HP 6268B Operating And Service Manual

5-13 Source Effect (Line Regulation).
Definition: The change AEq^j in the static value
of dc output voltage resulting from a change in ac input
voltage over the specified range from low line to high line,
or from high line to low line.
5-14 To check the source effect, proceed as follows:
a. Connect test setup shown in Figure 5-1.
b. Connect variable autotransformer between input
power source and power supply ac input.
c. Adjust autotransformer for 208Vac input.
d. Turn CURRENT controls fully clockwise.
e. Turn on supply and adjust VOLTAGE controls
until front panel meter indicates exactly maximum rated
output voltage.
f. Read and record voltage indicated on digital volt¬
meter.
g. Adjust variable autotransformer for 254Vac input.
h. Digital voltmeter reading should not differ from
reading recorded in step (f) by more than:
6259B, 6260B 1.2mV
6261B 2.2mV
6268B, 6269B 4.2mV
5-15 PARD {Ripple and Noise).
Definition: The residual ac voltage superimposed on
the dc output of a regulated power supply. Ripple and
noise measurements may be made at any input ac line
voltage combined with any dc output voltage and load
current within the supply's rating.
5-16 The amount of ripple and noise present on the
power supply output is measured either in terms of its
rms or (preferably) peak-to-peak value. The peak-to-peak
measurement is particularly important for applications
where noise spikes could be detrimental to sensitive loads
such as logic circuitry. The rms measurement is not an
ideal representation of the noise since fairly high output
noise spikes of short duration can be present in the ripple
without appreciably increasing the rms value.
5-17 Ripple Measurement Techniques. Figure 5-2A
shows an incorrect method of measuring peak-to-peak
ripple. Note that a continuous 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 internal jumper
connecting the power supply negative output and sensing
terminals, the wire between the negative sensing 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 Eq between the two ground points causes an I R
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 sensing 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 for the oscillo¬
scope in Figure 5-2A. 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 (1/1 20Hz) or 16.7
milliseconds (1/60Hz). Since the fundamental ripple
frequency present on the output of an HP supply is 120Hz
(due to full-wave rectification), an oscilloscope display
showing a 120Hz fundamental component indicates a
"clean” measurement setup, while the presence of a 60Hz
fundamental usually means that an improved setup will
result in a more accurate (and lower) value of measured
ripple.
POWER SUPPLY CASE OSCILLOSCOPE CASE
A INACCURATE METHOD USING SINGLE ENDED SCOPE
GROUND CURRENT Iq PRODUCES 60 CYCLE DROP IN
NEGATIVE LEAD WHICH ADDS TO THE POWER SUPPLY
RIPPLE DISPLAYED ON SCOPE. METHOD MAY BE ADEQUATE
UNDER CERTAIN CONDITIONS SEE TEXT.
POWER SUPPLY CASE OSCILLOSCOPE CASE
B. RECOMMENDED METHOL 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 a
SCOPE, AND SHIELDED TWO WIRE FURTHER REDUCES STRAY
PICK-UP ON SCOPE LEADS.
Figure 5-2. Ripple Test Setup
5-3

HP 6268B Specifications

General

Brand	HP
Model	6268B
Category	Power Supply
Language	English