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Optical RF signal transmission allows for long-range distribution of digital terrestrial and satellite television, DAB radio, security video etc. Fiber-optic SMATV systems provide much more flexibility compared to those based only on coaxial cable. In addition to the greater freedom in implementing long transmission lines, the installation of the systems is also easier thanks to replacing multiple coaxial cables by a single fiber-optic cable. It means that the deployments of the shared antenna systems are easier for implementation, especially in existing buildings equipped with narrow shafts or even without such conduits.
Currently, the most cost-effective solutions use optical technology at the front of the systems and for the long distribution lines run to optical nodes in buildings. The nodes convert the optical signals into electrical RF signals which are further distributed in the buildings by multiswitches and coaxial cables. An example is shown below.
The FibSZ-ODU32 converter/optical transmitter combines signals from the FibSZ-LNBF downconverter (SAT Input) and terrestrial antennas (DVB-T/DAB/FM Input). Both the FibSZ-LNBF and FibSZ-ODU32 are the main parts of the A9880 SAT-IF & DVB-T optical kit. The combined signal is converted into optical form and available at two FC/PC optical outputs (at the level of 3.5 dBm). The passive distribution network consists of two optical splitters, Opto-Spt2 BOX A98892 and Opto-Spt3 BOX A98893, distributing the optical signals via single-mode fibers to nodes located in 5 multi-family apartment buildings. The nodes in the buildings are FibSZ-QUATRO receivers/converters A9882 which replicate the satellite IF, DVB-T and FM/DAB signals coming in optical form via the PON. They have 5 outputs allocating the output RF signals as four SAT-IF polarization/band combinations (VL-HL-VH-HH) and terrestrial DVB-T + DAB + FM signals. The signals are then distributed by multiswitches deployed in the buildings. Each building installation consists of SS-510 R70516 SAT/TV tap used for sub-network partition and two MSV-532 R70732 multiswitches providing signals to 64 Signal R694100 outlets (so the total number of outlets in the SMATV system is 320).
Designing fiber-optic distribution system based on COMSAT devices
The basic task is to determine the optical signal levels at the inputs of the receivers, taking into account the output level of the transmitter and the total attenuation of each optical path. The power of the optical signal at the input of each receiver must be within the sensitivity range of the receiver, with a certain safety margin.
The attenuation (loss) of optical fibers, connectors, and splitters is expressed decibels (dB), while the output power of transmitters and the sensitivity of receivers are given in decibels with reference to a power of one milliwatt (dBm). Knowing the lengths and types of the fibers and parameters of other components of the distribution system, one can calculate the loss of signal at each point of the network, including the inputs of the receivers. This shows the following example:
Example of an optical network, with the red markings meaning:
| N S O Z | - Transmitter - Splitter - Receiver - Connector |
Each of the two optical links starts at the output of the A9880 transmitter (N) and ends with the A9882 receiver. One path includes the Opto-Spt2 BOX 2 A98892 splitter(S), the second the Opto-Spt3 BOX 3 A98893 splitter (S). Each of the lines uses four optical connectors (Z). The table below lists the parameters of the network components:
| Tx output power: Connector loss (termination): Optical fiber loss : Insertion loss of Opto-Spt2 BOX 2 splitter: Insertion loss of Opto-Spt3 BOX 3 splitter: Sensitivity of FibSZ-QUATRO receiver/converter: | 3.5 dBm 0.15 dB 0.4 dB/km 4 dB 5.5 dB min -15 dBm / max 0 dBm |
With these values, it is easy to calculate the optical power levels at the inputs of the receives. Let's first calculate the power level in the case of the optical path with the Opto-Spt2 A98892 splitter and the length of the optical fiber of e.g. 600 meters:
| loss of the fiber (1310nm): loss of the connectors: loss of the splitter: | 0.6 km x 0.4 dB/km = 0.24 dB 4 x 0. 15dB = 0.60 dB 4 dB |
The total attenuation of the transmission path is 4.84 dB. So, the power level at the input of the receiver is reduced by 4.84 dB from the initial 3.5 dBm value, to -1.34 dBm.
In the second case (the path with the Opto-Spt3 BOX 3 A98893 splitter and the same length of the fiber):
| loss of the fiber (1310nm): loss of the connectors: loss of the splitter: | 0.6 km x 0.4 dB/km = 0.24 dB 4 x 0.15 dB = 0.60 dB 5.5 dB |
The total attenuation of the transmission path is 6.34 dB. So, the power level at the input of the receiver is reduced by 6.34 dB from the initial 3.5 dBm value, to -2.84 dBm.
Both in the first and second case, the optical power at the input of the A9882 receiver is between the -15...0 dBm operational range. Below the lower limit, the receiver would not be able to decode the signal correctly, whereas above the upper limit it could be even destroyed (to avoid such situation an additional optical FC/PC attenuator should be used at the receiver's input).
If a SMATV system should only distribute satellite broadcasts, the good choice is the SAT-IF Optical Receiver/Converter Opto-QUATRO A9886. Thanks to two switchable sensitivity levels (SML/STD), it can be used in systems with very different lengths of the links and power levels. The SML PON mode is dedicated mainly for small networks (sensitivity range: -13 ... 0 dBm), and the STD PON mode for standard and larger networks (sensitivity range: -18 ...-14 dBm).
