Question of Elliptic-function filter

[YCChiou]

Hello everyone:

When I designed the elliptic-like filter, I would sometimes meet higher sidelobe problem. The higher sidelobe usually occurs at the higher band close to upper zero. Even I generate an additional zero in the stopband, it's still not easy to be suppressed. Please see the attached file and kindly give your comments.

Thanks a lot.

 

[RCinFLA]

You don't get something for nothing. The more you pull in the zero toward the passband the more flyback you are going to get.

You can always cascade with a non-eliptical low pass or bandpass filter.

 

[FvM]

Did you design it as an asymmetric filter? I would expect a symmetric frequency characteristic for the basic elliptical filter.

 

[vfone]

From what you mention, to me looks like somewhere is a parasitic resonance.

 

[Phytech]

What kind of topology is your filter? If it is cross coupled, it seems to me you have too strong cross coupling, and you can reduce the cross coupling which will move the zero up. Without knowing your exact design, it is difficult to give you a good advice.