fc, the center frequency,
BWp, the pass-band bandwidth and
BWs, the stop-band bandwidth, determine one of the five possible responses. For the Analog and IIR filters, the cut-off frequency (@
Ap or @
Asc) is given by
BWp, while for the FIRs,
ωc is given by the middle point of the transition width (e.g.
(BWp+BWs)/2 for LP).
With these in mind this is how the values of the three frequencies should be: for LP/HP
fc=0, for BP/BS
fc>0, while the ratio between
BWs determines whether it's a LP/HP, or a BP/BS. AP is a special case and only the all-pole Analog Active filters (Bessel, Butterworth and Chebyshev) and FIR_FS can achieve it. For that,
Satisfying the conditions is not enough, some care should be taken. E.g. if
BWs=2k it's a BP case, but the results will probably be unsatisfactory.
©Vlad, 2008 - 2015