Characteristics of an Ideal Filter

Ideal filters allow a specified frequency range of interest to pass through while attenuating a specified unwanted frequency range. The following filter classifications are based on the frequency range a filter passes or blocks:

The following figure shows the ideal frequency response of each of the preceding filter types.

In the previous figure, the filters exhibit the following behavior:

The frequency points fc, fc1, and fc2 specify the cut-off frequencies for the different filters. When designing filters, you must specify the cut-off frequencies.

The passband of the filter is the frequency range that passes through the filter. An ideal filter has a gain of one (0 dB) in the passband so the amplitude of the signal neither increases nor decreases. The stopband of the filter is the range of frequencies that the filter attenuates. The following figure shows the passband (PB) and the stopband (SB) for each filter type.

The filters in the previous figure have the following passband and stopband characteristics:

In practical applications, ideal filters are not realizable.