Miniature low-pass EMI filters

This paper describes miniature ceramic feedthru type electromagnetic interference (EMI) filters. Electrically, the devices function as low-pass filters with a 3-dB cutoff frequency in the low-megahertz range. The filters are mechanically rugged and in many applications can be placed in the I/O connector or substituted for the existing feedthrus. The monolithic distributed filter covered in the paper behaves as a section of transmission line at all frequencies and is essentially free of undesired resonances. The theory of lossy transmission line is used to show that with a distributed dissipative filter, the effect of terminating impedances is dramatically reduced. The effect of operating environment on the electrical characteristics is also presented.