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 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.
5-28 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 actual ripple measurement.
5-29 If the foregoing measures are used, the
single-ended scope of Figure 5-6Amay 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 5-6B. 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 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 difference in the ac potential between the power supply case and scope case. Before using a differential input scope in this manner, 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, 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, proceed 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 frequency spike measurement is being made, an instrument 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 is 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 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
