Fracarro OPT-TX51 Instructions D'emploi page 19

Measure the Fracarro OPT optical link with (audio/video digital programs only)
The measure of the Fracarro optical link can be done in three simple steps:
Step 1 – know the number of digital channels at the input and the optical attenuation of the link.
The first step is to know the number of digital channels that you want to transmit in the optical link. The number of chan-
nels to be considered for correctly measuring the optical link is the result of the addition of satellite transponders and
DTT bouquets. The number of SAT transponders can be fixed to 32 (the maximum number of transponders that can
be transmitted in a unique polarization). Therefore, to transmit 20 digital terrestrial channels, the total number of chan-
nels to be considered is given by (32CH SAT + 20 CH DTT) = 52CH. Furthermore, even the link's optical loss from
the transmitter to the receiver should be considered (consider the worst case). For example, if you are using a 2-way
optical splitter (-3.2 dB optical) and 5 km of fibre optics ((-0.4×5)= -2 dB optical), the optical attenuation will be 3.2 + 2
= 5.2dB (consider the possible losses of fibre connectorization).
Step 2 – determining the input signal level
The signal level that must be set at the transmitter input should be chosen according to the number of channels that
will be transmitted. If the number of channels that will be transmitted is 52 (32CH SAT + 20CH DTT), the input level
that will be set is 98dBµV. Example:
62
60
58
56
Step 3 – calculation of CNR and output value
If you have to transmit 52 channels with an optical attenuation of 15dB, the input level of each channel (satellite or ter-
restrial digital) should be 98 dBµV (Pin). Table 2 gives the CNR ratio of the digital satellite signals and table 3 gives the
CNR ratio of the terrestrial satellite signals. CNR will be about 21.6 dB in the event of satellite channels and 30.2 dB in
the event of digital terrestrial signals. The output level of the optical receiver will be 66 dBµV (in both cases).
82
80
78
76
74
72
70
68
66
64
62
60
58
56
17.6 18.0 18.4 18.6
16.0 16.4 16.6
14.4 14.6
12.6
94 96 98 100 102 104 106 108 110 112 Pin (dBuV )
126 80 50 32 20
Number of channels to
28.4 29.7 31.0 32.1 33.2 34.1 35.0 35.8 36.5 37.1
Output level of the
27.7 29.0 30.1 31.2 32.1 33.0 33.8 34.5 35.1 35.6 receiver: 66dBµV
27.0 28.1 29.2 30.1 31.0 31.8 32.5 33.1 33.6 34.0
26.1 27.2 28.1 29.0 29.8 30.5 31.1 31.6 32.0 32.4
25.2 26.1 27.0 27.8 28.5 29.1 29.6 30.0 30.4 30.6
24.1 25.0 25.8 26.5 27.1 27.6 28.0 28.4 28.6
23.0 23.8 24.5 25.1 25.6 26.0 26.4 26.6
21.8 22.5 23.1 23.6 24.0 24.4 24.6
20.5 21.1 21.6 22.0 22.4 22.6
19.1 19.6 20.0 20.4 20.6
17.6 18.0 18.4 18.6
16.0 16.4 16.6
14.4 14.6
12.6
94 96 98 100 102 104 106 108 110 112 Pin (dBuV )
126 80 50 32 20
Figure 3 – Exemple of CNR reading
Level of the input signal
to the transmitter
13 8 5 3
transmit at the input
CNR level: 21,6
Input channels: 52
TX input level: 98dBµV
13 8 5 3
19
2
2 Channels
1
15
16
17
18
2 Channels