Table of Contents
Advertisement
341
343A
to
the
high
voltage
power
supply.
This
allows
the
auxiliary
power
supplies
time
to
stabilize
and
reach
equillibrium before high voltage
is
applied.
Secondly,
the
control
circuit acts to
trip
the
instrument
to
STANDBY
when
the
output
terminals
are
shorted
at
voltage
settings
of
300
or
above on
the
1000
volt range.
The
control
circuit
consists
of an
AND
gate,
which
maintains the high
voltage
relay
in
an
energized condition
as
long
as
both
the delay
and
trip
inputs
are present.
3-11.
The
high voltage
power
supply
is
a
conventional
full-wave voltage doubler.
Output
voltage
is
determined
by
the
range switch
and
most
significant
decade
control
switch,
which
select
one
of
six
voltage taps
at
the
power
transformer secondary.
3-12.
CIRCUIT
ANALYSIS
3-13.
+15
VOLT REFERENCE
AND
AUXILIARY
SUPPLIES
3-14.
The +15
volt
reference
supply
is
a
conventional
series
regulated
power
supply,
employing
a
reference
amplifier
and
an
error
amplifier
to
provide
a stable, well
regulated output.
Filtered,
unregulated dc
voltage
for the
+15
volt
supply
is
provided
by CR22,
CR25,
and
C23.
Darlington
amplifier
Q15,
Q23
together with zener diodes
CR19
and
CR20
form
a
simple preregulator.
The
pre-
regulator
improves
line
regulation
and
reduces
noise
due
to
power
line
transients.
Reference
amplifier
Q14
is
a
single
device
containing
matched
zener
reference
and
amplifier elements,
thereby providing
a
voltage reference
that
is
extremely
stable
with
respect
to
temperature
variations.
Output
voltage variations are
sensed
at
the
input
of
differential
amplifier
Q15, Q17.
The
amplified
output of
the
differential
amplifier
is
applied to the input
of
the Darlington
amplifier
pair
Q20, Q19, which
varies
the
conduction
of
Q19
to
maintain
a
constant output
voltage.
Frequency
response
for
the
overall
servo loop
is
controlled
by
R14
and C20.
3-15.
Filtered
dc
voltage for the
-15
volt
auxiliary
supply
is
provided
by
CR23, CR24,
and C22.
The
voltage
reference
for
this
supply
is
the
+15
volt
reference supply
output.
The
error amplifier consists
of
differential,
ampli-
fier
Q16, Q18.
Series
control
of
the
output
is
provided
by
Darlington
amplifier
Q22, Q21.
Transistor
Q24
and
associated
components
comprise
a
constant current source
for
Q16,
thus achieving
a
high
overall
loop
gain.
Capacitor
C21 and
resistor
R6
control the
frequency
for
this
servo
loop.
3-16.
CHOPPER-STABILIZED AMPLIFIER
3-17.
The chopper
amplifier consists
of an input
modu-
lator,
a high gain amplifier,
a
synchronous demodulator,
and an output
filter
network. Input
signals are restricted to
less
than
30
Hz by
a
low-pass
filter
consisting
of
R172,
R171,and
C47.
The
resulting
baseband
signal,
dc
to
30
Hz,
is
applied
through
R170
to the drain
of
MOS
FET
chopper
Q39.
Q39
is
switched
off
and
on
at
a
215
Hz
rate
by
a
squarewave
signal
applied
to
its
gate,
thus
modulating
the
input
signal
and
producing
a
squarewave output
signal
having an amplitude proportional to the amplitude of
the
baseband
signal.
The
resulting signal
is
coupled
to the
in-
put
of
JFET
amplifier
Q38.
Use of
the
JFET
as
an input
amplifier provides high input
impedance and
assures
low
noise operation.
From
Q39,
the signal passes
through IC1,
an operational amplifier having
a
gain of
approximately
420
and
a
frequency
response of
20 Hz
to
10
kHz.
The
output of
IC1
is
inverted
in
Q37 and
appears
with
approxi-
mately
equal
amplitude
at
both
collector
and
emitter of
Q37.
The
collector
signal
is
synchronously
demodulated
by
shunt switch
Q36.
The
resulting
voltage
is
filtered
by
R129
and C31,
leaving
only
the
amplified dc
and low
frequency
signals.
The Q37
emitter
signal
is
applied
through
a
filter
network
to
C31
to
assist
in
reducing
chopper
ripple
and
to
cancel
any base
band
signal
that
is
present.
The
purpose
of
CR57
is
to
assist in
stabilization
and quick recovery
of the
amplifier
by
providing
a
quick
charge path
for
C36.
Diode
CR56
clamps
the voltage
at
the
emitter of
Q36
to a safe value.
3-18.
The
source of
the
215
Hz
chopper
drive
signal
is
multivibrator
Q2,
Q3
and
driver
Ql.
The
signal at
the
collector
of
Ql
is
coupled
to the drain
of
Q39
to
compen-
sate
for
the
dc
error
introduced
by
the gate-to-drain
capacitance of
Q39
and
is
adjusted
by
R162.
3-19.
The
preamplifier
consists
of
two
differential
impli-
fiers
and
an
output
emitter follower.
The
signal
passes
through
the
input
FET
amplifier
Q35,
through
a
second
differential pair
Q33, Q34,
and
out
the emitter
of
Q32
to
the
series
regulator.
The
purpose of Cl
and
C2
on
the
ref-
erence
board assembly
is
to
reduce
noise, principally
60
Hz
ripple,
by
increasing the ac
feedback
at
the
undesired
frequency.
3-20.
SERIES
REGULATOR
3-21
.
The
series
regulator
is
composed
of
a
preregulator,
a control
transistor,
and
a fixed
current
limiter.
The
pre-
regulator consists
of
four Darlington amplifiers
in
series
Q42
through
Q49.
Resistors
R58, R60, R61, and
R64
divide the
high voltage
equally
among
the four amplifiers
3-3
Advertisement
Table of Contents
