Siemens SIMATIC NET PROFIBUS, Owner'S Manual

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Configuring Networks
3-14
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
3.2.2 Optical Power Budget of a Fiber-Optic Transmission System
Optical Power Budget
The transmitted optical power P
out
and the received optical power P
rec
are
specified in dBm, the attenuation caused by connectors and the fiber is specified in
dB.
dBm is a reference unit and describes the logarithmic ratio of the power level to the
reference power P
0
=1mW. The following formula applies:
P
x
[in dBm] = 10*log(P
x
[in mW] / P
0
)
Examples:
Transmitter Power P
x
Transmitter Power as Logarithmic Power
Ratio P
x
to P
o
10 mW
+ 10 dBm
1 mW
0 dBm
1
µ
W
– 30 dBm
Transmitter
Depending on the fiber being used, the minimum and maximum optical power that
can be coupled into a fiber is specified. This power is reduced by the attenuation of
the connected transmission path resulting from the fiber itself (length, absorption,
scattering, wavelength) and the connectors used.
Receiver
The receiver is characterized by its optical sensitivity and its dynamic range. When
configuring an optical link, you should make sure that the power reaching the
receiver does not exceed its dynamic range. If the power falls below the minimum,
this increases the bit error rate (BER) due to the signal-to-noise ratio of the
receiver. If the maximum received power is exceeded, saturation and overload
effects increase pulse distortion and therefore also increase the bit error rate.