Keithley 7015-C, Инструкция по эксплуатации

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Operation
4-15
4.5.4 Magnetic fields
When a conductor cuts through magnetic lines of force, a
very small current is generated. This phenomenon will fre-
quently cause unwanted signals to occur in the test leads of a
switching system. If the conductor has sufficient length, even
weak magnetic fields like those of the earth can create suffi-
cient signals to affect low-level measurements.
Three ways to reduce these effects are: (1) reduce the lengths
of the test leads, (2) minimize the exposed circuit area, and
(3) keep test leads and equipment stationary. In extreme cas-
es, magnetic shielding may be required. Special metal with
high permeability at low flux densities (such as mu metal) is
effective at reducing these effects.
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.
4.5.5 Radio frequency interference
RFI (Radio Frequency Interference) is a general term used to
describe electromagnetic interference over a wide range of
frequencies across the spectrum. Such RFI can be particular-
ly troublesome at low signal levels, but it can also affect mea-
surements at high levels if the problem is of sufficient
severity.
RFI can be caused by steady-state sources such as radio or
TV signals, or some types of electronic equipment (micro-
processors, high speed digital circuits, etc.), or it can result
from impulse sources, as in the case of arcing in high-voltage
environments. In either case, the effect on the measurement
can be considerable if enough of the unwanted signal is
present.
RFI can be minimized in several ways. The most obvious
method is to keep the equipment and signal leads as far away
from the RFI source as possible. Shielding the switching
card, signal leads, sources, and measuring instruments will
often reduce RFI to an acceptable level. In extreme cases, a
specially-constructed screen room may be required to suffi-
ciently attenuate the troublesome signal.
Many instruments incorporate internal filtering that may
help to reduce RFI effects in some situations. In some cases,
additional external filtering may also be required. Keep in
mind, however, that filtering may have detrimental effects on
the desired signal.
4.5.6 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 instru-
mentation is connected to other instrumentation with more
than one signal return path such as power line ground. As
shown in Figure 4-13, the resulting ground loop causes cur-
rent to flow through the instrument LO signal leads and then
back through power line ground. This circulating current de-
velops a small but undesirable voltage between the LO ter-
minals of the two instruments. This voltage will be added to
the source voltage, affecting the accuracy of the measure-
ment.
Figure 4-13
Power line ground loops
Instrument 1 Instrument 2 Instrument 3
Power Line Ground
Signal Leads
Ground Loop
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