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4. USER’S INSTRUCTIONS
4.1 Dimensioning A Fibre Optic system
When designing a fibre optic system it is important to consider the optical input level to the OPT-
RX DT receivers, suitable attenuation is required and must be calculated at each point to ensure
the system falls within the required parameters. The optical output power from the OPT-TX DT
transmitter is fixed (+7.5dBm typical).
The OPT-RX DT receivers can guarantee that the correct level of TV and SAT signals are available
to the end users sockets. This guarantee is on the proviso that the input level of the optical
signal is between -8dBm and -14dBm therefore giving an optical attenuation range of 15
to 21dB.
The catalogue includes a range of optical line attenuators, splitters and dividers with different
attenuation values to enable the optimisation of the system, especially when considering a classic
cascade distributions, to obtain an optical signal level from the OPT-RX DT of between -8dBm and
-14dBm. Refer to the product specifications to calculate the optical attenuations for the devices
ATTENTION:
• Direct connections are not allowed between the OPT-TX DT and OPT-RX DT,
unless the correct line optical attenuators are used.
• An optical signal reader must be used to check the optical input level to the OPT-RX DT and
to prevent damaging the receiving photodiode (damage can be caused if the optical input
signal is higher than 0dBm).
• If the quality of the input signals are not optimised, or if a lot of signals have to be distributed
to a lot of sockets, the optical budget should be reduced or professional splitters should be
used with SC-APC connectors.
4.2 Requirements for the input signal to the OPT-TX DT optical transmitter
For the signals to be correctly transmitted through the fibre optic, the RF signals must respect the
following requirements:
TV (DTT/CATV)
To ensure the TV signals are transmitted and received correctly, the input signals to the transmitter
should be equalised, especially if a lot of channels are being transmitted.
As an indication, the input signals should respect the following requirements:
1. The transmitter signal level should be ≥ 74dBµV to guarantee correct reception by the OPT-
RX DT receivers, even after an optical attenuation of 21dB.
2. Overall input power: -19dBm (90dBµV)
3. The overall power ≤ -19dBm (90dBµV) is guaranteed in the following cases:
•40TVchannelsequalisedat74dBµV
•20TVchannelswithamaximumlevelof77dBµV
•16TVchannelswithamaximumlevelof78dBµV
•8TVchannelswithamaximumlevelof81dBµV
If the TV input signal quality is not optimal or not correctly equalised, then the system should be
designed considering an optical attenuation lower than 21dB to guarantee correct signal reception
at the socket (especially when a lot of channels are being transmitted).
FM and DAB
The FM and DAB signal levels should be at least 10dB lower than the overall input power of the
TV signal.
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