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Now with a useable error signal for the north path, I've been trying to track down the source of ringing in the north FFS control loop. Initially I though it was too much gain, but it turns out to be related to offset. With the current settings the fast monitor output is showing ringing at ~1 MHz. It turns out that this is because the offset is set to low, as it rails it set of this oscillation (pictured below) that persists until I reset the lock. Vpp of this oscillation is 1.08 V (min=-1.78 V, max=-0.680V). The upper rail (at 1.52 V) didn't have this oscillation.
I blocked the lasers, turned off the Marconi output and also added terminators to the test input on the RF stage (I don't know why this wasn't done before). Watching the voltage at the fast monitor output I adjusted the north offset until the fast voltage was zeroed. It was extremely sensitive at the zero crossing point. Even when I left the knob untouched it was flickering up and down. I tried the same thing on the south path with identical gain settings. The offset smoothly tuned from one rail to the other (only going into oscillations at the lower rail) and was stable. It didn't have any of the flickering level of the north path. To clarify the north path pretty much flipped from one rail to the other with very little adjustment, the south path was much less sensitive. I'm not sure what is going on here. Possibly a issue with grounding? Or something about the stages of gain has been altered in the north TTFFS box? Something is broken?
Maybe when I relock the loop the OLG will suppress this residual noise. But the sensitivity to offset knob tuning my mean that it will drift back to generating this 1 MHz oscillation, which is above the UGF of the loop. I think this is something that needs to be solved before a sensible beat note can be detected.
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In another unrelated problem I noticed that the ±24 V power supplies were flickering on the edge of current limit mode. We are pulling almost 1 amp @ 24V to run all the electronics in the rack. Seem surprisingly high. I turned up the current limit, the +24 V line is now giving full voltage as it should.
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