Monolithic LTCC SiP Transmitter for 6GHz Wireless Communication Terminals Young Chul Lee, Won-il Chang, and Chul Soon Park School of Engineering, Information and Communications University (ICU) 13-6 Munji-dong, Yuseong-gu, Daejeon, 35-714, Korea Abstract We demonstrate a 36 X 12 X.9mm 3 sized compact monolithic LTCC SiP transmitter (Tx) for 6GHz-band wireless communication terminal applications. Five GaAs MMICs including mixer, driver amplifier, power amplifier and two of frequency doublers have been integrated onto LTCC multilayer circuit which embeds a stripline and a microstrip patch antenna. A novel CPW-to-stripline transition has been devised integrating air-cavities to minimize the associated attenuation. The fabricated transmitter achieves an output of 9dBm at a RF frequency of 6.4GHz, an IF frequency of 2.4GHz, and a LO frequency of 58GHz. The up-conversion gain is 11.2dB; while the LO signal is suppressed below 33.4dBc, and the spurious signal is also suppressed below 27.4dBc. This is the first report on the LTCC SiP transmitter integrating both a and an antenna. A 6 GHz communication will be demonstrated. Index Terms LTCC, SiP, transmitter, vertical via, transition, air cavity. I. INTRODUCTION With the expanding need for multimedia data services in wireless communications, millimeter waves (mm-w) have recently evolved as a carrier frequency for communication. Notable research topics such as wireless Gigabit Ethernet link [1], video transmission systems [2], and wireless local area networks (W-LAN) have been areas of much interest. In order to implement terminals for mobile communications or W- LAN, a small size and lightweight radio transceiver is indispensable. Multi-layer LTCC based System-in-Package (SiP) technology [1]-[3], integrating monolithic microwave integrated circuits (MMICs) and passive devices, is one of the best candidates for mm-w radio system integration due to its low loss, integration capability, similar temperature coefficient of expansion (TCE) value to MMICs, and cost effectiveness. Using LTCC based SiP technology, some groups have developed compact 6GHz RF systems for wireless communication applications [1]-[3]. Although 6GHz-band LTCC SiPs [1-3] have achieved compact RF modules and good performances, two key elements for radio systems, the band pass filter and antenna, have yet to be fully integrated in the single LTCC SiP module. A flip-chip mounted [1] and waveguide interfaced antennas [1], [3] result in additional losses and bulky systems. In this paper, we first demonstrate a LTCC SiP transmitter integrating a and an antenna monolithically as well as bias circuits and MMICs for wireless mobile terminal applications. In order to cope with attenuation and interference at 6 GHz, we have proposed a new transition structure between coplanar waveguide and strip line and adopted isolation structure composed of ground vias and planes. The implemented compact radio transmitter is as small as 36 x 12 x.9 mm 3. The measured transmitter performance, including the overall gain and output power, is presented. II. MONOLITHIC LTCC SIP MODULE Fig. 1 shows the schematic concept of the MMICs integrated LTCC mm-w radio transmitter. This transmitter integrates a and an antenna as well as DC bias circuits and MMICs. The bandpass filter is integrated in the LTCC block monolithically, and the patch antenna is defined at the top layers. Then the active circuits of GaAs MMICs are integrated on top of the LTCC multilayer circuit. Each layer of the LTCC circuit is integrated together through the via hole interconnection, and the MMICs through wire bonding. The transmission lines within the LTCC block are strip line (SL) and that on top surface is conductor backed coplanar waveguide (CB-CPW). The issues in design and fabrication of the 6 GHz monolithic transmitter are the attenuation along the transmission line and discontinuities and crosstalk between circuits. PCB LO / IF Lid MTL PA MMICs Mixer DA GND DC Antenna Fig. 1 The 3D schematic concept of the mm-w LTCC SiP transmitter III. AIR CAVITIES INTEGRATED NOVEL LOW-LOSS TRANSITION Attenuation along transmission lines and embedded devices should be minimized for power efficiency and noise performance of the mm-w transceiver. Particularly in the 3D integrated radios, discontinuities at SL-to-CPW transitions for interconnection of embedded passive devices to surface Via -783-8846-1/5/$2. (C) 25 IEEE 115 Authorized licensed use limited to: University of Electronic Science and Tech of China. Downloaded on June 19, 29 at 11:12 from IEEE Xplore. Restrictions apply.
