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Design of Dual Notch Filter from UWB Bandpass Filter to Reject 5 GHz Narrowband WLAN Service

This paper presents a dual notch filter from UltraWideband (UWB) bandpass filter for the purpose of avoiding the interference between the UWB radio system and the existing WLAN system. The 5 GHz WLAN (IEEE 802.11a) exists in 5.15-5.35 GHz and 5.725-5.825 GHz which are called as lower and upper band respectively. The filter is based on ring resonator with interdigital coupling at both sides. The interdigital coupling is used instead of direct coupling for the reason of strong coupling and to introduce capacitance effect to obtain flat passband. The notched band can be designed to some specific frequency band(s) by introducing four open stubs at the coupling arms nearby the ring geometry. Design of the UWB bandpass and notch filters are proposed and group delay has also been obtained for both filters in this paper. As the simulated results, for bandpass filter, the insertion and return loss are about -1.3 dB and -45 dB respectively over the passband of 5.56 GHz. For notch filter, the dual notch is obtained at 5.1 GHz with -20 dB and 5.9 GHz with -9 dB. The overall size of the designed filter is 6×39.2×1.6 mm3. The proposed filters have also shown good outband performance that is better than Federal Communications Commission’s (FCC) mask.