After measuring few amplitude spectral densities of CPS and GS13 on the HAM-ISI, we were surprised by some narrow peaks on CPS between 10 and 100Hz (cf LHO_ISI_HAM10_ASD_m_CPS_GS13_L4C_Locked_vs_Unlocked_2011_12_05.pdf) and these peaks are less visible on GS13 spectra.  We were actually concerned by the grounding of the new shield installed on the CPS cables. We took measurements in different configurations to find the source of the peaks:

-          Fans ON vs Fans OFF: We only saw minor differences

-          We measured a spectra of a locked CPS using the jig  in several configurations:

o   Shield not connected to the ground

o   Shield connected to the ground

The two spectra (CPS_Noise_Investigation.pdf) were identical and without any features in the 10-100Hz bandwidth (Noise floor at 7e-10 m/sqrt Hz). It confirms that CPSs are not picking up electric noise but are actually seeing a real motion. Due to the passive isolation provided by the ISI above 1Hz, amplitudes of the narrow peaks (probably motors) are reduced (in the unlocked and the so-called locked configurations).