Compact dual-band BPF with wide stopbandusing stub-loaded spiral stepped-impedanceresonator

V. Singh✉, V.K. Killamsetty and B. Mukherjee

Techset Com

A compact dual-band bandpass filter (BPF) with wide stopband per-formance using stub-loaded spiral short-circuit λ/4 stepped-impedanceresonator is proposed. Spiral configuration has been used for compact-ness of filter. Both passbands can be controlled individually by chan-ging the geometric parameters of resonator. Multiple transmissionzeros provide high selectivity to both passband and extend stopbandup to 3 GHz. The filter has compact size of 0.06λg × 0.09λg. A dual-band BPF has been designed and fabricated for terrestrial trunkedradio (TETRA) band and global system for mobile communicationapplications.

Introduction: Owing to increasing demand for dual-band operation inwireless communication, dual-band filters having compact size, goodisolation between passbands, high selectivity, and wide stopband arerequired. Several methods are investigated to design dual-band bandpassfilter (BPF) [1–6]. In [1], series and parallel open stubs are used as reso-nators to design dual-band BPF. The filter has good selectivity, but thestopband rejection needs improvement and also has large size. In [2], asplit ring λ/4 resonator and a stepped-impedance resonator (SIR) wereused to design dual-band BPF. However, the selectivity and stopbandhave to be improved. Novel stub-loaded (SL) theory [3] was used todesign balanced dual-band BPF with independently controlled passbandfrequencies and bandwidths. Still selectivity of the second passbandneeds improvement and also size has to be miniaturised. In [4], dual-band performance was achieved without increasing the overall circuitsize. Here, first passband generated by ring resonator and second pass-band is introduced because of tightly coupled input and output struc-tures. In [5], open-/short-circuited SL resonators were used to builddual-band BPFs. Here, selectivity of passbands needs improvementand design should be compact.

In this Letter, two SL spiral short-circuit λ/4 SIR (SLS-SIR) are usedto design a dual-band BPF at central frequencies of f1 = 0.350 GHz andf2 = 0.900 GHz. Spiral configuration helps for the miniaturisation offilter. SIR is used for pushing the harmonics away up to 8.57f1(3.33f2). Two passbands are generated and controlled individually.Eight transmission zeros (TZs) offer high selectivity and wide stopband.

Filter design: Fig. 1 shows the configuration of the proposed dual-bandBPF. The filter consists of two SL short-circuit quarter wavelength SIRs(SL-SIRs), and for the miniaturisation of filter spiral configuration of theproposed resonator is used.

L2

G1

G4

G6

L8

L7

G8

K2

K1

K5

W2

W1

W0

L4

L6

G2

G3

G7

G5K4

K3

L5

L3

port 1 port 2

D

L1

Fig. 1 Configuration of proposed dual-band BPF

From [2], a short-circuit λ/4 SIR is designed at the centred frequencyof f1 (0.350 GHz) and then for dual-band operation of filter, a stub isloaded on an SIR as shown in Fig. 2. The input admittance for the pro-posed resonator can be calculated as

Yin = 1

Z2

Z1 K1 − tan u1 tan u2( ) + jZL K1 tan u1 + tan u2( )ZL 1− K1 tan u1 tan u2( ) + jZ1 tan u1 + K1 tan u2( )

[ ](1)

positionLtd, Salisbury

where

ZL = jK2 tan u3 cot u4K2 cot u4 − tan u3

(2)

and K1 = Z2/Z1 and K2 = Z4/Z1 are impedance ratios. Resonant frequencyof the proposed resonator can be calculated by setting Yin = 0. Therefore,resonant condition are given by (3a) and (3b)

Z1 K1 − tan u1 tan u2( )(K2 cot u4 − tan u3)− (K2 tan u3 cot u4)

× K1 tan u1 + tan u2( ) = 0 (3a)

tan u3 tan u4 = K2 (3b)

Equations (3a) and (3b) show that the proposed resonator gives tworesonating frequencies. Therefore, the proposed resonator is a dual-mode resonator and each mode decides one passband of dual-bandBPF separately.

ground

loadedstub

Z1,q3 Z1,q1Z2,q2 Yin

Z4,q4

Fig. 2 Schematic of proposed SL-SIR

Fig. 3 shows simulated results of single-band filters operating atdifferent centre frequencies and dual-band BPF. Passband-1 is createdwhen both resonators without loaded stubs are resonating at 0.35 GHzand passband-2 is created when loaded stub is behaving as λ/4 SIR atf2 (0.90 GHz) with shared path as shown in Fig. 4. The proposed dual-band BPF’ characteristics is the combination of each passband.

S21

–20

0

–40

–60

mag

nitu

de, d

B

–80

–100

–1200.5 1.0 1.5

0.90 GHz0.350 GHz 0.35/0.90 GHz

frequency, GHz

2.0 2.5 3.0

0.90 GHz

0.35/0.90 GHz

0.350 GHz

Fig. 3 Simulated frequency response of filters

l/4 at f1

l/4 at f2

shared path loaded stub

port 2port 1

Fig. 4 Schematic of dual-band BPF using proposed resonator

TZs (TZ1, TZ3, and TZ6) are generated due to mixed coupling [6],TZs (TZ2, TZ4, and TZ7) are generated because the lengths betweenthe tapped points and open-ends of the input/output resonators behaveas open-circuit λ/4 resonator at these frequencies and TZs (TZ5 and

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TZ8) are generated due to source-coupling [7] and Fig. 5 shows that TZ5

and TZ8 are varying with gap (G7).

