Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
1998-12-17
2002-04-30
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
Reexamination Certificate
active
06381292
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase synchronizing apparatus and a phase synchronizing method and is preferably applied to a disc drive for recording or reproducing data in or from a discoid recording medium such as a magnetic disc or an optical disc.
2. Description of the Related Art
The servo system for such a disc drive includes, for example, a servo-surface servo system for obtaining servo information from an exclusive servo surface, and an embedded servo system in which a data recording region and a servo information region are formed on the same disc surface in a time-division multiplex manner. To realize the above servo system, a disc drive is provided with a phase synchronizing circuit for generating a servo clock signal continuously synchronizing for one turn of a disc.
As shown in
FIG. 1
, a phase synchronizing circuit
1
is a so-called phase locked loop (PLL) circuit which consists of a phase comparator
2
, a loop filter
3
, a voltage control oscillator (VCO)
4
, and a frequency divider
5
.
A time reference signal S
1
obtained by reproducing a clock mark in a servo region formed on the surface of a disc at a regular time interval is supplied to one input terminal of the phase comparator
2
. Moreover, a clock signal S
CLK
output from the VCO
4
is supplied to the other input terminal of the phase comparator
2
via the frequency divider
5
after being frequency-divided into 1/N times (N: natural number). Thereby, the phase comparator
2
generates a voltage corresponding to the phase difference between the time reference signal S
1
and the clock signal S
CLK
, and supplies the voltage to the loop filter
3
as a phase error signal S
2
.
The loop filer
3
applies a predetermined filtering processing such as low-pass filtering to the phase error signal S
2
supplied from the phase comparator
2
and thereafter, supplies the processed signal to the VCO
4
. The VCO
4
outputs a clock signal S
CLK
having a frequency and a phase corresponding to the voltage level of the phase error signal S
2
obtained through the loop filter
3
.
Thus, the phase synchronizing circuit
1
can output the clock signal S
CLK
having a frequency N times higher than that of the phase error signal S
2
and synchronizing with the phase error signal S
2
from the VCO
4
, by feedback-controlling the phase difference between the phase error signal S
2
and the clock signal S
CLK
so as to keep it constant.
However, in case where the phase synchronizing circuit
1
is applied to such as a variable-medium-type disc drive or a disc drive using a disc in which a phase synchronizing mark is previously embedded in a servo region when the disc is manufactured, it is very difficult to generate a high-accuracy clock signal because the following problems occur.
Firstly, when chucking a disc to a spindle, the effective circumferential speed fluctuates to a high speed at a portion where the substantial radius of a track circle is large, and to a low speed at a portion where it is small, due to an eccentricity in which the center of a rotary shaft and the center of a track circle are offset. Therefore, a problem occurs that a large phase fluctuation occurs in the time reference signal S
1
reproduced from the track.
In case where, for example, twelve servo regions for one turn of a disc are formed at regular time intervals, a signal waveform of the time reference signal S
1
obtained by reproducing a clock mark in each of the servo regions is shown in FIG.
2
A. However, reproduced waveforms except for the clock mark, such as a position signal, track address, synchronizing pattern, and user data, are omitted.
In the time reference signal S
1
, the servo regions appear at regular time intervals shown by time t
0
-t
12
in the case when the disc has no eccentricity. However, if the disc has an eccentricity, the time when the servo region appears is advanced or delayed. The phase-shift value of each servo region in the above case is shown as the arrow attached to the lower stage of the signal waveform of the time reference signal S
1
.
Therefore, in the time reference signal S
1
, time intervals for the servo region to appear become dense nearby the time t
3
where a track circle has a large substantial radius, whereas they become nondense nearby the time t
9
where the track circle has a small substantial radius.
FIG. 2B
shows a graph obtained by arranging the phase-shift value of each servo region on the axis of ordinate. In this case, the sine-wave-shaped time reference signal S
1
is input to the one input terminal of the phase comparator
2
in the phase synchronizing circuit
1
shown in
FIG. 1
as described above.
The PLL of the phase synchronizing circuit
1
requires to accurately follow the phase-shift value of each of time t
0
-t
9
. However, since an open loop gain of passing through the phase comparator
2
, the loop filter
3
, the VCO
4
and the frequency divider
5
, generally takes a limited value, a phase of the clock signal S
CLK
returned to the other input terminal of the phase comparator
2
has an amplitude slightly smaller than that of the time reference signal S
1
and is shifted as shown in FIG.
2
C.
Therefore, the eccentricity-following residue shown in
FIG. 2D
occurs in the phase error signal S
2
output from the phase comparator
2
. For example, in the case where a disc having a radius of 16 mm has an eccentricity of 120 &mgr;m while rotating at 75 turns per second, the phase-shift value shows approximately 16 &mgr;s. Even if the open loop gain of the PLL at 75 Hz is 60 dB, an eccentricity-following residue of 16 nanoseconds remains.
When the low-frequency gain of the PLL is increased in order to completely follow the phase fluctuation due to the eccentricity, high-frequency noises are increased due to band widening accompanying with the increase of the low-frequency gain, for example. Therefore, the problem occurs that it is very difficult to make the PLL accurately follow a disc eccentricity.
Secondly, to band-wide a PLL without increasing noises, a method of raising the phase comparison frequency by increasing the number of servo regions has been proposed. However, because a lot of time is required to switch over between recording and reproducing particularly in case of a magnetic disc, the utilization efficiency of the data surface lowers when the number of servo regions for one turn of the disc is increased. As a result, it is practically very difficult to form hundreds of servo regions for one turn of the disc.
To solve the above first and second problems, for example, a feedforward compensation method is disclosed (the U.S. Pat. No. 5,615,191) in which eccentricity errors of the phase and frequency of an output clock signal are corrected by previously measuring the eccentricity value of a disc, adding the eccentricity value to a PLL, and thereby making the phase and frequency follow the eccentricity value.
However, according to the aforementioned method, it is necessary to use an eccentricity measurement circuit for measuring the eccentricity value of a disc, and a measurement time equivalent to at least one turn of the disc is required for measurement of the eccentricity value. Moreover, the measured eccentricity value is temporarily stored in a memory. However, because a phase fluctuation value due to eccentricity increases toward inner tracks of the disc, and decreases toward outer tracks of the disc, a large-capacity memory and a large-scale circuit are required if the measured eccentricity value is stored over the entire disc radius to be sought by a head.
Moreover, according to the above-described feedforward compensation method using the open loop, characteristics of the PLL are changed due to temporal changes of element constants of the analog circuit section of such a loop filter and thereby, errors occur in the phase and frequency of an output clock signal. Therefore, the conventional compensation method is insufficient for practical use.
SUMMARY OF THE INVENTION
In view of the foregoing,
Bocure Tesfaldet
Kessler Gordon
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