HP 6269B Operator’s, Organizational, Direct Support And General Support Maintenance Manua L page 35

TM 1 1 - 6 6 2 5 - 2 9 5 8 - 1 4 & P
wire between the negative output terminal of the
power supply and the vertical input of the scope,
and the grounded scope case. Any ground current
circulating in this loop as a result of the difference
in potential EG between the two ground points
causes an IR drop which is in series with the scope
input. This IR drop, normally having a 60Hz line
frequency fundamental, plus any pickup on the un-
shielded leads interconnecting the power supply
and scope, appears on the face of the CRT. The
magnitude of this resulting noise signal can easily
be much greater than the true ripple developed be-
tween the plus and minus output terminals of the
power supply, and can completely invalidate the
measurement.
5-18 The same ground current and pickup problems
can exist if an RMS voltmeter is substituted in
place of the oscilloscope in Figure 5-3. However,
the oscilloscope display, unlike the true RMS
meter reading, tells the observer immediately
whether the fundamental period of the signal dis-
played is 8.3 milliseconds (1/120 Hz) or 16.7 milli-
seconds (1/60 Hz). Since the fundamental ripple
frequency present on the output of an
120Hz (due to full-wave rectification), an oscillo-
scope display showing a 120Hz fundamental com-
ponent is indicative of a "clean" measurement set-
up, while the presence of a 60Hz fundamental
usually means that an improved setup will result in
a more accurate (and lower) value of measured rip-
ple.
5-19 Although the method shown in Figure 5-3A is
not recommended for ripple measurements, it may
prove satisfactory in some instances provided cer-
tain precautionary measures are taken. One meth-
od of minimizing the effects of ground current (IG)
flow is to ensure that both the supply and the test
instrument are plugged into the same ac power
buss.
5-20 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 input
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 in-
ducing a 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 (+) 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
supply is
actual ripple measurement.
5-22 If the foregoing measures are used, the
single-ended scope of Figure 5-3A 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
floating input as shown in Figure 5-3B. If desired,
two single-conductor shielded cables may be sub-
stituted in place of the shielded two-wire cable
with equal success.
5-4
Figure 5-3. Ripple Test Setup