Switchable bandpass filter with two state frequency responses

Shi A novel switchable bandpass filter with two-state frequency responses is presented, where dual-band bandpass and single bandpass character-istics can be conveniently switched by turni

Switchable bandpass filter with two-state

frequency responses

B Lui, F Wei and X Shi

A novel switchable bandpass filter with two-state frequency responses

is presented, where dual-band bandpass and single bandpass

character-istics can be conveniently switched by turning pin diodes on and off

The switchability results from the effect of different working modes

The filter can work as a dual-band bandpass filter obtaining two

centre frequencies, 1.8 and 3.5 GHz, which could be appropriately

used for GSM and WiMax systems In the meantime, it also can

operate only at 3.5 GHz, rejecting signal at 1.8 GHz The proposed

filter consists of two dual-mode resonators, using pin diodes to

switch the frequency responses An experiment was carried out to

validate the design concept, and the measured results agree well with

simulated results

Introduction: Electronically switchable and reconfigurable microwave

devices such as filters are in great demand for existing wireless

com-munication systems, and this trend will continue for future systems

To meet the increasing requirement of modern multi-functional

systems, bandpass filters (BPFs) should have multi-frequency response

Switchable BPFs can be used to control the spectrum of proposed

signals and support multiple information channels[1, 2]

In some cases, there is a requirement for switchable BPFs to have

dual-band bandpass and single bandpass characteristics Much research

work has been performed and various methods have been designed to

realise dual-band behaviour Among them, there are two typical

methods The first category is to combine sets of independent resonators

with common input/output ports[3, 4] The second is to utilise stepped

impedance resonators[5, 6] However, dual-band BPFs designed by

these methods cannot be conveniently switched to operate in one

pass-band In this Letter, a novel structure to implement dual-band behaviour

is proposed, and the filter employing this structure can be easily

switched to operate in one passband With the pin diodes on, a

short-circuited stub is loaded in the centre of the open-loop resonator to

obtain a dual-mode dual-band BPF By turning the pin diodes off, the

stub is disconnected with the open-loop resonator, and the filter has

only one passband The passband frequencies can be easily controlled

by tuning the length of the stub and the open-loop resonator Based

on the above principle, the switchable BPF operating at 1.8/3.5 and

3.5 GHz is designed and fabricated The EM simulated and measured

results are present to demonstrate the performance of the proposed filter

L1

s

L3

L2

w0

L5

w1

w2

L4

D1

D2

L R DC

g1

g2 DC

w3

Fig 1 Schematic of proposed filter

Z2 , L2

2Z2 , L2

Z ino

Z ine

Z1 , L1

Fig 2 Structure and equivalent circuit of dual-mode resonator

a Dual-mode with short-circuited stub

b Even-mode equivalent circuit

c Odd-mode equivalent

Filter design: The schematic of the proposed BPF is shown inFig 1

The filter is composed of a pair of coupled microstrip open-loop

resona-tors and two short-circuited stubs The external coupling is established

by edge coupling As depicted inFig 2a, the resonator with the

short-circuited stub loaded is symmetrical, and then odd- and even-mode

analysis can be applied to characterise it For even-mode and

odd-mode excitation, the equivalent circuit is shown in Figs 2b and c

The input impedance for even-mode and odd-mode can be expressed as

Zine= jZ1

Z1tan(bL1) + 2Z2tan(bL2)

Z1− 2Z2tan(bL1) tan(bL2) (1)

wherebis the propagation constant, and it is equal for even-mode and odd-mode The resonance condition is

By solving (3), the fundamental resonant frequency can be determined

as follows:

4(L1+ L2) √ , f1e odd= c

4L1 1

e

Apparently, the odd-mode resonant frequency fodd is larger than the even-mode resonant frequency feven.fodd is only determined by the length of the open-loop resonator and the length of the short-circuited stub only influences feven In our design, the odd-mode resonant fre-quency fodd is 3.5 GHz, and the even-mode resonant frequency feven is 1.8 GHz When the pin diodes are in the ‘ON’ state, the short-circuited stubs are connected The existence of both even-mode and odd-mode in the resonators results in a dual-band bandpass response Under this state, the filter obtains two passbands ( f1¼ 1.8 GHz, f2¼ 3.5 GHz) When the pin diodes are in the ‘OFF’ state, the short-circuited stubs are discon-nected with the open-loop resonators There is one mode in the resona-tors, and thus the filter has only one passband ( f1 ¼3.5 GHz) The bandwidths of the two passbands are mainly affected by the coupling structure, and the desired bandwidths can be acquired by choosing proper dimensions of the coupling structure Based on the above analy-sis, the filter is designed to operate in two different states by turning the pin diodes on and off Infineon pin diodes BAR63-02V in a SC79 package are used to connect the stubs, which have a capacitance of 0.22 pF, a parasitic inductance of 0.6 nH, and a resistance of 1.2 V at

