Hardware Reference
In-Depth Information
disk faster or by including more servo sectors. Each of these options comes
with its disadvantages. Larger is the number of servo sector, more is the
servo overhead. Increase in spindle speed, on the other hand, generates more
heat which in turn requires better cooling mechanism. Increased speed also
translates into larger rate of data transfer between media and electronics that
demands for expensive electronics in the read-write channel. Besides, higher
spindle speed pushes the spectrum of disturbances related to disk rotation
to higher frequency and, as a consequence, higher bandwidth is required for
actuator servo. As a result, the disk drive servomechanism remains as an
example of control system which demands for as large a bandwidth as possible
but comes with severe restrictions on the sampling frequency.
2.3.2 Servo Demodulation
A good estimate of the burst amplitude is the most important consideration
for reliable generation of the PES signal. Until very recently, the servo demod-
ulation used non-coherent analog method. In this method, the burst waveform
is first processed through a full-wave rectifier. Inamethodcalledthepeak-
detected servo demodulation, a circuit that can detect and hold the peak am-
plitude of the rectified burst waveform is used. Another method, known as the
area demodulation, finds the area under the rectified waveform. The analog
area detection method uses several precautionary measures to minimize detec-
tion error. The zero-crossings of the burst waveform are first detected. Once a
zero crossing is found in a burst, a demodulation window is opened. The win-
dow is closed after a pre-defined number of cycles of the burst waveform have
elapsed. The rectified waveform that falls inside the window controls a charge
pump that, in turn, charges a capacitor. The capacitor voltage is proportional
to the area of the burst falling inside the demodulation window. The process
is illustrated in Figure 2.19. Once the area of one burst is obtained, the ampli-
tude of the capacitor voltage is converted into a digital number and latched to
a register. The capacitor is then discharged to zero voltage before the demod-
ulation window for the next burst is opened. The same capacitor is used for
all four burst waveforms to eliminate the effect of component variances. The
first few cycles in the burst are not used for charging the capacitor. During
this period, the zero-crossing detector synchronizes the charging process. Each
burst is, therefore, made longer than the number of cycles actually used for
meaningful area detection. Moreover, inter-burst space is essential to provide
suļ¬cient time for analog-to-digital conversion of the capacitor voltage and for
discharging of the capacitor. The peak-detected servo demodulator uses sim-
pler circuit, but it is prone to error in presence of noise. Area detection, on the
other hand, is an averaging process and provide better immunity to broadband
noise.
Availability of higher processing power in modern disk drives encouraged
the use of digital algorithms to find the burst amplitude. The first approach
