Ground-Adjustable Inductor for Wide-Tuning VCO Design Wu-Shiung Feng, Chin-I Yeh, Ho-Hsin Li, and Cheng-Ming Tsao

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1 Applied Mechanics and Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Ground-Adjustable for Wide-Tuning VCO Design Wu-Shiung Feng, Chin-I Yeh, Ho-Hsin Li, and Cheng-Ming Tsao Graduate Institute of Electronic Engineering, Chang Gung University, Taoyuan, Taiwan, ROC Keywords: RF IC,, VCO, Abstract. A wide-tuning range voltage-controlled oscillator (VCO) with adjustable ground-plate inductor for ultra-wide band (UWB) application is presented in this paper. The VCO was implemented by standard 90nm process at 1.2V supply voltage and power consumption of 6mW. The tuning range from 13.3 GHz to 15.6 GHz with phase noise between and - 115dBc/Hz@1MHz is obtained. The output power is around -8.7 to -9.6dBm and chip area of 0.77x0.62mm 2. Introduction In recent years, wireless Communication system becomes an important part in many mobile applications, including cell-phone, ipad, LAN, mobile satellite communication system and personal communication system. VCO provides a precise frequency source to drive the transceiver system and many VCOs were designed for wireless communication. For better performance, some VCOs were fabricated by GaAs hybrid and phemt technologies. Of course, the fabrication costs of such devices are higher than the fine-line standard devices available today. Moreover, since future system-on-chip solutions will comprise tens of millions of digital gates as well as analog/digital and RF/ microwave circuits. These can be implemented in low-cost standard technology [1-2]. In general, designing the RF frontend sub- system of any wideband wireless transceiver illustrates a great challenge for the implementation of the whole transceiver due to the stringent requirement of the ultra-wideband (UWB) radio. Current research focuses on UWB IEEE a of GHz frequency band for short range, low power, and high data-rate applications. We try to separate the UWB bandwidth for six parts and the VCO realizes continuously each part of them. In this paper, we present a novel VCO with new variable-inductor switched-modes using a standard 90nm technology. Because the varactor is the major factor causing low Q and low phase noise [3], there were many paper had published for varactor displacement like switched capacitor array, variable inductor and intrinsic-tuning technique. The new variable-inductor switched-modes could develop in existing structure, so it needs no addition area. In Section 2, the design strategy and circuit simulation of the various VCO considerations are discussed in details. In Section 3, it summarizes the measurement results and some final conclusions are presented in Section 4. Design and Simulation Recently, many kinds of variactors are developed, such as p-n junction, N-MOS, accumulation A- MOS, to enhance tuning range. However, it still encounters nonlinear effects in the variation of capacitor value versus applied voltage. Fig.1 shows various kinds of variable inductors. The types one to three are the major architectures of variable inductors which had published [3-4] and the type 4 is our architecture. Type one is most common variable-inductor. It combined the varactor and inductor together, and changes its inductor value by mutual inductance. Type two is a MEMS structure. It moves the metal flat to cut the magnetic flux and changes the inductor value. Type three is an inductor with a control current, and changes the current to change the inductor value. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-08/05/16,23:52:08)

2 2374 Advances in Civil Engineering II Fig.1 inductors Fig.2 is our new design, and Fig.3 shows the architecture of this design. It is a cross-section view of the variable inductor. We use the top metal as the inductor and change the different ground point to obtain different value of inductor. When the current I flows cross the top metal clockwise, it induces a magnetic flux. The equation (1) of total magnetic flux φ shows the surface integral of magnetic field B on the normal direction of an infinitesimal area ds. ϕ = B ds (1) The value of an inductor for large-signal calculation shows in equation (2) L =ϕ / I (2) According to equations (1) and (2), we could find that if we change the magnitude of magnetic flux, the inductor value will be varied. If we change the ground area, we will cut the magnetic flux, and also take a change of inductor value. Fig. 3 shows the implemented schematic of an adjustable inductor with variable ground-area. Fig. 2 Adjustable inductor with ground-area variation... Fig 3 Schematic diagram of an adjustable inductor with different kinds of sequential switch operation

