2006 INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS, SEPTEMBER 12 - 14, 2006, PUTRAJAYA, MALAYSIA

Miniaturized Low Pass Filter Using Modified Unifolded Single Hairpin-LineResonator for Microwave Communication Systems

Samsul Haimi Dahlan' and Mazlina Esa21 Kolej Universiti Teknologi Tun Hussien Onn, 86400 Parti Raja, Batu Pahat, Johor, MALAYSIA2 Microwave/RF and Antenna Research Group, Department of Radio Communication Egineering,

Faculty of Electrical Engineering, 81310 UTM Skudai, Johor, MALAYSIAsamsulh Okuittho.edu.my, mazlina Ofke.utm.my

Abstract - A compact low pass filter (LPF) usingsingle folded stepped impedance hairpin resonator isproposed. It consists of an internally coupled-linesection which acts as a phi-connection capacitors, anda transmission line which act as a parallel connectedinductors. Together, an elliptic low pass filterconfiguration is achieved. The internal coupled line'slength is tuned to adjust the cutoff frequency as well asthe spurious frequency response. The length of theinductive transmission line section is maintained.Simulation investigations revealed that as the coupledline length increases, the magnitudes of the spuriousfrequency responses reduced and the locations wereshifted to higher frequencies. Hence, wider stopbandbandwidth area is achieved. It is also interesting tonote that small coupled line extension introduced at theoriginal coupled line has improved the profile further.Using electromagnetic simulation software, it is shownthat a compact LPF with 2 GHz cutoff frequency andband reject of 3 GHz to 10 GHz has been successfullydesigned and simulated.

Keywords: Folded stepped impedance hairpin resonator,Low Pass Filter, Elliptic filter response.

1. Introduction

Hairpin resonator has always drawn muchattention in the design of compact microwave devicesince its introduction in the early 1970s [1]. Intelecommunication system, small size and highperformance devices are always in high demand tominimize overall system size and reduce productioncost. The resonator has proved its usefulness inproducing small and compact microwave devices suchas bandpass filter, oscillator and mixer [2]. Theapplication is extended further in the development ofthe compact low pass filter. The filter was firstproposed by Kai Chang and Lung Hwa-Hsieh usingstepped-impedance hairpin resonator. It has widebandgap with elliptic response characteristic. Thedimensions of the low pass filter are synthesized from

the equivalent-circuit model with the publishedelement-value table. The stopband bandwidth,however, is limited by the spurious frequency due toharmonics. To overcome this problem, additionalattenuation poles were added to surpress the spuriousfrequencies. Cascades of stepped-impedance hairpinresonators were used to achieve sharp cutoff frequencyresponse [3].

The same type of filter can also be developedusing folded stepped-impedance hairpin resonator. Theresonator's design method and analysis are equivalentto that of the conventional stepped-impedanceresonator but, it offers smaller size due to the foldedopen-end area (internally coupled line). Based onprevious studies, the resonance frequency of theresonator can be tuned by varying the coupled linelength [1]-[7]. Besides, the capacitance effect due tothe internal coupled line shift the spurious frequencyfrom the integer multiples of the fundamental resonantfrequency. This helps widen the stopband bandwidth.Various researches have been done on hairpinresonator filters [8]-[16].

This paper present a compact LPF design usingsingle miniaturized folded stepped impedance hairpinresonator. The planar circuit design is as shown inFigure 1 and its equivalent circuit is as shown inFigure 3. A study was performed to see the effect onthe cutoff frequency and the spurious response due tothe variations of the coupled line length. All theseeffects were performed through simulation using theSonnetLite software [15].

2. Miniaturized Folded Hairpin-Line ResonatorDesign

Figure 1 shows the miniaturized folded steppedimpedance hairpin resonator design. By properly selectthe impedance ratio and length ratio betweenresonator's transmission line and the coupled line, acompact resonator can be designed [1]. To produce acompact resonator, the impedance ratio R =ZIZ,between the coupled line and the transmission line

0-7803-9745-2/06/$20.00 ©2006 IEEE. 16

should be selected at lower than 1. Figure 1 shows a

LPF planar circuit using miniaturized steppedimpedance internally coupled hairpin resonator withtwo direct tap at its transmission line end. Theresulting simulated response is shown in Figure 2. Thestopband of the LPF is limited by the spuriousresponse of the resonator. Supressing the spuriousresponse will help widen the stopband.

Af t~~~~~~zFigure 1: Miniaturized folded stepped impedance hairpinresonator with double tapped at the end of the transmission

line.

0

M

a

n

t

de

-10-

-20

-30

-40

-50-

-60

dB)

a

0 1 2 3 4 5 6 7 8 9 10

onnet Softvare Inc:. Frequency (GHz)

Figure 2: The LPF response with first and second spuriousresponse effects denotes by spl and sp2.

