How can I get -60dB in a low pass filter design?

[Scholl]

filter design help!!

a low pass filter,f0=2695MHz,rejection:
3800-3999MHz -25db min
4000-5000MHz -40db min
5000-14999MHz -60db min
15000-20000MHz -20 min
I used coaxial structure and synthesized and optimized it with senerade.
But -60dB seemed impossible to me.How can I get it?Please help me!

 

[borich_03]

Re: filter design help!!

You can use two filters. If you connect their in series.

 

[flyhigh]

Re: filter design help!!

I think you should use some other technology then coaxial on these frequencies (microstrip, stripline etc...). Try to use eliptical or so-called cross-coupled resonator filter to get to -60dB attenuation. You should use some more sofisticated synthesis tools to do so, it is quite a theory to read.

 

[VSWR]

Re: filter design help!!

I had roughly the same filter requirement once. Solved by implementation in a narrow channel stripline, giving sufficient suppression (minor leakage through the filter).

 

[microwaveboy]

dear flyhigh:
Can you tell me some about cross coupled filter?

 

[Debeli]

Re: filter design help!!

Your problem can be parasitic pass band since you must have more of 5 fc free. I suggest 2 filters in series if your IL can take it.
Here is excellent article about cross-coupled resonators, hope it can help.

D.

 

[AL0117]

Re: filter design help!!

Microwave boy,

below is a link for an article about cross coupled filters

hxxp://www.shively.com/tb-cross-coupling.pdf
(x=t)

rgds, Al