Partially interdigitated combline filter

Ceramic combline filters have been the preferred choice for duplexing the transmitter and the receiver to the antenna in wireless portable applications. However, it is also the biggest component in today's radios. Therefore, there is great interest in reducing the size of these filters. Low cost is also a big driver. One incremental solution is to find a way to eliminate the external metal shield that, typically, accounts for 10% to 20% the filter volume and for 5% to 10% of the manufacturing cost. However, the external metal shield helps to reduce the unwanted parasitic field coupling between combline resonators at the open circuit end. These fields terminate on the ground plane provided by the shield. The solution presented is to interdigitate some of the resonators in the filter and thereby distribute the ground plane sufficiently to eliminate the shield without adversely affecting the stopband attenuation performance.