As noticed by Paolo and others, the level of 56 MHz is slowly decreasing since last septembre, when we increased the modulation depth from 7dBm to 12 dBm. In order to confirm and evaluate this loss I have plotted the trend data of the 112MHz during this time. As it can be seen in the first three figures, we start with 0.035 and we end up with 0.023 in two months.
Then we recovered the level of the sidebands with the power increase (Figure 4).
Figures 5 and 6 show the level of the sidebands during the recovery of February and the present one, and the level has further decreased to 0.013.
The signal used for monitoring is coming from the B4 photodiode 2f signal. It depends on many parameters, but the trend over these months is clear.