# FIU ports available for connection to the OSC board

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An OSC board can be connected to the TM or RM port of an FIU board.

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FIU board isolation
Isolation refers to the ratio of the optical power leaked by local-frequency or RF signals to the input optical power, in the unit of dB. Isolation of a frequency mixer refers to the mutual isolation between frequency ports, including the isolation among local frequency, IF, and RF.

Insertion loss of the FIU board
The insertion loss of the IN-TM and RM-OUT ports on the FIU board is 1.5 dB, and that of the IN-TC and RC-OUT ports is 1 dB.

Functions of each port on the FIU board
An FIU board multiplexes and demultiplexes signals the main optical path and OSC signals. The RC port of the FIU board receives signals from the main optical path and the RM port receives signals from the OSC. The two types of signals are then multiplexed into one signal using the multiplexer module and are output from the OUT port. The IN port receives signals from the line and then sends the signals to the demultiplexer module. The demultiplexing module splits the signals into main optical path signals and OSC signals, which are then output through the TC and TM ports respectively.

Split ratio of the MON port on the FIU board
For TN11FIU, TN12FIU, TN13FIU01, TN14FIU, and TN21FIU boards, the split ratio between the MON port and OUT port is 1:99. In other words, the optical power of the MON port is 20 dB lower than that of the OUT port, which can be calculated using the following formula: Optical power (dBm) of the OUT port �?Optical power (dBm) of the MON port = 10 x lg(99/1) = 20 dB. For the TN13FIU02 board, the split ratio between the MON port and OUT port is 0.1:99.9. In other words, the optical power of the MON port is 30 dB lower than that of the OUT port, which can be calculated using the following formula: Optical power (dBm) of the OUT port �?Optical power (dBm) of the MON port = 10 x lg(99.9/0.1) = 30 dB.

Optical ports of an FIU board
An FIU board has a total of seven optical ports. The IN/OUT optical port is used to receive and transmit line optical signals. The RC/TC optical port is used to receive and transmit optical signals of the main optical channel. The RM/TM optical port is used to receive and transmit optical signals of the supervisory channel. The MON optical port is used to output optical signals to the spectrum analyzer board for online optical performance monitoring.

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