Practical Filter Design - Part 7

PRACTICAL FILTER DESIGN - PART 7 The major advantage of Bessel sections is their phase behaviour, which is more linear than that of any other type of filter - see Fig. 38. If the Bessel amplitude char- acteristic is projected on a linear scale, it is a straight descending line. It is only be- cause of its usual projection on a logarith- mic scale that the characteristic looks like a typical filter skirt. The transfer charac- teristic of a Bessel filter is, therefore, mod- erate. A roll-off of 6n dB per octave is not attainable: the curve is particularly poor around the cut-off frequency and this is not dependent on n (n is the order of the filter). Bessel tables The tables giving the data for the com- putation of Bessel filters are compiled si- milarly to those for Butterworth sections, described in detail last month, and they should therefore not present any problems. Again, the component values are given for a cut-off frequency of 1 Hz. Table 6 gives the pole locations of Bessel...