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Ripple and noise measurement can be made at any
input ac line voltage combined with any de output
voltage and load current within rating.
5-16 The amount of ripple and noise that is present
on the power supply output is measured either in
terms of the RMS or (preferably) peak-to-peak value.
The peak-to-peak measurement is particularly
im-
portant for applications where noise spikes could
be detrimental to a sensitive load, such as logic
circuitry. The RMS measurement is not an ideal
representation of the noise, since fairly high output
noise spikes of short duration could be present in
the ripple and not appreciably· increase the RMS
value.
5-17 The technique used to measure high frequency
noise or "spikes" on the output of a power supply is
more critical than the low frequency ripple and
noise measurement technique; therefore the former
is discussed separately in Paragraph 5-25.
AC
ACC
GND
AC
POWER
Sll'PLY CASE
+
~
OSCILLOSCOPE CASE
OSCILLOSCOPE CASE
+
- VERTICAL
... INPUT
8. A CORRECT METHOD USING A SINGLE-ENDED SCOPE.
OUT'FUT FlOOED TO BREAK GROUND CURRENT LOOP, TWISTED
PAIR REDUCES STRAY PICKUP ON
SCOPE LEADS.
. AC
ACC
GND
POWER SUPPLY CASE
OSCILLOSCOPE C.ISE
AC
ACC
GND
N;
ACC
GllD
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 ParENTIAL OF POWER SUPPLY ll
SCOPE; SHIELDED TWO WIRI:: FURTHER REDUCES STRAY ·
PICK-UP ON SCOPE LEADS.
Figure 5-4. Constant Voltage Ripple and Noise,
Test Setup
5-4
5-18 Ripple and Noise Measurements. Figure 5-4A
shows an incorrect method of measuring p-p 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 os cil-
loscope 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 EG between the two ground points
causes an IR drop which is in series with the scope
input. This IR drop, normally having a 60Hz line
frequency fundamental, plus any pickup on the un-
shielded 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
th~
true ripple developed be-
tween the plus and minus output terminals of the
power supply, and can completely invalidate the
measurement.
5-19 The same ground current and pickup problems
can exist if an RMS voltmeter is substituted in
place of the oscilloscope in Figure 5-4. However,
the oscilloscope display, unlike the true RMS
meter reading, tells the observer immediately
whether the fundamental period of the signal dis-
played is 8.3 milliseconds (l/120Hz) or 16.7 milli-
seconds (l/60Hz}. Since the fundamental ripple
frequency present on the output of an
~
supply is
120Hz (due to full-wave rectification), an oscillo-
scope display showing a 120Hz fundamental com-
ponent is indicative of a "clean" measurement set-
up, while the presence of a 60Hz fundamental us-
ually means that an improved setup will result
in
a
more accurate (and lower} value of measured ripple.
5-20 Figure 5-4B shows a correct method of mea-
suring the output ripple of a constant voltage pow-
er supply using a single-ended scope. The ground
loop _path is broken
by
floating the power supply
output.
Note that to ensure that no potential dif-
ference exists between the supply and the oscillo-
scope, it is recommended that whenever possible
they be plugged into the same ac power buss.
If
the same buss cannot be used, both ac grounds
must be at earth ground potential.
5-21 Either a twisted pair or (preferably) a shield-
ed two-wire cable should be used to connect the
output terminals of the power supply to the vertical
inp'Ut terminals of the scope. When using a twist-
ed pair, care must be taken that one of the
two
wires is connected to the grounded input tenninal
of the oscilloscope. When using shielded two-
wire, it is essential for the shield to be connected
to ground at one end only so that no ground cur-
•
rent will flow through this shield, thus inducing a
•
noise signal in the shielded leads.
, _
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