Microstrip even-mode half-wavelength SIR based I-band interdigital bandpass filter

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 9, October 0, pp Microstrip even-mode half-wavelength SIR based I-band interdigital bandpass filter Ram Krishna Maharjan* & Nam-Young Kim RFIC Lab, Electronic Engineering Department, Kwangwoon University, 447- Wolgye-dong, Nowon-gu, Seoul 39-70, Korea Received 6 July 0; accepted 3 July 0 In this paper, a new microstrip interdigital even-mode half-wavelength type stepped impedance resonator (SIR) based bandpass filter (BPF) for I-band applications is presented. The bandpass filter characteristic depends on the self-resonant frequency of the interdigital resonated arms. The combination of the interdigital arm structures behaves as an interdigital capacitance coupling between port and port. The self generated capacitive and inductive reactances of the interdigital resonators interacted in order to obtain a resonance at 9.4 GHz. The resonant frequency and the bandwidth of the filter are directly optimized as a result of the physical arrangement of the interdigital resonators. The measured insertion loss (S ) and the return loss (S ) are 0.85 db and 8.0 db, respectively, at the 9.4 GHz resonant frequency. Keywords: Microstrip stepped impedance resonator, Even-mode SIR, Half-wavelength SIR, Interdigital resonators, Bandpass filter, I-band communication systems Modern wireless communication systems currently use miniature radio frequency (RF) filter design technologies in order to achieve the required compact size and high quality performance. Microstrip filters are important in many microwave subsystems, particularly in the transmitter and receiver modules -3. Their purpose is to select and pass a desired band of frequencies and reject or stop the other unwanted frequencies. The interdigital and parallel-coupled line filters are traditional coupled-line structures to implement bandpass filter responses 4-8. Because of the unequal even and odd mode velocity, one of the intrinsic limitations of parallel-coupled-line is the spurious passband which occurs at harmonics of fundamental frequency. Unlike the traditional parallel coupled-line filter, interdigital filter takes the advantage of compact size (about /3 of a parallelcoupled ones with the same specification) and to avoid the nd harmonic spurious passband. But the higher order harmonic spurious passband is still concerned. To overcome the problem of spurious passband in parallel-coupled-line, the microstrip filter topology based interdigital capacitive coupling phenomena is proposed. Therefore, it is used to introduce the capacitance effect needed for resonance frequency tuning. *Corresponding author: ( rkmahajn@gmail.com) The designed filter characteristics are generated by using the concept of splitting the even-mode halfwavelength (λ/) stepped impedance resonator structure. In general, the BPF design procedure normally requires many resonators in order to obtain a steep attenuation rate outside of the passband, because it is necessary to have a high insertion loss outside of the passband and very low insertion loss in the desired passband frequency range 9-. In this paper, we report upon an interdigital BPF with a sharp and desired bandwidth for I-band communications. Design and Simulation SIR based interdigital structure The SIR is an improved version of the uniform impedance resonator (UIR). It was introduced to address the spurious frequency responses which normally appear at the integer multiple harmonics of the UIR fundamental resonant frequency. In addition, the SIR has the capability of shortening the overall resonator length without degrading the unloaded-q,. An essential parameter in SIR analysis is the impedance ratio R Z, the ratio between the transmission line impedance at the short-circuited plane and the open-circuited plane. This impedance ratio is generally used to determine the resonance of an SIR based on its length. Different SIR structures and their relevant design parameters are discussed

2 300 INDIAN J. ENG. MATER. SCI., OCTOBER 0 elsewhere,. There are two main basic structures for an SIR: the odd-mode and the even-mode. In this paper, an even-mode half-wavelength SIR is modified by splitting its structure into an interdigital form. A concept of design sequences for the proposed microstrip interdigital BPF is shown in Fig.. The SIR and its relevant design layouts are shown in this figure. Figure a shows the basic quarter-wavelength (λ/4) odd-mode structure. Figures b and c present odd-mode and even-mode half-wavelength (λ/) type SIR structures, respectively. Figure d mainly focuses on a half part of proposed interdigital filter that seems like a comb shape, it is depicted with black colour. When these similar type of two combs are placed in opposite as shown in Fig. e, that gives interdigital resonator bandpass filter. Fig. Basic structure of SIR: (a) Quarter-wavelength type, (b) Odd-mode λ/ type, (c) Even-mode λ/ type, (d) Highlighted with black colour left part of even-mode interdigital resonator and (e) Proposed interdigital even-mode filter Schematic design layout of the proposed interdigital filter with necessary dimensions is presented in Fig.. The proposed BPF is designed and fabricated on a Teflon substrate with a dielectric constant, ε r of.5 and a thickness of 0.54 mm. The filter consists of two identical comb shaped structures as shown in Fig. d to form compact SIR based evenmode interdigital bandpass filter. It is also noted that these two interdigital arms of the BPF are coupled to each other to realize interdigital capacitive-coupling and become a very compact size 3,4. For optimization, the filter designed and analyzed by a commercially available Sonnet electromagnetic (EM) simulation tool, then it was fabricated using photolithographic techniques and wet etching processes in the printed circuit technology on commercially low-cost Teflon substrate to verify the simulated results of our design. According to layout analysis and design, the resonant frequency and the bandwidth can easily be varied by a slight change in the spacing between the interdigital arms, i.e., the width, length, and the gaps in the separate parts of the structure. As the gaps, g and g are fixed at 0. mm Fig. Schematic design layout of the proposed filter