3. Low Pass Filter with Single MiniaturizedResonator

The equivalent lumped-element circuit ofconfiguration in Figure 1 is shown in Figure 3. It issimilar to that of the connection of an elliptic low passfilter design. The transmission line is inductive henceit is equivalent to an inductor and the parallel coupledline is equivalent to phi-circuit connection ofcapacitors [3]-[7], [16]-[18].

dC

Figure 3: Equivalent circuit of a stepped impedanceinternally coupled hairpin resonator [4].

The capacitance due to internal coupled-line isdenoted by Cg. Capacitance due to strip to ground isdenoted by Cgnd. Capacitance Cg will be dependent tothe size of the internal coupled-line gap and the length.Longer line and narrower gap will provide higher Cg.

For this elliptic LPF lumped element circuit,changes of L and Cg values will cause the cut-offfrequency to vary. The pole can be tuned by varyingthe value of Cgnd. Hence, the same effect is expected atthe planar circuit. To verify the planar circuit,simulation was performed using SonnetLite software.The circuit was simulated based on the RogersR0601OLM substrate with a relative dielectricconstant of 10.2, a loss tangent of 0.0023, and a

thickness of 0.254 mm. The study was to verify theeffect of extending the coupled-line length to thespurious and the fundamental operating frequencyresponse.

It can be observed that the increase of thecapacitance effect at the coupled line reducing themagnitude of the spurious signals of the filter, as

depicted in Figure 4. Point A shows the S1I magnitudeof the first spurious signal when a short coupled line isapplied, and point B is when longer (towardsmaximum) coupled line length is applied. Similarphenomenon is observed at points C and D for thesecond spurious signal. This has given an advantage tothe circuit whereby the stopband area of the LPFresponse can be widened.

In addition, the cut-off frequency shifted to theleft when the coupled-line length increased, as

depicted in Figure 5.

17

sp2

-4 Uo

* 0

10

M -10a

20-

n -3040

tu -50d -6o

(dB) -8--90

-1000 1 2 3 4 5 6 7

Frequency (GHz)

capacitor, C, referred to capacitor at the additionalcoupled-line and Cg, denotes its strip to groundcapacitor.

(a)

Figure 4: The return loss profile for the circuit with variouscoupled-line lengths. The magnitude of the spurious signal

reduces with the increase of the length.

2

1

ag -1n

-2t -3

-4de -5

dB) -6-7-8

1 1.5 2

Frequency (GHz)

(b)

fc +C_ gcp gx

X _7

fC + C_ _ gcp gx

7,Figure 5: The cut-off frequency of the LPF as the coupled-line length (I) increase. IC2 > IC1 -

The stepped impedance folded resonator,however, having the disadvantage of a limited spacefor the coupled-line due to the nature of the design. Iflonger coupled line were required to bring the cut-offfrequency further down, this will be the limit. Thisproblem, however, can be solved by introducingadditional coupled-line to the original design as shownin Figure 6(a). The equivalent lumped element circuitis also presented. Cc, denotes the capacitor at theinternal coupled-line, Cgcp is the strip to ground

(c)

Figure 6: (a) Stepped impedance hairpin resonator withadditional coupled line (b) 3D view of equivalent circuit (c)

2D view of equivalent lumped element circuit.

The additional coupled line is equivalent toanother phi-C circuit connected in parallel. Theconnection increases the effective capacitance of thecoupled-line and between the strip to ground. Based on

18

the resulting response as shown in Figure 7, theskirting becomes steeper and the cut off frequencyshifts to lower frequency.

Magn

tude

IB)

2

1

0

-1

-2

-3

-4

-5

-60 0.5 1 1.5

re lne- Frequency (GHz)2 2.5

Figure 7: The cut off frequency of the resonator withadditional coupled-line (lac), where lac2 > ad

A LPF with 2 GHz cut off frequency is designedfrom a stepped impedance resonator with originalfundamental frequency of 2.8 GHz. The coupled line isprolonged to maximum and an additional coupled lineis added to achieve the desired cut off target. Thedimensions of the circuit are as follows: coupled gap g= 0.1 mm, tap line lengthpl = 1.2 mm, transmissionline length p2 = 5 mm, p3 = 2 mm, p4 = 1.5 mm. Thecoupled line width is w = 0.6 mm for 35 Ohmcharacteristic impedance and the width of transmissionline is 0.25 mm for 50 Ohm characteristic impedance.The design is as shown in Figure 8 and the resultingresponse is depicted in Figure 9. The LPF responsehaving the cut off frequency of 2 GHz with 20 dBband reject maintain from 3 GHz to 10 GHz. Thisprovides a very good LPF performance with veryminimum circuit modification required.

