Tara and I spent some time looking at the TTFSS boards. The offset issues appear to be caused by bad choice of offset knob on the TTFSS interface boards. Previously, Tara and I had used the offset knob to null RFAM-induced offsets in the north PDH signal. The current thinking is that when Tara re-optimized the electro-optic elements on the north path, the RFAM-induced offset changed and was therefore no longer nulled.

We have now returned the offset knobs to their optimal values as follows: we set each TTFSS to use the TEST SMA input rather than the LO/PD + mixer input, we applied a 50 Ω terminator to this input, and we then watched TP4 (after the common VGA) on a scope while adjusting the offset knobs. The optimal knob positions are 526 for the south TTFSS and 506 for the north TTFSS. Varying the common gain causes the DC offset on TP4 to change only slightly (it stays within ±5 mV for both north and south). Varying the fast gain causes the DC offset on TP17 (after the fast VGA) to vary as well; on south, this also appears to stay within ±5 mV of zero, but on north it is as high as 20 mV when the common and fast gains are turned all the way up. However, since these VGAs are each +30 dB at maximum gain, this means that the offset referred to TTFSS OUT1 is more like 20 µV, which is negligible compared to an error signal that is something like 1 Vpp.