Just measured the PLL OLGTF.  Here is an ascii diagram.

                  EXC
                   |
    Beatnote --->( + )----->[H]-------> Error Signal
                   ^                |
                   |                |
Actuation Signal <----------[A]<-----

H is the SR560 preamplifier.  A is the Marconi RF signal generator.  To get the PLL OLG, first we measure from EXC to the error signal with the loop open.  Then we close the loop and measure again. 

The first measurement gives us H(f).  The second gives us H(f)/(1 - G(f)) where G(f) = H(f) A(f) is the PLL open loop gain.

I have a script in ctn_labdata/scripts which takes in open and closed loop measurements and gives OLGs called OLGCalculator.py.  Then I used iris.py to plot the OLGTF shown below.  The tar that iris.py produces is included.

Our phase locked-loop open loop gain unity gain frequency is around 30 kHz.  This should be okay for our purposes, with sufficient gain to suppress low-frequency (~300 Hz) noise.

Next step: calibrated spectra.