3 Applied Mechanics and Materials Vols A dual-cross couple LC-tank VCO based on adjustable inductor with MOS-inversion varactor combination was designed and its schematic is shown in Fig. 4. After the circuit design, this circuit was fabricated by using TSMC 90nm standard process. The proposed VCO is the complementary cross-coupled pair architecture. The advantages of complementary cross-coupled is relaxed start-up condition, differential-ended, easy implementation and more balanced output. It is also has some disadvantages like lower energy-transfer efficiency and poor phase-noise. We try to use the switched-modes variable inductor for phase-noise improvement. The NMOS transistors (M3/M4) and PMOS transistors (M1/M2) are cross-coupled pair providing a negative resistor for oscillating. The varactors (C1/C2) use for fine tuning. At the dual output (Terminals Va, Vb), we designed complementary buffer for output matching and protecting external effect. After detailed simulation, the output frequency f o of the VCO versus applied varactor voltage with different kinds of sequential switch operation is shown in Fig. 5, and can be expressed as, 1 f0 = 2π L1(i)C1(V1) (3) Where L1(i) is controlled by grounded switches for band selection i and C1(V1) is tuned by applied voltage V1of the varactor for process variation. VDD Vb... Va Vt Fig 4 VCO using variable inductor Fig. 5 Oscillator frequency range versus varactor voltage with different kinds of switch operation

4 2376 Advances in Civil Engineering II Experimental Results And Discuss The wide-tuning VCO circuit was measured using on-wafer micro-probes. Fig. 6 shows the implemented chip with die size of 0.77x0.62 mm 2 including pads and guard ring. The supply voltage of this circuit is 1.2 V with 5 ma, and the total dc power consumption is 6 mw. Fig. 7 shows the measured spectrum of the VCO circuit. The tuning range of this VCO is between 13.35GHz to 15.6GHz; the output power is between -2dBm to dBm. Fig. 8 shows the measured performance of phase-noise. The VCO is measured a phase noise of around -115 dbc/hz at 1-MHz offset. Fig. 6 Die microphotograph of purposed VCO, with a chip size of 0.77x0.62 mm 2 (a) (b) Fig. 7 The purposed VCO spectrum under (a) switch count = 0, i.e. all-of state, and (b) switch count = 5, all-on state

5 Applied Mechanics and Materials Vols Conclusions Fig. 8 Phase-noise of the purposed VCO performance Table I. Performance Benchmark [5] [6] [7] This Work Power Supply(V) Process 90nm 0.18µm 0.18µm TSMC 90nm Technique Mutual Mutual MEMS Switching Power Dissipation(mW) ~ ~ Frequency(GHz) 53.1~ ~ ~ ~15.6 Tuning Range(GHz) 8.2GHz (14.33%) 0.6GHz (5.45%) 1.47GHz (72.95%) 2.25GHz (15.6%) Phase Noise (dbc/hz@1mhz) (@10MHz) (@10MHz) FOM FOM(T) The voltage controlled oscillator for UWB application is investigated in this work. The VCO is designed and fabricated by using TSMC 90nm technology. The proposed VCO consists of a 5 variable switched-mode inductors, which obtain low phase noise and good performance. The UWB VCO achieves a tuning range between 13.35GHz to 15.6GHz, and output power is between -2dBm to dBm. The phase noise of this VCO is dBc/Hz at 1-MHz offset. The proposed switched-modes voltage controlled oscillator provides a practical good design and solutions for UWB applications. Acknowledgements The authors would like to acknowledge the fabrication support provided by Taiwan Semiconductor Manufacturing Company (TSMC) through the National Chip Implementation Center (CIC), and also appreciate to the CIC for signal source measurements. We are grateful to the national Center for High-Performance Computing (NCHC) and Green Technology Research Center (GTRC) for computer time and facilities.

6 2378 Advances in Civil Engineering II References [1] Piernas B, Nishikawa K, Nakagawa T and Araki K 2003 IEEE MTT [2] Ballweber B M, Kurdoghlian A, Sokolich M, Case M, Micovic M, Thomas S and Fields C H 2000 IEEE GaAs IC Symp [3] Raffaelli L, Stewart E, Borelli J and Quimby R1994 IEEE Sarnoff Symp. 0_28 0_34 [4] Hou J A and Wang Y H 2010 IEEE Microwave and Wireless Components Lett [5] Yu C Y, Chen W Z, Wu C Y and Lu T Y 2008 IEEE Asian Solid-State Circuits Conference [6] Leung L L K, Chui K W C and Luong H C 2005 IEEE Asian Solid-State Circuits Conference [7] Ohashi K, Ito Y, Yoshihara Y, Okada K and Masu K 2007 Asia and South Pacific Design Automation Conference 98-99

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