About Combline Filter

Hi all,

When we design the combline filter in microstrip technology, we usually can get one transmission zero at the higher edge of passband. How does it occur? I read the "wide-bandwidth combline filter with high selectivity" (please see the attached file), the author thought it is caused by inductive coupling between adjacent and non-adjacent resonators.

If the combline filer in microstrip structure is magnetic-dominant coupling circuit so that it usually has a zero at higher band?

Also, when we realize the combline filter in LTCC process, it could have two zeros at both higher and lower bands (Please see the file 2 and 3). If the multilayer process could provide significant electric coupling so that it can satisfy the condition of elliptic-function filter (shunt parallel-resonance + series parallel resonance)?

Thank you

To produce a transmission zero on the higher side for an inductively coupled combline filter, one needs to introduce an inductive coupling between resonator 1 and 3, (or 1 and 5, 1 and 7, ...), which is referred as cross coupling. If the cross coupling in this case is capacitive, the transmission zero would be on the lower side.

To produce two transmission zeros, one on the higher side and one on the lower side, one needs to introduce a cross coupling between resonator 1 and 4 with different signs as between resonator 2 and 3. If the signs between 1-4 and 2-3 are the same, no transmission zero will be produced and the rejection will become worse, however, the group delay will become flatter.