IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 23, NO. 4, APRIL 2013 193

Dual Band Band-Pass Filter With WideStopband on Multilayer Organic Substrate

Hai Hoang Ta, Student Member, IEEE, and Anh-Vu Pham, Senior Member, IEEE

Abstract—We present the design and development of a compact,dual-band, wide stop-band band-pass filter on amultilayer organicsubstrate. The filter is designed based on quasi-lumped hybridLC resonators that are coupled by shunt-inductor impedanceinverters. The experimental results show that the filter has twopassbands at 1.7 and 4.0 GHz with an out-of-band rejection levelbetter than 50 dB up to 16.5 GHz.

Index Terms—Band-pass filter (BPF), dual-band, multilayer,quasi-lumped.

I. INTRODUCTION

B AND-pass filters (BPFs) are important componentsin communication circuits. BPFs with wide stopband

and high selectivity are desirable to improve system perfor-mance. Recently, with the rapid growth of various wirelessservices, the demand for multi-band components has greatlyincreased. Dual-band BPFs have attracted the attention ofmany researchers. Conventional filter designs using distributedcomponents usually suffer from unwanted spurious responses.Many techniques have been proposed for the design ofdual-band BPFs that have a wide stopband [1]–[5]. In [1]–[3],stepped-impedance resonators with different topologies areused to design dual-band BPFs that have good stopband per-formance. In [4], net-type resonators are used to design adual-band BPF that has a rejection level of better than 20 dBup to 5.6 times of the first frequency. In [5], a dual-band BPFis designed by using dual-mode microstrip E-shape resonators.The stopband of this filter is extended by using two T-shapefeedlines and the coupling between them.In this letter, we present the design of a dual-band BPF that

achieves a wide stopband using hybrid quasi-lumped LC res-onators on a multilayer organic substrate. Impedance inverterswith a shunt inductor and a series transmission line are usedto couple the LC resonators to external circuits. A nine-poledual-band BPF is designed, fabricated and measured. The ex-perimental results show that the filter has dual passbands at 1.7and 4.0 GHz with out-of-band rejection better than 50 dB up to

the first center frequency. To the best of the authors’knowledge, this filter has the highest out-of-band rejection andwidest stopband compared to other printed-circuit board (PCB)dual-band BPFs reported to date.

Manuscript received August 06, 2012; revised January 29, 2013; acceptedFebruary 20, 2013. Date of publication March 12, 2013; date of current versionApril 04, 2013. This work is supported in part by the Vietnam Education Foun-dation, and Agilent Technologies.The authors are with the School of Electrical and Computer Engineering,

University of California at Davis, Davis, CA 95616 USA (e-mail: hhta@uc-davis.edu)Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LMWC.2013.2251617

Fig. 1. Circuit diagram and 3-D implementation of a one-cell dual-band BPF.

II. DUAL-BAND BANDPASS FILTER DESIGN

The circuit diagram and 3-D implementation of a one-celldual-band BPF is shown in Fig. 1. The filter is designed usinga hybrid LC resonator that consists of series componentsand as well as shunt components and . The hy-brid resonator is coupled to external circuits by two impedanceinverters that are composed of a series transmission line stuband a shunt inductor L. The hybrid resonator is realized on amultilayer Liquid Crystal Polymer (LCP) substrate which has adielectric constant of 2.9 and a loss tangent of 0.002. The multi-layer LCP substrate has 4 metal layers (0, 1, 2 and ground) and 3LCP layers as shown in Fig. 1. The thicknesses of the LCP layersare , and . The two se-ries capacitors are Metal-Insulator-Metal (MIM) structuresrealized by conductors on metal layers 0 and 1. The two seriesinductors are meandered conductors on metal layer 0. Theshunt capacitor is a MIM capacitor formed by conductorson metal layers 1 and ground. The shunt inductor is formedby a meandered conductor on metal layer 2.

1531-1309/$31.00 © 2013 IEEE

194 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 23, NO. 4, APRIL 2013

Fig. 2. Simulated S21 of the one-cell dual-band BPF with ,, , and at 2 GHz.

(a)- variation, . (b)- variation, .

TABLE IINDUCTANCE AND CAPACITANCE VALUES OF THE LC CELLS

Fig. 2 shows the simulated insertion loss of the circuit inFig. 1 as and are changed [Fig. 2(a)] and [Fig. 2(b)],respectively. As can be seen from the figures, the first passbandof this filter does not depend on and while the secondpassband can be adjusted by changing and . Therefore,the first passband of this filter can be designed by neglectingand . Without the shunt components and , the cir-cuit in Fig. 1 becomes a one-pole BPFwith a series LC resonatorand two impedance inverters. The procedures of designing thisfilter can be found in [6]. The second passband is then realizedby adding the shunt components and . The center fre-quency of the second passband can be designed by changing ei-ther or or both without affecting the first one. However,after adding and , another pole appears in the lower sideof the frequency band. This pole has much smaller bandwidthas compared to the two main passbands and can be suppressedby adding high-pass components or having a high-order filter aspresented in this work.This concept is extended to design a nine-pole dual-band

BPF. The schematic diagram of this filter is shown in Fig. 3. Thefilter is designed to have Chebyshev responses at two passbands,1.7 and 4.0 GHz, with the ripples of 0.1 dB for both bands. First,the initial values of the series inductances and capacitances aswell as the inverter characteristic impedances are determined byfollowing the procedures in [6] to design the first passband. Byusing the impedance inverter transformation [7], all the LC cells

