Design of wide-stopband microstripbandpass filter with assistance of spur lines

C.W. Tang, C.H. Teng and J.M. Sun

A novel microstrip bandpass filter with compact size is proposed. Withtwo short-circuited stubs, one open-circuited coupled line using sym-metrical feeding, and two spur lines, a wide passband can be obtained.Detailed design and synthesis procedures are provided for filter devel-opment. Moreover, the electromagnetic simulator IE3D is used; theprototype of the bandpass filter is consequently fabricated andmeasured. Agreement between measured results and the theoreticalprediction validates the proposed structure.

Introduction: Bandpass filters have been widely used in many micro-wave and wireless communication systems. Planar filters with printedcircuit technology are particularly attractive because of their easy fabri-cation, compact size, and low cost [1]. Because of its planar structureand easy design procedures, the parallel-coupled microstrip bandpassfilter has been one of the most commonly used filters for the RF frontend. However, the first harmonic of the conventional microstrip parallelcoupled filter appears at twice the fundamental passband. Therefore, therejection level in the upper stopband will be decreased.

Various methods have been developed to improve the performance inthe stopband of the planar microstrip bandpass filter. The sinusoidal ruleis used to eliminate spurious harmonics by continuously perturbing thewidth of the coupled lines in the microstrip wiggly-line filter [2]. In [3],capacitors are utilised to slow down the odd-mode phase velocity. Inaddition, with the stepped-impedance resonators adopted for planarmicrostrip bandpass filters [4], the second harmonic can shift to ahigher frequency band and a wide stopband will appear. Moreover, har-monic suppression can be carried out as well by employing the split-ringresonator [5], and the open stub and spur line [6].

In this Letter, we propose a novel microstrip bandpass filter, as shownin Fig. 1a. With the assistance of two short-circuited stubs, one open-circuited coupled line using symmetrical feeding, and two spur lines,the harmonic resonances are suppressed and a wider stopband comes up.

L2

L1

L4

W3

L5

L6

W5

W4

S

D

L3

G1W2 W1

a

b

Fig. 1 Proposed wide-stopband bandpass filter

a Schematic layoutb Spur line employed for input and output port

Filter design: If both the input and output impedance of the spur lineshown in Fig. 1b are 50 Ω, the analysis of the proposed wide-stopbandmicrostrip bandpass filter shown in Fig. 1a can then be simplified as twoshort-circuited stubs and one open-circuited coupled line with symmet-rical feeding. With the assistance of the immittance inverter, the equiv-alent circuit of the simplified filter can be obtained, as in Fig. 2.

ELECTRONICS LETTERS 17th January 2013 Vol.

Consequently, the design equations can be expressed as

Y0e3Y3

= 1− J12Y3

cotu3 (1)

Y0o3Y3

= 1+ J12Y3

cotu3 (2)

J01 = J23 =��������������������Y0F × FBW × b1A

g0 g1

√(3)

B1A = B2A = Y2(Y2tanu2 + Y3tanu3)

Y2 − Y3tanu2tanu3− Y1cotu1 (4)

B1B = B2B = Y2(Y2tanu2 − Y1cotu1)

Y2 + Y1cotu1tanu2+ Y3tanu3 (5)

where Yi’s is the inverse of Zi’s and Y3 is the corresponding admittanceof the symmetrical open-circuited coupled line.

J01

RS

J12 J23

RL

Y1A Y1B Y2B Y2A

Y2, q2 Y2, q2

Y1, q1Y1, q1Y3, q3Y1, q1

Fig. 2 Equivalent circuit of simplified wide-stopband bandpass filter

With the short-circuited stubs and the open-circuited coupled lineemployed for the bandpass filter, multiple transmission zeros show upso that a wide stopband appears. As for transmission zeros, fz1 and fz2result from the open-circuited coupled line and the short-circuitedstub, respectively. Besides, transmission zeros are located at

fz1 = nf0 × 900

u3, n = 1, 2, · · · (6)

fz2 = nf0 × 1800

u1, n = 1, 2, · · · (7)

where f0 is the central frequency of the passband.Moreover, 2.4 GHz, 0.01 dB, and 15% are adopted for the central fre-

quency, ripple, and fractional bandwidth, respectively, to develop thesimplified filter mentioned above with Chebyshev response as anexample. In addition, set the transmission zeros fz1 and fz2 at 4.5 and9 GHz, respectively. Furthermore, electrical length θ2 and characteristicimpedance Z2 are set as 24o and 110 Ω, respectively. Consequentlyaccording to (1)–(7), the design parameters are shown in Table 1.Accordingly, theoretical responses of the simplified bandpass filter arepresented as in Fig. 3. In addition, Fig. 3 shows the electromagnetic(EM) simulated responses converted from Table 1, with RogersRO4003, the substrate whose dielectric constant, loss tangent, andthickness are 3.55, 0.0027, and 0.508 mm, respectively.

