Proceedings of Asia-Pacific Microwave Conference 2007

An Active Resonator Using Defected GroundStructure with Islands

Mun-Su HwangDepartment of Electrical and Communication System Engineering, Soonchunhyang University

Asan, Chungnam, 336-645, Republic ofKoreaTel :82-41-530-1630, email: u zicman@hotmail.com

Seongmin OhDepartment of Electrical and Communication System Engineering, Soonchunhyang University

Asan, Chungnam, 336-645, Republic of KoreaTel :82-41-530-1630, email: steadfastfriend(nate.com

Jae-Jin KooDepartment of Electrical and Communication System Engineering, Soonchunhyang University

Asan, Chungnam, 336-645, Republic of KoreaTel :82-41-530-1630, email: jaejinkoo617ghotmail.com

Chunseon ParkDepartment of Electrical and Communication System Engineering, Soonchunhyang University

Asan, Chungnam, 336-645, Republic of KoreaTel :82-41-530-1630, email: dujigparan.com

Jongsik LimDepartment of Electrical and Communication System Engineering, Soonchunhyang University

Asan, Chungnam, 336-645, Republic of KoreaTel :82-41-530-1332, email: jslimgsch.ac.kr

Yongchae JeongDepartment of Electronic and Information Engineering and IDEC WG, Chonbuk National University

Jeonju, 561-756, Republic of Korea.Tel :82-63-270-2458, email: ycjeongWmail.chonbuk.ac.kr

Kwan-Sun ChoiDepartment of Electrical and Communication System Engineering, Soonchunhyang University

Asan, Chungnam, 336-645, Republic of KoreaTel :82-41-530-1329, email: cks l 329@sch.ac.kr

Dal AhnDepartment of Electrical and Communication System Engineering, Soonchunhyang University

Asan, Chungnam, 336-645, Republic of KoreaTel :82-41-530-1331, email: dahnkr@sch.ac.kr

Abstract- A new resonator using active device and defected which is one of key factors of the proposed activeground structure with islands (DGSI) is proposed. The resonator. The negative resistance is realized by the seriesactive device is used for supplying the negative resistance feedback circuit of the active device. The electrical

1-4244-0749-4/07/$20.00 @2007 IEEE.

characteristics of the proposed resonator with DGSI areimproved by combining the negative resistance and themicrostrip line with DGSI where the electric field is themost strong. It is shown that the measured improvement inS21 and Sit of the proposed active resonator with DGSIare 4.55dB and 0.32dB, respectively, at the resonantfrequency compared to the previous DGSI resonatorwithout active device.

Keywords-component; resonator, active resonator, DGS, DGSI,defected ground structure

I. INTRODUCTION

G Kd

sI d

Fed LdFig. 1 FET series RLC feedback circuit

G

Passive resonators have been widely used in design highfrequency filters. It has been one of interesting challenge todevelop resonators of which size and loss are minimized.Especially, highly integrated resonators are required in RFICand MMIC technologies. However, severely miniaturizedresonators have relatively increased loss, which is the cause ofdegraded Q-factor. So it is required to compensate the loss andimprove Q of passive resonators. One of methods is to adoptactive devices in resonator circuits, which is called activeresonators.

In this paper, a novel active resonator is proposed bycombining the existing microstrip line with DGSI (defectedground structure with islands) and the negative resistance ofactive devices [1-3]. The microstrip line and DGSI forms a

parallel line structure to which the negative resistance circuithaving the series RLC feedback circuit of FETs are connected.Due to the combined active device, the loss and Q ofresonators are improved.

The analysis for the negative resistance and measuredfrequency characteristics are presented to show the validity ofthe proposed resonator.

II. NEGATIVE RESISTANCE OF ACTIVE DEVICES

Fig. 1 and Fig. 2 show the RLC series feedback circuit ofa common-source FET topology and its equivalent circuit,respectively. The impedances Z1, Z2 and Zd in the equivalentcircuit are expressed by eqs. (1)-(3) [3-5].

The input admittance, Yin, is expressed by eq. (4). Yin can

be expressed by real and imaginary parts separately, and Fig. 2is simplified as Fig. 3. The real part of Yin is the negativeresistance (Rneg) of the FET series RLC feedback circuit. Rnegis mainly dependent of Ld and Cd. The larger real part of Yin,the smaller Rneg and the narrower bandwidth where thenegative resistance exists.

Fig. 2 Equivalent circuit of the FET series RLC feedbackcircuit

zi =I

= _jx1 (1)JWcgs

Z2= = _jx2 (2)

IWCgd

Z =R ~~=Rd+Xd (3)d =d + (d - ) =d + jXd (3CCt)d

Ceq

yi in

Fig. 3 Simplified equivalent circuit of Fig. 2

Rd+gm+ (Rd2+ (Xd X2)Xd)IN Rd +(Xd -X2)

1 Xd -X2 gmX2Rd )

Xi Rd2 + (Xd -X2)2

R +JO)Ceq

neg

(4)

D

n

III. PROPOSED ACTIVE RESONATOR USING DGSI

Fig. 4(a) shows the proposed active resonator having DGSIand active part for the negative resistance. DGSI is composedof one dumb-bell shaped DGS on the ground plane and twoislands at both sides of microstrip line on the top plane [1,2].The negative resistance is realized by FET series RLCfeedback circuit which is connected to one of islands on thetop plane. The connection point of RLC feedback circuit isplaced at the very high dense region of electric field.

Fig. 4(b) shows the equivalent circuit of the proposedactive resonator with DGSI. + represents the electrical lengthof islands, and L and C are the equivalent circuit elements ofDGSI. Rneg and Ceq are determined using Fig. 3 and eq. (4).The loss of passive DGSI resonator is compensated throughthe active negative resistance.

