Optimizing High-Rejection LTCC Filter Performance in Co-Planar Waveguide Implementations

Mini-Circuits’ BFHK-series of high-rejection LTCC filters have been characterized with stopband rejection floors on the order of 90 dB and higher with a combination of size, reliability, and cost currently unmatched by other filter technologies with comparable performance. Performance in the customer’s system, however, can vary depending on the specific implementation. The unique design of these devices features coaxial input and output pins on the bottom surface of the structure requiring blind vias to the conductive layer of a stripline circuit board. While many PCB manufacturers have developed the capability to build surface-mount assemblies with blind vias reliably, some designers still prefer to use coplanar waveguide (CPW) wiring boards where the contact between the conductive trace and the device ports is exposed on the top layer.

This paper describes the physical differences between stripline and coplanar waveguide implementations of Mini-Circuits’ high-rejection LTCC filters and related effects on performance. Channelization is proposed as an effective technique to achieve comparable performance to stripline implementations in CPW environments. Real test data from a leading customer’s evaluation of the new BFHKI series of these filters on interposer boards in a channelized housing is
then presented as proof of concept.