circuits generate significant amounts of radiation as well as reflection. The attenuations caused by radiations at the discontinuities and impedance mismatch along the vias are in general remedied by placing ground vias around the transitions. In this work, a stagger via (STV) structure and embedded air cavities [4] are adopted for reduction of discontinuity and shunt capacitance, respectively. The STV structure consists of three-stacked vias through the 5 th and 7 th layer. These three vias are connected through lines. For reduction of shunt capacitance, the air cavities are integrated through the 2 nd to 5 th layer below the 7 th layer via and the 2 nd to 3 rd layer below the 5 th layer via, respectively. Their diameter is 17µm. The conductor-backed CPWs (CB-CPW) are placed on the top layer and the SL is placed on the 4 th layer. In order to maintain the characteristic impedance of 5Ω for both CB-CPWs and the SL, the width and gap of the CB-CPWs are 244µm and 1µm, respectively and the SL is 135µm in width. S11/S22 [db] -1-2 -3 S22 S11 S11 S22 Con. Novel Novel Con. -4-4 5 52 54 56 58 6 62 64 66 Freq. [GHz] Fig. 2 Measured performances of the CPW-SL-CPW transitions using a novel CPW-to-SL transition and a conventional one (Con.: the conventional transition and Novel: the novel one) Fig. 2 presents the absolute value of the transmission coefficients of the proposed novel transition (a length of a CPW=35µm and a length of a SL=2,µm) in comparison with the conventional one using directly stacked three vias through the 5 th to the 7 th layer (a length of a CPW=335µm and a length of a SL=2,595µm). The fabricated novel one shows an insertion loss of -1.6 db compared to the 2dB of the conventional one at 6GHz. These values represent losses along the three-segment transmission lines (CPW-SL-CPW), and the two CPW-to-SL transitions. Considering the total losses of transmission lines (two CPWs and a stripline) with -.19dB, which is calculated by a line calculator, the transition loss is.75db per STV transition. The return loss for the novel structure shows -1dB over 6GHz that is improved by 5dB when compared to that of the conventional one. -1-2 -3 S21 [db] IV. FULLY EMBEDDED Fig. 3 shows a fully embedded stripline structured involving two CPW-to-CPW transitions and a CPW-to-SL transition at each end of a. CPW-to-CPW CPW-to-SL f -io Cut f -int P-R L_ P_R Fig. 3 A layout of a 6GHz stripline LTCC including transitions A dual-mode four-pole is designed in between the 2 nd and 7 th layer using dual-mode patch resonators [5]. The dual mode can be generated from a single-mode resonator by adding a perturbation (cut) at a point that is 45 degrees from the axes of coupling to the resonator [5]. For broad bandwidth, two resonators are on the 3 rd and 5 th layer, and two of their blocks are 684µm away from each other for four-pole operation as shown in Fig. 3. Feed lines, external coupling between the resonators on the 3 rd and 5 th layer, and internal coupling between their two blocks are on the 4 th layer. The, with the center frequency of 6.4GHz and 3.3% bandwidth (BW), is designed with a dielectric constant of 7.. This filter has to achieve LO rejection over 2dBc at 58GHz (the LO frequency). The side length of a resonator is about half a wavelength (613µm) [6]. The widths of the feed lines are 135µm. Changing the depth of the cut, coupling coefficients can be controlled, and an optimum cut length is calculated as 15µm. The external coupling distances on the 4 th layer are 14µm and the internal coupling is realized by an overlap of 4µm between two resonators on the 3 rd and 5 th layer. A 1µm thick MMIC and a 6µm thick stripline can cause an abrupt ground-plane discontinuity between them at interconnection. The CB-CPW for wire bonding of MMICs has backed ground plane on 6 th layer while the CB-CPW for CPW-to-SL transition has the ground plane on 2 nd layer. For gradual transition of the ground planes, another