500µVrms and 5mV p-p 1mVrms and 5mV p-p

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 in this man- ner, 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 accordance with the manufacturer’s instructions until proper common mode rejection is attain- ed.

5-23 To check the ripple output, proceed as follows :

a. Connect oscilloscope or RMS voltmeter as shown in Figures 5-3Aor 5-3B.

b. Turn CURRENT controls fully clockwise. c. Adjust VOLTAGE controls until front panel

meter indicates maximum rated output voltage. d. The observed ripple should be less than

the following:

6259B, 6260B, 6261B

6268B, 6269B

5-24 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.

5-25 The test setup illustrated in Figure 5-3A

is generally not acceptable for measuring spikes; a differential oscilloscope is necessary. Further- more, the measurement concept of Figure 5-3Bmust be modified if accurate spike measurement is to be achieved

1.As shown in Figure 5-4,twO coax cables must be substituted for the shielded two-wire cable.

2.Impedance matching resistors must be in- cluded to eliminate standing waves and cable ring- ing, and capacitors must be inserted 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 connection would give rise to a ground current path through the

TM 11-6625-2958-14&P

Figure 5-4.Noise Spike Measurement Test Setup

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 2.5mV p-p instead of 5mV p-p.

5-26 The circuit of Figure 5-4can also be used for the normal measurement of low frequency ripple: simply remove the four terminating resistors and the blocking capacitors and substitute a higher gain vertical plug-in in place of the wide-band plug-in required for spike measurements. Notice that with these changes, Figure 5-4becomes a two-cable version of Figure 5-3B.

5-27Transient Recovery Time.

Definition: The time "X" for the output voltage recovery to within "Y" millivolts of the nominal output voltage following a "Z" amp step change in load current, where: "Y" is specified as 10mV, the nominal output Voltage is defined as the dc level ‘halfway between the static output voltage before and after the imposed load change, and "Z" is the specified load current change of S amps or the full load current rating of the supply, whichever is less.

5-28 Transient recovery time may be measured at any input line voltage combined with any output voltage and load current within rating,

5-29 Reasonable care must be taken in switching the load resistance on and off. A ha rid-operated