I measured the TF between VCO feedback and PLL feedback. The result agrees with the TF of the VCO.

  The frequency noise measured from PLL feedback and VCO feedback do not have the same shape, even though

the coherence is ~1. There might be some devices that do not have flat frequency response and change the shape

of the frequency noise. 

    So I measure the TF between the feedback signal to VCO (ref, chA) and the feedback signal to Marconi (resp, chB).

The excitation is sent to In test 2 ch on FSS servo input which modulates the frequency of the laser to RCAV.

Then I measure the TF of the LIGO's VCO box.

The shape of the transfer functions agree well except a resonance peak around 100 Hz and a pole at 7 kHz.

 This TF can be used to calibrated the feedback signal to the VCO to real frequency noise between two cavities.