0.4 0.8

TZ2TZ4

TZ1

TZ6

3

2

1

freq

uenc

y, G

Hz

0

TZ5

TZ3

TZ7

TZ8

G7, mm

1.2

Fig. 5 Variation of TZs with gap (G7)

Fig. 6 shows that both passbands of dual-band BPF can be tuned indi-vidually by changing geometric parameters of resonator. The filter isoptimised and simulated using Computer Simulation Technology soft-ware. The prototype is designed and fabricated on Rogers RO3010dielectric sheet (having ɛr = 10.2, tan δ = 0.0022, height of substrate =1.28 mm, and thickness of metal = 0.017 mm). Filter having compactsize of 20.8 × 31 mm2 (0.06λg × 0.09λg), where λg is the guided wave-length at centre frequency of passband-1 (0.350 GHz) and the optimiseddimensions of the filter are given as: L1 = 9.0, L2 = 10.3, L3 = 22, L4 =1.8, L5 = 3.9, L6 = 7.9, L7 = 12, L8 = 8.8, K1 = 12.2, K2 = 0.38, K3 =7.25, K4 = 2.2, K5 = 4.4, G1 = 1.3, G2 = 0.8, G3 = 3.1, G4 = 0.5, G5 =1.85, G6 =G8 = 0.2, G7 = 1.14, W0 =W1 = 1.2, and W2 =D = 1 (unit:millimetres).

0

–20

–40

–60

|S21

|, dB

–800.3 0.6 0.9

frequency, GHz

L1 = 6 mm

L1 = 9 mm

L1 = 12 mm

L7 = 11 mmL7 = 12 mmL7 = 13 mm

1.2

0

–20

–40

–60

|S21

|, dB

–800.3 0.6

frequency, GHz

a b

0.9 1.2

Fig. 6 Passbands of dual-band BPF

a Independently tuning of passband-1b Independently tuning of passband-2

0

–30

–60

TZ1

TZ2

TZ4

TZ3

TZ5

TZ7

TZ8

TZ6

S11S21

mag

nitu

de, d

B

–90

1.00.5 1.5

simulated results

measured results

frequency, GHz

2.0 2.5 3.0

Fig. 7 Simulated and measured results of proposed filter

Results and discussion: Measurement of fabricated design is done byusing Agilent E5071C Vector Network Analyser. Fig. 7 shows simu-lated and measured results of the filter. Filter operates at 0.35/0.90 GHz, with fractional bandwidth of 10.57%/13.67%, respectively.Passband-1/passband-2 having a measured insertion loss of 1.6 dB/1.4 dB and measured return loss is better than 14 dB/17 dB. EightTZs are generated at 0.284, 0.471, 0.657, 1.04, 1.37, 1.68, 1.91, and2.90 GHz. These TZs provide high selectivity, good isolation betweenpassbands and improve the stopband performance up to 2.3 GHz with>19 dB rejection level and from 2.3 to 3 GHz with >12 dB rejectionlevel. A comparison of the proposed structure with other references isas tabulated in Table 1 and it shows that this work has compact sizeand both passbands have good selectivity.

Table 1: Comparisons of proposed design with previous designs

Ref.

f0, GHz/3 dB FBW

(%)

Insertionloss, dB

Size(λg × λg)

Selectivity ofbands

[2]

2.49/3.85, 3.85/4.4 1.62/1.31 0.18 × 0.18 Yes/No

[3]

1.8/8.64, 5.8/5.35 1.2/2 0.37 × 0.28 Yes/No

[4]

2.4/9.2, 5.2/9.5 1.4/2.7 0.18 × 0.18 Yes/Yes

[5]

2.4/4.63, 5.8/3.6 1.35/1.97 0.39 × 0.25 No/Yes

This work

0.35/10.57, 0.90/13.67 1.6/1.4 0.06 × 0.09 Yes/Yes

Conclusion: A compact dual-band BPF with wide stopband is pre-sented using SLS-SIR for TETRA band and global system for mobilecommunication applications. The filter has compact size of 0.06λg ×0.09λg due to spiral configuration. The filter has good selectivity andwide stopband due to generation of eight TZs generated. It has been ver-ified by simulated and measured results. The proposed BPF has signifi-cant improvement in selectivity of both passband and size reduction incomparison with the references presented in Table 1.

© The Institution of Engineering and Technology 2016Submitted: 23 August 2016doi: 10.1049/el.2016.2838One or more of the Figures in this Letter are available in colour online.

V. Singh, V.K. Killamsetty and B. Mukherjee (Department ofElectronics and Communication Engineering, PDPM Indian Instituteof Information Technology, Design and Manufacturing, Jabalpur,Madhya Pradesh, India)

✉ E-mail: vivekchauhan10@gmail.com

References

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