[regenfreak]

Here are the results of two super-wide 330KHz ceramic filters in cascade with 12 pole-LC filters; a 5-pole 300kHz Cherbyshev and a 280kHz 7-pole Butterworth:

Attachment 1: set-up with two LA1222 IF amps
Attachment 2: S21 and S11 of the cascade
Attachment 3: comparison of the group delay for the 7-pole Butterworth (280kHz) and 5-pole (300kHz) Cherbyshev.

I have had lots of fun tuning 12-pole LC filters. The flatness Butterworth of the 7-pole Butterworth is not really significantly better than the 5-pole Cherbyshev.

At the moment, I am building the group delay equalizer (attachment 4) taken from the Sansui TU517. It is based on the idea that the typical group delay of ceramic filters have double humps like the back of a camel and a one-pole LC resonator has group delay like a bell shape. By adjusting the value of the shunt damping resistor for the LC resonator, it will smooth out the gap between the two humps of the group delay curve. The closest analogy I can think of is the Area Rule in the design of transonic and supesonic fighter jets.