On 2021 July 26 JST, we reported this on Slack but forgot to put it here. A strange bamboo shoot! This is a rare event but, once occurred, it continued for ~ hours to be calm down. (Someone please check in a summary page?)
Left top panel in Fig1 shows the comparison between nominal noise level of PLL-X and one in excited time.
There is more than 10 times excess from nominal level around 300Hz.
On the other hand, environmental noise didn't change in time (see fig bottom left and right panels)
and there is no coherence except higher harmonics of power line at 60Hz, 180Hz and 300Hz.
As I checked the summary page, this strange behavior started from June 24 2021 (Sat.) around 15:00 JST. But on k-log, there is no related work on that day.
I attach the example on June 26. I could not find the coincident event in other conponents (PMC, IMC, PEM sensors) on summary page.
