HP 3400A Service Manual

SECTION IV
THEORY
OF
OPERATION
4-1. INTRODUCTION
4-2.
This
section
contains
the
theory
of
operation
of
the
Model
3400A
RMS
Voltmeter.
In-
cluded
is
a
general
and
detailed
description
of
the
theory
of
operation.
4-3. GENERAL DESCRIPTION
4-4.
The
operation
circuitry
of
the
Model
3400A
consists
of
two
attenuators,
an
impedance
converter,
a
video
amplifier,
a
chopper
op
amp,
a
thermocouple
pair,
and
a
direct
reading
meter.
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
out-
put
of
the
input
attenuator
is
applied
to
impedance
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
pro-
vide
12
ranges
of
attenuation.
4-6.
The
output
of
the
second
attenuator
is
amplified
by
video
amplifier
A4.
The
video
amplifier
is
a
wideband,
five
stage
amplifier.
The
overall
gain
of
the
video
amplifier
is
con-
trolled
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
applied
to
the
chopper
op
amp
(U601)
which
modulates
the
voltage.
The
resultant
modulated
signal
is
then
amplified
and,
after
the
amplification,
the
signal
is
demodulated
back
to
de
volts.
This
voltage
is
the
output
of
the
chopper
op
amp.
The
value
of
the
voltage
is
proportional
to
the
dc
input
voltage.
4-8.
The
output
of
the
chopper
op
amp
is
applied
to
a
filter
and
also
to
TC402,
the
second
thermocouple
of
the
thermocouple
pair.
The
output
of
the
filter
is
applied
to
an
output
amp
(A4)
which
is
used
to
drive
the
meter
(M1).
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
chopper
op
amp.
The
difference
in
the
heating
effect
of
these
voltages
is
the
dc
input
to
the
chopper
op
amp.
This
difference
input
is
amplified
and
is
fed
to
TC402
and
also,
after
going
through
the
output
amplifier
(U602),
to
the
meter
(M1).
This
voltage
represents
the
rms
value
of
the
ac
Signal
applied
to
the
3400A’s
INPUT
jack.
By
using
two
“‘matched’’
thermocouples
and
measuring
the
dif-
ference,
the
output
of
the
chopper
op
amp
will
be
linear.
Using
two
thermocouples
also
pro-
vides
temperature
stability.
4]

HP 3400A Specifications

General

Brand	HP
Model	3400A
Category	Measuring Instruments
Language	English