Hardware Reference
In-Depth Information
Figure 5.7: Noise and disturbances in a clock propagation process.
and electronic detection noise, d(z) be the write noise which is the random
difference between the position where a write is commanded and where it is
actually written, n(z) be the slow timing variation due to HDA geometry which
depends on radial position, and electronics delay time drift p(k)representsthe
delay at each step. A control strategy similar to that used for containment of
radial error prorogation can be developed to stop the propagation of timing
error.
It is reported in [171] that v(z) is one of the dominant noise contributors
for propagation of timing error. Use of a narrower di-bit generally reduces
the trigger noise. d(z) is due to a combination of head, media, and electronic
noise. Approximately one quarter of the
fi
nal alignment noise power in this
system can be traced to this source. Similar to the track error propagation
case, F(z) in the timing loop can also be designed such that the timing error
does not propagate. With suitably designed detector and reasonable system
write noise, it is possible to achieve an alignment error with σ≤ 1.
5.4.4 Concluding Remarks
1. Self-servo track writing (SSTW) uses the amplitude of the read signal
from a previously written track as a measurement for the distance of the
head from the center of the previously written track. This is used as
a feedback signal in a closed loop system that controls the position of
the read/write head when a new track is created. Using such a control
system structure, the disk and spindle vibrations are corrected by the
servo loop. The transitions from the previous track are also used by a
PLL to produce timing reference for placement of transitions in the new
track.
2. SSTW radial direction step size needs to be calculated based on periodic
calibration of geometrical error in the radial direction across the whole
disk radius. Use of wrong step size may cause one track to erase the
other tracks.
