Design of a Tunable Bandpass Filter With the Assistance of ModifiedParallel-Coupled Lines

Chien-Tai Tseng�, Ching-Wen Tang�, Shih-Chieh Chang�, and Yuan-Chih Lin�

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Abstract — A tunable bandpass filter with a wide tuning range is presented. The proposed filter is composed of two modified parallel-coupled lines and one varactor-loaded stub. The measured results show that a broad tuning range of 41.7%, from 0.95 to 1.45 GHz, was achieved. Within the tuning range, the insertion loss is from 2.4 to 2.9 dB and the return loss is greater than 15dB.

Index Terms — Bandpass filter, tunable filter, varactor.

I. INTRODUCTION

Recently, wireless communication systems have been developed for a wide variety of applications such as mobile multimedia communication, near-body wireless healthcare, and wireless surveillance. These emerging applications drive the RF transceiver to posses characteristics of multi-bands and multi-functions. Therefore, the band-selected bandpass filter with a broad tunable frequency range is essential for the RF front end.

The planar tunable bandpass filters can be realized by RF micro-electro-mechanical system (RF-MEMS) devices [1]-[3] or varactor diodes [4]-[7]. In particular, the RF-MEMS capacitor performs better than the varactor diode in terms of the quality factor and insertion loss. However, the varactor diode gains much attention due to the higher tuning speed and lower fabrication cost.

In this paper, a compact tunable bandpass filter with a wide tuning range is developed as shown in Fig. 1. With the open stub and varactor diode added, the electrical length of the modified parallel-coupled line can be miniaturized. Moreover, the varactor diode can vary the capacitance to tune the resonant frequency. Furthermore, the varactor-loaded stub is employed to increase the slope around the passband skirt.

II. DESIGN THEORY

The tunable bandpass filter consists of two modified parallel-coupled lines and one varactor-loaded stub as shown in Fig. 1(b). The modified parallel-coupled line, Block 1/Block 3, is utilized for central frequency tuning. On the other hand, the varactor-loaded stub, Block 2, is introduced to tune the transmission zero around the lower/higher passband skirt.

(a)

(b)Fig. 1. Proposed tunable bandpass filter. (a) Aspect. (b) Schematic diagram.

With the current-voltage method, the Z-matrix of the modified parallel-coupled line can be analyzed as

2 2 2 211 2 12 ( )Z a bcd a b d b Z d Z� �= + − + − − Δ� � (1)

2 212 21 1 2( ) 2 ( )Z Z c c b d abd bd Z Z� �= = + + − − + Δ� �

(2)

2 2 2 222 1 22 ( )Z a bcd a b d b Z d Z� �= + − + − − Δ� � (3)

Port 2

Block 1 Block 3Block 2

Z0e, Z0o, θ1

Port 1 Z3, θ3

Z1, θ2Z1, θ2

Z0e, Z0o, θ1C1 C1

C2

978-1-4673-2141-9/13/$31.00 ©2013 IEEE

where

0 0 12 ( ) cotje oa Z Z θ= − + (4)

0 0 12 ( ) cotje ob Z Z θ= − − (5)

0 0 12 ( ) cscje oc Z Z θ= − − (6)

0 0 12 ( ) cscje od Z Z θ= − + (7)

2 21 2 1 2( )a a Z Z Z Z cΔ = + + + − (8)

1 11Z j Cω= (9)

2 2cotcZ jZ θ= − (10)

Therefore, one ABCD matrix of the modified parallel-coupled line, Block 1, will be derived by

1 1 11 21 11 22 12 21 21

1 1 21 22 21

( )1

A B Z Z Z Z Z Z ZC D Z Z Z

−� � � �=� � � �

� � � � (11)

Moreover, the other ABCD matrix of the modified parallel-coupled line, Block 3, can be obtained by exchanging the varactor’s location with the open stub’s. Furthermore, the ABCD matrix of the varactor-loaded stub, Block 2, can be expressed as

2 2

2 2

1 01 1in

A BZC D

� �� �= � �� �

� � � � (12)

where

2 3 33

2 3 3

tan 1tanin

C ZZ jZC Z

ω θω θ

−=+

(13)

Consequently, the ABCD matrix of the proposed tunable bandpass filter can be obtained by multiplying the ABCDmatrix of Block 1, 2, and 3. As a result, the tunable filter’s transmission zero around the lower/higher passband skirt can be derived as

2 3 3

12 tanzf

C Zπ θ= (14)

where Z3 and θ3 denote the characteristic impedance and the electrical length of the varactor-loaded stub, and C2 is the loaded capacitance of the varactor. In addition, the central frequency of the tunable filter approximates to

01 0 0 1

12 tane o

fC Z Zπ θ

≈ (15)

III. SIMULATION AND MEASUREMENT

In this paper, there is a wide tuning range, 0.95-1.45 GHz, for the central frequency of the proposed tunable bandpass filter. Moreover, the tunable transmission zero is located around the lower/higher passband skirt. With the central frequency of 0.95 GHz as a design example, the transmission zero is located at 1.06 GHz. 57o is consequently selected for the electrical length θ3. Moreover, with 36.6o and 33.1o

selected for the electrical length θ1 and θ2 of Block 1/Block 3 in Fig. 1(b), impedances Z0e, Z0o, and Z1 are hence 44, 24, and 15 Ω, respectively, to obtain the optimal length for the modified parallel-coupled line. Furthermore, according to (14) and (15), the capacitances C1 and C2 can be derived as 11.3 and 6.9 pF, respectively. As a result, Fig. 2 depicts the simulated responses of the proposed tunable bandpass filter. Particularly, with 1.45 GHz as the central frequency, Fig. 2 indicates that with transmission zero at 1.3 GHz, the capacitances C1 and C2 are 2.1 and 3.9 pf, respectively.

