Appendix F. Time Series and Spectrum Graph Information
F.2 Good Sensors with Noise
The measurements graphed in Figure F.2-1 and Figure F.2-2 are made by the
same sensor used for Figure F.1-1 and Figure F.1-2. However, for Figure F.2-1
and Figure F.2-2, a drill is running about ½ inch away from the sensor. This
shows the effects of narrowing the begin/end frequency to deal with noise
generated by an electric motor. The narrow frequency range in F.2-1 has
reduced the effects of the noise source and has yielded a signal-to-noise ratio of
4 times better than F.2.-2. Notice that if the begin frequency in Figure F.2-2
was much less than 450 Hz, the 60 Hz harmonic would have been the dominate
frequency.
Holding the drill ½ inch away from the sensor is an invasive noise source.
When the sensor is measured with the drill a few inches away, the harmonics
of the 60 Hz are a lot less and are not more dominate than the wire’s natural
frequency. Sensors with a frequency range that are below 450 Hz should work
fine even in the presence of a 50 or 60 Hz noise source, however they should
be characterized.
FIGURE F.2-1. Good Sensor with Noise (600 to 1800 Hz)
F-3
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