Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 101)

Publication GMC-RM001A-EN-P — July 2001
Measuring Noise Reduction Effectiveness 11-11
Field Strength Meters
Field strength meters (RF sniffers) are commercially available to aid in
the alignment of transmitters and antennae. They are small hand-held
instruments with a row of LED’s or a meter to indicate level and
respond to a wide range of signal frequencies. They appear to be an
ideal instrument for measuring noise, but are not. They are designed
to respond to a continuous sine wave signal, however the typical
noise signal is comprised of a ringing effect lasting a few
microseconds and repeated at the PWM switching frequency, usually
a few kHz. As an example, consider the noise trace shown in the
figure below.
Figure 11.7
Typical noise trace
The peak to average ratio of this waveform is 35:1, but this trace has
been altered to show noise pulses from both positive and negative
going edges of a PWM output. In practice the distance between these
two events would be ten times greater. The actual peak to average
ratio would then be 350:1.
Monitoring for Noise
By adding simple monitors to a system, you can check the noise level
of your clean zones and the on-going effectiveness of the system HF
bonding.