CB-CPW having the backed ground plane on 4 th layer has been inserted between the two different CB-CPWs. Fig. 4 shows the fabricated including two transitions and its measured performance. The size of the is as small as 1.8 x 3.5mm 2, and the total area including the whole transitions at both ends is 3.5 x 6.5 x.7mm 3. Measured center frequency and bandwidth are 6.44GHz and 3.5%, respectively. The input and output return losses are below - 16dB at the pass band. The LO rejection is 23.9dBc. The f -io Stripline structured Transitions Authorized licensed use limited to: University of Electronic Science and Tech of China. Downloaded on June 19, 29 at 11:12 from IEEE Xplore. Restrictions apply. 116
overall filter insertion loss, including the whole transitions at both ends, is 4.98dB, while the insertion loss through both ends transitions is 1.6 db and the filter insertion loss is 3.38dB. [db] -5-1 -15-2 -25-3 Transitions Cavity for MMIC mount S21/S12-35 57 58 59 6 61 62 63 (b) Fig. 4 Fabricated (a) and its measured performance (b) V. MULTILAYER 2X2 ARRAY PATCH ANTENNA A 2x2 array patch antenna is implemented with a LTCC microstrip (MSL) structure. Fig. 5 (a) and (b) show the schematic structure and the photograph of the fabricated LTCC antenna. The total of LTCC layers is 3. The radiating patches are placed on the 3 rd layer, the feeding network on the 2 nd layer, and the ground plane is on the back side of the 1 st layer from the bottom. Fig. 6 shows the measured radiation pattern as well as the simulated one. The antenna reveals a gain of about 7dBi and a 3-dB beam width of 36 o. The antenna size is as small as 1 x 1 x.3mm 3. (a) (a) Freq. [GHz] Cavity for MMIC mount Transitions S22 S11 Patch antenna Embedded MSL GND Fig. 5 (a) the structure of the antenna (the patch size: 645x1, 2µm) and (b) the fabricated transmitter antenna of 2x2 array (the size: 1 x 1 x.3mm 3 ) (b) R/C SL Relative gain(db) L9 5-5 -1-15 -2-25 -3-9 -6-3 3 6 9 Angle(deg) L8 ECPW Fig. 6 Radiation patterns at 6.4GHz (H-plane) DC feed lines VI. ISOLATION BETWEEN CIRCUITS Planar mm-w circuits may suffer from parasitic modes due to power leakages from transmission lines [7] and unexpected radiation at discontinuities [8]. These parasitic modes can propagate to other parts of the circuits through DC bias feed lines, IF path and/or signal lines, and finally result in unexpected cross talk and feedback. These internal cross talk and feedback effects have a crucial influence on the circuit stability. In order to maintain the stability, the DC bias lines and long IF feed lines are shielded with isolating ground planes and vias as will be shown in Fig. 7. The high frequency noise from bias is effectively bypassed only next to the RF circuitry that employs appropriate resistors and capacitors. CPW-to-SL Stripline structured CPW-to-CPW via MMIC L7 P-R L6 I-GND SL L5 L4 P-R L3 I-GND Simulation simulation Measurement measure Air cavities E-GND Fig. 7 Schematic cross section of the SiP transmitter (Lx: the number of LTCC layers, I-GND: the internal ground plane, E-GND: the external ground plane, ECPW: the embedded CPW, and P_R: the patch resonator) VII. FABRICATED SIP TRANSMITTER Fig. 8 shows a block diagram of the transmitter for 6GHz wireless communication applications. The transmitter consists of a stripline, a patch antenna, a HBT up-converting mixer, and four.15µm GaAs PHEMT MMICs: two frequency multipliers (MTLs), a drive amplifier and a power amplifier. The LO signal of the transmitter is supplied by multiplying the external LO source of 14.5GHz by 4 to the mixer. Fig. 9 shows the fabricated monolithic LTCC SiP L2 L1 Authorized licensed use limited to: University of Electronic Science and Tech of China. Downloaded on June 19, 29 at 11:12 from IEEE Xplore. Restrictions apply. 117