MIct tMrp ihne onthe top ptane

II

DGS on thebOfOm PlAne

resonator frequency depends on Ctotal rather than C, and Ctoialis greater than C.

C -C+ eq gc C +Ctotal e gCeq +

(5)

IV. SIMULATION AND MEASURED RESULTS

The proposed resonator composed of DGSI and activedevice has been fabricated, as shown in Fig. 5, measured andcompared to the previous passive resonator realized by DGSIonly. The FET device adopted is Fujitsu FHX35LG to which2.5V of Vds and lOmA of Ids have been applied for biasing it.The element values of Rd, Ld, Cd, and Cg for the negativeresistance circuit are 39Q, 8.2nH, 3pF, and lpF, respectively.

lslands on thetop plane

i ,,

Ib~ I_

;1 X

tlwsI LI,<. Rd

(a

(a)

(a) (b)Fig. 5 Photograph of the fabricated active resonator (a) Topview (b) Bottom view

'T / /I

% eqf L C"

0;--

Ri ieg

(b)

Fig. 4 Proposed active resonator using DGSI and active device(a) layout and schematic (b) equivalent circuit (Er=2.2,substrate thickness=3 Imils, a=b=5mm, c=15mm,d=W50=2.4mm, g=0.2mm, s=0.5mm)

Total capacitance of the active resonator, Ctotal, can becalculated by eq. (5). Here Cg is the DC block capacitorconnected to the gate of FET. Therefore it is noted that theresonant frequency, Fo, of the active resonator shifts downfrom that of passive DGSI resonator, because the resultant

Fig. 6 shows simulated S-parameters of two resonators, thepassive resonator composed of only DGSI and proposedresonator having DGSI and active device. Agilent ADSmomentum has been used for electromagnetic (EM)simulation, and the required Co-simulation between EM- andcircuit-simulation has been performed on ADS. It is shownthat the simulated S21 and S11 of the proposed activeresonator are improved by 5.6dB and 0.62dB, respectively, atthe resonant frequency compared to DGSI passive resonator.

Fig. 7 illustrates the measured S-parameters of the DGSIresonator and proposed active resonator. The measuredimprovement in S21 and 511 are 4.55dB and 0.23dB,respectively. Even though there are some minor discrepanciesbetween simulation and measurement, it is definitely verifiedthat the frequency response and fractional 3dB bandwidthversus Fo, which is the measure of Q, have been improved.The minor discrepancies are expected to be caused by thepractically inserted additional lines in the negative resistancecircuit for connecting of Rd, Ld, Cd, Cg, and FET device. Table1 summarizes the simulation and measurement data.

Isfff -I-e

V. CONCLUSION

-30

1.0 2.0 3.0

Frequencyl[GHz]

Fig. 6 Simulated S-parameters of the DGSIactive resonator with DGSI and FET

m -10

c

;t -15

-20 X

-30

1.0 2.0 3.0 4.0 4.7

Frequency[GHz]

Fig. 7 Measured S-parameters of the DGSI resonator and

active resonator with DGSI and FET

Table 1. Dataresonator

A new active resonator using DGSI and FET negativeresistance has been proposed and measured. It has beenverified that the proposed active resonator has superiorcharacteristics to the passive resonator composed of onlyDGSI. The measured improvement in S21 and S11 of theactive resonator are 4.5dB and 0.3dB, respectively. Theimprovement has been caused by the compensation of thenegative resistance of the FET series feedback circuit. It isexpected that the proposed active resonator can be applied to

4.0 4.7 microwave circuits such as oscillators and filters and so on,

which require microwave resonators.

resonator and

REFERENCES

[1] J. U. Kim, K. S. Kim, S. J. Lee, J. S Lim, K. H. Park, andK. S Kim, and D. Ahn, "New Defected Ground Structurewith Islands and Equivalent Circuit Model," APMC 2005Digest, Dec. 2005, pp. 458-46 1..

[2] J. U. Kim, J. S Lim, K. S Kim, and D. Ahn, "Effect of a

Lumped Element on DGS with Islands," IEEE MTT-S,Symp. Dig, ppI 145-1148, Jun. 2006.

[3] Y.-H. Chun, J.-R. Lee, S.-W. Yun, J.-K, Rhee, "Design ofan RF low-noise bandpass filter using active capacitancecircuit," IEEE trans. Microwave Theory Tech., vol.MTT-53, No.2, pp687-695, Feb. 2005.

[4] J.-R. Lee, Y.-H. Chun, and S.-W. Yun, "A novel bandpassfilter using active capacitance," in IEEE MTT-S Int.Microwave symp. Dig., vol. 3, 2003, pp. 1747-1750.

[5] C. Y. Chang and T. Itoh, "Microwave active filters basedon coupled negative resistance filter." IEEE Trans.Microw. Theory Tech., vol. 38, no. 9, pp. 1879-1884,Sep. 1990.

of DGSI resonator and proposed active

DGSI only Active ResonatorSimulati Measur Simulati Measuron ement on ement

F0 [GHz] 3.2 3.018 2.8 2.635(m3) (m7) (m4) (m8)

S21 [dB] -23.614 -25.024 -29.219 -29.577(ml) (m5) (m2) (m6)

S11 [dB] -0.625 -0.697 -0.003 -0.370(m3) (m7) (m4) (m8)

3dB BW 1%] 71.9 53.5 56.25 33.85F [o

m4

--D- S21 [Active Resonator]-2!..- Sl [Active Resonator]-X- S21 [DGSI only]

6 -o- Sl [DGSI only]

f