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2.7 MEASUREMENT CONSIDERATIONS
Many measurements made with the Model 7072-HV con-
cern low-level signals. Such measurements are subject to
various types of noise that can seriously affect low-level
measurement accuracy. The following paragraphs dis-
cuss possible noise sources that might affect these mea-
surements.
2.7.1 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 matrix system. If the conductor has sufÞcient
length, even weak magnetic Þelds like those of the earth
can create sufÞcient signals to affect low-level measure-
ments.
Two ways to reduce these effects are: (1) reduce the
lengths of the test leads, and (2) minimize the exposed
circuit area. In extreme cases, magnetic shielding may be
required. Special metal with high permeability at low
ßux densities (such as mu metal) are effective at reducing
these effects.
Even when the conductor is stationary, magnetically
induced signals may still be a problem. Fields can be pro-
duced by various signals such as the AC power line volt-
age. Large inductors such as power transformers can
generate substantial mangetic Þelds, so care must be
taken to keep the switching and measuring circuits a
good distance away from these potential noise sources.
2.7.2 ELECTROMAGNETIC INTERFERENCE
(EMI)
The electromagnetic interference characteristics of the
Model 7072-HV Semiconductor Matrix Card comply with
the electromagnetic compatibility (EMC) requirements of
the European Union as denoted by the CE mark. Howev-
er, it is still possible for sensitive measurements to be af-
fected by external sources. In these instances, special
precautions may be required in the measurement 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
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• impulse sources as in the case of arcing in high-volt-
age environments
The effect on instrument performance can be considerable
if enough of the unwanted signal is present. A common
problem is the rectiÞcation by semiconductor junctions of
RF picked up by the leads.
The equipment and signal leads should be kept as far
away as possible from any EMI sources. Additional
shielding of the measuring instrument, signal leads, and
sources will often reduce EMI to an acceptable level. In ex-
treme cases, a specially constructed screen room may be
required to sufÞciently attenuate the troublesome signal.
Many instruments incorporate internal Þltering that may
help to reduce RFI effects in some situations. In some cases,
external Þltering may also be required. Such Þltering, how-
ever, may have detrimental effects on the desired signal.
2.7.3 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 occure when sensi-
tive instrumentation is connected to other instrumenta-
tion with more than one signal return path such as power
line ground. As shown in Figure 2-19, the resulting
ground loop causes current to ßow through the instru-
ment LO signal leads and then back through power line
ground. This circulating current develops a small but un-
desirable voltage between the LO terminals of the two in-
struments. This voltage will be added to the source
voltage, affecting the accuracy of the measurement.
SIGNAL LEADS
INSTRUMENT 1
INSTRUMENT 2
GROUND
LOOP
CURRENT
Figure 2-19. Power Line Ground Loops
Figure 2-20 shows how to connect several instruments to-
gether to eliminate this type of ground loop problem.
SECTION 2
Operation
INSTRUMENT 3
POWER LINE GROUND
2-23
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