As I outlined in this post, the primary PDH box needed some modifying if we are to see any significant improvement from the new setup. I made these changes:

• Remove the AD8336 variable gain stage and bypass it altogether
• Reduce the overall gain of the servo by ~40 dB

The input-referred noise is now at the level explained by the LISO model---unlike it was before---though this calls for a funny story:

When I plotted the noise and divided by the transfer function I measured, I got a noise level that was exactly 10x higher than predicted by LISO. After quite a while of scratching my head and checking my code, I realized that I mistakenly changed the gain of the wrong stage (despite writing down the right one in my notes), causing the noise from the nasty LF356 switchable stage to have 10x the influence. I will correct it and re-measure tomorrow, but the important thing is that we have removed the noisy 8336 from near the input.

Here is a picture of the modified portion of the board and a LISO prediction of the noise when I've corrected the mistake. You can compare with the plot in the link above, but this corresponds to a true low-frequency voltage noise betterment of 20 dB at low frequency, which adds to the 40 dB improvement in the servo's contribution from the expected optical gain increase. This actually puts the servo noise below the aLIGO requirement between about 100-350 mHz.