E-6
Measurement Considerations
Model 2750 Multimeter/Switch System User’s Manual
Magnetic fields
When a conductor loop cuts through magnetic lines of force, a very small current is gener-
ated. This phenomenon will frequently cause unwanted signals to occur in the test leads of
a test system. If the conductor has sufficient length or cross-sectional area, even weak
magnetic fields such as those of the earth can create sufficient signals to affect low-level
measurements.
Three ways to reduce these effects are: (1) reduce the lengths of the connecting cables, (2)
minimize the exposed circuit area, and (3) change the orientation of the leads or cables. In
extreme cases, 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 prob-
lem. Fields can be produced by various sources such as the AC power line voltage. Large
inductors such as power transformers can generate substantial magnetic fields, so care
must be taken to keep the Model 2750 voltage source and connecting cables a good dis-
tance away from these potential noise sources.
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 partic-
ularly troublesome at low signal levels, but it can also affect measurements at high levels if
the fields are of sufficient magnitude.
RFI can be caused by steady-state sources such as radio or TV signals, or some types of
electronic equipment (microprocessors, 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 Model
2750 voltage source and signal leads as far away from the RFI source as possible. Addi-
tional shielding of the instrument, signal leads, sources, and other measuring instruments
will often reduce RFI to an acceptable level. In extreme cases, a specially-constructed
screen room may be required to sufficiently attenuate the troublesome signal.
The Model 2750 digital filter may help to reduce RFI effects in some situations. In some
cases, additional external filtering may also be required. Keep in mind, however, that fil-
tering may have detrimental effects such as increased settling time on the desired signal.
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Summary of Contents for 2750
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