Cascaded Feedthrough Capacitors

Feedthrough capacitors built of a multiplicity of stacked disks are practically useless filter elements because they behave essentially like tubular feedthrough capacitors, namely exhibiting pronounced parallel resonances. However, if washers of suitable, lossy ferrite are interspaced in the structure, the effective coupling impedance for the lower frequencies is identical with the 1/ωC, whereas at medium and higher frequencies the effective coupling impedance is reduced by a factor of 1,000 to 10,000 or more, as compared with the best discoidal feedthrough capacitors available now. In fact, the effective coupling impedance is so low that resonances of the disks (even large ones) themselves become negligible. The advantage of the discussed structures is the absence of dc losses. The mathematical theory for all cases above is developed by matrix algebra and verified by experiments. The selection of the correct ferrite is analyzed. The practical significance of this development for extremely effective miniaturized low-pass filters is presented.