Design of a UWB Low Insertion Loss Bandpass Filter with Spurious Response Suppression

Fig. 1 Dual-mode, wideband bandpass filter.

Recently, more attention has been paid to applications of ultrawideband (UWB) technology on wireless communication. It offers advantages of decreased cost and increased capabilities, compared with other conventional radio technologies. Standards activities for UWB systems are promoting a global perspective for not only technology but also regulations. UWB has the characteristics of low cost, bulk data transmission rate and very low power consumption that make it attractive in local area networks, position location and tracking, and radar systems. Planar bandpass filters, with a wide bandwidth of 3.1 to 10.6 GHz, are highly suitable for integration of UWB front-ends. A planar bandpass filter, based on a microstrip structure, can provide the advantages of easy design, lower fabrication cost and compact size, and has been widely used. In addition, a dual-mode resonator is often built into the microstrip related bandpass filter design to improve the passband and rejection. Planar dual-mode filters have been introduced, which offered significant size, weight and cost advantages over cavity and dielectric resonator designs.1 Several authors, dealing with the advances of novel materials and fabrication technologies,2–5 have discussed the filtering characteristics of different configurations. These types of filters were ideally suited for narrow bands. A new generation of microwave dual-mode filters, using square ring resonators with enhanced L-shaped coupling arms, was later discussed.6 Advantages of low insertion loss, sharp rejection band and wide passband were obtained. Subsequently, the area of the overall filter structure was reduced and a sharper passband was provided. However, the presence of spurious harmonics was a fundamental drawback.7 A number of improved structures, such as an I/O taping line, defected ground structure (DGS), split ring resonator (SRR) and photonic band-gap (PBG), have proven to be effective in suppressing spurious harmonics. These techniques either increase the complexity of fabrication or enlarge the component’s size. It has been proven that a filter, incorporating two spur-lines at the input and output feed lines of the coupled ring, can increase the rejection band of the bandpass filter.8 The ideas demonstrated in this article are to implement spur-line structures in the square ring resonator to suppress the spurious harmonics without altering the passband response of the filter.

Fig. 2 Simulated results for the dual-mode, wideband bandpass filter.

Dual-mode, Wideband Microstrip Bandpass Filter

A novel bandpass microstrip filter, based on a ring resonator, is shown in Figure 1. The input and output ports are directly connected to the ring resonator at 180° and 270°. Two stepped-impedance tuning stubs are implemented within the resonator at 0° and 90°, and a 0.5 by 0.5 mm perturbation is positioned at 45°. The circumference lr of the ring resonator is given by Equation 1, where n is the mode number and ?g is the guided wavelength

The ring resonator and the ?/8 stepped-impedance stubs are designed for a resonance frequency of 4.2 GHz and fabricated on an RT/duroid 6010.2 substrate with a thickness h = 25 mil and a relative dielectric constant ?r = 10.2. The dimensions of the filter are l = 7.4 mm, m = 3.55 mm, n = 0.7 mm, t = 2.6 mm, w = 0.6 mm and p = 0.5 mm. The simulated results of the filter are shown in Figure 2. The perturbation stubs can generate two transmission zeros, or dual modes, on either side of the passband within the 2.68 to 2.87 and 5.56 to 5.99 GHz bands. The filter has a 3 dB fractional bandwidth greater than 56 percent and an insertion loss better than 0.3 dB.