The insertion loss caused by the DCM module between the TDC and RDC is too large. So TN11OAU/TN12OAU reports the VOA_ADJUST_FAIL alarm and the attenuation setting range is abnormal.
Products
Fault Symptom
The TN11OAU or TN12OAU board reports a VOA_ADJUST_FAIL alarm. In addition, the nominal gain for the board is abnormal.
Cause Analysis
The insertion loss caused by the DCM module between the TDC and RDC is too large.
A good understanding of the concept of "maximum settable gain" helps when analyzing this issue.
You must set the nominal gain before using an OAU board. The sum gain of the PA and BA is the maximum gain of an OAU board. The nominal gain is achieved by adjusting the intermediate insertion loss, which consists of the attenuation of the internal VOA and the insertion loss caused by the DCM module between the TDC and RDC ports. If the insertion loss between the TDC and RDC ports is large, the nominal gain will be small and the gain spectrum form will deteriorate.
Maximum settable nominal gain = Maximum gain – VOA attenuation – Insertion loss between the TDC and RDC ports
The attenuation of the internal VOA is automatically adjusted based on the software computation.
The TN11OAU101 board, whose maximum gain (PA + BA) is 31 dB, is used as an example to illustrate how to set the gain for a board:
- If an insertion loss is caused between the TDC and RDC ports, for example a 7 dB DCM module installed between the TDC and RDC ports, set the gain according to the following principles:Maximum settable nominal gain = 31 dB – 7 dB = 24 dBIf you set the nominal gain to 24 dB, the attenuation of the internal VOA is automatically adjusted to 0 dB.If you set the nominal gain to 22 dB, the attenuation of the internal VOA is automatically adjusted to 2 dB.
- When the TDC and RDC ports are directly connected, the maximum settable nominal gain is 31 dB.You need to consider a certain insertion loss (no more than 0.5 dB) between the TDC and RDC ports. In addition, keep the TDC and RDC ports clean.If the intermediate insertion loss exceeds the maximum intermediate insertion loss of a board, the actual board gain may not reach the set nominal gain. In this case, the board locks the gain based on the intermediate insertion loss, which is obtained by the nominal gain upper threshold minus the actual gain upper threshold.
Fault Isolation Procedure
Replace the DCM module with a DCM module that has a smaller insertion loss and clean the related pigtails and optical ports.
Conclusion and Suggestion
When using the TN11OAU or TN12OAU board, you must set the nominal gain for the board on the NMS. If the nominal gain setting range is abnormal, verify the insertion loss of the DCM module between the TDC and RDC ports.
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