Theory Of Operation; Input Attenuator Assembly; Impedance Converter Assembly - HP 3400A Operating And Service Manual

Model 3400A
Section IV
SECTION
IV
THEORY OF OPERATION
i
4-1.
INTRODUCTION.
4-2. This
section contains the
theory
of
operation
of
the
Model 3400A
RMS
Voltmeter.
Included
is
a
general
and detailed description
of
the
theory
of
operation.
4-3.
GENERAL DESCRIPTION.
4-4.
The Model 3400A comprises two
attenuators,
an
impedance
converter,
a video amplifier, a photo-
conductor chopper (modulator/demodulator), a chop-
per amplifier,
an emitter follower,
a thermocouple
pair,
and a direct
reading meter. (See
Figure 4-1.)
4-5.
A
signal
being
measured
with the
Model 3400A
is
applied to input
attenuator
Al
through the
INPUT
jack,
located
on
the
Model 3400A
front panel.
The
input attenuator
has an
input
impedance
of
over
10
megohms and provides two ranges
of
attenuation.
The
output of
the input attenuator
is
applied
to
imped-
ance converter A2.
The impedance converter
is
a
non-inverting unity voltage
gain amplifier.
It
presents
a high
impedance
to
the
input signal
and provides a
low
impedance
output to drive the
second attenuator
A3.
The second attenuator
provides
6
ranges
in
a
1,
3,
10
sequence.
The
two
attenuators are
switched
to
provide
12
ranges
of
attenuation.
4-6.
The
output
of
the
second attenuator
is
amplified
by video amplifier A4.
The video amplifier
is
a wide-
band, five
stage amplifier.
The overall gain
of the
video amplifier
is
controlled
by an ac feedback
loop.
The
ac output of
the amplifier
is
applied to
TC401;
one
of the
thermocouples
of
the
thermocouple
pair.
4-7.
The
dc output
of
TC401
is
modulated by
mod-
ulator
A5.
The modulator
comprises two photocells
which are
alternately illuminated by
two neon lamps
which
in
turn are controlled by the oscillator
located
on power supply
assembly
A7.
The
output
of
the
modulator
is
a
square
wave whose amplitude
is
pro-
portional to the dc input level.
4-8.
The
square
wave
output
of
the
modulator
is
amplified by
chopper amplifier A6.
The chopper
amplifier
is
a high gain ac amplifier.
Its
output
is
applied to
demodulator A5.
The demodulator
output
is
a dc level
whose magnitude
is
proportional to the
amplitude
of the
ac
input.
The demodulator
output is
applied to
two
direct
coupled emitter
followers.
The
emitter follower
is
used
to
make
the
impedance trans-
formation
from the high
impedance output
of the
de-
modulator to the
low
impedance
of
the direct reading
meter
Ml
and TC402;
the
second thermocouple
of
the
thermocouple pair.
4-9.
The
thermocouple
pair
TC401
and
TC402
acts
as a
summing
point for the ac
output
of
the
video
amplifier
A4
and
the dc output of
the
emitter followers.
The difference
in
the
heating
effect of
these voltages
is felt
as a dc input to
modulator
A5.
This
difference
input is
amplified and
is
fed
to
TC402 and
to
meter
Ml. This
amplified dc voltage represents
the
rms
value
of
the ac signal applied
at
the
INPUT
jack.
By
using
two "matched" thermocouples and measuring
the
difference, the output to the
modulator
will
be linear.
Using two thermocouples also
provides
temperature
stability.
4-10.
The dc voltage driving
meter
Ml
is
also
available
at
the
DC OUT
jack,
located
at
the
rear
of
the
Model
3400A.
4-11.
DETAILED DESCRIPTION.
4-12.
INPUT
ATTENUATOR ASSEMBLY
Al.
4-13. The
input
attenuator
assembly
is
a capacitive-
compensated attenuator
which provides
two ranges
of
attenuation for the 12 positions
of
the
RANGE
switch.
See input
attenuator
schematic diagram
illustrated
on
Figure 6-1.
4-14.
When
the
RANGE
switch
is
positioned
to
one
of
the six
most
sensitive
ranges
(.
001
to
.
3
VOLTS),
the attenuator output voltage
is
equal
to the input
voltage.
When
the
RANGE
switch
is
positioned
to
one
of
six highest
ranges
(1
to
300
VOLTS),
the input sig-
nal
is
attenuated 60
dB
(1000:
1
voltage division)
by
the resistive voltage divider consisting of
R101, R103,
and R104.
Trimmer
C102
is
adjusted
at
100 kHz,
and
R104
is
adjusted
at
400
Hz
to
provide constant atten-
uation
over
the input
frequency range.
4-15.
IMPEDANCE CONVERTER ASSEMBLY
A2.
4-16.
The impedance
converter
assembly
utilizes a
nuvistor tube cathode follower
circuit to
match the
high output
impedance
of
the input
attenuator
to
the
low
input
impedance
of
the
second attenuator.
The
cathode follower circuit
preserves
the
phase
relation-
ship
of
the input
and output signals
while maintaining
a gain of unity.
See
impedance converter
assembly
schematic diagram
illustrated
on Figure 6-1.
4-17.
The
ac
signal input to the
impedance
converter
is
RC
coupled
to the
grid
of
cathode follower
V201
through C201 and R203.
The
output signal
is
devel-
oped by
Q201 which
acts
as a
variable resistance
in
the
cathode circuit
of
V201.
The bootstrap feedback
from
the cathode of
V201
to
R203
increases the
effective
resistance
of
R203 to the input signal.
This
prevents
R203 from loading the input signal
and pre-
serves the high input
impedance
of
the
Model 3400A.
The
gain
compensating feedback from
the plate of
V201 to the
base
of
Q201 compensates
for
any varying
gain
in
V201 due to
age or
replacement.
4-18.
Breakdown
diode
CR201 controls the grid bias
4-1