Triple-Mode Ceramic Cavity Filters With Wide Spurious-Free Performance

A triple-mode silver-plated ceramic cavity filter with wide spurious-free response is presented. It uses single mode ceramic slabs as interfacing resonators with the cube to give wide spurious-free performance up to 4 GHz for an 1800-MHz base station filter. Three square apertures at the slab-cube interface give simple and flexible control of transmission zero placement while maintaining low insertion loss. The parallel nature of the slab-to-cube coupling allows a simple cuboid structure without chamfers or intermode coupling elements. A seven-pole filter in the DCS-1800 transmit band is built using a slab-slab-cubeslab-slab structure and measured. Along with wide spuriousfree performance, the filter achieves around 1 dB of band edge insertion loss with more than 50 dB of attenuation, 20 MHz from the band edges, and from an 11-cm(3) package. The near band performance shows all of the seven poles in the return loss. The insertion loss is 1.1 dB on the low side (1805 MHz), 1.0 dB on the high side (1880 MHz), and 0.6 dB at the band center. The wideband response shows more than 40 dB of attenuation up to the frequency 3.7 GHz.