100 MHz

The filter is fabricated on a substrate with dielectric permittivity 1r¼ 2.65 and thickness h ¼ 0.8 mm The fabricated filter with DC bias is shown inFig 3 The final optimised parameters of the filter are listed

as follows: L1¼ 1.6 mm, L2¼ 5.2 mm, L3¼ 3 mm, L4¼ 2.5 mm,

L5¼ 11.8 mm, w0¼ 2.2 mm, w1¼ 1 mm, w2¼ 0.9 mm, w3¼ 0.5 mm, g1¼ 0.7 mm, g2¼ 1.1 mm, s ¼ 0.18 mm, L¼ 27 nH, R ¼

1 kV The overall size of the proposed filter is 30× 30 mm2

Fig 3 Photograph of fabricated filter

Simulated and measured results: Simulation and measurement were carried out with EM simulation software HFSS 11.0 and an Agilent vector network analyser N5230A Fig 4 shows the simulated and measured scattering parameters when the pin diodes are in the ‘ON’ state It can be seen that the proposed filter can operate at 1.8 and 3.5 GHz, and the measured fractional bandwidths are 9% in the lower passband and 7% in the upper passband.Fig 5shows the simulated and measured results when the pin diodes are in the ‘OFF’ state According toFig 5, the filter obtains only one passband The measured results agree with the simulated results, which validates the design Insertion loss is nearly 3 dB in the lower passband, a little higher than that of the simulated results, which mainly results from the resistances

of the pin diodes

–20

–40

–60

simulated measured

S 11

S 21

frequency, GHz

Fig 4 Simulated and measured results with pin diodes on

–20

0

–40

–60

simulated measured

frequency, GHz

S 11

S 21

Fig 5 Simulated and measured results with pin diodes off

Conclusion: A switchable bandpass filter with two-state frequency

responses has been developed The proposed filter can be conveniently

tuned in two states, operating as a dual-band bandpass filter, or just having one passband Experimental results agree well with simulated results, and show good performance Because of its simple structure, compact size, and good performance, the proposed filter is attractive for use in GSM and WiMax systems

#The Institution of Engineering and Technology 2011

7 November 2010 doi: 10.1049/el.2010.3156 One or more of the Figures in this Letter are available in colour online

B Lui, F Wei and X Shi (National Key Laboratory of Science and Technology on Antennas and Microwaves, Xidian University, Box

223, No 2 South Taibai Road, Xi’an, 710071 Shaanxi, People’s Republic of China)

E-mail: baby2884419@163.com References

1 Wong, P.W., and Hunter, I.C.: ‘Electronically reconfigurable microwave bandpass filter’, IEEE Trans Microw Theory Tech., 2009, 57, (12),

pp 3070 – 3079

2 Tu, W.-H.: ‘Swithable microstrip bandpass filters with reconfigurable on-state frequency responses’, IEEE Microw Wirel Compon Lett., 2010,

20, (4), pp 208 – 210

3 Zhang, X.Y., and Xuei, Q.: ‘Novel dual-mode dual-band filters using coplanar-waveguide-fed ring resonators’, IEEE Trans Microw Theory Tech., 2007, 55, (10), pp 2183 – 2190

4 Dai, G.-L., Guo, Y.-X., and Xia, M.-Y.: ‘Dual-band bandpass filter using parallel short-ended feed scheme’, IEEE Microw Wirel Compon Lett.,

2010, 20, (6), pp 325 – 327

5 Shi, J., and Xuei, Q.: ‘Novel balanced dual-band bandpass filter using coupled stepped-impedance resonators’, IEEE Microw Wirel Compon Lett., 2010, 20, (1), pp 19 – 21

6 Tsengand, C.-H., and Shao, H.-Y.: ‘A new dual-band microstrip bandpass filter using net-type resonators’, IEEE Microw Wirel Compon Lett., 2010, 20, (4), pp 196 – 198