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RF and microwave microstrip BPFs with high selectivity, low insertion loss, suppression of spurious sideband and wide stopband are widely used in recent wireless communication systems.¹ In 2002, the IEEE extended the 802.11b standard in the 2.4 to 2.4835 GHz frequency range; the fast development of WLAN communications has also made high performance essential for RF BPFs.² However, many microstrip planar BPFs have spurious resonant frequencies, which may be close to the operating frequencies, with severe influence on the system.³ Although quarter-wavelength resonator filters have their first spurious passband at 3ƒ₀, where ƒ₀ is the center frequency, they require short circuit connections with via holes, which creates a parasitic effect difficult to cancel.⁴ BPFs using low temperature co-fired ceramic and stepped impedance resonators are able to control the spurious responses, but they can only be implemented in certain filtering configurations with high insertion loss.^5,6 The open loop resonator filter has a pair of attenuation poles at finite frequencies, making it a viable intermediate between the Chebyshev and elliptic-function filters.^7,8 Defected ground structures (DGS) have been applied to improve the spurious response of the microstrip open loop resonator BPF; it also results in some problems such as high insertion loss and electromagnetic compatibility interference.⁹

In this article, a 0° feed open loop resonator WLAN BPF is presented, using a spurline and quarter-wavelength open-circuited stubs to improve the out of band response. The use of the spurline and quarter-wavelength open-circuited stubs realizes a wide range of spurious response suppression without changing the passband performance. By modifying the dimensions of spurline and quarter-wavelength open-circuited stubs, different stopband frequencies are obtained. The proposed 0° feed open loop resonator BPF shows stopband attenuation losses better than 20 dB from 2.6 to 10 GHz and insertion loss as low as 1.05 dB. This BPF is characterized by two attenuation poles, low insertion loss and high out of band rejection.

Spurline and Quarter-Wavelength Open-Circuited Resonators
Figure 1 shows the configuration of conventional asymmetric spurlines, which are etched on the microstrip transmission line.^10,11 This is different from the DGS, which is located on the ground side. The spurline is etched as an inverted "L" shape slot on the microstrip transmission line. Generally, the spurline dimension parameters include slot width s, slot length l and slot height h, where the slot gap provides a capacitive effect while the narrow microstrip line exhibits an inductive effect.^12,13