Keithley 7999-4, Инструкция по эксплуатации

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3-6 Operation
Switching considerations
Signals switched by the Model 7999-4 may be subject to various effects that can seriously
affect their integrity. The following paragraphs discuss these effects and ways to minimize
them.
Connector integrity
As is the case with any high-resistance device, the integrity of connectors can be damaged if
they are not handled properly. If connector insulation becomes contaminated, the insulation
resistance will be substantially reduced, affecting high-impedance measurement paths. Refer to
Section 4 for cleaning information.
Oils and salts from the skin can contaminate connector insulators, reducing their resistance.
Also, contaminants present in the air can be deposited on the insulator surface. To avoid these
problems, never touch the connector insulating material. In addition, use the relay only in
clean, dry environments to avoid contamination.
Voltage standing wave ratio
The Voltage Standing Wave Ratio (VSWR) is a measurement of mismatch in a cable,
waveguide, or antenna system. The measurement is shown as ratio to 1, e.g., a VSWR of 1.2 is
actually the ratio of 1.2:1. Refer to the specifications located at the front of this manual for
Model 7999-4 VSWR information (specifications are shown with a 50
Ω
load).
Path isolation
The path isolation is the equivalent impedance between any two test paths in a measurement
system. Ideally, the path isolation should be infinite, but the actual resistance and distributed
capacitance of cables and connectors results in less than infinite path isolation values for these
devices.
Path isolation resistance forms a signal path that is in parallel with the equivalent resistance
of the DUT. For low-to-medium device resistance values, path isolation resistance is seldom a
consideration; however, it can seriously degrade measurement accuracy when testing high-
impedance devices. The voltage measured across such a device, for example, can be substan-
tially attenuated by the voltage divider action of the device source resistance and path isolation
resistance. Also, leakage currents can be generated through these resistances by voltage sources
in the system. Refer to the specifications located at the front of this manual for Model 7999-4
isolation information.