An Embedded RF Lumped Element Hybrid Coupler Using LTCC Technology

Transcription

1 An Embedded RF Lumped Element Hybrid Cupler Using LTCC Technlgy Ke-Li Wu, Chi-Kit Yau and Kwk-Keung M. Cheng Dept. f Electrnics Eng., The Chinese University f Hng Kng, NT., Hng Kng, PRC klwu@ee.cuhk.edu.hk Abstract A nvel embedded RF lumped element 3-dB hybrid cupler is prpsed by a 3D vertical architecture using LTCC technlgy. The innvative architecture deliberately inherits the parasitic capacitance that is inevitable t embedded inductrs fr size reductin. A prttype is build and tested t prve the cncept. This wrk demnstrates the advantages f LTCC technlgy fr reducing the size and manufacturing cst f RF circuits. I. Intrductin The latest wireless prducts demand ever-greater functinality, higher perfrmance and lwer cst in smaller and lighter frmats. Due t the verlng wavelength f current ppular wireless frequency bands, a distributed RF circuit has its inherent limitatin in size reductin. Lumped element circuits, n the ther hand, prvide an effective means t alleviate the predicament, prvided that an apprpriate lw lss multilayer substrate is emplyed. Lw Temperature Cfired Ceramic (LTCC) technlgy, with its lw dielectric lss, high flexibility fr a 3D circuit cnfiguratin, and lw manufacturing cst fr high vlume prductin, is certainly an excellent candidate fr the purpse [1,2]. 3-dB hybrid cupler is ne f the mst ppular cmpnents used in RF/micrwave engineering [3,4]. Hwever, fr RF applicatins, a distributed element hybrid cupler ccupies a large real estate due t the nature f quarter wavelength branch lines. A significant size reductin can be realized by using a high Q lumped element circuit withut nticeable degradatin in electrical perfrmance. In this paper, a nvel 3D vertical architecture fr embedded lumped element 3-dB hybrid cupler is prpsed. The layut is deliberately designed in the way that the parasitic capacitance is beneficially used t reduce the circuit size. A prttype f the prpsed cupler is built in a 7-layer LTCC substrate t prve the cncept. A gd agreement between the measured and the designed can be bserved. II. The Lumped Element 3-dB Cupler

2 In rder t replace a quarter wavelength transmissin line f impedance Z by a lumped element π netwrk as shwn in Fig. 1(a) and t determine the values f the L and C elements, the ABCD matrix f the quarter wavelength transmissin line is equated t that f the lumped element π netwrk at the center frequency [5], i.e. B X = Z r L = Z ω (1a) C L = 1 Z r C =1 ωz (1b) The required cmpnent values fr the lumped element 3dB branch line cupler at the center frequency f 1.88GHz can be determined by equatin (1) and they are listed in Table 1. Table 1 Cmpnent values fr a lumped element 3dB branch line cupler at center frequency f 1.88GHz Z (Ω) L (nh) C (pf) Tw cnsideratins are taken int accunt fr chsing the π netwrk: (1) it is preferable t cnstruct serial inductrs and shunt capacitrs in a multilayer circuit because f less influence f parasite; and (2) the parasitic capacitances f the serial inductr can be absrbed by the tw shunt capacitrs at the ends f the inductr. This cncept is illustrated in Fig. 1(b). Having understd the basic 'pi' netwrk, a schematic lumped element circuit f the 3-dB branch line cupler with center frequency f 1.88 GHz can be easily cnstructed as shwn in Fig. 2. The lumped element 3-dB cupler is implemented in a 7-layer LTCC substrate. The thickness fr each layer is 90 µ m and dielectric cnstant is 7.8. As shwn in Fig.3, the inductrs are built n the upper layers f the π netwrk t reduce the lss induced n grund plane. Due t the parasitic capacitance t grund f the inductrs, the required value f the shunt capacitrs is reduced frm 4.09pF t 3.2pF. Cnsequently, this reductin helps t reduce the real estate f the circuit. The x-y size f the cupler is 5.3mm 4.7mm. The verall structure can be designed using an Electrmagnetic (EM) simulatin tl (fr example, IE3D f Zeland in this case). The designed LTCC lumped element 3-dB cupler has been prttyped and measured. As shwn in Fig. 4, althugh the measured center frequency is mved up abut 150 MHz, the measured electrical perfrmance matches t the predicted respnse very well. It is

3 wrthy t mentin that the insertin lss f the lumped element cupler is better than 0.3 db at resnant frequency. jxl jbc jbc (a) Electric mdel fr LTCC series inductr Lmain C main Cparasitic1 Cparasitic2 Cmain Electric mdel fr LTCC shunt capacitrs (b) Fig.1. (a) A lw-pass π netwrk fr replacing quarter wavelength transmissin line. (b) The π netwrk with pssible parasitic capacitance nh Prt 1 Prt nh 4.23 nh 50Ω 2.99 nh Prt 3 Fig. 2. The schematic f the lumped element 3-dB hybrid cupler with center frequency f 1.88GHz.

4 Lad prt Fig.3. The tp and perspective views f the layut f the lumped element cupler in a 7-layer LTCC substrate. 0 Sij db S13 S12 EM Designed Measured S Frequency (GHz) Fig.4. The measured and the EM designed perfrmance f the LTCC lumped element 3-dB hybrid cupler. III. Cnclusins An innvative RF embedded lumped element 3-dB hybrid cupler is prpsed and implemented, fr the first time, in a 3D vertical cnfiguratin using LTCC technlgy.

5 The schematic circuit and the LTCC layut are ingeniusly designed in the way that the parasitic effects are fully used fr reducing the circuit size. Since an accurate EM simulatin is used, all the parasitic cuplings are accunted in designing the verall layut. As the result, the measured perfrmance agrees well with that f the designed. This new cnfiguratin f embedded passives can be used in many ther applicatins such as lumped element Wilkinsn's pwer dividers and lw pass filters with reduced size and manufacturing cst. ACKNOWLEDGEMENT The authrs wuld like t thank LTCC Sci&Tec Inc., Lake Frest CA, USA fr LTCC prttyping supprt. REFFERENCES [1] L. K. Yeung and K.-L. Wu, A cmpact secnd-rder LTCC band-pass filter with tw finite transmissin zers," IEEE Trans. Micrwave Thery Tech., Vl.51, N.2, pp , [2] W.-Y. Leung, K.-K. M. Cheng and K.-L. Wu, "Multi-layer LTCC band-pass filter design with enhanced stp-band characteristics," IEEE Micrwave and Wireless Cmpnents Letters, Vl. 12, N.7, pp , July [3] I. Ohta, X.-P. Li, T. Kawai, and Y. Kkub, "A design f lumped-element 3 db quadrature hybrids," Prceedings f Asia-Pacific Micrwave Cnference, pp , Dec [4] Y.-C. Chiang; C.-Y. Chen, "Design f a wide-band lumped-element 3-dB quadrature cupler," IEEE Trans. n Micrwave Thery and Techniques, Vl. 49, N. 3, pp , Mar [5] S. J. Parisi, 180 lumped element hybrid, IEEE MTT-S Digest, pp , 1989.