Eaton’s Crouse-Hinds Business
Cooper Crouse-Hinds Pte Ltd
No. 2 Serangoon North Ave 5 #06-01
Singapore 554911
tel: + 65 6645 9888
fax: +65 6297 4819
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noise due to the random movement of electrons conducting current, and even
radiation from solar flare or from the cosmos.
Unwanted noise may appear over specific frequencies, or across the maximum
possible frequency bandwidth that the physical media supports.
The level of noise depends on the quality and type of the cable used, the
earthing/grounding concept and implementation, the environment, and many other
factors. The F809F identifies low frequency noise, in-band frequency noise (i.e. noise
in the frequency that Foundation Fieldbus uses), and high frequency noise.
It differentiates between peak and average noise levels.
Note that noise levels only have upper limits, as only high noise represents a problem.
The lower the noise, the better.
Average values are typically lower than 20 mV, and peak values are typically lower
than 60 mV.
The Fieldbus physical layer specification IEC 61158-2 specifies maximum values for
noise that a fieldbus device has to reject
1
. These maximum values are set as high-limit
and high-high-limit (out of specification) in the F809F module.
1
IEC61158-2:2004, section 12.5.1, specifies receiver noise rejection limit as 75 mV. This
value was applied to in-band frequency noise. Section 12.7.3 allows significantly higher limits
of up to 2000 mV for low frequency noise and 1600 mV for HF noise. These limits are
however frequency-dependent. As F809F can only have one limit per LF and HF noise, MTL
selected 150 mV as limits, which appear to be sufficient to avoid nuisance alarms.
