KWS Electronic VAROS 109, Руководство пользователя

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Chapter 13 - Optical Receiver
10911 V01.06
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Chapter 13
Optical Receiver (optional)
13.1 Introduction
RF signals are increasingly being transmitted via fibre optic cables. Optical transmission in
broadband networks is gaining importance. While optical transmission in most existing networks still
occurs exclusively at network level 2, the trend is moving towards fibre optic distribution up to the
subscriber terminals. Even in the field of SAT-IF distribution, solutions are already available for
optical transmission.
Optical fibres:
The optical fibre is the medium through which the light signal is transmitted. There are 2 different
fibre types. With multi-mode fibres, the light can move through the optical fibres on multiple
“paths” (modes). This results in modal dispersion (distortion), which limits the bandwidth and the
transmission distance. With single-mode fibres, on the other hand, the light can only move through
the fibres on a single path, preventing modal dispersion and resulting in higher bandwidths. At the
moment, almost all the fibres used are single-mode.
They have a core diameter of 9 µm and a sheath diameter of 125 µm. A single-mode optical fibre
has an attenuation of approx. 0.3 dB/km.
Optical plug connection:
There are 2 different kinds of fibre optic plug connection. The first one has a straight polish. This
version, called PC (physical contact), has a somewhat worse return loss. Connectors with APC
(angled physical contact) have an interface with an angle of 8°. PC connectors have blue markings,
whereas APC connections have green ones.
Fibre optic plug connections are available in various forms such as FC (threaded connection), SC
(plug connection), and E2000 and LC (both with snap/plug connection).
An SC/APC plug connection is built into the measuring instrument.
The measuring instrument is equipped with an optical receiver that converts the light signal back into
an RF signal. After the optical receiver, the RF signal behaves as if it had been supplied via the coax
input of the measuring receiver. This means that all the measurements available through the RF
input can also be taken via the optical input.
The optical receiver itself is not wavelength-selective. In some systems, light with different
wavelengths is transmitted via one and the same optical fibre. This is known as a wavelength
division multiplex. In this type of system, the wavelengths must be separated again before the
optical receiver because otherwise the signals from the two wavelengths would be mixed in the
optical receiver, leading to interference. A patch cable with an integrated wavelength filter should be
used for this type of application. But generally, only one wavelength is used, making this
unnecessary. In most cases, wavelengths of 1310 nm, 1490 nm and 1550 nm are used.
Optical input power
The measuring instrument does not have an integrated adjustable optical attenuator element.
As a result, the measuring instrument’s optical receiver can be operated with up to 8 dBm of
continuous power. However, the optimal range for the receiver is from -7 dBm to +3 dBm. At lower
power levels, the reception quality is reduced because of the receiver noise.
At higher input power levels, the intermodulation products have a negative effect on the
performance. In this case, optical attenuation elements should be used.
Example:
Measurements must be taken on an optical transmitter with an output power of 8 dBm.
The optical power can be measured directly. However, an attenuation element of 5 to
10 dB should be connected between the transmitter and the receiver to determine the
signal quality.