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HP 120A - Vertical Channel

HP 120A
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Sect,
m
Page
2
e.
Cathode-ray
Tube
-
The
cathode-ray
tube
is
a
5AQP
mono-accelerator
type.
It
is
normally
supplied
with
a
Pl
phosphor
screen
but
is
available
in
the
P7
and
Pll
phosphors
also
and
P2
upon
special
order.
P2
is
not
recommended
because
of
low
acce
lerating
/oltage.
All
are
electrically
interchangeable
and
the
tube
is
easily
changed.
The
mono-accelerator
anode
makes
possible
a
simple
astigmatism
adjust
ment
which
requires
no
resetting
when
adjusting
the
FOCUS
or
INTENSITY
controls.
The
deflection
plate
terminals
are
connected
through
removeable
jumpers
at
the
rear
of
the
instrument
so
that
direct
connec
tions
to
the
plates
can
be
made
easily.
f.
Calibrator
-
This
oscilloscope
has
a
built-in
calibrator
with
which
you
can
calibrate
the
sensi
tivity
of
die
vertical
amplifier
ranges.
The
calibrator
output
is
a
60mv
peak-to-peak
square
wave.
3-3
VERTICAL
CHANNEL
The
vertical
amplifier
consists
of
three
parts,
AC-DC
switch,
the
input
attenuator
and
the
amplifier
sec
tion
proper.
a.
Input
Attenuator
-
The
Input
Attenuator
is
a
six
position
switch
having
CALIBRATOR,
10
and
100
MILLIVOLTS/CM,
1
and
10
VOLTS/CM
and
OFF
po
sitions.
When
the
switch
is
in
the
CALIBRATOR
po
sition
the
input
of
the
amplifier
is
directly
connected
to
the
output
of
the
calibrator.
Since
the
square
wave
output
of
the
calibrator
is
set
to
60
millivolts,
the
gain
of
the
amplifier
should
be
adjusted
to
show
a
pattern
height
erf
6
centimeters,
with
the
VERNIER
in
the
CALIBRATED
position.
This
will
calibrate
the
basic
sensitivity
of
the
oscilloscope.
When
used
on
the
other
ranges,
precision,
frequency
adjusted
at
tenuators
are
inserted
ahead
of
the
vertical
ampli
fier.
These
attenuators
give
a
ten
to
one
attenuation
between
adjacent
ranges.
The
sensitivity
may
be
varied
continuously
between
ranges
by
means
of
the
VERNIER
control.
This
control
reduces
the
sensiti
vity
of
any
range
down
to
at
least
ten
to
one.
Nor
mally,
the
VERNIER
control
will
be
calibrated
only
in
the
CALIBRATED
switch
position.
However,
it
may
be
calibrated
to
have
a
known
fraction
of
the
sensitivity
of
the
ranges
by
setting
the
VERTICAL
SENSITIVITY
range
switch
to
the
CALIBRATOR
po
sition
and
adjusting
the
VERNIER
control
to
the
de
sired
fraction
of
the
normal
six
centimeter
pattern.
As
long
as
the
VERNIER
control
is
left
at
this
set
ting
all
ranges
will
have
the
same
fractional
sensi
tivity.
The
original
sensitivity
may
be
restored
by
merely
rotating
the
VERNIER
to
the
CALIBRATED
position.
For
AC
coupling
a
.
1/xfd
condenser
is
switched
into
the
signal
path
by
means
of
the
AC
-
DC
switch.
This
coupling
has
a
low
frequency
cut-off
at
approximately
2
cycles
per
second.
However,
degradation
of
input
pulses
and
other
complex
waveforms,
such
as
square
waves,
occurs
long
before
the
cut-off
frequency
is
reached.
Therefore,
use
DC
coupling
for
any
such
waveforms
below
about
200
cycles
per
second
or
whenever
degradation
of
waveform
is
suspected.
Balanced
input
may
be
used
by
removing
the
jumper
to
the
ground
terminal.
Balanced
input
will
be
found
useful
in
applications
where
it
is
desired
to
amplify
the
out-of-phase
(differential)
signal
and
attenuate
the
in-phase
(common
mode)
signal
at
the
same
time.
This
rejection
is
an
inherent
property
of
differential
amplifiers
such
as
are
used
in
the
vertical
amplifier.
Thus
you
can
reject
by
better
than
100
times
any
common-mode
portion
of
the
input
signal
while
at
the
same
time
passing
and
amplifying
the
differential
por
tion
of
the
signal.
This
will
prove
to
be
advantageous
very
often.
Many
of
the
unwanted
signals
picked
up
along
the
wanted
differential
signals
are
of
the
com
mon-mode
type.
Noise,
hum,
etc.
,
are
in
this
class.
By
the
use
of
balanced
input
to
the
differential
ampli
fiers
the
picture
obtained
on
the
screen
of
the
oscil
loscope
will
improve
considerably
over
that
obtained
using
single-ended
input.
The
common-mode
signal
rejection
is
at
least
40
db
(100
times).
When
using
balanced
de
input
certain
limitations
must
be
met.
The
proper
operating
points
must
be
maintained
on
the
input
amplifier.
The
COM
MON-MODE
SIGNAL
VOLTAGE
EITHER
POSITIVE
OR
NEGATIVE
ON
EITHER
GRID
MUST
NOT
EX
CEED
3
VOLTS
MINUS
THE
PEAK
AMPLITUDE
OF
THE
DIFFERENTIAL
SIGNALj(see
page
2-17)
In
addition,
it
may
be
found
desirable
to
use
differ
ential
input
with
a
lower
sensitivity
input.
The
easiest
way
to
desensitize
up
to
one
order
of
magnitude
(ten
times)
is
to
use
SENSITIVITY
VERNIER.
The
ampli
fier
is
still
balanced
and
signals
up
to
1
volt
peak-to-
peak
can
be
easily
handled
provided
the
maximum
common-mode
signal
is
not
exceeded.
b.
Vertical
Amplifier
-
The
vertical
amp
lifier
con
sists
of
three
sets
of
balanced
differential
am
plifiers
in
cascade.
The
last
two
stages
are
neutral
ized
by
plate-to-grid
cross-neutralization.
The
first
stage,
with
a
12AU7,
has
the
balance
and
gain
ad
justments.
The
balance
adjustment
is
a
potentiometer
in
the
cathode
circuit
which
adjusts
the
current
dis
tribution
between
the
two
halves
of
the
stage.
The
two
gain
adjustments
consist
of
potentiometers
connected
between
the
two
plates
which
adjust
the
resistance
between
the
plates.
The
screwdriver
adjustment,
VERT.
GAIN,
R18,
adjusts
the
basic
gain
of
the
am
plifier.
The
front
panel
control,
VERTICAL
SENSITI
VITY
VERNIER,
controls
the
gain
over
a
ten-to-one
range
and
varies
the
gain
between
step
ranges
of
the
main
VERTICAL
SENSITIVITY
switch.
The
second
balanced
differential
amplifier,
V2,
a
12AU7
,

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