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A high stopband-rejection LTCC filter with multiple transmission zeros

A low-temperature-cofired ceramic (LTCC) bandpass filter with high stopband rejection is presented for multistandard coexisted wireless communication applications, such as the integrated wireless local area network (WLAN)/wavelength code-division multiple access handset, the dual-band triple-mode WLANs, and the global system for mobile communications/global positioning system receivers. By improving the filter cell structure, 2n transmission zeros can be generated to achieve wide-band suppression from cascading n filter cells. The presented method provides the design flexibility of locating these transmission zeros distributed in the lower and upper stopbands. An LTCC bandpass filter with four transmission zeros has been implemented for experimental demonstration. The measured insertion loss is less than 1.5 dB at 2500 MHz, and four transmission zeros are obtained at 1.64, 1.88, 4.36, and 5.32 GHz, respectively. These result in 48-59 dB for lower stopband suppression and 38-55 dB for higher stopband reduction. This paper demonstrates that the proposed filter is extremely suitable for the multiband RF transceivers where the cross-band interference must be adequately reduced.