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5-32. DEDICATED SERVO AGC CIRCUIT.
Included in this circuit are a dedicated AGC
reference circuit and an AGC integrator. In opera-
tion, the AGe reference demod(ulator) switch
connects Servo Gain Reference
(SGR),
a current
representing th,e gain of the servo head, to the
dedicated AGC reference circuit. The switch is
controlled
by
Gap
(GAP-L),
which causes
SGR
to
be applied to the dedicated AGC reference circuit
during the period between embedded servo gaps on
the data
surfact:~.
In the AGC reference circuit, the
current is converted to a reference voltage level.
At the AGC integrator, the reference is integrated
with Dedicated Level
DLVL
from the dedicated
servo position demodulator and the resulting out-
put from the integrator is Dedicated AGC
(DAGC).
This voltage is fed back to the dedicated servo
AGC amplifier in read/write PCA-A2.
5-33. SAMPLED SERVO AGC CIRCUIT. The
sampled servo AGC circuit is similar to the dedi-
cated servo AGC circuit described above.
In
operation,
the
AGC
reference
demod(ulator)
switch switches Servo Gain Reference
(SGR),
a
current represt:mting the gain of the selected
read/write head to the sampled AGC reference
circuit. In this case,
GAP-L
connects
SGR
to the
sampled reference circuit for the duration of the
sampled servo gap. The reference voltage is in-
tegrated in the sampled AGC integrator with
Sampled Level
(SL VL)
from the sampled servo
position demodulator and the resulting output is
Sampled AGC
(SAG
C). This voltage is fed back to
the sampled servo AGC amplifier in read/write
PCA-A2.
5-34. SECTOR MARK DECODER.
The sector
mark decoder identifies two unique dibit patterns
on the dedicated servo tracks. These two patterns,
sector mark and guard band mark, are used to
synchronize the timing functions of the drive to
the dedicated servo surface.
The sector mark decoder monitors the Dedicated
Servo signal
(DSRVP)
looking for missing dibits, or
zeros.
It
is the a.bsence of these dibits which iden-
tifies the sector and guard band marks. The actual
decoding is performed by a zero detector and a
16-bit shift register. The basic timing for the servo
mark decoder is generated by the dedicated servo
timing circuit.
Functional Description
7936 and 7937
When the servo head is over the data area, the sec-
tor mark decoder is searching for a sector mark.
The absence of both in-phase dibits within a cell
defines a sector mark zero.
The Dibit Gate
(DBGT-H)
signal gates the in-phase dibits into the
zero detector. At the end of each cell, the output
of the zero detector is clocked into the shift regist-
er by Zero Clock
(ZCLK). If
both in-phase dibits
are missing. a zero is clocked into the register.
When the unique 16-bit sequence of ones and zeros
which define a sector mark is clocked into the
shift register. the decoder generates Sector Mark
Detect
(SM DT -H),
which is clocked out of the
decoder by Sector Mark Clock
(SMCLK).
When the servo head is over the guard band area,
a change in the
ZCLK
frequency causes the
decoder to search for the guard band mark. In the
guard band, the absence of either in-phase dibit
within a cell defines a zero.
Therefore.
ZCLK
must sample the output of the zero detector twice
during each cell. Once again, the output of the
zero detector is clocked into the shift register by
ZCLK.
However, the decoder is now looking for a
different 16-bit pattern of ones and zeros. When
the guard band mark is detected, the decoder out-
puts Guard Band Detect
(GBD-H)
to the dedicated
servo timing.
5-35. DEDICATED
SERVO
CIRCUIT.
Components in the dedicated servo circuit include
a filter, amplifier, position demodulator, and PLL
and timing circuit.
See figure 5-14 for circuit
timing diagram.
5-36. Dedicated Servo Filter.
The input to the
dedicated servo low-pass filter is differential
Dedicated Servo
(DEDP, DEDN)
from the dedi-
cated servo AGC amplifier in read/write PCA-A2.
The fundamental frequency of
DEDP, DEDN
is
288 kHz and the filter has a cut-off of 3.5 MHz.
The filter is needed for noise suppression.
5-37. Dedicated Servo Amplifier. The dedicated
servo amplifier is an integrated circuit operational
amplifier connected to operate as a buffer
amplifier with a gain of 20. The differential out-
put of the amplifier is labeled Dedicated Servo
(DSRVP, DSRVN).
The
DSRVN
line is connected
5-17
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