Fracarro OPT-TX54 Instrucciones De Funcionamiento página 16

Evaluation of the signal/noise ratio (CNR)
Use table 2 and table 3 to determine the CNR value (signal/noise ratio) of the optical link. The output level and the CNR
corresponding to any possible input level are shown as regards to each level of optical attenuation. The Pin(dBµV) line must
be considered as follows: if you have to transmit N channels, each channel must be set to a level of X dBµV at the optical
transmitter input.
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
channels 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
channels 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:
65
63
61
59
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 93 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 22.6 dB in the event of satellite channels and 31.2 dB in
the event of digital terrestrial signals. The output level of the optical receiver will be 67 dBµV (in both cases).
83
81
79
77
75
73
71
69
67
65
63
61
59
20.6 21.0 21.4 21.6
19.0 19.4 19.6
17.4 17.6
15.6
91 93 95 97
80 50 32 20
Number of channels to
transmit at the input
30.7 32.0 33.1 34.2 35.1 36.0 36.8 37.5 38.1
Output level of the
receiver: 66dBµV
30.0 31.1 32.2 33.1 34.0 34.8 35.5 36.1 36.6
29.1 30.2 31.1 32.0 32.8 33.5 34.1 34.6 35.0
28.2 29.1 30.0 30.8 31.5 32.1 32.6 33.0 33.4
27.1 28.0 28.8 29.5 30.1 30.6 31.0 31.4 31.6
26.0 26.8 27.5 28.1 28.6 29.0 29.4 29.6
24.8 25.5 26.1 26.6 27.0 27.4 27.6
23.5 24.1 24.6 25.0 25.4 25.6
CNR level: 22.6
22.1 22.6 23.0 23.4 23.6
20.6 21.0 21.4 21.6
19.0 19.4 19.6
17.4 17.6
Input channels: 52
TX input level: 98dBµV
15.6
91 93 95 97 99
80 50 32 20 13
Figure 3 – Exemple of CNR reading
Level of the input signal
to the transmitter
99 101 103 105 107 Pin (dBuV )
13 8 5 3 2
101 103 105 107 Pin (dBuV )
8 5 3 2
16
2
Channels
1
15
16
17
18
Channels