Filiberto, Sheila, Richard,  Robert

Improved grounding of the ISC Common chasis at EX was shown to reduce coupling to DARM (LLO-65907 and comments). To better understand how close the ground noise was to the DARM floor, I made a ground injection by driving current between the ISC common chassis and a good building ground (see the spectral comb in Figure 1). The coupling of these ground injections to DARM predicted the height of the 95ish peak in DARM from the ESD power monitor before the grounding improvements. The reduction in the coupling of my injections into DARM was, with just my grounding clips, about a factor of 3, and after the new grounding straps that Fil added at the end of last week, was about a factor of 5.3. With this improvement, based on my injections, I would expect the 95 Hz peak to be at about 0.3 of DARM. The 2000 average, 0.2 Hz BW coherence in Figure 2 suggests that the 95ish Hz peak and the noise at other frequencies might now be 0.1 or less of DARM (see Figure 2). I don’t understand the source of the 0.3 vs. 0.1 discrepancy so the story may be more complex. Figure 2 shows, in addition, photos of the grounding straps that made this latest improvement.

Figure 1 shows that when Sheila reduced the ETMX ESD bias by a factor of about two we got a reduction in my injected 11 Hz comb that we think was consistent with a linear relation between bias and coupling.