I checked a 3-hour block of time after the work described in 72241, and it appears that the 1.6611 Hz comb was successfully removed. I've attached pre (Aug 15 10:00 UTC) vs post (Aug 16 06:00 UTC) plots, each with an averaging time of 3 hours for direct comparison. The 1.6611 Hz comb is the structure around 280 Hz marked with yellow triangles on the first plot, and absent on the second plot. There are also some untagged lines belonging to the comb around 180 Hz, which also disappear. Note that the small line still present around 269.7 Hz is not part of the comb, and the blue squares are an unrelated comb at 4.98 Hz.
Tagging ISC and CDS. Nice work Keita and Ansel!
We can leave the temporary setup for a while (it's jury-rigged to use DC power supply). But the question is what to do next.
E2100049 shows that the Beckhoff voltage output comes out of EL4134 and is supposed to be directly connected to the positive and negative input of the driver chassis (pin 6 and pin 19 of the DB25, respectively, see D2000212).
EL 4134 is a 4-channel DAC module and its output is single ended ("O" terminal and GND terminal). If you bother to download the catalog from Beckhoff web page, it turns out that all GND terminals are connected together inside. These GND terminals are not connected to GND terminals of neighboring Beckhoff modules, Beckhoff power ground nor Beckhoff chassis (I checked this with Marc in the shop). It seems as if the Beckhoff GND output is floating relative to everything else.
I don't know why there's a 1.66Hz comb between the Beckhoff GND terminal and the driver ground (pin 13 on DB25) but maybe we can connect them together? (Unfortunately E2100049 doesn't show which Beckhoff terminal is connected to which pin on the driver chassis. I assume that the GND terminal goes to negative input (pin 19) but not sure. We have to make sure whch is GND before making any connection.) However, if we do that, probably we don't want to repeat that for the second T-SAMS in the future assuming that the second DAC output in the same EL4134 module will be used, or we'll be making a ground loop.
Anyway, if the noise comes back by doing that, we could add an adjustable resistive divider using a trim pot inside the driver chassis to supply necessary voltage as a kind of inconvenient mid-term solution. We could even try to connect the Beckhoff cable back to the driver chassis to regain readback after disconnecting the DAC output inside the Beckhoff chassis.
I'll be gone for a month and cannot do these things, so it's up to Daniel.
Drawings and wiring tables for this Beckhoff chassis can be found here E1200377.
We should also check that the noise isn't propgated through the shield of the wire.