4. Conclusion

A compact LPF using single stepped impedancefolded hairpin resonator is proposed. Circuit responsecan be adjusted by varying the coupled line section ofthe filter. Additional coupled line can be used toincrease capacitance effect to tune the cut offfrequency. The high capacitance effect helps to supressand extend the spurious signals further and make theband reject area wider. The resulting circuit has goodLPF performance and of simple design.

g14*- *l

4F- 8f

Figure 8: Low Pass Filter design for 2 GHz cut off frequency

10 -

0 -

a 10-g -20 -

nI-30-tu -40 -de

-60 -

(dB)-70 -

-80

0 1 2 3 4 5 6Frequency (GHz)

7 8 9 10

Figure 9: The Low Pass filter response with 2 GHz cutofffrequency and with 20 dB band rejection from 3 GHz to 10

GHz.

References

[1] Sheng Yuan Lee and Chih Ming Tsai, "Newcross-coupled filter design using improved hairpinresonator, " IEEE Trans. Microwave Theory Tech.,vol 48,pp. 2482- 2490, Dec.2000.

[2] Mitsuo Makimoto and S. Yamashita, MicrowaveResonators and Filters for WirelessCommunication Theory, Design and Application,Springer, New York, 2000.

[3] L.H. Hseih and Kai Chang, "Compact lowpassflter using stepped impedance hairpin resonator,"Electron. Lett., vol.37, no 14, pp. 899-900, July2001.

19

"I S21

S11i ''RX~~~~~~~\ ....

Sil \k '''

r r~~

[4] Samsul Haimi Dahlan, Compact microwavehairpin line band pass filter using foldedquarterwave resonator, Master Dissertation,Universiti Teknologi Malaysia, March 2005.

[5] Mazlina Esa and Samsul Haimi Dahlan,Miniaturised Hairpin Line Filter, Proc of 2005Malaysia Science and Technology Congress(MSTC2005), Cititel Midvalley, Kuala Lumpur,Malaysia, 18-20 April 2005.

[6] Samsul Haimi Dahlan and Mazlina Esa, Design ofFolded Quarter Wave Resonators, Proc of 2005Asia Pacific Conference on AppliedElectromagnetics (APACE2005), Hyatt Regency,Johor Bahru, Johor, Malaysia, 20-21 December2005.

[7] Samsul Haimi Dahlan and Mazlina Esa,Miniaturised Hairpin-Line Bandpass Filter, Procof 2005 Asia Pacific Conference on AppliedElectromagnetics (APACE2005), Hyatt Regency,Johor Bahru, Johor, Malaysia, 20-21 December2005.

[8] Jia-Sheng Hong and Michael J. Lancaster, "Cross-Coupled Microstrip Hairpin-Resonator Filters",IEEE, vol. 46, No. 1, pp.1 18-122, Jan 1998.

[9] JS Yuk, JS Park, Dal Ahn, KS Choi and JunoKim," A Novel Accurate Design Method For TheHairpin Type Coupled Line Bandpass Filter",IEEE MTT-S Digest, 2001.

[1O] Cheng Chun Chen, Yi-Ru Chen and Chi-YangChang, "Miniaturized Microstrip Cross-CoupledFilters using Quarter-Wave or Quasi-Quarter-Wave Resonators", IEEE, vol. 51, No. 1, pp.120-131, Jan 2003.

[11] Chi-Yang Chang and Cheng Chun Chen, "ANovel Coupling Structure Suitable for Cross-Coupled Filters with Folded Quarter-WaveResonators", IEEE, vol. 13, No. 12, pp.517-519,Dec 2003.

[12] Kuo, J. T., Hsu, W. H. and Huang, W. T. (2002).Parallel coupled microstrip filters withsuppression of harmonic response. IEEEMicrowave Wireless Comp. Lett., 12, 383-385.

[13] Ikhwan Peranggi Pohan, Mazlina Esa, JasmyYunus, Noor Asniza Murad, "Filter Candidate forthe Front-End Rectenna Prototype of a WPTReception", Proceedings of InternationalElectronics Seminar, Indonesia, Dec. 2005.

[14] Ikhwan Peranggi Pohan, A Compact S-BandFront-End Rectenna for Wireless Power TransferApplication, unpublished Master dissertation,Universiti Teknologi Malaysia, Jan. 2006.

[15] Sonnet 8.51-Lite Plus User's Guide, SonnetSoftware Inc. USA.

[16] D. M. Pozar, 3rd edition, Microwave Engineering,New York, Wiley, 2005.

[17] Morikazu Sagawa and Kenichi Takahashi,"Miniaturized Hairpin Resonator Filters and TheirApplication to Receiver Front-End MIC's", IEEETransactions on Microwave Theory andTechniques, Vol 37, No 12, Dec 1989 pp. 1991-1997.

[18]L.H. Hseih and Kai Chang, "Compact Elliptic-function Low-Pass Filters Using MicrostripStepped-Impedance Hairpin Resonators", IEEETransactions on Microwave Theory andTechniques, Vol. 51, No.1, Jan 2003 pp. 193-198.

20