Ultra-compact dual-band WLAN filter using independent band stop resonators

In this paper, a highly miniaturized dual-band band pass filter using independent band stop resonators has been newly designed and fabricated by embedding all the passive lumped elements into a low-temperature co-fired ceramic (LTCC) substrate for 2.4 to 2.5 GHz (802.11b) and 5.15 to 5.85 GHz (802.11a/g) wireless LAN system applications. In order to reduce a size/volume of the filter and avoid electromagnetic parasitic couplings between the embedded passive elements, the proposed filter was designed by using 3^rd order Chebyshev circuit topology and J-inverter transformation technology. The 3^rd order Chebyshev band pass filter was firstly realized for 802.11b frequency band selection and it was transformed by using finite transmission zeros technology to minimize the electromagnetic coupling. Finally a dual-band filter was realized by constructing 802.11a/g WLAN band selection filter by adding independent band stop resonators into the 802.11b filter circuit. The measured maximum insertion losses in the lower and higher pass-bands were better than 2.0 dB and 1.3 dB with minimum return losses of 15 dB and 14 dB, respectively. The overall size/volume of the fabricated filter was 2.7 × 2.3 × 0.68 mm³ which was the smallest one in the previously reported devices.