Re: Microwave Filter Design
Hello boy and all others.
I would like to raise some more points that may help.
1-No eliptical filters, why?
The coupling between lines can be normaly modeled as a capacitor that will just be an imitance converter in the modern filter design theory. With a single capacitor in series we can only realize transmission zeroes in the infinity. Because of this, filters based in coupled line structures normally cannot have finite transmission zeroes. Since the eliptical filters have finite transmission zeroes it is normaly not possible to realize them using coupled line structures. That is why you don't find examples of eliptical coupled line filters in the literature......
2-Other polinomial functions that have no finite transmission zeroes, like butherworth, chebishev, gaussian, etc. can be realized.
3-What analog filter to use to have the best group delay reponse?
The best filter concerning group delay response is always the eliptic filter. The reason is, because of its very abrupt transition from pass-band to stop-band (low pass example) all the poles are higher Q and are near the transition. This means that the group delay will be very much disturbed near the cut-off frequency comparing to other types of filters, but much less in the pass-band, why?
Now, assuming that we have all filters adjusted to satisfy our filter requirements (filter orders of each type of filter adjusted to satisfy the same specification) you will see that the eliptic filter (the lower order one) will have the worse group delay around the cut-off frequency, but it will have the best group delay performance in the pass-band comparing to all others (assuming the same filter specs). The eliptic filter is then the best choice concerning group delay in the pass-band. The chebyshev filter will have a similar pass-band group delay performance as the eliptic one.
4-In your case, for a microwave filter based in coupled line structures, the choice is then made. Chebyshev. You will just have to put the cut-off frequency far enough so that the pass-band group delay performance satisfies your requirements.
6-Another remark: If your process is not very precise, i. e., may have variations like , substrate thickness, epsylon, track width (due to under eaching for example), etc.. than I would sugest that you try a butherworh filter . It will be much more tolerant to process variations. Unfortunately you will have to put the cut-off frequency further than in the chebyshev case, etc.. etc..
Hope it helps.
Greetings to all
S.