3 MAHARJAN & KIM: SIR BASED I-BAND INTERDIGITAL BANDPASS FILTER 30 and 0.4 mm spacing including other parameters kept constant as Fig., the bandwidth quickly increased and reflection coefficient was also drastically reduced at central resonant frequency. As well as, transmission coefficient was improved noticeably with maintaining desired bandwidth and resonance condition. The proposed BPF behaves as a series RLC circuit. Therefore, an approximate equivalent circuit model of the designed filter is represented as in Fig. 3, where C P and C P models are considered as pad capacitances, C S is the total series capacitance effect of the interdigital arms, L S is the inductance model of the given interdigital structure, and R S models the overall pure resistive effect of the given structure. The self-resonant frequency is generated when the interdigital arm resonators are electro-magnetically coupled each other. While design analysis taken place, the electric field intensity was checked out in desired band and out-of-band ranges, ultimately design optimization concluded surface current distributions were resulted as Fig. 4. Interdigital I-band filter design An interdigital microstrip bandpass filter is designed from the interdigital resonator elements by splitting the even-mode SIR structure. This is a concept applied to the designed filter in order to get a specified resonant frequency. The impedance ratio is used to determine the resonance of the SIR based on its length. The characteristic impedances and the corresponding electrical length of the transmission lines between the open and short circuited ends are defined as Z and Z, and θ and θ, respectively. The electrical parameter which characterizes the given SIR is the ratio between the two transmission line impedances Z and Z ; the impedance ratio, R Z -3 can therefore be defined as: R Z Z Z L The resonance equation for the SIR structure is : () Z i Z tanθ + Z tanθ jz Z Z tanθ tanθ () If Z i 0, the resonance condition is derived as: Fig. 3 Equivalent circuit model of the proposed filter Z tanθ tanθ Z R Z (3) Fig. 4 Current distributions in the proposed filter: 9.0 GHz, (b) 9.5 GHz, (c) 0.0 GHz and (d) 0.5 GHz frequencies The interdigital resonators each resonate at a 9.4 GHz center frequency with approximately 900 MHz effective bandwidth in the 3 db passband range. This interdigital BPF may gain popularity due to its compact size, cost effectiveness, and fabrication simplicity; it can be used extensively in low-power to medium-power RF transceiver applications. The observed differences between the simulations and measurements can be attributed to the fabrication tolerances. The conductor loss, the dielectric loss and the non-ideal microstrip coaxial line transitions are thought to contribute to the little bit higher insertion loss seen in the physical measurements compared to the simulation. The current distributions in the proposed resonator at the different operating frequencies are shown in Fig. 4. The current distributions at (a) 9.0 GHz, (b) 9.5 GHz, (c) 0.0 GHz and (d) 0.5 GHz were investigated using an EM