0.6 0.8 1 1.2 1.4 1.6 1.8Frequency (GHz)

-60

-50

-40

-30

-20

-10

0

10

Mag

nitu

de (d

B)

0.95 GHz_Circuit0.95 GHz_Meas.

1.45 GHz_Circuit1.45 GHz_Meas.

Fig. 2. Comparison between theoretical calculation and measure-ment of the proposed tunable bandpass filter.

The tunable bandpass filter is fabricated on the RT/duroid 6010 substrate, whose dielectric constant, loss tangent, and layer thickness are 10.2, 0.0023, and 0.635 mm, respectively. The theoretical parameters need to be transformed into circuit dimensions for fabrication. Because unequal parallel-coupled line widths result in better passband performance, the circuit dimensions can be modified as shown in Fig. 1(a). Speci-

978-1-4673-2141-9/13/$31.00 ©2013 IEEE

0.6 0.8 1 1.2 1.4 1.6 1.8Frequncy (GHz)

-50

-40

-30

-20

-10

0

10

|S11

| & |S

21| (

dB)

950 MHz1050 MHz1150 MHz

1250 MHz1350 MHz1450 MHz

fically, L1, L2, L3, W1, W2, W3, W4, and G1 are 8.8, 10, 16, 4.4, 1.6, 0.9, 0.2, and 0.1 mm, respectively. Moreover, the varactor diode employed for this design is MA46H202 made by M/A-COM. Furthermore, the adopted dc-blocking capacitance and chip inductance are 18 pF and 10 nH, respectively. Fig. 2 compares theoretical calculated and measured responses of the proposed tunable bandpass filter, whose central frequency rages from 0.95 to 1.45 GHz. The measured insertion loss is less than 2.8 dB and the return loss is better than 16 dB at 0.95 GHz. Specifically, with 0.95 GHz as central frequency, the measured transmission zero is located at 1.09 GHz; the measured transmission zero is located at 1.28 GHz with 1.45 GHz as central frequency. In addition, measurement matches well with theoretically calculated simulation.

(a)

(b) Fig. 3. Fabrication of the proposed bandpass filter with controll-able passbands. (a) Photograph. (b) Measured results of |S11| and |S21|.

Fig. 3(a) shows the photo of the fabricated tunable bandpass filter, whose size is 24.3 mm × 31.4 mm. As for the measured responses, Fig. 3(b) illustrates the tuning performance of the developed tunable bandpass filter. Particularly, with a dc bias from 1.5 to 20V, the central frequency of this bandpass filter ranges from 0.95 to 1.45 GHz; that is, there is a wide tuning rage of 41.7%. Within the tuning range, 0.95-1.45 GHz, the measured insertion loss is less than 3 dB while the measured return loss is greater than 15 dB. Moreover, the 3-dB bandwidth ranges from 80 to 100 MHz.

IV. CONCLUSION

A compact tunable bandpass filter is developed for the emerging wireless communication systems with multi-bands and multi-functions. The modified parallel-coupled lines with varactors are utilized to make the tunable resonant element and the varactor-loaded stub is employed to generate the transmission zero around the lower/higher passband skirt. The measured results show that within the wide tuning, 0.95-1.45 GHz, the measured insertion loss is between 2.4 and 2.9 dB and the return loss is better than 15 dB.

ACKNOWLEDGEMENT

This work was supported in part by the National Science Council of Taiwan, under Grant NSC 100-2628-E-194-007-MY3 and NSC 101-2221-E-194-041.

REFERENCES

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[4] I. C. Hunter and J. D. Rhodes, “Electronically tunable microwave bandpass filters,” IEEE Trans. Microw. Theory Tech., vol. 30, no. 9, pp. 1354–1360, Sep. 1982.

[5] Y. H. Chun and J. S. Hong, “Electronically reconfigurable dual-mode microstrip open-loop resonator filter,” IEEE Microw. Wireless Compon. Lett., vol. 18, no. 7, pp. 449–451, Jul. 2008.

[6] J. Long, C. Li,W. Cui, J. Huangfu, and L. Ran, “A tunable microstrip bandpass filter with two independently adjustable transmission zeros,” IEEE Microw. Wireless Compon. Lett., vol. 21, no. 2, pp. 74–76, Feb. 2011.

[7] X. G. Wang, Y. H. Cho, and S. W. Yun, “A tunable combline bandpass filter loaded with series resonator,” IEEE Trans. Microw. Theory Tech., vol. 60, no. 6, pp. 1569–1576, Jun. 2012.

978-1-4673-2141-9/13/$31.00 ©2013 IEEE