Beckhoff EtherCAT EK112 Series Manual page 39

Further influences on the signal transmission
In addition to the main influences (attenuation and dispersion) that limit the transmission link, care must be
taken when installing and maintaining fiber-optic transmission links.
Sharp kinks and micro-bends in the fiber-optic lead to additional reflections in the fiber, as a result of which
the influences of the attenuation and dispersion are increased. The specified bending radii of fiber-optic
cables must be adhered to.
Poorly installed connectors also have a great influence on the signal quality. In case of poor connections, the
fiber-optic may be too far away from the connecting piece, so that the light waves do not enter the fiber at the
right angle of entry.
The third influence on the signal transmission that needs to be observed is soiling of, or damage to the ends
of optical fibers. Due to the size of the fibers, often just 125 µm, dirt or damage cannot be discerned with the
naked eye. Only a microscope with a sufficient magnification (at least factor 100) enables the fiber ends to
be checked. To prevent soiling, the cable cap supplied with the cable should always be fitted to the fiber end.
Power and attenuation budget
The power budget specifies the minimum power present between transmitter and receiver. The attenuation
budget, conversely, describes the attenuation present between transmitter and receiver due to the three
attenuation influences - fiber, connectors and splices - described above.
Transceivers (from the words transmitter and receiver) are installed in fiber-optic transmitters and/or
receivers. This transceiver is a combined transmitting and receiving device. The data sheet for the
transceiver contains two values that are necessary for the calculation of the power budget. These values are
the minimum output power of the transmitter and the maximum sensitivity of the receiver. Therefore, the
worst case, i.e. the lowest power between transmitter and receiver, is always considered. Both values are
often specified in the unit decibel milliwatt (dBm). dBm describes a power level in relation to a reference
value of 1 mW.
• L [dB] = 10*log (P
P 10
0 dBm then corresponds to a power value of 1 mW, positive dBm values indicate power values >1 mW and
negative dBm values indicate power values <1 mW.
The difference between the maximum output power and the minimum sensitivity at the input results in the
power level.
• Power level = minimum output power - maximum sensitivity
The attenuation level results from the influences on the attenuation described above.
• Attenuation level [dB] = fiber loss attenuation [dB] + connector insertion attenuation [dB] + splice
insertion attenuation [dB]
EK112x, EK15xx
/1 mW)
1
Version: 3.6
Commissioning/application notes
39