[HELP]Meet Problems in designing Microwave filter

understanding hairpin topology filters

Hello guys,

I am supposed to design several filters for one project. The only specification given is the frequency band. (Others will be decided according to the design.)
1. 0.80 to 0.96GHz
2. 1.565 to 2.315GHz
3. 2.55 to 3.8GHz
4. 3.95 to 6.45GHz

Anyone can give me some hint to design them? I have tried to design with microstrip coupled line in ADS and momentum. They more or less met the spec. But my supervisor said my design were too "cut-and-dry" or ponderous. Anybody know some more popular or novel design of microwave filter in industry or academy suitable for my case?

Thanks in advance,
alard

For your frequency range, i feel if using microstrip, your filter may be a bit too big... I will prefer to use lump element, but since you are requesting microstrip...
How about try some of the following topology... Hairpin, interdigital?
There are more compact than edge-couple kinda filter...

Thank you for your quick reply. Actually I have tried other topologies as well. But they didn't performed as good as edge-couple filter. Maybe you have seen that the transition of the filter is quite sharp compared to the bandwidth, especially for filter 3 and 4.
Do you think I need to look for some ideas in IEEE explore(The papers really sucks) or you think let me try again hairpin, interdigital kinda filter?

Personally, i also dun really like read paper becos i am lazy (Honest ans)... but is still good to read some papers to give u an idea what are the type of topolopies that are around and give u more options to choose from.

Before you go into other topologies like interdigital or hairpin, perhap u like to ask ur supervisor what does he want. I see that your filters meet the specification more or less, but your supervisor doesn't seem to be impress with it, and likely, he expecting something more than that unless he is the kinda that want you to think think think through so that you will appreciate and learn. It better to communicate to him first.

Thank you very much JOhn, you really helped me a lot.
I think my supervisor want me to find some novel and compact designs, not just pick a traditional one from the BOOKS. Then I will have to compare different topologies in my final report.
I do find some nice designs in the papers. But the problem is that the writers usually come out a new idea and only realize it in the specific frequency band. It is very hard to summarize a method from such kinda paper to apply it to my case. What can I do?

You lack several important filter specification parameters to judge what topology to use:

- lower and upper stopband rejection
- passband amplitude ripple
- group delay requirements
- insertion loss
- input/output VSWR
- mechanical requirements (max size, interfaces/connections, etc)

Thank VSWR for your remind. Actually till now there are no such kinda requirements in the Spec. Then I just set a goal by myself for example the lower and upper stopband rejection better then -15dB and realize it as good as I can. The specific goals I have set are:

- lower and upper stopband rejection (-15dB)
- passband amplitude ripple (-0.1dB)
- group delay requirements (TBD)
- insertion loss (1dB)
- input/output VSWR (matched to 50 Ohms)
- mechanical requirements (max size, interfaces/connections, etc) (the length should be less than 15mm or even smaller, SMA)

Waiting for your further instruction.

Hi, alard,

I have a hobby in filter design as well. Personally I would suggest a well good book to read which I believe has a comprehensive explanation of filter design. It's Microstrip Filter Design for RF/Microwave Applications by Lancaster and Jia Shenghong. Inside it explains various common filter topologies as well as basic foundamentals useful for filter design. You could focus on Chapter 5 and 8 and 10.

I have designed hairpin with 20% FBW. For your case, I don't know whether it can match 40% FBW. Stub filter definitely can, but too large.

You could try on multilayer coupled filters too increase its coupling, for this, you might need to understand how resonator works and how to couple the resonators to get required FBW.

regards

JY

Hello guys,

Really thank you for your warm help.

I want to design the filters with good electric performance and compact(the largest one 10cm at most). In the last several weeks, I am quite interested in the Microstrip SIR(Stepped Impedance Resonantor) filters with tapped input/output because it has the advantage of higher second passband than the conventional couple line filter(mostly more than 3f0) and then I will have the possibilty to save a low-pass filter which is used to filter out the spurious response. And I will bend the structure to make it more compact.

But the problem is that the S11 is very bad when I am simulating the first filter(fc=880MHz). And when I am trying the third filter(2.55 to 3.8GHz ), the requirement of bandwidth can not be reached even with 7 orders. Is there someone who can tell me how to improve it or some new idea about design my filters.

Best regards,
kevin

the first filter :0.80 to 0.96GHz .
You can try it with lump elements on microstrip or use lump element to improve its parameters.
the higher freq filter I thik you can try to use stripline to design for High Q, for microstrip Q is so poor

Hello folks,

I am back again.

Thank all of you for your discussions and suggestions, which makes me have a deeper understanding of designing a good filter.

Recently I started to design the last BPF 3.95 to 6.45GHz (48.1%). I have the specifications as follows:
-3.875 GHz, S21<-25dB
3.95-6.45 GHz, S21>-1.5dB
6.525- GHz, S21<-25dB
Inside passband, S11<-15dB

I tried a lot of methods from the papers but thought it was quite unrealizable because of the wide-band and sharp-transition. But today my supervisor showed me a filter they bought from BSC , which is packed in a black metal box with a dimentions around 8cm*3cm*1cm(L*W*H). And the measurement results were really impressive.

-3.875 GHz, S21<-48dB
3.95-6.45 GHz, S21>-1dB
6.525- GHz, S21<-52dB
Inside passband, S11<-14.8dB

Particularly the transition is almost a vertical line.... I almost cannot believe my eyes. Could any of you please spare me some minutes to make a guess of what kind of technology or topology they propably used (Till now I have only tried microstrip)? Thanks in advance.

Best wishes,

kevin

Wow, It is really impressive to see a filter like your supervisor showed to you. As your request specs, the filter will be really really sharp at stopband.
Maybe you can try interdigital filter and see if it matches your requirements. As I know, they have good performance to sharper the stopband.

BTW, why don't you just open that black metal box and see what kind of filter is inside

Best regards
K