It is necessary
to
count down the output of the high-frequency voltage controlled
oscillator (VCO)
in order
to
obtain the desired number
of
pulses per revolution for
sectoring. This is accomplished by the Upper Sector Countdown Counter and the Lower
Sector Countdown Counter shown in zones
10 through 12. Also associated with these
counters are the Upper Electronic Sector Programming array and the Lower Electronic
Sector Programming array that are installed in J126 and J127, respectively (zone
C,
E,
10,
11,12). These programming arrays determine the particular number
of
sectors.
Since it is necessary
to
synchronize the countdown with the index for the respective
platter, there is a synchronizer for each counter. These are used
to
synchronize the count
with the index pulse. The index pulse is obtained from the respective Time Demultiplexer.
In
the case of the lower platter, the index notch on the Phase Lock Ring generates a pulse
which is separated by the Time Multiplexer and
applied
to
the Lower Synchronizer. In the
case of the upper platter, the index
slot
or
notch
will
generate a pulse which is separated
as
necessary by the Upper Time Demultiplexer and applied
to
the Upper Synchronizer.
The output of the respective Countdown Counter is a pulse, which is the raw sector pulse,
for use by the pulse formers and the Sector Number Counter when electronic sectoring is
employed.
The
Sector Phase Lock Loop will
be
discussed first, then one
of
the countdown counters
and synchronizers
will
be
discussed since the Upper Synchronizer and Lower Synchronizer
function
in
the same manner. This is also true
of
the Upper Sector Countdown Counter and
the Lower
Sector Countdown Counter. Therefore only the Lower Synchronizer and Lower
Sector Countdown Counter
will
be
discussed in
this
document.
The
Sector Phase Lock Loop is, in essence,
an
electronic servo loop designed to servo the
voltage controlled oscillator frequency to phase lock that frequency to the pulse-train
derived from the demultiplexed phase lock ring notches.
The
Sector Voltage controlled Oscillator (Sector
VCO)
consists
of
03,
R66,
R71,
U227,
C22,
C23,
C24,
R67,
and
R68
shown in zones
Band
C17.
C25
and
C26
are used only to
decouple the power supply voltages that are supplied
to
U227. The voltage controlled
oscillator is a
R-C
oscillator with part
of
the resistive component comprised of
03,
which
is made variable by the voltage applied
to
the gate of
03.
The output of the oscillator is
derived from pin 7
of
U227
and coupled into a SCHMITT TRIGGER
BY
C27
and
R69.
The
Schmitt trigger consists of one section
of
U268
with
its
threshold established
by
R70.
The
output of the
VCO
is at
U268,
pin
4,
which also connects
to
TP16
(zone C15). Thus,
U268
converts the output of the oscillator into a digital signal that is suitable for use by the
counters.
In
order
to
phase lock the
VCO
frequency
to
the phase locked pulses derived
from the spindle,
it
is necessary
to
detect the phase difference between the Sector
VCO
and the phase lock pulses,
to
produce a control voltage
to
servo the frequency of the
Sector
VCO
to
the proper value. This is accomplished using a Phase Comparator and Filter
shown
in
zones 817 through
20.
The phase lock pulses are applied
to
the clock input
of
the Phase Lock Flip-Flop
U129
(zone
C20).
The Phase Lock Flip-Flop divides the frequency of the phase-lock pulse train
by
a factor of 2 and converts it into a square wave. This square wave is then applied to the
Phase Comparator and Filter. Additionally. the Phase Lock Flip-Flop,
0 output (LPLFF),
is
also
fed
to the Spindle Speed Control Electronics as previously described.
The
other input
to
the phase comparator is the output
of
the
VCO,
suitably counted down.
The
countdown
of
the sector
VCO
is accomplished by the Sector PLL Countdown Counter
(zone
813,14,15).
The
particular division ratio obtained with this countdown counter is
determined by the programming array installed in J124 (Sector PLL Countdown
Programming).
5-41
6300
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