Siemens SIMATIC NET PROFIBUS manual Optical Power Budget of a Fiber-Optic Transmission System

Configuring Networks

3.2.2Optical Power Budget of a Fiber-Optic Transmission System

Optical Power Budget

The transmitted optical power Pout and the received optical power Prec 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 P0=1mW. The following formula applies:

Px [in dBm] = 10*log(Px [in mW] / P0)

Examples:

Transmitter Power Px

10 mW

1 mW

1 ∝ W

Transmitter Power as Logarithmic Power Ratio Px to Po

+10 dBm

0 dBm

– 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.

3-14	PROFIBUS Networks SIMATIC NET
	6GK1970-5CA20-0AA1 Release 2 05/2000