effectively improved. 
   Referring to  FIG. 2 , there is shown a structure of a clock generation system in accordance with a second embodiment of the invention. 
   In the clock generation system shown in  FIG. 2 , the first PLL circuit  20   a  is supplied with the second reference frequency clock Fr 2  (33.8688 MHz). Based on this second reference frequency clock Fr 2 , clocks of the first reference frequency series Fr 1   s  including the first reference frequency clock Fr 1  and clocks of a third reference frequency series Fr 3   s  including a third reference frequency clock Fr 3  are generated. Although the first through the third PLL circuits  20   a,    20   b,  and  20   c  and the respective frequency dividers have different frequencies and division ratios, they are the same in fundamental structure and function as the first embodiment shown in  FIG. 1 . 
   The first frequency divider  21   a  of the first PLL circuit  20   a  divides the input clock Fr 2  by a factor of 14, and supplies the resultant clock to one comparative input P 1  of the PD  23 . The S/N ratio of this comparison input P 1  is theoretically 103 [dB] (=80+20 log 14), which, in actuality however, turns out to be 90 [dB] due to the limitation of the noise floor. The second frequency divider  22   a  divides the output of the PLL circuit  20   a  by 50, and supplies the resultant clock to the other comparison input P 2  of the PD  23   a.  The S/N ratio also turns out to be 90 [dB]. 
   The output frequency of the VCO  25   a  has been converted to 120.96 (=33.8688×50/14) MHz according to the division ratios of the frequency dividers  21   a  and  22   a.  The S/N ratio of the output of VCO  25   a  is 56 [dB] (=90−20 log 50). The output of VCO  25   a  is again frequency-divided by frequency divider  26   a  by 3 to obtain a second intermediate-frequency clock Fim 2  (40.320MHz). The S/N ratio of the clock Fim 2  becomes 65.7 [dB] as a consequence of frequency division by the frequency divider  26   a  by 3. 
   This clock Fim 2  (40.320 MHz, 65.7 [dB]) is used as a common input clock to the second PLL circuit  20   b  and the third PLL circuit  20   c.    
   The second PLL circuit  20   b  is composed of a first frequency divider  21   b  (division ratio 1/14), a second frequency divider  22   b  (division ratio 1/75) and PD  23   b,  LPF  24   b,  and VCO  25   b,  and operates in the same way as the first PLL circuit  20   a,  though its frequency division ratios differ. 
   This second PLL circuit  20   b  is supplied with the clock Fim 2  to generate a first-base clock of 216 MHz (=40.320×75/14) according to the division ratios of the frequency dividers  21   b  and  22   b.    
   The S/N ratio of the output of the first frequency divider  21   b,  that is, the first input P 1  of the PD  23   b,  is not affected by the noise floor, and is 89 [dB] (=65.7+20 log 14). The S/N ratio at the input of the second frequency divider  22   b,  that is, the S/N ratio of the first-base clock, becomes 51.1 [dB] (=89−20 log 75). 
   The first-base clock (216 MHz, 51.1 [dB]) is frequency-divided by a frequency divider  27   b  by 8, outputting a first reference clock (27 MHz, 69.1 [dB]). The first reference clock is further frequency-divided by a 1/4 frequency divider  26   b  and a 1/16 frequency divider  28   b,  outputting clocks of 54 MHz (63.1 [dB]) and 13.5 MHz (75.1 [dB]) clocks, which are associated with the first-frequency clock Fr 1 . These clocks are outputted as clocks of the first reference frequency series Fr 1   s.    
   The third PLL circuit  20   c  is composed of a first frequency divider  21   c  (division ratio 1/35), a second frequency divider  22   c  (division ratio 1/128), a PD  23   c,  an LPF  24   c,  and a VCO  25   c,  and operates in the same way as the first PLL circuit  20   a,  though its frequency division ratios differ. 
   Like the second PLL circuit  20   b,  the third PLL circuit  20   c  is supplied with the clock Fim 2  to generate a third-base clock of 147.456 MHz (=40.320×128/35) according to the division ratios of the frequency dividers  21   c  and  22   c.    
