Keithley 7999-2, Instruction Manual

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3-4 Operating Considerations
Insertion loss
Insertion loss indicates signal lost while passing through the card. This loss occurs in the
various signal path components through the card (connectors, PC board traces, and relays). The
Model 7999-2 has an insertion loss figure of <0.1dB at audio frequencies up to 20kHz (50
Ω
source and load impedances), which should not be a consideration in most cases.
Crosstalk
The crosstalk figure indicates the amount of signal leakage between channels on the card.
With similar power levels applied to the various channels, crosstalk will be of little conse-
quence. With widely different power levels, however, crosstalk may result in undesired results.
Note that the Model 7999-2 has crosstalk figure of <-40db @ 1MHz (50
Ω
load).
RFI/EMI
RFI (Radio Frequency Interference) and EMI (Electromagnetic Interference) are general
terms used to describe electromagnetic interference over a wide range of frequencies across the
spectrum. Such interference can be particularly troublesome at low signal levels, but is can also
affect measurements at high levels if the problem is of sufficient severity.
EMI can be caused by steady-state sources such as radio or TV broadcast 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 desired signal can be considerable if enough of the unwanted signal is
present.
EMI 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 switching card, sig-
nal 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.
E
OUT
=
R
DUT
R
PATH
+
E
DUT
R
PATH
R
DUT
E
DUT
R
PATH
Figure 3-2
Voltage attenuation by
path isolation
resistance