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5-19
Although the method shown in Figure 5-2A is
not recommended for ripple measurements, it may prove
satisfactory in some instances provided certain precautions
are taken. One method of minimizing the effects of ground
current flow (Iq) is to ensure that both the supply and the
test instrument are plugged into the same ac power bus.
5-20
To minimize pickup, a twisted pair or (preferably)
a shielded two-wire cable should be used to connect the
sensing terminals of the power supply to the vertical input
terminals of the scope. When using a twisted pair, take
care that the same wire is connected both to the grounded
terminal of the power supply and the grounded input
terminal of the oscilloscope. When using shielded two-
wire cable, it is essential for the shield to be connected to
ground at only one end to prevent any ground current
flowing through this shield from inducing a signal in the
shielded leads.
5-21
To verify that the oscilloscope is not displaying
ripple that is induced in the leads or picked up from the
grounds, short the (+) scope lead to the (-) scope lead
at the power supply terminals. The ripple value obtained
when the leads are shorted should be subtracted from the
indicated ripple measurement.
5-22
If the foregoing measures are used, the single-
ended scope of Figure 5-2A might be adequate to eliminate
extraneous ripple components so that a satisfactory
measurement can be obtained. However, in stubborn cases
or in measurement situations where it is essential that
both the power supply case and the oscilloscope case be
connected to ground (if both are rack-mounted, for
example), it may be necessary to use a differential scope
with floating input as shown in Figure 5-2B. If desired,
two single-conductor shielded cables may be substituted
in place of the shielded two-wire cable with equal
success. Because of its common mode rejection, a
differential oscilloscope displays only the difference in
signal between its two vertical input terminals, thus ignoring
the effects of any common mode signal produced by the
difference in the ac potential between the power supply
case and scope case. Before using a differential input scope,
however, it is imperative 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, then 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 accor¬
dance with the manufacturer's instructions so that proper
common mode rejection is attained.
5-23
Ripple Measurement Procedure. To check the
ripple output, proceed as follows:
a. Connect oscilloscope or rms voltmeter as shown in
Figures 5-2A or 5-2B.
b. Turn CURRENT controls fully clockwise.
c. Turn on supply and adjust VOLTAGE controls until
front panel meter indicates maximum rated output voltage.
d. The observed ripple should be less than:
6259B, 6260B, 6261 B
500mV rms, 5mV p-p
6268B, 6269B
1 mV rms, 5mV p-p
5-24
Noise Spike Measurements. An instrument of
sufficient bandwidth must be used when making a high
frequency spike measurement. An oscilloscope with a
bandwidth of 20MHz or more is adequate. Measuring noise
with an instrument that has insufficient bandwidth may
conceal high frequency spikes that could be detrimental
to the load.
5-25
The test setup illustrated in Figure 5-2A is generally
not adequate for measuring spikes; a differential oscilloscope
is necessary. Furthermore, the measurement technique of
Figure 5-2B must be modified as follows if accurate spike
measurement is to be achieved:
1. As shown in Figure 5-3, two coax cables must be
substituted for the shielded two-wire cable.
2. Impedance matching resistors must be included to
eliminate standing waves and cable ringing, and capacitors
must be connected to block dc current.
3. The length of the test leads outside the coax is critical
and must be kept as short as possible. The blocking capacitor
and impedance matching resistor should be connected
directly from the inner conductor of the cable to the power
supply sensing terminal.
4. Notice that the shields at the power supply 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 and result in an
erroneous measurement.
Figure 5-3. Noise Spike Measurement Test Setup
5-4
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