Filter Basics

First, a reminder about basic filter characteristics. Figure 13.9 shows an ideal lowpass
filter. Below cutoff frequency, ωC, inputs are passed with no attenuation. Above ωC,
inputs are infinitely attenuated. The transition from passband to stopband is infinitely
narrow.
An ideal lowpass filter is impossible to obtain. In practice, unwanted products will
occur a distance from the filter passband (harmonics, etc.), and so an infinitely narrow
transition band is not required. A more practical specification will have a finite width
transition band with tolerances and achievable limits.