Clock Generation Circuit of a Sampled Servo Optical Disk Drive

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There are two ISO standards for 5.25 in. servo format. One is the continuous servo format, and the other is the sampled servo format. The two formats have a different clock generation system and tracking servo error detection system. In the sampled servo system, the clock is always generated by the clock generation circuit even during accessing. This system uses embedded clock pits which are prerecorded on the disk as shown in Fig. 1. In the case of 1800 rpm disk rotation, an 11 MHz clock is generated from a 40kHz clock pit signal. Fig. 2 shows a block diagram of the clock generation circuit. The circuit is divided into two parts: Clock pit detection and the PLL(Phase Locked Loop). PLL is simpler than continuous servo system, and the design theory is already well known. Clock pit detection detects and extracts the clock pit signal from the read out signal by counting the unique distance against the generated clock. Thus, pit detection and clock generation are mutually dependent. Before the first clock pit is detected(when the disk begins to rotate), the clock must be generated with better than 2.7% accuracy. So, the PLL pull-in circuit is necessary for start up. We developed a new clock generation circuit which is linked to the spindle motor control circuit, and which has fast pull-in characteristics.