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Operation
Figure 2-17
Two cards in daisy chain configuration
2.7
Measurement considerations
Most measurements made with the Model 7174A concern
low-level signals. Such measurements are subject to various
types of noise that can seriously affect low-level measure-
ment accuracy. The following paragraphs discuss possible
noise sources that might affect these measurements.
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 sufficient
length, even weak magnetic fields like those of the earth can
create sufficient signals to affect low-level measurements.
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. Spe-
cial metals with high permeability at low flux 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
produced by various signals such as the AC power line
voltage. Large inductors such as power transformers can also
generate substantial magnetic fields, so care must be taken to
2-22
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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 7174A comply with the electromagnetic compatibil-
ity (EMC) requirements of the European Union (EU) direc-
tives as denoted by the CE mark. However, it is still possible
for sensitive signals to be affected by external sources. In
these instances, special precautions may be required in the
test setup.
Sources of EMI include:
• Radio and TV broadcast transmitters.
• Communications
transmitters,
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 Model 7174A, signal source, measuring instrument, and
signal leads should be kept as far away as possible from any
EMI sources. Additional shielding of the card, signal leads,
sources, and measuring instruments will often reduce EMI to
an acceptable level. In extreme cases, a specially constructed
screen room may be required to sufficiently attenuate the
troublesome signal.
Many instruments incorporate internal filtering that may
help to reduce EMI effects in some situations. In other cases,
additional external filtering may be required. Keep in mind,
however, that filtering may have detrimental effects, such as
increased settling time, on the measurement.
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 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 2-18, the resulting ground loop causes
unwanted signals to flow through the instrument LO signal
leads and then back through power line ground. This circu-
lating current develops a small but undesirable voltage
between the LO terminals of the two instruments. This volt-
age will be added to the source voltage, affecting the accu-
racy of the measurement.
including
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