Keithley 2701, Руководство пользователя

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E-6 Measurement Considerations Model 2701 User’s Manual
Very often, cooling the source is the only practical method available to reduce noise.
Again, however, the available reduction is not as large as it might seem because the
reduction is related to the square root of the change in temperature. For example, to cut the
noise in half, the temperature must be decreased from 293K to 73.25K, a four-fold
decrease.
Magnetic fields
When a conductor loop 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 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
problem. 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 2701 voltage source and connecting cables a good
distance 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
particularly 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 signals, 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 2701 voltage source and signal leads as far away from the RFI source as possible.
Additional 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 2701 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
filtering may have detrimental effects such as increased settling time on the desired signal.
2701-900-01.book Page 6 Wednesday, August 3, 2011 9:43 AM