   The S/N ratio of the first frequency divider  21   c,  that is, the S/N ratio of the first input P 1  of the PD  23   c,  is theoretically 96.6 [dB] (=65.7+20 log 35). In actuality, however, it is limited to 90 [dB] by the noise floor. The S/N ratio of the input to the second frequency divider  22   c,  or the S/N ratio of the third-base clock, is 48.0 [dB] (=90−20 log 128). 
   The third-base clock (147.456 MHz, 48.0 [dB]) is frequency-divided by a frequency divider  26   c  by 4 to output a third reference frequency clock Fr 3  (36.864 MHz, 60.0 [dB]). In addition, the third reference clock is further frequency-divided by a 1/6 frequency divider  27   c,  a 1/8 frequency divider  28   c,  and a 1/12 frequency divider  29   c  to generate clocks having frequencies of 24.576 MHz (63.5 [dB]), 18.432 MHz (66.0 [dB]), and 12.288 MHz (69.5 [dB]) belonging to a third reference frequency series Fr 3   s  associated with the third reference frequency clock Fr 3 . 
   Further, the second reference frequency clock Fr 2  (33.8688 MHz, 80 [dB]) and a clock of 16.9344 MHz (86 [dB]) that is obtained by frequency-dividing the second reference frequency clock Fr 2  by the frequency divider  26   d  by 2 are output, constituting the clocks of a second reference frequency series Fr 2   s.    
   The clocks of the first through the third reference frequency series Fr 1   s –Fr 3   s  have little influence of the noise floor and have much higher S/N ratios as compared with the clock of cited reference, though they are partially limited by the noise floor. 
   Referring to  FIG. 3 , there is shown a structure of a clock generation system in accordance with the third embodiment of the invention. The clock generation system shown in  FIG. 3  receives as its reference clock a 36.864 MHz third reference frequency clock for sound system (especially for DVD) and outputs clocks of the first 27 MHz reference frequency series for video system and clocks of a second 33.8688 MHz reference frequency series for audio system (especially for CD). 
   In this clock generation system of  FIG. 3  the third reference frequency clock Fr 3  (36.864 MHz) is supplied to the first PLL circuit  30   a.  Based on the third reference frequency clock Fr 3 , clocks of a first reference frequency series Fr 1   s  that includes the first reference frequency clock Fr 1  and clocks of a second reference frequency series Fr 2   s  that includes the second reference frequency clock Fr 2  are generated. It is noted that the first through third PLL circuits  30   a,    30   b,  and  30   c  and the frequency dividers included in this embodiments are basically the same in structure and functions as those of  FIG. 1 , except for the frequencies it generates and the frequency division ratios used. 
   In the first PLL circuit  30   a,  a first frequency divider  31   a  frequency-divides the input clock Fr 3  by 16, and supplies it as one comparison input P 1  to a PD  33   a.  The S/N ratio of the comparison input P 1  is theoretically 104 [dB] (=80+20 log 16), but in actuality it is reduced to 90 [dB] by the limitation of the noise floor. A second 1/60 frequency divider  32   a  frequency-divides the output of the PLL circuit  30   a  by 60 and provides the resultant clock as the other comparison input P 2  to the PD  33   a.  The S/N ratio of the comparison input P 2  is also 90 [dB]. 
   The frequency outputted from a VCO  35   a  is converted to 138.24 (=36.864×60/16) MHz according to the division ratios of the frequency dividers  31   a  and  32   a.  The S/N ratio of the output of the VCO  35   a  is 54.6 [dB] (=90−20 log 60). The frequency of the output of the VCO  35   a  is further divided by 2 by a frequency divider  36   a  to obtain a third intermediate-frequency clock Fim 3  (69.120 MHz). The S/N ratio of the clock Fim 3  becomes 60.6 [dB] after the frequency division by a factor of 3 by the frequency divider  36   a.    
   This clock Fim 3  (69.120 MHz, 60.6 [dB]) is used as the common input clock to the second and third PLL circuits  30   b  and  30   c,  respectively. 
