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Read Circuits
General.
The read circuits (Figure 3-46) process the
read data and are described in the foll.owing
discussion.
Preampl.ifier
A read preamplifier provides prel.iminary
ampl.ification of the vol.taqe induced in the
read coil..
This voltage is induced
in
the
read head
by
the magnetic fiel.d stored in the
disk oxide.
The frequency of the analog
voltage is proportional to the maqnetic field
fl.ux transitions qenerated while writing.
The.preampl.ifier is enabled whenever the
positioner is On Cylinder (applicable head
is selected) and the unit is not writing.
Note that a Read or Read Header command is
not needed at this point.
AGe Amplitude Stage
The analoq read data is applied to a low
pass filter in the AGe amplifier state (Piq-
.~
A
ANAL
REA
OAT
LOW
PASS
FILTER
TTME
~"
-
CONS
TAN T--...
CONTROL
AGe
AMPLI-
FIER
i
AGe:
GATN
~
CONTlmL
ure 3-47).
This filter attenuates the high
unwanted frequencies (noise) in the read
data signal and provides a linear phase re-
sponse over the read data. frequencies.
The output of the fil.ter is applied to the
AGe amplifier
(A8Sl).
The AGe amplifier pro-
vides a relativel.y constant output from a
wide amplifier range on the input.
This is
ac:compl.ished by the
AGe
Control circuit
which varies the gain of the AGe amplifier
as the output varies.
The output of the AGe amplifier is amplified
and
differentiated and then applied to the
Data
Latch
and Level Detection circuits.
Level. Detector and
Time
Constant Control
The Level Detector and Time Constant Control
circuit contains an Amplitude Enable pul.se
generator, a data detector to detect the
address
mark
gap, and a c:irc:ui t to control
the
~ima
constants of
the
AGe
ampl.ifier and
level.
detector.
BUFFER
AMPLI-
FIER
TO COliTIIOL ... 1I
DIFFER-
ENTIATOR
TO
DATA
LATCH
AND
LEVEL
DETECT
~
7M93
Figure 3-47.
AGC Amplified Storage (Simplified Logic)
3-92
83318200
A
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