Compact WLAN dual-band filter using independent band stop resonators combined with diplexer

In this paper, a highly miniaturized dual-band band pass filter has firstly been developed by embedding all the passive lumped elements into a LTCC substrate for 2.4-2.5 GHz (802.11b) and 5.15-5.85 GHz (802.11a/g) WLAN applications. In order to reduce a size/volume of the filter and improve its performances, the proposed filter was newly designed by using J-inverter transformation technology, independent transmission zero technology and independent band-stop resonators. The J-inverter transformation technology was highly effective to minimize electromagnetic parasitic coupling between the large inductors. The newly applied independent band-stop resonator helped to reduce the number of passive elements in the filter circuit. Finally, designed dual-band filter was combined with simple LC diplexer. The measured maximum insertion losses of the fabricated dual-band WLAN filter were better than 2.3 and 1.3 dB in the lower and upper pass-bands, respectively. Return loss was higher than 15 dB. The overall size/volume of the fabricated filter module was 4.7 × 2.3 × 0.68 mm³.