Siemens SIMATIC NET PROFIBUS Owner's Manual page 248

Testing PROFIBUS
Arrangement for Measuring Attenuation
The arrangement for measuring attenuation consists of a light source and an
optical meter. The light source is first connected to the receiver via a reference
fiber. The optical power measured at the receiver is the reference value for a link
without attenuation. Following this, the reference fiber is opened and the link to be
measured is inserted. The meter compares the optical power received now with
the previously measured reference value and calculates the attenuation of the
inserted link on this basis.
Light source
Light source
Figure A-2
Evaluating the Results of the Attenuation Measurements
An optical power budget is available between an optical transmitter and optical
receiver. This identifies the difference between the minimum power launched by
the transmitter and the minimum optical power required at the receiver. The optical
power budget is normally specified in dB. The measured total attenuation of the
fiber-optic link must be below this optical power budget.
The greater the difference between the total attenuation and the optical power
budget, the greater the operating reliability and long-term stability of the optical
link. The difference between the optical power budget and the total attenuation is
known as the link power margin of an optical link. For multimode glass-fiber links,
this link power margin should not be below 3 dB and for single-mode glass-fiber
links should not be below 2 dB.
A-8
Step 2: Link measurement
Measuring the Total Attenuation of a FO Link
Step 1: Reference measurement
Optical power meter
Optical power meter
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000