HP 6236A Operating And Service Manual page 18

5-19 Source Effect (Line Regulation)
Definition: The change,
b.
E OUT ' in the static
value of dc output vcltage resulting from a change in ac
input voltage over the specified range from low line (typi-
cally 104 Vac) to high line (typically 127 Vac), or from
high line to low line.
rectification), an oscilloscope display showing a 120 Hz
fundamental component is indicative of a "c1ean" 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.
5-24 Figure 5-48 shows a correct method of measuring
the output ripple of a constant voltage power supply using
a single-ended scope. The ground loop path is broken by
floating the power supply output. To ensure that no poten-
tial difference exists between the supply and the oscilloscope,
it is recommended that they both be plugged into the same
ac power bus. If the same bus cannot be used, both ac
grounds must be at earth ground potential.
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.
POWER SUPPLY CASE
•
OSCILLOSCOPE CASE
Figure 5-4. Ripple and Noise, Test Setup
AC-+---
ACC
GND
,..---f-..,.,....,~ AC
ACC
GND
I
I
I
VERTICAL I
.xr-+----~~G
INPUT
I
:--~~EG
I
L
~_~~
__
---_J
A. INCORRECT METHOD - GROUND CURRENT IG PRODUCES 60
CYCLE DROP IN NEGATIVE LEAD WHICH ADDS TO THE POWER
SUPPLY RIPPLE DISPLAYED ON SCOPE.
POWER SUPPLY CASE
OSCILLOSCOPE CASE
AC
f,C
ACC
ACC
GND
GND
+
+
- VERTICAL
G G
INPUT
POWER SUPPLY CASE
OSCILLOSCOPE CASE
AC
AC
ACC
ACC
GND
GND
+ +
- VERTICAL
G G
INPUT
B. A CORRECT METHOD USING A SINGLE -ENDED SCOPE.
OUTPUT FLOATED TO BREAK GROUND CURRENT LOOP,
TWISTED PAIR REDUCES STRAY PICKUP ON SCOPE
LEADS.
C. A CORRECT METHOD USING A DIFFERENTIAL SCOPE WITH
FLOATING INPUT. GROUND CURRENT PATH IS BROKEN;
COMMON MODE REJECTION OF DIFFERENTIAL INPUT SCOPE
IGNORES DIFFERENCE IN GROUND POTENTIAL OF POWER
SUPPLY 8 SCOPE, SHIELDED TWO WIRE FURTHER REDUCES
STRAY PICKUP ON SCOPE LEADS.
5-21 PARD (Ripple and Noise)
Definition: The residual ac voltage which is super-
imposed 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 Pop 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 sup-
ply 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 G
between the two ground points causes an IR drop which
is in series with the scope input. This IR drop, normally
having a 60 Hz I ine frequency fundamental, plus any pick-
up on the unshielded leads interconnecting the power sup-
ply and scope, appears on the face of the CRT. The magni-
tude of this resulting signal can easily be much greater than
the true ripple developed between the plus and minus out-
put terminals of the power supply and can completely in-
validate 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
5-4