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Why Individual Optical Power Levels in LANE BERT Do Not Add Up to the Total Power Displayed?
The total (composite) optical power in dBm displayed by the test set is indeed different than the direct sum of the individual lane power levels in dBm. That is normal.
Beginners frequently question, "Why is the total power 7.39 dBm instead of 5.58 dBm?" This confusion arises because they simply try to sum the power of all lanes using basic linear arithmetic, which is not the proper method for calculating total power.
Power in dBm, which is a logarithmic scale, cannot just be added directly. The individual power levels must be converted into Watts or milliwatts (mW) in order to use simple arithmetic summation. Then, the total power in Watts or milliwatts can be converted back to dBm. Then the results would match.
Take the TX Current values in dBm (with their respective conversion to Watts) as an example.
The total sum of power in Watts = 0.00548, or 5.48 mW
Convert it to dBm = 10 x log (5.48) = 7.387806, which is rounded up to 7.39 dBm in total shown on the test set's results.
If needed, online dBm addition calculation tools are available to ease the work.