What is Dual-mode Filter?

Hi,
I am planning to design some microstrip filters and I have observed lot of papers on Dual-mode and Triple-mode filters.

I know in a waveguide, there are like TE and TM modes. But what do they mean by "modes" for a planar filter?
And what are the applications of these dual/triple-mode filters?

Thanks,
sv.

Dual mode microstrip filter have two passband, such as one passband is 800~900MHz, and the other is 1800~1900Mhz. Obviously this dual mode has advantage in such as GSM etc.

Dual mode refers to the degenerated mode in a cavity, which has the same resonant frequency, such as HEM11n mode. When perturbation was introduced, the degenerated mode will be split into two modes with different resonant frequencies. By using dual mode design, one can realize a two pole filter with only one (physical) resonator, and realize a four-pole filter with two resonators. Another advantage for dual-mode design is the easiness to introduce cross coupling to realize elliptic or quasi elliptic filters. However, tuning dual mode filter is much more difficult than tuning single mode filter, (except for 2-pole filter), especially when the number of poles become big (e.g. 8 poles). Triple mode filter is yet find another mode with similar resonant frequency for the same resonator, and couple them to realize a 3-pole filter.

Filters with two passbands are referred as dual-band filters.

Hi tony_lth,

I am pretty sure that dual-mode filter doesnot have two passbands. Dual-mode filter is different from Dual-band filter. The example that you mentioned is a Dual-band filter.

Hi Phytech,
Thanks a lot for the explanation.

-sv