   The second PLL circuit  30   b  is composed of a first frequency divider  31   b  (frequency division ratio of 1/32), a second frequency divider  32   b  (frequency division ratio of 1/50) and a PD  33   b,  an LPF  34   b,  and a VCO  35   b,  and have different frequency division ratios from the first PLL circuit  30   a.  However, the two PLL circuits are the same in operation. 
   The second PLL circuit  30   b  is fed the clock Fim 3 , and outputs a first-base clock of 108 (=69.120×50/32) MHz in accordance with the division ratio of the frequency dividers  31   b  and  32   b.    
   Although the S/N ratio of the output of the first frequency divider  31   b  is theoretically 90.6 [dB] (=60.6+20 log 32), it is limited to 90 [dB] by the noise floor. The S/N ratio at the input end of the frequency divider  32   b,  i.e. the S/N ratio of the first-base clock becomes 56.1 [dB] (=90−20 log 50). 
   This first-base frequency clock (108 MHz, 56.1 [dB]) is frequency-divided by a frequency divider  37   b  by 4 to output a first reference frequency clock Fr 1  (27 MHz, 68.1 [dB]). In addition, the first reference frequency clock is further frequency-divided by a 1/2 frequency divider  36   b  and a 1/8 frequency divider  38   b,  resulting in clocks having frequencies of 54 MHz (62.1 [dB]) and 13.5 MHz (74.1 [dB]) belonging to a first reference frequency series Fr 1   s  associated with the first reference frequency clock Fr 1 . 
   The third PLL circuit  30   c  is composed of a first frequency divider  31   c  (division ratio of 1/50), a second frequency divider  32   c  (division ratio of 1/98), a PD  33   c,  an LPF  34   c,  and a VCO  35   c,  and have division ratios different from those of the first PLL circuit  30   a.  However, the PLL circuit  30   c  operates in the same manner as the PLL circuit  30   a.    
   Like the second PLL circuit  30   b,  the third PLL circuit  30   c  is fed the clock Fim 3  to generates a second-base clock of 135.4752 (=69.120×98/50) MHz clock in accordance with the division ratios of the frequency dividers  31   c  and  32   c.    
   The S/N ratio of the output of the first frequency divider  31   c,  i.e. the S/N ratio of the first input P 1  to the PD  33   c,  is theoretically 94.5 [dB] (=60.6+20log50), it is in actuality 90 [dB] as it is limited by the noise floor. The S/N ratio at the input end of the second frequency divider  32   c,  i.e. the S/N ratio the second-base clock becomes 50.3 [dB] (=90−20 log 98). 
   The second-base clock (135.4752 MHz, 50.3 [dB]) is further frequency-divided by a 1/4 frequency divider  36   c  by 4 to generate a second reference frequency clock Fr 2  (33.8688 MHz, 62.3 [dB]). In addition, the frequency of the second reference clock is further frequency-divided by a 1/6 frequency divider  37   c,  a 1/8 frequency divider  38   c,  and a 1/12 frequency divider  39   c  into frequencies of 22.5792 MHz (65.8 [dB]), 16.9344 MHz (68.3 [dB]), and 11.2896 MHz (71.8 [dB]), respectively. These frequencies constitute a second reference frequency series Fr 2  specifically associated with the second reference clock Fr 2 . 
   In addition, the third reference frequency clock Fr 3  (36.864 MHz, 80 [dB]) supplied to the first PLL circuit  30   a  as the reference clock thereof is further frequency-divided by 2 by a frequency divider  36   d  to generate 18.432 MHz (86 [dB]) clock, which is output together with the third reference frequency clock Fr 3  to form the clocks of a third reference frequency series Fr 3   s.    
   The clocks of the first through the third reference frequency series Fr 1   s –Fr 3   s  have little influence of the noise floor and have much higher S/N ratios as compared with clocks of cited reference, though their S/N ratios are partially limited by the noise floor. 
