3-32
Even when the conductor is stationary, magnetically induced
signals may still be a problem. Fields can be produced by
various signals such as the AC power line voltage. Large in-
ductors such as power transformers can generate substantial
magnetic fields, so care must be taken to keep the switching
and measuring circuits a good distance away from these po-
tential noise sources. At high current levels, even a single
conductor can generate significant fields. These effects can
be minimized by using twisted pairs, which will cancel out
most of the resulting fields.
3.13.5 Electromagnetic Interference (EMI)
The electromagnetic interference characteristics of the
Models 2000/2001/2002 Multimeters comply with the
electromagnetic compatibility (EMC) requirements of
the European Union (EU) directives as denoted by the
CE mark. However, it is still possible for sensitive mea-
surements to be affected by external sources. In these
instances, special precautions may be required in the
test setup.
Sources of EMI include:
•
Radio and television /broadcast transmitters.
•
Communications transmitters, including cellular
phones and handheld radios.
•
Devices incorporating microprocessors and high-
speed digital circuits.
•
Impulse sources as in the case of arcing in high-
voltage environments.
The instrument, measurement leads, and other cables
should be kept as far away as possible from any EMI
sources. Shielding measurement leads and the multim-
eter may reduce EMI to acceptable levels. In extreme
cases, a specially constructed screen room may be re-
quired to suf
Þ
ciently attenuate troublesome signals.
Many instruments incorporate internal
Þ
ltering that
may help reduce EMI effects. In some cases, additional
external
Þ
ltering may be required. Keep in mind, how-
ever, that
Þ
ltering may have detrimental effects on the
measurement.
3.13.6 Ground loops
When two or more instruments are connected together, care
must be taken to avoid unwanted signals caused by ground
loops. Ground loops usually occur when sensitive
instrumentation is connected to other instrumentation with
more than one signal return path such as power line ground.
As shown in Figure 3-9, the resulting ground loop causes
current to flow through the instrument LO signal leads and
then back through power line ground. This circulating
current develops a small but undesirable voltage between the
LO terminals of the two instruments. This voltage will be
added to the source voltage, affecting the accuracy of the
measurement.
Figure 3-10 shows how to connect several instruments
together to eliminate this type of ground loop problem. Here,
only one instrument is connected to power line ground.
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 all instrumentation in the test
setup.
Figure 3-9
Power line ground loops
Instrument 1
Instrument 2
Instrument 3
Power Line Ground
Signal Leads
Ground Loop
Current
Figure 3-10
Eliminating ground loops
Instrument 1
Instrument 2
Instrument 3
Power Line Ground
Summary of Contents for 2001-TCSCAN
Page 1: ...Contains Operating and Servicing Information Model 2001 TCSCAN Scanner Card Instruction Manual...
Page 63: ...Operation 3 34...
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