TM 11-6625-2958-14&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 unshielded 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 between 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 displayed 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 
supply is 120Hz (due to full-wave rectification), an oscilloscope display showing a 120Hz fundamental component 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 5-3Ais not recommended for ripple measurements, it may prove satisfactory in some instances provided certain precautionary measures are taken. One method 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 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, the (+) scope lead should be shorted to the (-) scope lead at the power supply terminals. The ripple value obtained when the leads are shorted should be subtracted from the
Figure 5-3.Ripple Test Setup
actual ripple measurement.
5-22 If the foregoing measures are used, the single-ended scope of Figure 5-3Amay 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 situations where it is essential that both the power supply 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 substituted in place of the shielded two-wire cable with equal success.
