Keithley 7013-S Instruction Manual Download

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Operation

4-11

4.5.2 Magnetic fields

When a conductor cuts through magnetic lines of force,
a very small current is generated. This phenomenon
will frequently cause unwanted signals to occur in the
test leads of a switching matrix system. If the conduc-
tor 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. 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 inductors 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
potential noise sources.

At high current levels, even a single conductor can gen-
erate significant fields. These effects can be minimized
by using twisted pairs, which will cancel out most of
the resulting fields.

4.5.3 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 particularly troublesome at low signal lev-
els, but is can also affect measurements 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
(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 equipment and signal
leads as far away from the RFI source as possible.
Shielding the 7013 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
sufficiently 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.

4.5.4 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 instru-
mentation with more than one signal return path such
as power line ground. As shown in Figure 4-11, 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 termi-
nals of the two instruments. This voltage will be added
to the source voltage, affecting the accuracy of the mea-
surement.

Figure 4-12 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 instru-
ments 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.