   It will be understood that the frequency dividers  16   a,    26   a  and  36   a  used in the foregoing embodiments to generate the intermediate-frequency clocks Fim 1 –Fim 3  may be omitted by adapting other frequency dividers in other PLL circuits to generates these intermediate clocks. 
   Referring to  FIG. 4 , there is shown a configuration of a clock generation system in accordance with the fourth embodiment of the invention. 
   In the clock generation system shown in  FIG. 4 , besides the first through the third reference frequency clocks Fr 1 –Fr 3 , use is made of another reference clock Fr 0  suitable for forming the respective reference frequencies. 
   The reference clock Fr 0  (34.560 MHz, 80 [dB]) is supplied to a first PLL circuit  40   a,  a second PLL circuit  40   b,  and a third PLL circuit  40   c.    
   The first PLL circuit  40   a  has a first frequency divider  41   a  having a frequency division ratio of 1/16 and a second frequency divider  42   a  having a frequency division ratio of 1/50, outputting a first-base clock of 108 MHz (56.1 [dB]). The first-base clock is frequency-divided by a frequency dividers  46   a,    47   a,  and  48   a  having frequency division ratios 1/2, 1/4, and 1/8, respectively, to generate clocks of 54 MHz (62.1 [dB]), 27 MHz (68.1 [dB]), and 13.5 MHz (74.1 [dB]), respectively. These frequencies constitute a first reference frequency series Fr 1   s.    
   The second PLL circuit  40   b  has a first frequency divider  41   b  and a second frequency divider  42   b  having frequency division ratios 1/15 and 1/64, respectively, to generate a third-base clock of 147.456 MHz (54.0 [dB]). The frequency of this third-base clock is frequency-divided by four frequency dividers  46   b,    47   b,    48   b,  and  49   b  having a frequency division ratios 1/4, 1/6, 1/8, and 1/12, respectively, to obtain a third reference frequency series Fr 3   s  that include frequencies of 36.864 MHz (66.0 [dB]), 24.576 MHz (69.5 [dB]), 18.432 MHz (72.0 [dB]), and 12.288 MHz (75.5 [dB]), respectively. 
   The third PLL circuit  40   c  has a first frequency divider  41   c  and a second-base divider  42   c  having frequency division ratios 1/25 and 1/98, respectively, to generates a second reference clock (135.4752 MHz, 50.3 [dB]). The second-base clock is frequency-divided by frequency dividers  46   c,    47   c,    48   c,  and  49   c  having frequency division ratios of 1/4, 1/6, 1/8, and 1/12, respectively, to generate clocks having frequencies of 33.8688 MHz (62.3 [dB]), 22.5792 MHz (66.8 [dB]), 16.9344 MHz (68.3 [dB]), and 11.2896 MHz (71.8 [dB]). These frequencies constitute a second reference frequency series Fr 2   s.  Reference numerals  43   a – 43   c  stand for PDs,  44   a – 44   c  for LPFs, and  45   a – 45   c  for VCOs. 
   It will be apparent, in comparison with the example of  FIG. 7 , that each of the clocks of the first through the third reference frequency series Fr 1   s –Fr 3   s  of the embodiment shown in  FIG. 4  have sufficiently large S/N ratios (over 60 dB), which are adequate for use as system clocks. This has been achieved by the use of an appropriate common reference clock Fr 0  (34.560 MHz) to form the first through the third reference frequency series Fr 1   s –Fr 3   s,  and by selection of appropriate division ratios of the respective PLL circuits  40   a – 40   c  to eliminate the limitation or suppress the influence of the noise floor. 
   Referring to  FIG. 5 , there is shown a structure of a clock generation system in accordance with the fifth embodiment of the invention. As shown in  FIG. 5 , the system receives a 27 MHz clock as a first reference frequency clock and generates clocks of a 27 MHz series (first reference frequency series) for video system, clocks of a 33.8688 MHz series (second reference frequency series) for audio system (especially for CD), and clocks of a 36.864 MHz series (third reference frequency series) for audio system (especially for DVD). In this regard, this clock generation system is the same as the first embodiment shown in  FIG. 1 . Since a high-quality quartz oscillator having a frequency of 27 MHz is available on the market at low price, the clock generation system that receives a 27 MHz clock as the first reference frequency clock is advantageous. 