10

0

–10

–20

–30

–40

–50

–600 2 4

fz1 fz1 fz1 fz1fz2 fz2

6 8 10 12 14

frequency, GHz

16 18 20

mag

nitu

de o

f S11

and

S21

, dB S11

S21

spur linetheoryEM simulation

Fig. 3 Theoretical and EM simulated responses of simplified bandpass filterand effect of spur line

49 No. 2

Table 1: Parameters of simplified wide-stopband bandpass filter

Z1 θ1 Z2 θ2 Z0e3 Z0o3 θ332.57Ω 48o 110Ω 24o 135.14Ω 69.44Ω 48o

Experimental results: Fig. 3 indicates that harmonics appear at 14.5 and19 GHz. The spur line introduced in Fig. 1b is thus connected at theinput/output port for harmonic suppression. Consequently, Table 2shows the dimensions of the proposed wide-stopband bandpass filterincluding two spur lines, fabricated on Rogers RO4003.

Table 2: Dimensions of fabricated bandpass filter

L1 L2 L3 L4 L5 L6 W1

9.52 5.75 9.81 1.5 3 2.1 2.4

W2 W3 W4 W5 G1 D S

0.25 0.25 0.3 0.4 0.18 0.4 0.2

unit: mm

Fig. 4 presents the fabricated filter and compares EM simulation andmeasurement. In particular, the harmonic suppression reaches the levelof 20.5 dB and the stopband extends to 20 GHz.

0

–10

–20

–30

–40

–50

–60

–70

–80

mag

nitu

de o

f S11

and

S21

, dB

0 2 4 6 8 10 12 14

frequency, GHz

16 18 20

EM simulationmeasurement

Fig. 4 Comparison between EM simulation and measurement of proposedwide-stopband microstrip bandpass filter

ELECTRON

Conclusion: A new approach to generate a wide stopband in the micro-strip bandpass filter has been proposed. Methods for filter design andanalysis have been introduced. The fabricated filter has demonstratedthe potential for harmonic suppression with the assistance of two short-circuited stubs, one open-circuited coupled line using symmetricalfeeding, and two spur lines. Moreover, good agreement betweentheoretical and measured results has validated the proposed structure.

Acknowledgment: This work was supported in part by the NationalScience Council, Taiwan (grants NSC 100-2628-E-194-007-MY3 andNSC 101-2221-E-194-041).

© The Institution of Engineering and Technology 201323 September 2012doi: 10.1049/el.2012.3384One or more of the Figures in this Letter are available in colour online.

C.W. Tang, C.H. Teng and J.M. Sun (Department of CommunicationsEngineering, National Chung Cheng University, Chiayi, Taiwan)

E-mail: cwtang@ccu.edu.tw

References

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2 Lopetegi, T., Laso, M.A.G., Hernández, J., Bacaicoa, M., Benito, D.,Garde, M.J., Sorolla, M., and Guglielmi, M.: ‘New microstrip wiggly-line filters with spurious passband suppression’, IEEE Trans. Microw.Theory Tech., 2001, 49, (9), pp. 1593–1598

3 Bahl, I.J.: ‘Capacitively compensated high performance parallel coupledmicrostrip filters’. Dig. IEEE MTT-S, Long Beach, CA, USA, 1989,pp. 679–682

4 Tang, C.W., and Liang, H.H.: ‘Parallel-coupled stacked SIRs bandpassfilters with open-loop resonators for suppression of spurious responses’,IEEE Microw. Wirel. Compon. Lett., 2005, 15, (11), pp. 802–804

5 García, J.G., Martín, F., Falcone, F., Bonache, J., Gil, I., Lopetegi, T.,Laso, M.A.G., Sorolla, M., and Marqués, R.: ‘Spurious passband sup-pression in microstrip coupled line band pass filters by means of splitring resonators’, IEEE Microw. Wirel. Compon. Lett., 2004, 14, (9),pp. 416–418

6 Tu, W.H., and Chang, K.: ‘Compact microstrip bandstop filter using openstub and spurline’, IEEE Microw. Wirel. Compon. Lett., 2005, 15, (4),pp. 268–270

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