Siemens SIMATIC NET PROFIBUS manual Arrangement for Measuring Attenuation

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.

Step 1: Reference measurement

Light source

Optical power meter

Light source

Step 2: Link measurement

Optical power meter

Figure A-2 Measuring the Total Attenuation of a FO Link

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	PROFIBUS Networks SIMATIC NET
	6GK1970-5CA20-0AA1 Release 2 05/2000