Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 100)

Publication GMC-RM001A-EN-P — July 2001
11-10 Measuring Noise Reduction Effectiveness
Identifying the Noise Source
Two methods for identifying the source of a noise spike are listed
below.
• Disable each potential source in turn until the spike disappears.
• Correlate the noise signal with PWM sources by displaying the
PWM waveform on a second channel. If the PWM source is the
culprit, the noise signal will remain synchronized to the edges of
the PWM waveform. Use a differential probe (phase to phase) or
high-pass filter (phase to ground), connected to the suspect drive
terminals, to display the PWM edges.
Intermittent Noise
If noise from mechanical contacts (e.g., a motor contactor) is
suspected, the technique is a little different. Because of the variable
nature of the peak amplitude, the best method is to operate the
suspect device (for example) ten times in quick succession by
overriding the control system. If this is not practical, monitor the
device long enough to observe a number of operations. Progressively
increase the trigger level as before.
General Guidelines for
Measuring Noise This section contains general guidelines for measuring noise. Tips on
understanding acceptable noise levels, noise measurement methods
that don’t work, and system monitoring methods are discussed.
What are Acceptable Noise Levels?
No national or international standards for instantaneous peak voltage
levels are known, but a very conservative approach would be to
assume that a TTL gate may be closely coupled to a nominally clean
circuit. Then, the noise immunity of a TTL gate (around 1.0V)
becomes the critical level. This implies an allowable maximum of (for
example) 500mV to allow for some margin of safety.