Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 95)

Publication GMC-RM001A-EN-P — July 2001
Measuring Noise Reduction Effectiveness 11-5
Measuring AC Circuits
Line voltage AC circuits are more difficult to measure since 50/60Hz
AC waveforms will swamp the noise signals if a standard 10x or 100x
scope probe is used.
Professional noise probes include a 150kHz high-pass filter to
attenuate power frequency signals, but such a filter is easily built
(refer to Figure 11.3).
Figure 11.3
High-pass filter circuit, 150kHz, 1 pole
With this filter installed between a 1x scope lead and the scope input,
AC lines may be examined for noise. The 50/60 Hz waveform will be
reduced to around 200 mV peak.
Note: Note that a 10x probe will attenuate far more than 10x in this
situation and should not be used.
1000 pF
2 kV
Shielded enclosure
BNC plug to scope
BNC socket
from probe
1000 Ω
0.25 W
ATTENTION
!
To avoid personal injury or damage to equipment,
the capacitor must be rated at 2 kV or higher.