Removing line noise

Problem. A sharp peak at 50 or 60 Hz and its multiples dominates the spectrum.

The pipeline

1. Notch — freq 50 or 60, harmonics 3, width 1 Hz.
2. Look at the spectrum again before doing anything else.

What to watch

Do not reach for a band-stop that is wider than it needs to be. Line noise is narrow; a wide notch takes real signal with it, and at 50 Hz that means the lower gamma band.

If the peak persists after notching, the cause is usually a bad ground or a single misbehaving channel rather than the mains. Look at channels individually.