Broadband Microstrip band pass filters using triple-mode resonator

Transcription

1 Broadband Microstrip band pass filters using triple-mode resonator CH.M.S.Chaitanya (07548), M.Tech (CEDT) Abstract: A broadband microstrip band pass filter using a triple-mode resonator is presented. Triple-mode resonator contains a half-wave resonator and a shunt open circuited stub. Two filters using this approach with different center frequencies are designed, fabricated and tested to validate the proposed design approach. In the pass band, the measured return loss for the filter 1 is greater than 11 db and insertion loss is less than 1dB. For the second filter the measured return loss is greater than 15 db and insertion loss is less than 1dB. presented. It provides three transmission poles and two transmission zeros. The two transmission zeros beside the pass band achieve a sharp cut-off response. The resonator consists of a three-line quarter wave enhanced structure with a shunt open circuited stub. In section 2, the working of triple mode resonator is provided. In section 3, the proposed resonator is used to build two filters with center frequencies 3GHz and 3.5GHz and the measured and the simulated results are presented. 1. Introduction: Band-pass filters are the key components in RF systems. They should provide small insertion loss and large attenuation in the pass band and stop band respectively. The conventional way of designing a band pass filter is by cascading low-pass and high-pass filters [2]. They can also be realized by short circuited stubs and multimode resonators [3, 4]. With a proper coupling structure in multimode resonators high fractional bandwidth can be achieved [5]. In this paper, a broadband microstrip band pass filter with a triple mode resonator is 2. Triple-mode resonator: Fig. 1 shows the triple-mode resonator, where Z1 is the characteristic impedance of a half-wave resonator with electrical length 2 x ɵ1= 180, and Z2 is the characteristic impedance of the shunt open-circuited stub with electrical length ɵ2. For the simplicity Z1and Z2 are taken as 50 ohm and the centre frequency is 1 GHz. Fig. 2a shows the simulated S-parameters of the proposed resonator when ɵ2=0. When ɵ2=0, the proposed resonator becomes a conventional half-wave resonator. Besides the fundamental frequency f0, the spurious

2 frequencies are marked as fn = nf0 (n= 1, 2, 3...). Fig. 2b shows the S-parameters when ɵ2= 180. For the modes with n = 0, 2, 4, 6 the resonant frequencies are unchanged. It introduces two transmission zeros. Therefore with the unchanged fundamental mode at f0, one can have a triple-mode resonator with two transmission zeros, where the three modes are f1, f0 and f3. Also Z2 should be of low impedance for narrow and high impedance for wide bandpass filters. Fig. 2(b) Figure 2 Simulated S21 of the proposed resonator with different ɵ2, a) ɵ2=0, b) ɵ2= Filter design: Figure 1 Schematic of the proposed triple-mode resonator. The proposed triple-mode resonator is now used to design band pass filters. The schematic is shown in figure 3. The filters are built on a 1.524mm ARLON AD-350 substrate with a relative dielectric constant of 3.5 and a loss tangent of With the given fractional band width the location of three transmission poles can be given as [5] Fig. 2(a) where is the fractional bandwidth, f0 is the center frequency and k is the pole number.

3 determined as: W1= 0.2 mm, L1=16 mm; W2=0.2 mm, L2= 32 mm with a coupling gap of 0.2mm. Fig. 4 shows the simulated and measured S-parameters. The measured return loss is greater than 11 db and insertion loss is less than 1dB. Figure 3 Schematic of the proposed bandpass filter. To design the filter, initially half-wave resonator at central frequency is to be designed. For the width of the resonator, it is desirable to have a narrow width for tight coupling. The length of the resonator is of quarter wave length at the center frequency. The width of the shunt open circuited stub can be selected such that other two transmission pole locations are as calculated. The length of the shunt open circuited stub is of half wavelength at the center frequency. The coupling gap can be selected as small as possible to have wider 3-dB bandwidth and small ripple. With the above approach of design the two band pass filters with different center frequencies are designed. Filter1: The filter is designed with the specifications of 90% fractional bandwidth and the center frequency of 3 GHz with 0.3 db ripple. With this fractional bandwidth and center frequency the three transmission poles are at 1.8, 3, 4.8 G Hz. Therefore the dimensions of the triple mode generator are Figure 4 Simulated and measured S- parameters a) S12, b) S11.

4 Figure 5 Photograph of the first bandpass filter Filter2: The filter is designed with the specifications of 90% fractional bandwidth and the center frequency of 3.5 GHz with 0.3 db ripple. With this fractional bandwidth and center frequency the three transmission poles are at 2.1, 3.5, 4.9 G Hz. Therefore the dimensions of the triple mode generator are determined as: W1= 0.3mm, L1= 13.5mm; W2=0.3mm, L2= 27mm.with a coupling gap of 0.2mm. Fig. 6 shows the simulated and measured S-parameters. The measured return loss is greater than 15 db and insertion loss is less than 1dB. Figure 6 Simulated and measured S- parameters a) S12, b) S11. Figure 7 Photograph of the second bandpass filter Conclusions:. A triple mode generator approach for designing band pass filters is presented. Based on the approach two band pass filters with different center frequencies are designed and tested. Simulation as well as measured results are compared and found to be validated.

5 References: [1] W.-H. Tu.: Broad band microstrip band pass filters using triple-mode resonator. IET Microw. Antennas Propag., 2010, vol. 4, Iss. 9, pp [2] HSU C.-L., HSU F.-C., KUO J.-T.: Micro strip band pass filters for ultra-wideband (UWB) wireless communications. IEEE MTTS Int. Microw. Symp. Dig., 2005, pp [3] HONG J.-S., SHAMAN H.: An optimum ultra-wideband micro strip filter, Microw. Opt. Tech. Lett., 2005, 47, (3), pp [4] ZHU L., SUN S., MENZEL W.: Ultrawideband (UWB) bandpass filters using multiple-mode resonator, IEEE Microw. Wirel. Compon. Lett., 2005, 15, (11), pp [5] CHIOU Y.-C., KUO J.-T., CHENG E.: Broadband quasi- Chebyshev bandpass filters with multimode stepped impedance resonators (SIRs), IEEE Trans. Microw. Theory Tech., 2006, 54, (8), pp