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HP 400D Operating And Service Manual

HP 400D
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T.O.
33A1-12-349-1
Section
IV
Paragraphs
4-1
to
4-11
SECTION
IV
CIRCUIT
DESCRIPTION
4-1.
BLOCK
DIAGRAM.
4-2.
The
electrical
circuits
of
the
voltmeter
are
shown
in
the
block
diagram
in
figure
4-1;
they
consist
of
an
input
voltage
divider
controlled
by
the
RANGE
switch,
a
cathode
follower
input
tube,
a
precision
step
attenuator
controlled
by
the
RANGE
switch,
a
broadband
amplifier,
an
indicat-
ing
meter,
and
a
regulated
power
supply.
The
voltage
applied
to
the
INPUT
terminals
for
measurement
is
divided
by
1000
before
application
to
the
input
cathode
follower
when
the
RANGE
switch
is
set
to
the
1-volt
range
and
higher;
the
input
voltage
is
applied
directly
to
the
cathode
follower
on
the
lower
ranges.
The
voltage
from
the
cathode
follower
is
divided
in
the
precision
attenuator
to
be
less
than
1
millivolt
for
application
to
the
voltmeter
amplifier.
The
output
of
the
amplifier
is
rectified
in
a
full-wave
bridge
rectifier
with
a
d-c
milliammeter
across
its
midpoints.
The
resultant
direct
current
through
the
meter
is
directly
proportional
to
the
input
voltage.
4-3.
INPUT
VOLTAGE
DIVIDER
AND
STEP
ATTENUATOR.
4-4,
The
input
voltage
divider
limits
the
signal
level
applied
to
the
input
cathode
follower
to
less
than
0.3
volt
rms
when
voltages
above
this
level
are
measured
with
the
RANGE
switch
set
at
the
1-volt
range
or
above.
The
divider
consists
of
a
resistive
branch
with
one
element
made
adjustable
to
obtain
exact
1000:1
division
at
middle
frequencies
and
a
parallel
capacitive
branch
with
one
element
made
adjustable
to
maintain
exact
1000:1
division
to
beyond
4
megacycles.
The
input
impedance
of
the
voltmeter
is
established
by
this
divider
and
is
the
same
for
all
positions
of
the
RANGE
switch.
On
the
six
low-voltage
positions
of
the
RANGE
switch,
the
input
divider
provides
no
attenuation
of
the
input
voltage.
(See
figure
5-10
for
the
complete
schematic.)
4-5.
The
step
attenuator
in
the
cathode
circuit
of
the
input
cathode
follower
reduces
the
voltage
to
be
measured
to
1
millivolt
or
less
for
application
to
the
voltmeter
amplifier.
Each
step
of
the
attenuator
lowers
the
signal
level
by
exactly
10
db
(1:Â¥10).
The
attenuator
consists
of
six
precision
wirewound
resistors
which
are
selected
to
very
high
accuracy
and
carefully
mounted
on
a
12-
position
rotary
switch.
The
RANGE
switch
rotor
has
two
contactors
(see
figures
5-9 and
5-10);
the
first
contacts
each
resistor
in
turn
while
the
input
divider
is
in
the
non-attenuating
position;
the
second
rotor
finger
repeats
these
contacts
while
the
input
attenuator
is
in
the
attenu-
ating
position.
On
the
.001-volt
range
a
fixed
capacitor
(C15)
is
automatically
connected
to
provide
flat
frequency
response
beyond
4
megacycles,
In
the
.003-
and
the
.01-
volt
ranges,
separate
adjustable
capacitors
(C14,
C16)
are
automatically
connected
to
the
attenuator
to
permit
setting
the
frequency
response
at
4
megacycles.
C14
and
C16
are
also
connected
to
the
attenuator
on
the
3-
and
10-volt
ranges.
Fixed
capacitor
C106
(permanently
connected)
flattens
frequency
response
on
the
.03-
and
30-volt
ranges.
00102-2
4-6.
Cathode
