1. Strong wide peak, slow frequency sweep
We finally determined this is an issue with the MEG electronics warming up. It only occurs soon after system startup, and lasts only a few minutes. So always start the MEG electronics 5-10 minutes before doing an initial recording, typically the empty room noise collection.
It has to do with the electronics clock being sensitive to temperature, and interacting with environmental noise, likely the 60 Hz power line noise. It’s very noticeable as increased high frequency noise in many channels. On the spectrum, it is a wide peak with side lobes, that sweeps down, “bounces” at 0 Hz and then sweeps up until it disappears past our filter and sampling rate limits.
