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HP 120A
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Sect.
II
Page
17
ADDITIONAL
WAYS
OF
USING
YOUR
MODEL
120A
OSCILLOSCOPE
INTERMEDIATE
CALIBRATION
It
may
be
desired
to
set
the
calibration
of
the
verti
cal
amplifier
to
a
known
fraction
of
its
engraved
calibration.
Proceed
as
shown
in
Figure
2-10.
BALANCED
INPUT
(other
than
10
MILLIVOLTS/CM
range).
Normal
instructions
for
balanced
input
mentions
its
use
of
the
10
MILLIVOLTS/CM
range
only,
since
this
range
is
the
only
one
that
has
a
balanced
input
as
the
attenuator
is
in
only
one
side
of
the
signal
path.
The
easiest
way
to
desensitize
the
balanced
ampli
fier,
up
to
ten
times
the
engraved
sensitivity,
is
to
use
the
VERNIER
control.
This
desensitization
may
be
calibrated
by
means
of
the
procedures
described
in
Figure
2-10.
However,
under
certain
conditions
the
attenuator
may
be
used
alone.
For
instance,
if
you
wish
to
compare
two
signals,
one
of
which
varies
over
a
greater
range
than
the
other,
connect
the
lead
from
the
wide
variation
signal
to
the
upper
red
terminal
and
use
the
attenuator.
Connect
the
less
widely
varying
signal
to
the
lower
red
terminal
and
the
common
return
to
the
black
terminal.
If
ranges
other
than
the
10
MILLIVOLTS/CM
range
are
used
with
balanced
input,
an
additional
attenua
tor
will
be
required.
This
attenuator
must
be
fre
quency
compensated.
Connect
this
attenuator
bet
ween
the
signal
source
and
the
bottom
red
terminal.
For
ease
of
operation
this
attenuator
should
have
the
same
attenuation
ranges
as
in
the
oscilloscope
itself.
The
most
convenient
attenuator
would
be
one
which
is
a
copy
of
the
input
attenuator
in
the
Verti
cal
Amplifier.
In
operation,
the
balanced
signal
should
be
attenua
ted
equally
in
both
paths
by
switching
both
attenua
tors
simultaneously.
This
will
keep
the
amplifier
balanced
and
will
enable
you
to
operate
the
ampli
fier
with
balanced
input
in
the
same
manner
as
single-ended
operation.
With
any
type
of
ope
ration
the
operating
range
of
the
grids
of
the
input
amplifiers
must
not
be
exceeded.
THE
SIGNAL
APPLIED
TO
THE
GRIDS
MUST
NOT
EXCEED
3.
0
VOLT
PEAK
AT
ANY
TIME.
Thus,
the
arithmetic
sum
of
the
peak
common-mode
(in-
phase)
signal
and
the
peak
differential
(out-of
phase)
signal
must
not
exceed
3.
0
volts.
This
spec
ification
is
for
voltage
on
the
grids
or
for
the
input
voltage
when
on
the
straight-through
range
(10
MIL
LIVOLTS/CM).
When
the
attenuators
are
used
the
voltage
limit
at
the
input
may
be
increased
by
the
same
amount
as
the
sensitivity
was
reduced.
Figure
2-15.
Types
of
Input
for
Differential
Amplifiers.
TROUBLE
-SHOOTING
An
oscilloscope
is
a
very
versatile
tool
for
trouble
shooting
not
only
other
equipment
but
also
itself.
When
part
of
the
oscilloscope
is
not
operating
other
sections,
that
are
operating,
may
be
used
to
find
the
fault
in
the
non-operating
section.
For
instance,
if
the
sweep
is
not
operating
but
the
vertical
ampli
fier
is,
use
the
vertical
amplifier
as
a
voltmeter
to
trouble-shoot
the
sweep
generator.
Numerous
other
special
uses
are
possible,
such
as
using
regulated
voltages
for
calibration
if
the
cali
brator
is
not
working.
These
situations
will
come
to
mind
as
the
situation
demands
if
it
is
kept
in
mind
that
an
oscilloscope
consists
of
a
number
of
sec
tions
which
may
be
used
independently.
ADDITIONAL
COMMON-MODE
REJECTION
If
greater
than
40db
(100
times)
common-mode
(in-
phase)
signal
rejection
is
desired,
connect
a
low-
resistance
balance
potentiometer
between
the
supply
ends
of
the
plate
resistors
of
the
input
tube
of
the
vertical
amplifier.
The
arm
of
the
potentiometer
goes
to
the
+
99
volt
supply.
In
operaticn,
this
po
tentiometer
is
adjusted
for
maximum
rejection
of
the
common-mode
signal.

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