Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 98)

Publication GMC-RM001A-EN-P — July 2001
11-8 Measuring Noise Reduction Effectiveness
• Use a high-pass filter between the probe and the scope input
when checking AC circuits. Refer to Figure 11.4 for filter
construction details.
Note: Before installing the high-pass filter, check that the signal
does not overload the voltage probe at the chosen division
ratio.
Figure 11.5
Typical differential voltage probe
Differential Scope Inputs
Refer to these guidelines using an oscilloscope with two inputs in
differential mode.
• The trigger must also operate in differential mode. Check your
user manual for compatibility and instructions.
• Use two matched high-pass filters (one for each probe) for AC line
checks as described above. Refer to Figure 11.4 for filter
construction details.
• Connect both probe tips to the same point in the circuit under test.
The residual noise signal should be much smaller than the
measured value if the differential function is working. Refer to the
section Checking Your Method for Effectiveness for details.
• Avoid forming large loops with your probes by twisting the two
leads together as far as possible.