Ceramic transverse-electromagnetic-mode filter having a waveguide cavity mode frequency shifting void and method of tuning same

A method of tuning a dielectric ceramic block filter having four vertical edges comprising the steps of:

• removing metallization material from an edge defined by a top surface of the filter block and a resonator through-hole;
• repeating for each resonator until a desired transverse-electromagnetic-mode passband is achieved;
• removing metallization material from a side surface substantially in proximity to at least one of the four vertical edges of the block to provide a waveguide cavity mode frequency shifting void;
• checking a frequency response curve of the filter to confirm that the introduction of the waveguide cavity mode frequency shifting lowers the frequency of a set of parasitic passbands by a predetermined frequency while simultaneously maintaining a desired transverse-electromagnetic-mode passband;
• and
• enlarging the size of the waveguide cavity mode frequency shifting void until all unwanted parasitic passbands are shifted to meet a predetermined filter specification.

A ceramic duplex filter, comprising:

• a filter body comprising a block of dielectric material and having top, bottom and four side surfaces including vertical edges, and having a plurality of metallized through-holes extending from the top to the bottom surfaces defining transverse-electromagnetic-mode resonators, the surfaces being substantially covered with a conductive material defining a metallized layer, with the exception that the top surface is substantially uncoated, and with an additional exception that at least one vertical portion in proximity to the vertical edges of the block on at least one of the side surfaces is unmetallized defining a waveguide cavity mode frequency shifting void;
• the waveguide cavity mode frequency shifting void extending substantially vertically in proximity to the vertical edges of the block and with a substantially uniform width and extending a distance to shift a set of parasitic spurious responses in the filter frequency response curve to a lower frequency while simultaneously maintaining a desired transverse-electromagnetic-mode passband;
• and
• first and second and third input-output pads comprising an area of conductive material on at least one of the side surfaces and substantially surrounded by an uncoated area of the dielectric material.