TABLE IIINDUCTANCE AND ELECTRICAL LENGTH VALUES OF THE IMPEDANCE

INVERTERS

TABLE IIICOMPARISONS WITH OTHER PROPOSE DUAL-BAND FILTERS. (RL: RETURN

LOSS; IL: INSERTION; : FIRST CENTER PASSBAND FREQUENCYAND IS THE FREE-SPACE WAVELENGTH OF )

are made identical. The shunt components of the LC cells arethen determined so that the filter has a second passband at thedesired frequency. The impedance inverters are now realized byshunt inductors, and series transmission lines. The equations todesign a shunt-inductor impedance inverter can be found in [6],[7]. The values of the inductances and capacitances of the LCcells are summarized in Table I. The values of the shunt induc-tances and the transmission line electrical lengths are summa-rized in Table II. All the transmission lines have a characteristicimpedance of 50 ohm. The dimensional parameters of an LCcell are , , ,

, , , ,, , and .

Based on the values of , , and , the hybrid LCcell is built on a multilayer LCP substrate as shown in Fig. 1.The dimensions of the MIM capacitors are estimated using theparallel plate capacitor equation while the dimensions of themeandered-line inductors are estimated based on the Richardtransformation [8]. The shunt inductors L1–L5 are realized byshort-circuited high-impedance microstrip transmission lines.The dimensions of these transmission lines are also estimatedby using the Richard transformation [8].

III. EXPERIMENTAL RESULTS

The filter is simulated and verified by using High FrequencyStructure Simulator (HFSS) [9]. The complete filter prototypewith dimensions of 9.8 mm 70 mm is shown in Fig. 4.

TA AND PHAM: DUAL BAND BAND-PASS FILTER WITH WIDE STOPBAND ON MULTILAYER ORGANIC SUBSTRATE 195

Fig. 3. Schematic diagram of the filter.

Fig. 4. Fabricated prototype of the filter on a multi-layered LCP substrate.

Fig. 5. Simulated and measured results of the filter (a)-Insertion loss; (b)-Re-turn loss.

Coplanar-waveguide probe pads are added for probe measure-ments. The electrical performance of the filter was measured ona Cascade Microtech RF probe station with an Agilent E83642-port network analyzer. The probes were calibrated using thestandard Thru-Reflect-Line calibration on a Picoprobe CS-9substrate [10].The measured and simulated return loss and insertion loss of

the filter are shown in Fig. 5. The simulation and measurementresults are well correlated. The filter has measured passbandsat 1.7 and 4.0 GHz with minimum insertion losses of 6.7 dBand 6.5 dB, respectively and 3 dB bandwidths of 20% and 18%,respectively. Due to the non-periodical property of the quasi-lumped LC cells, the filter has a very wide stopband. As shownin Fig. 5(a), the rejection level of the filter is better than 50 dB

up to 16.5 GHz which is the first center frequency.The isolation between two bands is . The measuredreturn loss is greater than 10 dB within the two passbands andis less than 5 dB out-of-band up to 16.5 dB. Table III showsthe comparisons of different dual-band filter designs in termsof dimensions and stopband performance. As can be seen inTable III, this filter has a comparable size while it has the wideststopband and an out-of-band rejection which is 20 to 30 dBbetter than the other dual-band filter references.

IV. CONCLUSION

We present the design and development of a dual-band BPFon a multilayer organic substrate. The filter is designed basedon quasi-lumped LC resonator cells that are realized on a four-metal layer LCP substrate. The experimental results show thatthe filter has two pass bands at 1.7 and 4.0 GHz with a fractionalbandwidth of 20% and 18%, respectively. The filter has a widestopbandwith a rejection level better than 50 dB up to 16.5 GHz.The dimensions of the filter are 9.8 mm 70 mm 0.36 mm.

REFERENCES

[1] J.-T. Kuo and H.-P. Lin, “Dual-Band bandpass filter with improvedperformance in extended upper rejection band,” IEEE Trans. Microw.Theory Tech., vol. 57, no. 4, pp. 824–829, Apr. 2009.

[2] H.-W. Wu, Y.-F. Chen, and Y.-W. Chen, “Multi-Layered dual-bandbandpass filter using stub-loaded stepped-impedance and uni-form-impedance resonators,” IEEE Microw. Wireless Compon. Lett.,vol. 22, no. 3, pp. 114–116, Mar. 2012.

[3] W. S. Chang and C. Y. Chang, “Analytical design of microstrip short-circuit terminated stepped-impedance resonator dual-band filter,” IEEETrans. Microw. Theory Tech., vol. 59, no. 7, pp. 1730–1739, Jul. 2011.

[4] C. H. Tseng and H. Y. Shao, “A new dual-band microstrip bandpassfilter using net-type resonators,” IEEE Microw. Wireless Compon.Lett., vol. 20, no. 4, pp. 196–198, Apr. 2010.

[5] J. Wang, L. Ge, K. Wang, andW.Wu, “Compact microstrip dual-modedual-band bandpass filter with wide stopband,” Electron. Lett., vol. 46,no. 4, pp. 263–265, Feb. 2011.

[6] G. Mattaei, L. Young, and E. M. T. Jones, Microwave Filters,Impedance-Matching Networks, and Coupling Structures. Dedham,MA, USA: Artech House, 1980.

[7] R. E. Collin, Foundations for Microwave Engineering, 2nd ed. NewYork: Wiley, 2000.

[8] D. M. Pozar, Microwave Engineering. New York: Wiley, 1997.[9] [Online]. Available: http://www.ansoft.com/products/hf/hfss/[10] [Online]. Available: http://www.ggb.com/