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TECHNICAL REFERENCE MANUAL
6.1.3.3. Aliasing-free Bandwidth
In the previous chapters we have learned that an ideal filter with infinitely sharp filter characteristics is
not possible in real-world applications.
The frequency response in the image below “Bandwidth vs. Aliasing-free Bandwidth” shows the
bandwidth (-3 dB point) at about half of the sampling rate (Nyquist criteria). All frequencies higher than
the Nyquist frequency will be folded back. But the frequencies above the Filter Stop-band are already
attenuated so much, that they are negligible. Thus, the only guarantee that can be made, is that all
frequencies in the aliasing free bandwidth are correct.
Bandwidth vs. Aliasing-free bandwidth
The illustration oben shows the filter characteristics relative to the sample rate. To make it easier to
understand, let's make an example with some concrete values:
Example
Let's assume a sampling rate of 1000 S/sec or 1000 Hz. The bandwidth of this low-pass filter is at
500Hz. All input frequencies between 500 Hz and 680 Hz (= 0.68 * 1000 Hz) can be folded back
to the 320 Hz to 500 Hz range. All input frequencies higher than 680 Hz are rejected by the filter
anyway and can be ignored. Thus, the aliasing free bandwidth is 320 Hz (since the range of
320Hz and 500 Hz can already be due to the aliasing effect).
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