Multichannel Systems MC_Rack Manual - Manual (Page 142)

MC_Rack Manual
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Signal distortion
Though filters present an easy and convenient way to remove unwanted signals from your data,
filters should be used with care. All filters (analog and digital) distort signals, especially if the
signal frequencies lie near the cutoff frequency. You should be aware of the fact that the signal
amplitude may be modified by a filter, and that the signal may be shifted as well.
In this screen shot, you see regular signals (from a signal generator) with a frequency of about
70 Hz (green traces). The cutoff frequency of the low pass filter has been set to 200 Hz. A low
pass filter removes high frequency components of signals. You can clearly see that the amplitude
of the filtered data (magenta) is decreased and the phase of the signal has been shifted slightly.
The distortion increases with increasing signal frequency. If the frequency is set near the cutoff
frequency, the shape of the signal has changed to a sinus waveform.
If the frequency is increased far beyond the cutoff frequency, for example 1 kHz, the signals
are almost completely removed (which is, of course, the purpose of a filter).