Ideal Filters

Filters alter or remove unwanted frequencies. Depending on the frequency range that they either pass or attenuate (reject), they can be classified into the following types:

•A lowpass filter passes low frequencies but attenuates high frequencies.

•A highpass filter passes high frequencies but attenuates low frequencies.

•A bandpass filter passes a certain band of frequencies.

•A bandstop filter attenuates a certain band of frequencies.

The ideal frequency response of these filters is shown in Figure 16-1.

Figure 16-1. Ideal Frequency Response

The lowpass filter passes all frequencies below fc, whereas the highpass filter passes all frequencies above fc. The bandpass filter passes all frequencies between fc1 and fc2, whereas the bandstop filter attenuates all frequencies between fc1 and fc2. The frequency points fc, fc1 and fc2 are known as the cut-off frequencies of the filter. When designing filters, you need to specify these cut-off frequencies.