   However, the clock generation system in the form of the first embodiment shown in  FIG. 1  cannot make the 54 MHz frequency clock for video system from the first 27 MHz reference frequency clock. Moreover, although the 54 MHz frequency clock for video requires the highest S/N ratio, any of the clock generation systems shown in  FIGS. 2–4  is not necessarily adequate to provide a sufficiently high S/N ratio. 
   The clock generation system of  FIG. 5  can provide the 54 MHz first reference frequency clock for video with a sufficiently high S/N ratio, using the reference clock of 27 MHz as the reference frequency clock. 
   In the clock generation system of  FIG. 5 , the first reference frequency clock Fr 1  (27.0 MHz) is provided to a first PLL circuit  50   a.  Based on this first reference frequency clock Fr 1 , the clock generation system generates clocks of a first reference frequency series Fr 1   s  (54 MHz series) including the first reference frequency clock Fr 1 , clocks of a second reference frequency series Fr 2   s  including the second reference frequency clock Fr 2 , clocks of a third reference frequency series Fr 3   s  including the third reference frequency clock Fr 3 . 
   Although the first through the third PLL circuits  50   a,    50   b,  and  50   c  and the respective frequency dividers have different frequencies and division ratios from those of the first embodiment shown in  FIG. 1 , they are the same in fundamental structure and function as the first embodiment. 
   In the first PLL circuit  50   a,  the first frequency divider  51   a  frequency-divides the input clock Fr 1  by 4 to generate one comparison input P 1  to the PD  53   a.  The S/N ratio of the comparison input P 1  is theoretically 92 [dB] (=80+20 log 4). However, in actuality it is limited to 90 [dB] by the noise floor. The second frequency divider  52   a  frequency-divides the output of the PLL circuit  50   a  by 32 to generate the other comparison input P 2  of the PD  53   a.  The S/N ratio of the comparison input P 2  is also 90 [dB]. 
   The frequency of the output of the VCO  55   a  is converted to 216.0 (=27.0×32/4) MHz in accordance with the division ratios of frequency dividers  51   a  and  52   a.  The S/N ratio of the output of the VCO  55   a  is 60 [dB] (=90−20 log 32). The output of the VCO  55   a  is further frequency-divided by a frequency divider  56   a  by 5 to obtain a fifth intermediate-frequency clock Fim 5  (43.2 MHz). The S/N ratio of this clock Fim 5  is 74.0 [dB] after the frequency division by the frequency divider  56   a.    
   The clock Fim 5  is used as a common input clock to the second PLL circuit  50   b  and the third PLL circuit  50   c.    
   In addition, the output of VCO  55   a  is further frequency-divided by another frequency divider  57   a  by  4  to obtain a 54 MHz clock for video. The S/N ratio of the 54 MHz frequency clock is 72.0 [dB] after the frequency division by the frequency divider  57   a.  It should be appreciated that this S/N ratio is much larger as compared with the S/N ratios (about 60 [dB]) obtained in other embodiments shown in  FIGS. 2–4 . 
   The second PLL circuit  50   b  is composed of a first frequency divider  51   b  (frequency division ratio of 1/125), a second frequency divider  52   b  (frequency division ratio of 1/392), a PD  53   b,  an LPF  54   b,  and a VCO  55   b.  Although the division ratios are different, operations of the second PLL circuit  50   b  are the same as those of the first PLL circuit  50   a.    
   The second PLL circuit  50   b  is supplied with a clock Fim 5  and generates a second-base clock of 135.4752 MHz (=43.20×392/125) in accordance with the frequency division ratios of the frequency dividers  51   b  and  52   b.    
   Although the theoretical S/N ratio of the first frequency divider  51   b,  i.e. the S/N ratio of the first input P 1  of the PD  53   b,  is 114.3 [dB] (=74.0+20 log 125), it is in actuality 90 [dB] due to the limitation by the noise floor. The S/N ratio of the input signal to the second frequency divider  52   b,  i.e. the S/N ratio of the first-base clock, is 38.3 [dB] (=90−20 log 392). 