transmitter with surface mounting structure. The transmitter includes the five MMICs, a and an antenna, and is implemented with a total of 9-layer LTCC. Each layer is 1µm thick and its relative dielectric constant is 7. at 6GHz. The whole size of the transmitter is as small as 36 x 12 x.9mm 3. IF 2.4GHz Up-Mixer MTLX2 MTLX2 LO=58GHz LO Source=14.5GHz Drive Amp. ANT Power Amp. RF= 6.4GHz 3D LTCC SiP Tx Fig. 8 Block diagram of the 6GHz transmitter Pout[dBm]/Gain[dB] 13 12 11 1 9 8 7 6 5 4 3 2 1-1 -8-6 -4-2 2 4 6 8 Pin[dBm] Pout Gain [dbm] -3 57 58 59 6 61 62 63 (a) (b) Fig. 1 Output power (a) and frequency spectrum (b) at the output 15 1 5-5 -1-15 -2-25 LO 33.44dBc RF Freq. [GHz] LO+2IF MTLx2 MTLx2 Mixer DA PA Antenna ACKNOWLEDGEMENT R and C for Bias This work was financially supported by the Ministry of Science and Technology of Korea and KISTEP. Fig. 9 Fabricated 6GHz LTCC SiP Transmitter At the output port of the power amplifier, output power and frequency spectrum are measured using on-wafer probing. Fig. 1 (a) demonstrates the RF output power and the power gain as a function of 2.4GHz IF input power to the transmitter. With an IF power of -1.dBm, the 6.4GHz RF power at a 1- db gain compression point is 9dBm while the up-conversion gain of the transmitter is 11.2dB. Fig. 1 (b) shows the measured frequency spectrum of the LO, RF, and a spurious signal (LO+2IF) at the output port of the power amplifier. The isolation level between the LO and RF signal is less than - 33.4dBc and the spurious level is less than -27.5dBc. VIII. CONCLUSION We present a 36 X 12 X.9mm 3 sized monolithic LTCC SiP transmitter for 6GHz-band wireless communication terminals. This is the first report on the 6 GHz monolithic transmitter SiP including both embedded and antenna, as far as authors know. The transmitter includes active circuit of five GaAs MMICs on the monolithic LTCC circuit. Signal attenuation has been minimized with a newly proposed lowloss transition using air-cavities, and isolation between circuits has been obtained with dedicated shielding structure. The fabricated transmitter achieves an output of 9dBm at a RF frequency of 6.4GHz, an IF frequency of 2.4GHz, and a LO frequency of 58GHz. The up-conversion gain is 11.2dB; while the LO signal is suppressed below 33.4dBc, and the spurious signal is also suppressed below 27.4dBc. Wireless communication result will be presented. REFERENCES [1] K. Ohata, K. Maruhashi, M. Ito, S. Kishimoto, K. Ikuina, T. Hashiguchi, K. Ikeda, and N. Takahashi, 1.25Gbps wireless Gigabit Ethernet link at 6GHz-band, IEEE MTT-S International Microwave Symposium Digest, Vol.1, pp.373-376, June 23 [2] A. Yamada, E. Suematsu, K. Sato, M. Yamamoto, and H. Sato, 6GHz ultra compact transmitter/receiver with a low phase noise PLL-oscillator, IEEE MTT-S International Microwave Symposium Digest, Vol.3, pp. 235-238, June 23 [3] Young Chul Lee, Won-il Chang, Yun Hee Cho, and Chul Soon Park, A Very Compact 6GHz Transmitter Integrating GaAs MMICs on LTCC Passive Circuits for Wireless Terminal Applications, IEEE Compound Semiconductor Integrated Circuit Symposium Technical Digest, pp.313-316, October 24 [4] Young Chul Lee and Chul Soon Park, A Novel High-Q LTCC Stripline Resonator for Millimeter-Wave Applications, IEEE Microwave and Wireless Components Letters, Vol. 13, No. 12, pp.499~54, 23 [5] J. A. Curtis and S. J. Fiedziuszko, Miniature Dual Mode Microstrip Filters, IEEE MTT-S Int. Microwave Symposium Digest, pp.443-446, 1991 [6] I. J. Bahl and P. Bhartia, Microstrip Antennas, Artech House, 1982 [7] William H. Haydl, On the Use of Vias in Conductor-Backed Coplanar Circuits, IEEE Trans. on Microwave Theory and Techniques, Vol.5, No.6, pp.1571~1577, 22 [8] T. Krems et al., Avoiding Cross Talk and Feedback effects in Packaging Coplanar Millimeter-wave Circuits, IEEE MTT-S Int. Microwave Symposium Digest, Vol. 3, pp.191~194, 1998 Authorized licensed use limited to: University of Electronic Science and Tech of China. Downloaded on June 19, 29 at 11:12 from IEEE Xplore. Restrictions apply. 118
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