TM 11-6625-2965-14&P
5-27 To minimize pick up, a twisted pair or (pref-
erably) a shielded two-wire cable should be used
to connect the output terminals of the power supply
to the vertical input terminals of the scope. When
using a twisted pair, care must be taken that one
of the two wires is connected both to the grounded
terminal of the power supply and the grounded in-
put terminal of the oscilloscope. When using
shielded two-wire cable, it is essential for the
shield to be connected to ground at one end only to
prevent any ground current flowing through this
shield from inducing a signal in the shielded leads.
5-28 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.
5-29 If the foregoing measures are used, the
single-ended scope of Figure 5-6A may be adequate
to eliminate non-real components of ripple so that
a satisfactory measurement can be obtained.
How-
ever, in stubborn cases or in measurement situa-
tions where it is essential that both the power sup-
ply case and the oscilloscope case be connected
to ground (e. g. if both are rack-mounted), it may
be necessary to use a differential scope with float-
ing input as shown in Figure 5-6B. 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 common
mode rejection is attained.
5-30 To check the ripple and noise output, pro-
ceed as follows:
a.
Connect the oscilloscope or RMS volt-
meter as shown in Figures 5-6A
or 5-6B.
b. Adjust VOLTAGE control until front pane 1
meter indicates maximum rated output voltage.
c. The observed ripple and noise should be
less than 200µVrms and lmV p-p.
5-31 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 20 MHz or more
is adequate. Measuring noise with an instrument
that has insufficient bandwidth may conceal high
frequency spikes detrimental to the load.
5-32 The test setup illustrated in Figure 5-6A is
generally not acceptable for measuring spikes; a
differential oscilloscope is necessary. Further-
more, the measurement concept of Figure 5-6B
must be modified if accurate spike measurement
to be achieved:
1. As shown in Figure 5-7, two coax ca-
bles, must be substituted for the shielded two-
wire cable.
2. Impedance matching resistors must be
included to eliminate standing waves and cable
is
ringing, and the capacitors must be connected to
block the dc current path.
3. The length of the test leads outside the
coax is critical and must be kept as short as pos-
sible; 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 connec-
tion would give rise to a ground current path
through the coax shield, resulting in an erroneous
measurement.
5.
Since the impedance matching resistors
constitute a 2-to-1 attenuator, — the noise spikes
observed on the oscilloscope should be less than
0.5mV p-p instead of lmV.
5-33 The circuit of Figure 5-7 can also be used
for the normal measurement of low frequency ripple
and noise;
simply remove the four terminating re-
Figure 5-7,
CV Noise Spike, Test Setup
5-6
