Yes, saturation was the issue.

I turned down the input excitation to -23 dBm and checked with test clips all the way through the the test points on both paths.  Looking with the oscilloscope The only point saturating was TP13, the last stage of the EOM path.  For this data point I turned the power down to -50 dBm; I have a feeling this is too low and maybe we need to injected an excitation at at the start of the EOM to get a decent TF.

I've attached TF for both north and south field boxes. 

South box (2009:007):

There is still a little saturation on stage 3 of the PZT path, it seemed fine looking with the oscilloscope so I'm not sure what is going on there: its possible I was looking with a 100 kHz excitation and that it looked ok, but was not at lower frequencies.  Something to check again. It also looks like EOM stage 2 is saturating.  On the third stage EOM, I turned the excitation right down (as mentioned above).  This looks not right, but might need more excitation closer to that point in the circuit. 

North path (2009:005):

As you can see, there is a fault at the first stage of the common path.  Craig has since diagnosed this to be a fault with the common path circuit.  The pad has peeled off for R4 on the pin2 side of the op amp. Craig has more detailed pictures of his before and after fix of this. Some time in the past R4 had been removed from both of the FSS boxes in the PSL lab and replaced with with a regular metal film through hole resistor pictured below. The resistor sits in vertical pin holes soldered to the pads. I guess the motivation was to have some programmable gain, but the resistors has been knocked and now we have some damage on the north (2009:005 unit).

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All data and plots are committed into ctn_labdata repo in /data/20171020_Ser7_South_FSSTFs and /data/20171020_Ser5_North_FSSTFs

​Plotting notebooks are commited into ctn_noisebudget repo under /TTFSS_lisomodel