   This second-base clock (135.4752 MHz, 38.3 [dB]) is further frequency-divided by a frequency divider  56   b  by 4, outputting a second reference frequency clock Fr 2  (33.8688 MHz, 50.3 [dB]). In addition, the second reference clock is further frequency-divided by a 1/6 frequency divider  57   b,  a 1/8 frequency divider  58   b,  and a 1/12 frequency divider  59   b,  generating clocks of 22.5792 MHz (53.8 [dB]), 16.9344 MHz (56.3 [dB]), and 11.2896 MHz (59.8 [dB]) belonging to a second reference frequency series Fr 2   s  associated with the second reference frequency clock Fr 2 . 
   The third PLL circuit  50   c  is composed of a first frequency divider  51   c  (frequency division ratio of 1/75), a second frequency divider  52   c  (frequency division ratio of 1/256), a PD  53   c,  a LPF  54   c,  and a VCO  55   c.  Although the division ratios are different, the third PLL circuit  50   c  operates in the same manner as the first PLL circuit  50   a.    
   As in the second PLL circuit  50   b,  this third PLL circuit  50   c  is supplied with the clock Fim 5 , and generates a third-base clock of 147.456 MHz (=43.200×256/75) in accordance with the division ratios of the frequency dividers  51   c  and  52   c.    
   The S/N ratio of the output of the first frequency divider  51   c,  i.e. the S/N ratio of the first input P 1  of the PD  53   c,  is theoretically 111.5 [dB] (=74.0+20 log 75), it is in actuality limited to 90 [dB] by the noise floor. The S/N ratio of the input signal to the second frequency divider  52   c,  i.e. the S/N ratio of the third-base clock, is 42.0 [dB] (=90−20 log 256). 
   This third-base clock (147.456 MHz, 42.0 [dB]) is further frequency-divided by a frequency divider  56   c  by 4, outputting a third reference frequency clock Fr 3  (36.864 MHz, 54.0 [dB]). In addition, the third reference clock is further frequency-divided by a 1/6 frequency divider  57   c,  a 1/8 frequency divider  58   c,  and a 1/12 frequency divider  59   c,  to generate clocks of 24.576 MHz (57.5 [dB]), 18.432 MHz (60.0 [dB]), and 12.288 MHz (63.5 [dB]) belonging to a third reference frequency series Fr 3   s  associated with the second reference frequency clock Fr 3 . 
   In addition, the first reference frequency clock Fr 1  (27 MHz, 80 [dB]), which is supplied to the first PLL circuit  50   a  as a reference clock, and the clock that results from the frequency division of the reference clock by a 1/2 frequency divider  56   d  (13.5 MHz, 86 [dB]) are also output as clocks of the first reference frequency series Fr 1   s.  The 54 MHz clock outputted from the frequency divider  57   a  also belongs to the clocks of the first reference frequency series Fr 1   s.    
   It is noted that the frequency divider  56   a  generating the intermediate-frequency clock Fim 5  may be omitted by adapting the frequency dividers  51   b  and  51   c  of other PLL circuit to generate the clock Fim 5 . 
   The clocks of the first through the third reference frequency series Fr 1   s –Fr 3   s  are far less influenced by the noise floor, though they are partially limited by the noise floor, so that they have sufficiently high S/N ratios as compared with the cited reference frequency. The clock generation system of  FIG. 5  provides a 54 MHz clock having a sufficiently high S/N ratio together with the first reference frequency clock of 27 MHz. 
     FIG. 6  shows in tabulated form clocks and their S/N ratios belonging to the respective frequency series as described in the first through the fifth embodiments, along with a reference. Entries in the table denoted as “NO OUTPUT” indicate cases where a clock cannot be output. Entries in the table denoted as “NO OUTPUT*” indicate cases where a clock with a duty ratio of 50% cannot be output, but can be output with other duty ratios, for example a duty ratio of 66%.