Prototype Filter - Low-pass Prototype

Low-pass Prototype

The prototype is most often a low-pass filter with a 3dB corner frequency of angular frequency ω_c' = 1 rad/s. Occasionally, frequency f' ' = 1 Hz is used instead of ω_c' = 1. Likewise, the nominal or characteristic impedance of the filter is set to R ' = 1 Ω.

In principle, any non-zero frequency point on the filter response could be used as a reference for the prototype design. For example, for filters with ripple in the passband the corner frequency is usually defined as the highest frequency at maximum ripple rather than 3dB. Another case is in image parameter filters (an older design method than the more modern network synthesis filters) which use the cut-off frequency rather than the 3dB point since cut-off is a well defined point in this types of filter.

The prototype filter can only be used to produce other filters of the same class and order. For instance, a fifth order Bessel filter prototype can be converted into any other fifth order Bessel filter, but it cannot be transformed into a third order Bessel filter or a fifth order Tchebyscheff filter.