4 30 INDIAN J. ENG. MATER. SCI., OCTOBER 0 simulator. The figure also shows the current density conditions of the designed resonator filter at the significant states around the resonance condition. Figure 4b shows the desired state of a good and almost equal current distribution at the 9.5 GHz resonant frequency, compared to the other nearby frequencies. At resonance, the reactive components cancel each other and leave only the resistance, resulting in a maximum current flow in that condition rather than that found in the other frequency bands. Fabrication and Measurement Results The proposed interdigital bandpass filter was fabricated on a Teflon substrate with the dielectric constant; ε r of.5 and a thickness of 0.54 mm. The fabrication process was accomplished by photolithographic techniques and wet etching process, then after the proposed filter was developed. The digital photograph of the fabricated filter is shown in Fig. 5. The overall physical size of the illustrated bandpass filter has an area of mm. Following the aforementioned design process, the geometric dimensions (Fig. ) were determined to be: W 3.8 mm, W.5 mm, W mm, L 5 mm, L 7.9 mm, L mm, s 0.7 mm, s 0.9 mm, s mm, s mm, h 0. mm, h 0.8 mm, g 0. mm and g 0.4 mm. Because of the unavoidable deviations due to fabrication tolerances, it was necessary to analyze the sensitivity. The performance of the passband is affected by the capacitive coupling between the interdigital arm resonators. Furthermore, the coupling gaps g and g are narrow and therefore remarkably sensitive to fabrication deviations compared to the other parameters. As a result, the gap was chosen as the parameter for the performance of the sensitivity analysis. The gap can be subject to random errors of up to 6.7% or ± 0.05 mm, which is usually considered to be the maximum allowable fabrication tolerance when fabricating a microstrip device 5,6. The measured insertion loss (S ) and return loss (S ) were about 0.85 db and 8.0 db, respectively, at the 9.4 GHz center operating frequency. The fabricated resonator was measured using an Agilent 850C vector network analyzer (VNA). The measured results were compared and analyzed in regards to the simulated results in terms of the S and S parameters, shown in Figs 6 and 7. Figure 6 shows the broadband response of the S-parameters in both the simulated and measured cases. The simulated and measured narrowband S-parameter responses are illustrated in Fig. 7. At the 9.4 GHz resonant frequency, this design was shown to have an approximately 900 MHz 3 db effective bandwidth. Fig. 6 Broadband simulated and measured S-parameter responses of the proposed filter Fig. 5 Photograph of the proposed fabricated-filter Fig. 7 Narrowband simulated and measured S-parameter responses of the proposed filter

5 MAHARJAN & KIM: SIR BASED I-BAND INTERDIGITAL BANDPASS FILTER 303 Conclusions A novel compact bandpass filter has been presented in the form of an interdigital structure using split techniques applied to an even-mode half-wavelength SIR structure. The design is based on the self-resonance frequency of the interdigital arm resonators and the EM coupling effects. The experimental results, presented by the optimization of the interdigital arm arrangement, show the improved performance of bandpass filter at 9.4 GHz resonant frequency. The filter can also be possible to fabricate in the similar pattern in MMIC design technology if its physical size will be further reduced significantly with indeed minor modifications for the applications of higher frequencies. Proposed device can be applied in various areas, such as terrestrial microwave networks and I-band wireless applications. Acknowledgment This research was supported by the National Research Foundation of Korea (NRF) and a Grant from the Korean Government (MEST) (No ) and (No. 0RAA004366). This work was also supported by a Research Grant of Kwangwoon University in 0. References Makimoto M & Yamashita S, Microwave Resonators and Filters for Wireless Communication Theory, Design and Application, (Springer, Berlin), 00. Hong J S & Lancaster M J, Microstrip Filters for RF/Microwave Applications, (Wiley-Interscience Publication, John Wiley & Sons Inc.), Sagawa M, Makimoto M & Yamashita Sadahiko, IEEE Trans Microwave Theory Tech, 45 (997) Gue L, Yu Z Y & Zhang L, Microwave Opt Technol Lett, 33 (0) Zhu L, Devabhaktuni V, Wang C & Yu M, IEEE Microwave Wireless Components Lett, 8 (008) Hung C Y, Weng M H, Su Y K, Yang R Y & Wu H W, Microwave Opt Technol Lett, 48 (006) Tang C W & Lu L P, IET Electron Lett, 5 (008). 8 Park H J, Jung W C, Lee J C, Kim J H, Lee B & Kim N Y, Microwave Opt Technol Lett, 8 (00) Weng M H, Wu H W & Hung C Y, Microwave Opt Technol Lett, 47 (005) Tu W H & Chang K, IEEE Microwave Wireless Components Lett, 5 (005) Her M L, Ko W, Hsu M W, Wang Y L & Hsu C J, Microwave Opt Technol Lett, 48 (006) Crute J R & Davis L E, Microwave Opt Technol Lett, 34 (00) Zhu L, Bu H & Wu K, IEEE MTT-S Int. Microwave Symp. Dig., Boston, MA, (Jun 999) Weng M H, Wu H W & Hung C Y, Microwave Opt Technol Lett, 47 (005) Lancaster M J, Passive Microwave Device Applications of High Temperature Superconductors, (Great Britain: Cambridge University Press), Bogatin E, IEEE Trans Components, Hybrids Manuf, (988)