よくある質問(FAQ)

Why is an input filter necessary when digitizing a baseband signal?

Baseband sampling implies that the signal to be sampled lies in the first Nyquist zone. It is important to note that with no input filtering at the input of the ideal sampler, any frequency component (either signal or noise) that falls outside the Nyquist bandwidth in any Nyquist zone will be aliased back into the first Nyquist zone. For this reason, an antialiasing filter is used in almost all sampling ADC applications to remove these unwanted signals. In the diagram below the antialiasing filter passes signals from dc to fa while attenuating signals above fa.

The diagram also shows how full-scale frequency components above fs – fa are aliased back into the bandwidth dc to fa. These aliased components are indistinguishable from actual signals and therefore limit the dynamic range to the value on the diagram which is shown as DR. For more on this subject see MT-002