Measurement Considerations C-7
instrument LO signal leads and then back through power line ground. This circulating current
develops a small but undesirable voltage between the LO terminals of the two instruments. This
voltage will be added to the source voltage, affecting the accuracy of the measurement.
Figure C-2
Power line ground loops
Figure C-3 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 instruments like the Model 2182 that have
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.
Figure C-3
Eliminating ground loops
Instrument 1 Instrument 2 Instrument 3
Signal Leads
Ground
Loop
Current
Power Line Ground
Instrument 1 Instrument 2 Instrument 3
Power Line Ground
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