Compact High Isolation Base-Station Duplexer Using Triple-Mode Ceramic Cavities

A microwave duplexer with high isolation is presented. The device is based on triple mode filters that are built using silver plated ceramic cuboids. To create a six-pole, six-transmission zero filter in the DCS-1800 band, which is utilized in mobile communications, two cuboids are cascaded. To shift spurious harmonics, low dielectric caps are placed on the cuboid faces. These caps push the first cuboid spurious up in frequency by around 340 MHz compared to the uncapped cuboid, allowing a 700 MHz spurious free window. To verify the design, a DCS-1800 duplexer with 75 MHz wide bands is built. It achieves around 1 dB of insertion loss for both the receive (Rx) and transmit (Tx) ports with around 70 dB of mutual isolation within only 20 MHz band separation, using a volume of only 30 cm3.