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3.19 Typical system configuration with fixed gain modules
The diagram below illustrates a typical communications system configuration.
The link gain for the fixed gain systems depends solely on the optical fibre loss from transmitter to
receiver. There is a Received Light Level (RLL) analogue monitor output on the receiver modules
which can be used to measure the light from the transmitter reaching the receiver during operation.
The RLL threshold alarm (which is triggered when the RLL drops below a preset level) can be used to
determine if the optical link has been damaged or degraded.
3.20 Commissioning of a communications link
This commissioning procedure illustrates the processes required to install and set up a
communications link with gain control. The example describes the commissioning of a C-Band HTS
inter-facility link.
We will be considering the installation of the following system.
A LNA provides an output signal in the C- band. The signal must be conveyed over 1500m of fibre,
through a bulkhead at each station, to the modem. An alarm must trigger if the optical path is
damaged.
1. Install the link, connecting all optical patch cords and cross-site fibre optic cables. Clean ALL
optical connectors BEFORE mating with the modules.
2. Power up the equipment and allow 15 minutes to warm up.
3. Ensure that the RF power into the transmitter module is set to optimum for your system. Use a
broadband RF power meter for this measurement. Typically this is the input level at which the
link’s intermodulation distortion (IMD) is ~ -40dBc. This value of input power can be calculated
from your datasheet, it is typically 8dB less than the input P1dB.
4. Calculate the approximate optical attenuation in the fibre path. In our case, we have two
bulkhead connectors @ 0.2dB each, 1500m of optical fibre @ 1310nm = 0.6dB, giving a total of
1dB of optical loss. The total RF gain of the system should be the nominal link gain minus 2x the
optical loss.
Modem
ViaLiteHD
chassis
Cable Termination box
Patch cables
SSPA and LNA
Patch cables
Cable Termination box
ViaLiteHD
chassis
Cross site cable
Cross site cable
RF feeds for Antenna
