Racal Instruments 9901 Maintenance Manual page 47

4.14 The a.c.
amplifier has
a
high
impedance FET
inpuf
(Q1)
with overload protection
by
the
clipping
resistor R1
together with the gate-drain junction
and diode D1
.
The
output from
the
source of
Q1
drives
a
wideband
amplifier IC21,
which
feeds via
C8
into
emitter
follower
Q3
.
Transistor
Q2
is
turned off
in all
modes except
PERIOD.
In
PERIOD mode
the
removal of the earth via switch
SI
turns
on
Q2,
to restrict
the
bandwidth by
the introduction
of capacitor
C9
.
4.15
Transistor
Q3
drives the
Schmitt
Trigger
shaper
Q4/Q5.
Potentiometer R11
sets
the
bias
on
Q3
to
ensure that the
mean
signal level
is
correctly set relative
to
the
two
triggering levels
of
the
Schmitt circuit, thus ensuring
maximum
sensitivity.
From
Q5
the
shaped
signal
is
fed via
Q7
and
Q8
to
the
counter
at
IC2/22.
4.16
When
the
AC/DC
switch
is
set
to
DC, and
also
when
CHECK
is
selected, the
AC
amplifier
is
disabled (except
the
output stage Q8)
by
a applied
to
the base of
Q6
via the inverter
IC14d.
D.C. AMPLIFIERS
4.17
The
'A'
and
'B'
channel d.c.
amplifiers
are basically
identical
.
The
description of
the
'B'
amplifier
and
contact
bounce
protection,
given below,
applies
to
both
circuits.
'B'
Channel
D.C.
Amplifier
4.18
The
'B'
input
is
fed
in
via pin 12 on
the
main p.c.b. and
emitter-follower
Q1
1
to
the input of
IC22d which
is
a dual
line
receiver
adapted by feedback
to
operate
as a Schmitt
trigger
shaper.
4.19 The shaped output from IC22c/9
is
fed via the slope selection
gate IC19b and
inverters
IC14b and IC14c
to the
output stage
Q12
which
feeds the signal
to
the
Start
input of the
CDl Chip
at IC2/20. Simultaneously,
the
output from IC22c/9
is
fed via
IC19a, IC18b, lC18c and
Q10
to
the Stop input at
IC2/21
.
On
single line
operation IC2
interprets the positive
edge
of
the
first
pulse received as the
start,
and
the positive
edge
of
the
second
pulse
as the stop.
For single line
(B-B)
operation
the
'B'
channel Stop path
is
enabled
at IC18b/3 by a applied
via inverter
IC14a
from
an
earth at switch 1S2.
For
double
line
(B-A) operation
IC18b
is
disabled
by
a
'O'
from
inverter
IC14a.
4.20
The
emitters
of
QIO
and
Q12
are
connected
to
OV
via transistor
Q13
which
is
normally conducting, except
when
manual
start/stop
is
used
(see
para. 4.29).
The
states of the
Schmitt
triggers
at
lC22b/4 and IC22c/9
are
fed
to
the
data output connector
at pins
Rand
P for
external information.
Bounce
Protection
4.21 Fixed
bounced
protection
is
provided
when
the
Pulse/Contact
switch
is
at 'Cpntact'.
Referring
to
the
'B'
amplifier,
for
example,
if
the external timing contact
is
open.
4-4
9901 Vol
.
2