When there is no vertical deflection, the beam in an oscilloscope should trace a horizontal line that is parallel with the horizontal lines of the CRT graticule.
The magnetic field present in the environment of the CRT can cause the trace to be twisted.
There are two schemes for correcting this misalignment:
- Mechanical trace rotation: The CRT is rotated a few degrees in its mounting clamps. This only works if the CRT has an external graticule. The 545A uses this technique.
- Electromagnetic trace rotation: Current is passed through a coil near CRT, creating a magnetic field that counterbalances the magnetic field in the environment. The 647 and most more recent Tek scopes uses this technique.