Racal Instruments 9901 Maintenance Manual page 52

Frequency Doubler
4.45 The
5
MHz
reference from
the oscillator
is
fed via
C32
to
a shaper formed by IC3d
with L2
and
diodes
D5/D6.
The square
wave
signal
is
then fed via |C3a
to
the
monostables IC4a and
IC4b.
A
positive
monostable
pulse
is
produced from each edge
of the
input
and
fed
from
IC4a/4and
IC4b/12
respectively
to
the
Nand
gate IC3c,
thus
producing
a 10
MHz
input
to
the
CDI
Chip at IC2
pin 5.
In
earlier models
(refer to
handbook
Issue
1)
the
shaped
signal
is
fed via
an
inverter IC4a
to
the
two branches of a
digital
doubler
with output at IC3a/3
.
By a
process
which delays
the signal
in
one
path
by a
series
of
inverters, then
by an Exclusive-OR arrangement,
the
two
signal paths
produce a doubling
effect.
1
MHz
Reference Output
4.46
The
1
MHz
reference
is
fed from
IC^6
via the buffer ICI5d
to
the
1
MHz BNC
output socket
on
the rear panel
.
External Standard
Input
4.47
A
1
MHz
reference can be
fed
In
via the
'B'
input socket
as
this
socket
is
not used
in
Frequency and
Period modes.
The input path
is
via the
'B'
channel
amplifier
and
the
Start
gates ICI9b etc
to
Q12
and
thence
to the
input of
the
CDI Chip at
IC2/20.
POWER
SUPPLY
4.48 The a.c. supply
is
fed
in
via
a 3-pin
fixed
plug on
the rear
panel, thence
via the
anti-surge
fuse
FS50
to
the transformer T50. The
transformer tappings are arranged
to
suit the local supply voltage
as
shown
in
the
diagram on page 5-3.
The output
at
secondary
winding
'A'
is
rectified
to
provide
-t5V
by
the potted bridge
rectifier
D50, which
is
mounted on
the rear
panel
for
adequate
heat
dissipation.
The
-5V
supply
is
provided by
the discrete
diode
bridge
D11-D14.
Smoothing
and
reservoir
capacitors are
C49, C51, C53
and C54.
Capacitor
C50
ensures that
any
ripple
in
the +5V
line will not affect
the
-5V
stabilizing comparator IC5b, and C52 and
C60
stabilize the system.
+5V
Stabilization
4.49
The reference
is
provided from
the
+5V
rail
by zener diode
DIO
which
is
connected
to the
comparator IC5a/2.
The
other input
to
the
comparator (IC5a/l)
is
from
the
OV
rail.
The comparator
function
is
to
ensure that the
OV
rail
is
maintained
5..
IV below
the
voltage of
the
+5V
rail
.
Any
change
from
this
state will
cause IC5a
Jo
apply a
signal
to
Q33
which
will regulate the current
in
Q50
to
restore
the correct condition.
-5V
Stabilization
4.50
TTie
-5V
stabilizing
comparator
is
IC5b,
in
which
both
inputs
should be
at
OV when
conditions are correct. IC5b/7
is
connected
to
the
OV
rail
and IC5b/6
to
the
mid-
point of a
potential divider
(R104,
R105 and
R106)
which
is
connected between
the stabilized
+5V
rail
and
the
-5V
rail.
If
the level at
IC5b/6
departs from OV
then a correcting
signal
4-9
9901 Vol
.
2