Supplemental Circuits - Wavetek 142 Instruction Manual

3.2.9
Output
Attenuator
The Output
Attenuator consists of four
pi
attenuators
with
some
modification to
minimize
the
number
of
components
0
0
OdB
Figure 3-6
-
Simplified Output Attenuator
used.
The
four attenuators employed
are
—
lOdB,
—
20dB,
and
two
—30
dB
attenuators;
all
with
500
input
and
output
impedances.
When
the
OUTPUT
ATTENuator
con-
trol
is
used to
select
—
10 dB,
—20
dB,
or
—30
dB; only one
pi
attenuator
is
used.
When —40
dB,
—50
dB, or
—60
dB
is
selected;
the second
30 dB
pi
attenuator
is
used
in series
with the appropriate attenuator
to yield
the desired
total
attenuation.
3.2.10
Power Supply
AC
voltage
is
coupled from the transformer secondary
to
the bridge
rectifiers
CR1
through
CR8.
Filtering
is
pro-
vided
by Cl, C2, CIO, and Cl
2.
Q9
is
connected and
operated
as
a zener reference for
the
-i-15
V
supply. IC1
is
a
comparator
difte-ential amplifier
whose
output
is
used
to drive transistor amplifier Q4,
whose
output
at
the collector
is
used
to drive a Darlington connected
pair consisting of
Q1
and Q2. The
Darlington connected
pair
provides
added
current
gain;
Q3
is
a
current
limiting
device.
The
base voltage of
Q3
is
biased
by
voltage divider
R5
and
R7
to be
slightly
below
-M5
volts.
Therefore, for
normal
operation
Q3
is
reverse biased and no current flows
through
its
collector.
However, when
the output current
from
the
-H5
volts exceeds
a certain
limit,
the voltage
drop
across
R12
will
increase to
a
point exceeding the
total voltage across
R5
plus the base-emitter voltage of
Q3, forward
biasing
Q3.
The
current
driving
the base of
Q2
will
be
diverted to the collector of Q3, causing the
power
supply
voltage to
drop below normal
operating
voltage.
As
the
power
supply
voltage drops, voltage
divider
R5
and
R7
has
more
effect
on
the base of Q2;
eventually turning
it
off
completely.
When
the overload
condition
is
removed,
the
normal condition
will
be restored.
The
diode,
CR9,
across the input terminals of IC1
pre-
vents
latch-up.
Potentiometer
R21
is
used to
calibrate the
-1-15 volt supply.
The
—15
volt,
and
+28
volt supplies have
similar circuitry.
The
—15
volt
supply
is
referenced to the
+15
volt
supply while the
+15
volt
supply
is
referenced to
the
—15
volts.
The
—28
volt supply
is
referenced to the
—15
volts,
and the
+28
volts
is
referenced
to the
+15
volt supply.
3.3
SUPPLEMENTAL
CIRCUITS
There
are
no unique
or
supplemental
circuits
within the
Model 142
HF
VCG
Generator
to
be separately described as
"Supplemental
Circuits".
The
preceding
circuit
descriptions
adequately describe the circuitry of the
Model 142
to the
degree required by
a
competent
technician.
3-7
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