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SIGNAL LEADS
INSTRUMENT 1
INSTRUMENT 2
GROUND
LOOP
CURRENT
POWER LINE GROUND
Figure 2-18
Power line ground loops
Figure 2-19 shows how to connect several instruments to-
gether to eliminate this type of ground loop problem. Here,
there is only one connection to power line ground.
INSTRUMENT 1
INSTRUMENT 2
Figure 2-19
Eliminating ground loops
Ground loops are not normally a problem with instruments
having isolated LO terminals. However, all instruments in
the test setup may not be designed in this manner. When in
doubt, consult the manual for each instrument in the test set-
up.
2.7.4 Keeping connectors clean
As is the case with any high-resistance device, the integrity
of connectors can be damaged if they are not handled prop-
erly. If the connector insulation becomes contaminated, the
insulation resistance will be substantially reduced, affecting
high-impedance measurement paths.
Oils and salts from the skin can contaminate connector insu-
lators, reducing their resistance. Also, contaminants present
in the air can be deposited on the insulator surface. To avoid
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INSTRUMENT 3
INSTRUMENT 3
POWER LINE GROUND
these problems, never touch the connector insulating mate-
rial. In addition, the matrix card should be used only in clean,
dry environments to avoid contamination.
If the connector insulators should become contaminated,
either by inadvertent touching or from air borne deposits,
they can be cleaned with a cotton swab dipped in clean meth-
anol or an HCFC. After thorough cleaning, they should be
allowed to dry for several hours in a low-humidity environ-
ment before use, or they can be dried more quickly using dry
nitrogen.
2.7.5 Noise currents caused by cable flexing
Noise currents can be generated by bending or flexing coax-
ial, triaxial, or quadraxial cables. Such currents, known as
triboelectric currents, are generated by charges created
between a conductor and insulator caused by friction.
Low-noise cable can be used to minimize these effects. Such
cables have a special graphite coating under the shield to pro-
vide lubrication and to provide a conduction path to equalize
charges.
Even low-noise cable generates some noise current when
flexed or subjected to vibration. To minimize these effects,
keep the cables as short as possible, and do not subject them
to temperature variations that could cause expansion or con-
traction. Tie down offending cables securely to avoid move-
ment, and isolate or remove vibration sources such as motors
or pumps.
2.7.6 Shielding
Proper shielding of all unguarded signal paths and devices
under test is important to minimize noise pickup in virtually
any switching matrix system. Otherwise, interference from
such noise sources as line frequency and RF fields can seri-
ously corrupt a measurement.
In order for shielding to be effective, the shield must com-
pletely surround the source, measure and guard signals. It
must also have a low impedance path to chassis ground. This
is shown pictorially in Figure 2-20. The shield also functions
as a safety shield since it is connected to chassis ground.
WARNING
Hazardous voltage may be present if LO
on any instrument is floated above
ground potential.
Operation
2-23
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