HP 6177B Operating And Service Manual
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,
used.
Figure
5-4A
shows
a
correct
method
of
measuring
the
output
ripple
of a
constant
current
supply
using
c.
single-ended
scope.
Ground
loop
paths
are
broken
by
floating
the
outpu:
terminals
0:
the
suppl'),.
5-20
Either
a
twisted
pair
or
(preferably)
a
shield-
ed
two-wire
cable
should
be
used
to
connect
the
output
terminals
of
the
power
supply
to
the
verti-
cal
inpu:
terminals
of
the
scope.
Vlhen
using
shielded
two-wire,
it
is
essential
for
the
shield
to
be
connecte:':
to
ground
at
one
end
oniy
so
that
no
grounci
current
will
flovv'
through
this
shield,
thus
inducing
a nOise
signal
in
the
shielded
leads.
5-21
To
verify
that
the
oscilloscope
is
not
dis-
playing
ripple
that
is
induced
in
the
leads
or
pick-
ed
up
from
the
grounds,
the
C
..
~
..
)
scope
lead
should
De
shorted
to
the
(-)
scope
lead
at
the
power
sup-
ply
terminals.
The
ripple
value
obtained
when
the
leads
are
shorted
should
be
subtracted
from
the
actual
ripple
measurement.
5-22
In
most
cases,
the
single-ended
scope
method
of
Figure
5
-4A
will
be
adequate
to
eliminate
non-real
components
of
ripple
and
noise
so
that
a
satisfactory
measurement
may
be
obtained.
How-
ever,
in
more
stubborn
cases,
or
in
measurement
situations
where
it
is
essential
that
both
the
power
supply
case
and
the
oscilloscope
case
be
connect-
ed
to
ground
(e.
g.
if
both
are
rack-mounted),
it
may
be
necessary
to
use
a
differential
scope
with
float-
ing
input
as
show,n
in
Figure
5-4B.
If
desired,
two
single
conductor
shielded
cables
may
be
substi-
tuted
in
place
of
the
shielded
two-wire
cable
with
equal
success.
Because
of
its
common
mode
re-
jection,
a
differential
oscilloscope
displays
only
the
difference
in
signal
betvveen
its
two
vertical
input
terminals,
thus
ignoring
the
effects
of
any
common
mode
signal
introduced
because
of
the
dif-
ference
in
the
ac
potential
between
t}-je
power
sup-
ply
case
and
scope
case,
Before
using
a
differen-
tial
input
scope
in
this
manner,
however,
it
is
im-
perative
that
the
common
mode
rejection
capability
of
the
scope
be
verified
by
shorting
together
its
two
l:nput
leads
at
the
power
supply
and
observing
the
trace
on
the
CRT.
If
this
trace
is
a
straight
line,
the
scope
is
properly
ignoring
any
common
mode
signal
present.
Ii
this
trace
is
not
a
straight
line,
then
the
scope
is
not
rejecting
the
ground
signal
and
must
be
realigned
in
accordance
with
the
manufacturer's
instructions
until
proper
com-
mon
mode
rejection
is
attained.
5-23
To
check
the
high
frequency
noise
output,
proceed
as
follows:
a.
Connect
test
setup
shown
in
Figure
5-4A
or
5-4B.
b.
Set
front
pane
1
and
external
range
switch-
es
to
1:Jighest
current
range.
5-5
OSCILLOSCOPE
CASE
A.MEASUREMENT
METHOD
USING A
SINGLE.-ENDED
SCOPE
OUTPUT
TERMINALS
OF
SlJPPl
Y ARE
FLOATED
TO
BREAK
GROUND
CURRENT
LOOPS,
TWISTED
PAIR
REDUCES
STRAY
PICKUP
or,
SCOPE
L::ADS.
RANG::
SWITCH
La
RL3
POWER
SUPPLY
~2
~
CASE
n,~'
I R
53
I
~
o-r--t
I
RS2
•
OSCI
LLOSCOPE
CASE
~=+--,.-,:-
AC
i-ACC
rr-
____
-.J'-GND
VER
C-
__
~~~
__
~INPUT
B
BETTER
METHOD USING DIFFERENTIA:..
SCOPE
WITH
FLOATING
INPUT,
GROUND
CUR.ItENT
PATH
IS BROKEN; COMMON
MODE
REJECTION
OF
DIFFERENTIAL
INPUT
SCOPE
IGNORES
LIKE
POLARITY
INPUTS,
SHIELDED WIRE FURTHER REDUCES
STRAY PICKUP
ON
SCOPE
LEADS
NOTES:
FOR
VALUES
OF
LOAD RESISTORS
(RL
AND
Rsl
SEE FIGURE5-3
OR
TABLE
5-1.
FOR
HIGH FREOUENCY MEASUREMENTS,
LOW INDUCTANCE
RES
ISTORS SHOULD 9E
USED.
Figure
5-4.
High
Frequency
Ripple
and
Noise,
Test
Setup
c.
Set
meter
switch
to
AMPS
and
adjust
CURRENT
control
for
maximum
rated
output
current.
d.
Observed
noise
should
be
less
than
25mV