5 Conclusions
This paper reviews state-of-the-art single-mode DR cavity filters for wireless base stations. The cavity characteristics of three TM modes at 2 GHz are analyzed using HFSS. The design of DR cavity filter for high, medium, and low Q operation is compared with the design for TE01 DR cavity modes. TE01 mode is suitable for high Q (>12,000) applications, dielectric combline is suitable for medium Q (8000-12,000) applications, and TM010 mode is suitable for lower Q (<8000) applications. A variety of cross-coupling schemes for implementing symmetric and asymmetric transmission zeros are presented, and topology based on cascading canonical asymmetric building blocks is discussed.
Designs with excellent measured performance are presented, with 6 and 8-pole quasi-elliptic-function filters taken as examples. An 8-pole filter with two transmission zeros on each stop band is an example of how, by cascading asymmetric building blocks, symmetric transmission zeros can be better balanced. This kind of 8-pole filter is high Q (around 30,000), and represents state-of-the-art high Q dielectric material technology. A 5-pole DR TE01 mode canonical asymmetric filter is an example of how three low-side transmission zeros can be implemented by three non-adjacent coupling irises. For a DR TM010 mode cavity filter, three non-adjacent coupling irises can yield three high-side transmission zeros. The combining of a DR TM010 mode cavity filter with a 3-pole elliptic function filter highlights the progress that has been made in single mode DR cavity filters for wireless base stations.
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