follower
V1
provides
a
constant,
high
input
impedance
to
the
input
voltage
divider
and
INPUT
ter-
minals
of
the
voltmeter
and
provides
a
relatively
low
impedance
in
its
cathode
circuit
to
drive
the
step
at-
tenuator.
The
voltage
gain
factor
across
V1
is
0.95.
4-7.
BROADBAND
VOLTMETER
AMPLIFIER.
4-8.
Amplification
of
the
signal
voltage
is
provided
by
a
four-stage
stabilized
amplifier
consisting
of
tubes
V2
through
V5
and
associated
circuits,
The
amplifier
provides
between
55-
and
60-db
gain
with
about
55
db
of
negative
feedback
at
mid-frequencies.
The
feedback
signal
is
taken
from
the
plate
of
the
output
amplifier
(V5)
through
the
meter
rectifiers
and
gain-adjusting
circuit
to
the
cathode
of
the
input
amplifier
(V2).
Variable
resistor
R107
in
the
feedback
network
adjusts
the
negative
feedback
level
to
set
the
basic
gain
of
the
amplifier
at
mid-frequencies,
while
adjustable
capacitor
C102
permits
setting
amplifier
gain
at
4
megacycles.
Variable
resistor
R118
in
the
coupling
circuit
between
V4
and
V5
permits
adjusting
the
gain
of
the
amplifier
at
10
cycles
per
second
by
controlling
the
phase
shift
of
low-frequency
signals
between
these
two
stages
(increasing
phase
shift
decreases
degeneration
and
increases
gain).
4-9.
Variable
resistor
R119
in
the
grid
return
path
for
V3,
V4,
and
V5
adjusts
the
total
transconductance
of
these
tubes
in
order
to
restrict
the
maximum
gain-
bandwidth
product
of
the
amplifier.
The
gain-bandwidth
product
must
be
restricted
to
give
a
smooth
frequency
response
rolloff
above
4
megacycles
and
to
prevent
possible
unstable
operation
at
frequencies
far
above
4
megacycles
when
tubes
having
unusually
high
trans-
conductance
are
used
(tube
transconductance
tolerances
during
manufacture
permit
wide
variations
in
new
tubes;
the
adjustment
permits
the
use
of
such
tubes).
The
plate
voltage
from
V5
is
rectified
by
the
meter
rec-
tifiers
and
drives
the
feedback
network.
The
cathode
voltage
of
V5
is
fed
to
the
meter
OUTPUT
terminals
for
monitoring
purposes.
The
current
through
V5,
and
thus
the
signal
voltage
at
the
cathode,
is
affected
by
the
loading
of
the
meter
rectifiers.
For
signal
levels
causing
third-scale
or
more
meter
deflection,
this
dis-
tortion
consists
of
a
very
small
irregularity
near
0
volts
on
the
waveform
as
each
diode
begins
conduction.
4-10.
INDICATING
METER
CIRCUIT.
4-11,
The
meter
rectifier
circuit
consists
of
two
silicon
diodes
and
two
capacitors
connected
as
a
bridge
with
the
indicating
meter
across
the
mid-points
as
shown
in
figure
4-2.
The
diodes
provide
full-wave
rectification
of
the
signal
current
for
operating
the
meter.
Electron
flow
through
the
meter
is
supplied
in
the
following
manner
(see
figure
4-2).
During
the
positive-going
half
cycle
of
plate
voltage
on
V5,
rectifier
CR1
conducts
electrons
from
both
C32
and
C33
back
to
the
B+
buss.
The
portion
of
electrons
from
C33
flows
through
the
meter
on
the
way
to
B+.
At
this
point
in
the
cycle,
both
C32
and
C33
are
charged
to
the
potential
of
B+
less
some
small
drop
in
R51
and
R52.
4-1

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HP 400D Specifications

General IconGeneral
BrandHP
Model400D
CategoryMeasuring Instruments
LanguageEnglish

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