R. Cavalieri, F. Nocera, P. Spinicelli

Brief recap on recent events and current status concerning our sideband generators a.k.a. LNFS (Spectradynamics' LNFS-100).

We have purchased a total of four identical 3-output synthesizers over the years (starting in 2011) and we have been using two of them at the same time to generate our 4 sideband frequencies for a very long time (the entire O4 and even before that), namely 6, 8, and 56 MHz (Synth1) and 22 MHz (Synth2).

Following Feb 28th blackout, one of the two used syntesizers (Synth1) failed and had to be swapped with a spare (https://logbook.virgo-gw.eu/virgo/?r=68788), let's call it Synth3. The failure consisted in a problem at the power on which has not been observed ever since during the following months of tests and monitoring in the Lab.

Starting on Jul 11th, Synth3 showed "additional noise" on both 6 and 56 MHz (https://logbook.virgo-gw.eu/virgo/?r=69359) and, for that reason, was in turn swapped back with Synth1, on Jul 15th. No tests in the lab have been performed on Synth3 since to verify and understand whether or not the problem was reproducible.

Synth1 too proved to be noisier than one would have liked (https://logbook.virgo-gw.eu/virgo/?r=69656) and therefore the idea of putting in a different one in its place popped up. This was what we set out to do yesterday.

First attempt: we tried Synth3 which worked just fine when in "Local Mode" (i.e., when not controlled via RS-232) but did not when in "Remote Mode", behavior never seen so far. 
Second attempt saw the swap of Synth3 with Synth4, the one we use in our Lab setup to test and characerize RF devices before intergation in Virgo, which failed miserably since the device did not even turn on.

The solution found to allow commissioning activities to go on has been to use Synth2 to generate 6, 8, and 56 and accept temporarily a slightly degraded performance (which has no impact on the ITF performance) on the 22 MHz, now generated with Synth1.

For completeness, it is useful to remind the original (AdV) 22 MHz sideband does not need to have the low phase-noise performance offered by the LNFS-100 and used to be generated with a different, garden variety DDS; we could revert to that at any time if required. As it often happens, at some point in the past we tried out the solution with the LNFS for the 22 "for a test" and we never went back.