HP Lab Series User Manual

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5-22
To
verify
that
the
oscilloscope
is
not
dis-
playing
ripple
that
is
induced
in
the
leads
or
pick-
ed
up
from
the
grounds,
the
(+)
scope
lead
should
be
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.
5-23
In
most
cases,
the
single-ended
scope
method of Figure 5-4B
will
be
adequate
to
elimi-
nate
non-real
components
of
ripple
and
noise.so
that
a
satisfactory
measurement
may
be
obtained.
However,
in
·more
stubborn
cases
it
may
be
nec-
essary
to
use
a
differential
scope
with
floating
in-
put
as
shown
in
Figure
5-4C.
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
between
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
the
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
input
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.
If
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.
5-24
To
check
the
ripple
and
noise
output,
proceed
as
follows:
a.
Connect
the
oscilloscope
or
RMS
volt-
meter
as
shown
in
Figures
5-4B
or
5-4C.
b.
Adjust
VOLTAGE
control
until
front
panel
meter
indicates
maximum
rated
output
voltage.
c.
The
observed
ripple
and
noise
should
be
less
than
500µ.Vrms
and
SOmV
p-p.
5-25
Noise
Spike
Measurement.
When a
high
fre-
quency
spike
measurement
is
being
made,
an
in-
strument
of
sufficient
bandwidth
must
be
used;
an
oscilloscope
with
a
bandwidth
of 20MHz
or
more
is
adequate.
Measuring
noise
with
an
instrument
that
has
insufficient
bandwidth
may
conceal
high
fre-
quency
spikes
detrimental
to
the
load.
5-26
The
test
setups
illustrated
in
Figures
5-4A
and
5-48
are
generally
not
acceptable
for
measuring
spikes;
a
differential
oscilloscope
is
necessary.
Furthermore,
the
measurement
concept
of
Figure
S-4C
must
be
modified
if
accurate
spike
measure-
ment
is
to
be
achieved:
5-5
50tl
TERMINATION
POWER
SUPPLY
T•CONN~CTOR
OSCILLOSCOPE
CASE
CASE
0.0lui
:±
+
~
);
l/ERTICAL
----
L1<
I
INPUT
-·~
r.<
VERTICAL
.0~
~INPUT
$011.
T·CONNECTOR
5011.
TERMINATION
Figure
5-5.
CV
Noise
Spike,
Test
Setup
1.
As
shown
in
Figure
5-5,
two
coax
cables,
must
be
substituted
for
the
shielded
two-wire
cable.
2. Impedance
matching
resistors
must
be
in-
cluded
to
eliminate
standing
waves
and
cable
ring-
ing,
and
the
capacitors
must
be
connected
to
block
the
de
current
path.
3.
The length
of
the
test
leads
outside
the
coax
is
critical
and
must
be
kept
as
short
as
possi-
ble;
the
blocking
capacitor
and
the
impedance
matching
resistor
should
be
connected
directly
from
the
inner conductor
of
the
cable
to
the
power
supply
terminals.
4.
Notice
that
the
shields
of
the
power
sup-
ply end of
the
two
coax
cables
are
_not
connected
to
the
power supply
ground,
since
such
a
connection
would
give
rise
to
a
ground
current
path
through
the
coax
shield,
resulting
in
a·n
erroneous
measurement.
5. Since
the
impedance
matching
resistors
constitute
a
2-to-l
attenuator-the
noise
spikes
ob-
served
on
the
oscilloscope
should
be
less
than
20m
V
p-p
instead
of
40mV
p-p.
5-27
The
circuit
o!
Figure
s·-s
can
also
be
used
for
the
normal measurement
of
low
frequency
ripple
and
noise;
simply remove
the
four
terminating
resistors
and
the
blocking
capacitors
and
substitute
a
higher
gain
vertical
plug-in
in
place
of
the
wideband
plug-
in
required for
spike
measurements.
Notice
that
with
these
changes,
Figure
5-5
becomes
a
two-cable,
version
of Figure
5-4C.
5-28
Transient
Recovery
Time.
Definition: The
time
"X"
for
output
voltage
recovery
to
within
"Y"
millivolts
of
the
nominal output
voltage
following
a
"Z"
Amp
step
change
in
load
current-where:
"Y"
is
specified
as
10
millivolts.
The
nominal
output voltage
is
defined
as
the
de
level
half
way
between
the
static
output
voltage
before and
after
the
imposed
load
change.
"Z"
is
the
specified
load
current
change,
which
is
5
Amperes.

HP Lab Series Specifications

General

Brand	HP
Model	Lab Series
Category	Power Supply
Language	English