Clock generator for magnetic disk drive that switches between preamble and data portions

Claim 1

A clock generator for a magnetic disk drive, in which data are recorded by the zone-bit recording method, a read signal is read from a magnetic disk in which a training signal is recorded in a preamble portion of the data and is sampled, after partial equalizing, to thereby generate a sampling read signal from which data and a clock are reproduced, said clock generator comprising: a sample-holder circuit for generating a sampling signal from a read signal; a partial-response phase-comparator comprising: a ternary detection circuit for detecting three magnitude values +1, 0 and -1 of said sampling signal to thereby generate a detection signal; first delay means for generating a delayed sampling signal which is delayed in time by 1 symbol from said sampling signal; second delay means for generating a delayed detection signal which is delayed in time by 1 symbol from said detection signal; a first multiplier for multiplying said delayed sampling signal and said detection signal; a second multiplier for multiplying said sampling signal and said delayed detection signal; and a subtracter for calculating the difference between the products which are obtained in said first and said second multipliers and and for detecting a phase difference between a sampling clock timing and a signal point of a read signal waveform; a loop filter for smoothing the phase difference which is detected; a voltage-controlled oscillator the frequency of which is controlled by the phase difference which is smoothed; a frequency divider for frequency dividing a signal from the voltage-controlled oscillator; a clock extraction circuit for extracting a clock component from said training signal as it is partially equalized; a phase-comparator for comparing the phase of an output signal of the frequency divider with the phase of an output signal of the clock extraction circuit to thereby obtain a phase difference; and a change-over switch which is disposed between said partial-response phase-comparator, said phase-comparator and said loop filter, said change-over switch connecting said phase-comparator to said loop filter during reproduction of the preamble portion of the data and for connecting said partial-response phase-comparator to said loop filter during reproduction of a data portion of the data.