CAMBRIDGE CED 1902 - Use of the Notch Filter

Details of operation
43
Clamping before stimulus
The 1902 incorporates a mains-frequency notch filter, set at
time of manufacture to be either 50 Hz or 60 Hz, depending on
the country of destination.
It is important to note that a notch filter is a diagnostic tool for
use during the set-up phase of laboratory work, and is not
intended for use during the main recording. If you suspect
mains hum is corrupting your signals, you should compare the
amplified signal with and without the notch filter in circuit, and
then make adjustments to the electrode cable routing,
positioning of the subject relative to overhead lighting etc. The
aim would be to minimize the waveform differences with and
without the notch filter in place.
Note also that interference from sources such as fluorescent
lights can be rich in harmonics of the mains frequency, and a
notch filter will only remove the fundamental component. This
can leave components at 150 Hz, 250 Hz etc., and hence the
need for reduction of interference coupling at source by making
improvements to the cable routing and the positioning of the
subject.
Use of the
notch filter
Mains harmonics