BASF 6106 - Page 36

ACTIVE
DIFFERENTIATOR
AND
COMPARATOR
Both
circuits
are
part
of the
read
LSI MC3470.
A
simplified
circuit
is
shown
on Fig.
The
active
differentiator
is
implemented
by
a
differential
amplifier with
coupled emitters
by a
capacitor.
The
current
trough
this
capa-
citor
and also
through
the
collector resistor
will
be
a
derivative
of the
input
voltage.
dV
in
(t)
dt
Also
the output voltage
Vo
of the
differential
amplifier will be
a
derivative
of the
input
vol
tage
.
dV.
rt
,
V
=
2 R
•
I
c
=
2RC
—ULL11
dt
The output voltage
Vo
is
applied to
the
com-
parator wnicn provides
zero
crossing detec-
tion
of the
waveform.
Since the capacitor
shifts the current
—
'90°
to the input volta-
ge peak detection
of the input
voltage is per-
formed.
Fig.
2-40
shows a
timing diagram
of
the
differentiator and comparator
circuit.
Time
Domain
Filter
and
Crossover
Detector
The
purpose
of
the
time
domain
filter
is
tu
suppress
false
crossovers
of
the
comparator
caused
by
shouldering
in
the
differentiated
read
signal.
This
can
happen
on
outer
tracks
of
high
resolution
disks
when
high
resolution
heads
are
used.
The
time
domain
filter
com-
prises
a puis
generator,
the
time
domain
one
shot
and
the
time
domain
flipflep
(
see
Fig.
2-39
)
and
is
part
of
the
integrated
read
LSI
.
The
puis
generator
generates
a short
pulse
for
every
transition
on
its
input.
These
pulse
are
used
to
trigger
the time
domain
one
shot.
The
pulse
duration
of the
time
domain
one shot
is
determined
by an
external
Re-
combination
and
is
set to
2
usee
for
the BASF
6106.
The state
of
the
comparator
output
is
loaded
into
the
time
domain-
flipflop
by the
trailing
edge
of
the time
domain
one
shot
2
usee
later
(see Fig.
2-40
). Because
false
zero
crossings
always
exists
for a
shorter
time,
the
time domain
flipflop
will
not
change
when
it is
clocked
by
false
crossovers.
The
crossover
detector
consists
of
a
bidirec-
tional
one shot
which
is
triggered
by
each
transition
of
the
time domain
flipflop.
The
pulse
wide
of
the
crossover
detector
can
be
adjustet
by
external
elements.
For
the BASF
6106
the
output
pulses
(READ
DATA/)
of
the
crossover
detector
are
set to
500
nsec.
MC 34
70
FROM
FILTEK
NETWORK <
'Y/
^y\
DIFFERENTIATOR
FIGURE
2
-
38
.
ACTIVITY
ACTIVE DIFFENTIATOR AND
COMPARATOR
2-26