LTCC Wideband Filter Design With Selectivity Enhancement

A new design scheme for the wideband filters with the low temperature co-fired ceramic (LTCC) technology is proposed in this letter. Based on the traditional second-order combline filter, an extra shunt capacitor is added in the middle of the coupled line, and a new transmission zero is thus obtained. The proposed design not only possesses excellent selectivity at both sides of the passband, the fractional bandwidth can also be as wide as 86%. To validate the design scheme, the proposed technique is applied to the third-order combline filter and realized by using the LTCC technology. The wide-band filter with passband of 3.1-5.1 GHz is obtained. With four transmission-zeros, the selectivity and out-band rejection of the third-order filter is further improved from those of the second-order designs.