HP 6236B Operating And Service Manual page 18

Figure 5-3. Output Current, Test Setup
5-19 Source Effect (Line Regulation)
Definition: The change,
value of dc output voltage resulting from a change in ac
input voltage over the specified range from low line
(typically 104 Vac) to high line (typically 127 Vac), or
from high line to low line.
5-20 To test the source effect:
a. Connect a variable autotransformer between the in
put power source and the power supply line plug.
b. Connect a full load resistance and a digital voltmeter
across the output of the +20V supply.
c. Adjust the autotransformer for a low line input.
d. Turn on the power, adjust the output of the supply
to its maximum rated voltage, and record the DVM indica-
tion.
e. Adjust the autotransformer for a high line input and
recheck the DVM indication. It should be within .01% plus
2mV of the reading in step (d).
f. Repeat steps (b) through (e) for each of the
remaining supply outputs.
5-21 PARD (Ripple and Noise)
Definition: The residual ac voltage that is
superimposed on the dc output of a regulated power
supply. Ripple and noise may be specified and measured
in terms of its rms or peak-to-peak value.
5-22 Measurement Techniques. Figure 5-4A shows an
incorrect method of measuring p-p ripple. Note that a con-
tinuous ground loop exists from the third wire of the input
power cord of the supply to the third wire of the input
power cord of the oscilloscope via the grounded power
supply case, the 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 E between the two ground points causes an IR
G
drop that is in series with the scope input. This IR drop,
normally having a 60 Hz line frequency fundamental, plus
any pickup on the unshielded leads interconnecting the
power supply and scope, appears on the face of the CRT.
∆
E OUT in the static
5-4
The magnitude of this remaining 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.
5-23 The same ground current and pickup problems can
exist if an rms voltmeter is substituted in place of the oscil-
loscope in Figure 5-4. However, the oscilloscope display,
unlike the true rms meter reading, tells the observer imme-
diately whether the fundamental period of the signal dis-
played is 8.3 milliseconds (1/120 Hz) or 16.7 milliseconds
(1/60 Hz). Since the fundamental ripple frequency present
on the output of an HP supply is 120 Hz (due to full-wave
rectification). an oscilloscope display showing a 120 Hz
fundamental component is indicative of a "clean" measure-
ment setup, while the presence of a 60 Hz fundamental
usually means that an improved setup will result in a more
accurate (and lower) value of measured ripple.
Figure 5-4. Ripple and Noise, Test Setup