HP A Series - Page 20

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 circulat¬
ing in this loop as a result of the difference in potential
Eq 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 unshielded leads interconnecting
the power supply and scope, appears on the face of the
CRT. The magnitude of this resulting 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.
POWER SUPPLY CASE OSCILLOSCOPE CASE
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
B. A CORRECT METHOD USING A SINGLE-ENDED SCOPE.
OUTPUT FLOATED TO BREAK GROUND CURRENT LOOP,
TWISTED PAIR REDUCES STRAY PICKUP ON SCOPE
LEADS-
POWER SUPPLY CASE OSCILLOSCOPE CASE
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 a SCOPE, SHIELDED TWO WIRE FURTHER REDUCES
STRAY PICKUP ON SCOPE LEADS,
figure 5-3. Ripple and Noise, Test Setup
5-25 The same ground current and pickup problems
can exist if an RMS voltmeter is substituted in place of
the oscilloscope in Figure 5-3, However, the oscillo¬
scope display, unlike the true RMS meter reading, tells
the observer imrriediately whether the fundamental
period of the signal displayed is 8.3 milliseconds
{1/120Hz) or 16.7 milliseconds {1/60Hz). Since the
fundamental ripple frequency present on the output of
an HP supply is 120 H 2 (due to full-wave rectification),
an oscilloscope display showing a 120Hz fundamental
component is indicative of a "clean” measurement set¬
up, while the presence of a 6 OH 2 fundamental usually
means that an improved setup will result in a more
accurate (and lower) value of measured ripple.
5-26 Figure 5-3B shows a correct method of measur¬
ing 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. Note that
to ensure that no potential difference exists between
the supply and the oscilloscope, it is recommended that
whenever possible they both be plugged into the same
ac power buss. If the same buss cannot be used, both
ac grounds must be at earth ground potential.
5-27 Either a twisted pair or (preferably) a shielded
two-wire cable should be used to connect the output
terminals of the power supply to the vertical input term¬
inals of the scope. When using a twisted pair, care must
be taken that one of the two wires is connected to the
grounded input terminal of the oscilloscope. When using
shielded two-wire, it is essential for the shield to be con¬
nected to ground at one end only to prevent ground
current flowing through this shield from inducing a
signal in the shielded leads.
5-28 To verify that the oscilloscope is not displaying
ripple that is induced in the leads or picked up from the
grounds, the (+) scope lead should be shorted to the
(—) scope lead at the power supply terminals. The ripple
value obtained when the leads are shorted should be
subtracted from the actual ripple measurement.
5-29 In most cases, the single-ended scope method of
Figure 5-3B will be adequate to eliminate non-real com¬
ponents of ripple so that a satisfactory measurement
may be obtained. However, in more stubborn cases, (or
if high frequency noise up to 20MHz must be measured),
it may be necessary to use a differential scope with
floating input as shown in Figure 5-3C. If desired, two
single-conductor shielded cables may be substituted in
5-4