The attached plots show the evolution of the RF min and max, and the 2f signals on the B4 sensors (PD1, PD2, QD1, QD2), during an example of lock acquisition on Nov 23rd. The RF channel saturates when the min/max reaches 8191 counts.

One can see that the B4_PD2 RF channel saturates during the lock acquisition, as well as the quadrant RF channels.

The B4_PD1 RF channel does not saturate, and therefore this photodiode can be used as observable for the monitoring of the 12 and 112 MHz trends. Note that the channels B4_12MHz and B4_112MHz are already built from the B4 PD1 photodiode.

Concerning the QPD, it may be possible to prevent this saturation by reducing the VGA gain.