IN3LBQ on the Hühnerspiel (Amthorspitze) - photo by IN3OTD

BPFILT example 2

Here is a brief comparison between the minimum loss, equiripple and maximally flat filter types. As in the previous example a bandpass filter for the LF band with 3 resonators, having a Q of 350, centered at 137.7 kHz and with a 26.3 kHz bandwidth at an attenuation of 60 dB was designed, this time with three bandpass response types.

The bpfilt input screen is:

    Enter Center Frequency [Hz]    : 137.7e03
    Enter Bandwidth [Hz]           : 26300
    Enter Stopband Attenuation [dB]: 60
    Enter Number of Resonators     : 3
    Enter Resonators Unloaded Q    : 350
    Enter Inductance L [H]         : 1.031e-03
    Enter Source Impedance [ohm]   : 50
    Enter Load Impedance [ohm]     : 50

Frequency responseHere is a graph with the frequency response of these three filters:

  1. Cohn
  2. Tchebyscheff
  3. Butterworth

where it can be seen that they have the same bandwidth at -60 dB.

Frequency responseHere is a graph with the close-in frequency response. As expected the Cohn type has the lowest losses, even though the Tchebyscheff is quite close and the Butterworth not much worse.
Actually the latter two filter types should be called here pseudo equi-ripple and pseudo maximally flat because, in presence of losses, they do not keep the passband response of their low-pass lossless prototype. There are also exact designs which take into account the effect of losses and yield an exact equi-ripple or maximally flat characteristic but require unsymmetrical structures and suffers from further losses, due to the bandpass shape constraint.