J. Kissel, D. Barker, E. von Ries, R. McCarthy

Continuing investigations of the OMC0 DuoTone coupling to the ADC0 card (see LHO:77579 and LHO:78218), Erik installed some software this morning (LHO:78237) giving us the ability to turn off the DuoTone signal in the h1omc0 IO chassis in two different ways:
    (1) Change the output of the Timing Card from the standard 960 and 961 Hz DuoTone signal to a constant 2.5 [V].
    (2) Triggering the ADC DuoTone relay in the backplane interface adapter card for the low-noise 524 kHz ADC (D2100491) to turn OFF the DuoTone input to the last channel (H1:IOP-OMC0_MADCO_TP_CH31, or the 32nd channel)

The first attachment is a screen grab of the h1omc0 page of the recently updated as-built drawings of the IO chassis D1301004. This serves as a visual aide for where things are.

As such, I measured the same differentially-shorted channel I had in LHO:78218, H1:IOP-OMC0_MADCO_TP_CH17, in the following three different configurations because (again according to LHO:78218) it sees the DuoTone signal just as loudly as H1:IOP-OMC0_MADCO_TP_CHCH0, one of the channels that informs the average of four, the OMC0-DCPD_A0 gravitational wave channel. This way, we're not confused by the OMC DCPD signal chains' electronics noise.
    (a) BLACK The nominal configuration, with the standard DuoTone signal coming out of the timing card, and the interface card relay set to "ON" allowing the DuoTone into CH31.
    (b) MAGENTA Switching the timing card out to a constant 2.5 V, but leaving the interface card relay set to "ON," and
    (c) RED The standard DuoTone signal coming out of the timing card, but the interface card relay set to "OFF."

Attached is the ASD comparing the three different configurations. 

Both "OFF" configurations show no DuoTone signal. This rules out three of the suspect coupling paths -- 
    (i) direct coupling of the timing card to the ADC channels via the Adneco mother board they both reside on or 
    (ii) the ribbon cable that connects the timing card to the interface card somehow radiating the DuoTone signals to the rest of the chain
    (iii) The backplane itself (D2000297) mixing the timing signal in with interface cards that it houses . 

This differentiates between which two things that LLO did in (LLO:71315) that mitigated the issue. Remember, with the LLO the arrangement of their non-segregated h1lsc0 IO chassis, they both 
    - moved the channels off the card *and* 
    - migrated the card off the same Adneco board as the timing card . 
Because we did NOT see and DuoTone on ACD0 CH17 in configuration (c), with the timing signal ON, but the relay piping it into ADC0 CH32, that means indirect coupling across the Adneco board is ruled out, and moving the monitor channel off the card that was their real solution.

That leaves the following suggested coupling sites:
    (iv) The flat, non-twisted pair ribbon cable that connects the interface card to the ADC (less likely, given that we've proven that the same type ribbon cable from the timing card to the backplane doesn't radiate)
    (v) On the ADC card itself, direct coupling from CH31 to all its other channels

Given that (v) seems most likely, the options forward that seem most promising are:
    (I) Reduce the amplitude of the DuoTone timing signal (hopefully easy software change)
    (II) Trust that the timing system hasn't failed us, and won't fail us again, and turn off the DuoTone signal all together (dubious)
    (III) Do as LLO has done, and move the DuoTone Monitor signal OFF of this ADC by installing another ADC card, and moving the low-noise ADC to another slot on the Adneco board (expensive)

My vote (if it's not clear from my parenthetical assessment of each) is (I) because it's "the easiest." However, the CW and Stochastic groups are in the business of integrating forever, so it'll merely reduce the problem for them rather than eliminate.
The discussion will continue ... 

I've left the system in nominal configuration (a), with the DuoTone ON at it's full 5 [V_pp] amplitude, and the backplane interface card relay ON, allowing that DuoTone signal to flow into ADC0's CH31