Wide Tuning Range LC-VCO Using Variable

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PAPER

Special Section on Analog Circuit Techniques and Related Topics

Wide Tuning Range LC-VCO Using Variable Inductor for Reconfigurable RF Circuit Yoshiaki YOSHIHARA†a) , Hirotaka SUGAWARA†b) , Hiroyuki ITO†c) , Student Members, Kenichi OKADA†d) , and Kazuya MASU†e) , Members

SUMMARY This paper presents a novel wide tuning range CMOS Voltage Controlled Oscillator (VCO). VCO uses an on-chip variable inductor as an additional variable element to extend the tuning range of VCO. The fabricated variable inductor achieves the variable range of 35%. The VCO was fabricated using 0.35 µm standard CMOS process, and can be tuned continuously from 2.13 GHz to 3.28 GHz (tuning range of 38%) without degradation of phase noise. Wide tunable LC-VCO using a variable inductor is one of the key circuits for reconfigurable RF circuit. key words: VCO, oscillator, variable inductor, reconfigurable RF circuit

1.

Introduction

In the recent development of radio communication technology, chip-scale integration and multi-band/mode functions are required for analog RF circuits [1]. While single chip integration can reduce circuit area and cost of RF system, circuit design becomes difficult. Circuit performance and design manufacturability of analog RF circuit have to be improved for the future radio technology. One of the promising technologies is the reconfigurable RF circuit design [2], [3]. This technology can reconstruct analog RF circuits by controlling the various bias voltages of RF circuits and variable passive devices, and by switching a circuit block, which is achieved by the collaboration with digital control circuits. Reconfigurable RF circuit architecture can realize the multiband/mode RF circuit in a single chip for the software defined radio, and achieves considerable reduction of circuit area and power consumption. Furthermore, it can obtain robust RF circuits by the dynamic reconfiguration for the process fluctuation, the dynamic change of temperature, etc. To realize reconfigurable RF circuit, analog circuits (such as VCO, LNA, Mixer, and so on) have to extend the frequency range. In this paper, a wide tuning range CMOS VCO is presented, which is one of the key circuits for reconfigurable RF circuit design [4]. VCO utilized in the recent wireless technology is required to achieve wide tuning range, low phase noise, low Manuscript received June 20, 2004. Manuscript revised September 17, 2004. Final manuscript received October 24, 2004. † The authors are with Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama-shi, 226-8503 Japan. a) E-mail: [email protected] b) E-mail: [email protected] c) E-mail: [email protected] d) E-mail: [email protected] e) E-mail: [email protected]

power consumption, and so on. However, there is a trade-off between tuning range and phase noise. VCO using switched capacitors has been proposed [5]. Although it achieves wide tuning range, the switch resistance degrades phase noise characteristics. In this paper, a novel tuning architecture for LC-VCO is proposed. The proposed VCO uses an onchip variable inductor as an additional variable element to achieve the wide tuning range and to satisfy the above tradeoff requirement. The principle and measured results of the variable inductor are explained in the next section. Section 3 describes the proposed wide tuning range LC-VCO using the variable inductor, and Sect. 4 concludes this paper. 2.

Variable Inductor

Wide-range variable passive components in RF circuits are necessary to realize the reconfigurable RF circuit. The switched capacitor and switched inductor has been proposed [5]. The variable resistance using switched capacitor is also proposed [6], [7]. However, they have large series resistance caused by the on-resistance in transistor channel, so it is difficult to achieve high-quality RF circuit. Therefore, it is important to realize the low-loss variable passive components. This section describes about the variable inductor which has quite different mechanism to change the inductance and achieves high quality characteristics. 2.1 Principle of Variable Inductor Figure 1(a) shows a structure of a variable inductor, which

(a) Structure of variable inductor

(b) Spiral inductor

Fig. 1 Variable inductor consists of a conventional spiral inductor, a metal plate and a MEMS actuator. Inductance varies by moving the metal plate vertically above the spiral inductor.

c 2005 The Institute of Electronics, Information and Communication Engineers Copyright


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loss. 3. The metal plate decreases the magnetic flux that penetrates the spiral inductor, resulting in the variation of inductance. Furthermore, the inductance can be continuously varied according to the bias voltage. 4. The variable range depends on the amount of eddy current in the metal plate, so large eddy current can extend the variable range. Therefore, the thickness of over a few µm is required for the metal plate and the resistivity of the metal plate has to be small [8]. (a) Magnetic flux before inserting plate

(b) Magnetic flux after inserting plate Fig. 2 Change of magnetic flux simulated by electromagnetic simulation. Magnetic flux is decreased by the metal plate after inserting plate.

was proposed by our group [8]. The variable inductor consists of a conventional planar spiral inductor, a metal plate and a MEMS actuator. The metal plate is placed above the spiral inductor and suspended by the arm. In Fig. 1(a), h is the distance between the spiral inductor and the metal plate. The metal plate can be moved vertically using a MEMS parallel-plate actuator [9]. By generating DC bias voltage between the stationary electrode and the movable electrode (the metal plate), the metal plate can be moved vertically. The stationary and movable electrodes are supposed to be separated from the variable inductor by the high resistance. Less than 10 V of DC bias voltage can be used for the variable inductor. The parallel plate constructs a capacitor, so it does not consume power dynamically. Furthermore, the distance between parallel plate can be varied from 1 µm to 100 µm, with an increment of 20 nm, by the recent MEMS actuators [9], [10]. The variable inductor changes inductance by moving the metal plate above the spiral inductor. Figure 2 shows a mechanism of the change of the magnetic flux simulated by the electromagnetic simulation (Ansoft HFSS) before and after inserting the metal plate. In the figure, the magnetic flux decreases after inserting the metal plate, so inductance also decreases. These phenomena can be explained as follows. 1. When the magnetic flux of the spiral inductor penetrates the metal plate, an eddy current flows in the plate. 2. The eddy current induces a counteractive magnetic field according to Lenz’s law, which causes magnetic

We have previously proposed other type of variable inductors [11], [12]. It was supposed to use a comb-type MEMS actuator, which provides horizontal movement [13]. The horizontal type of variable inductors can vary the inductance by inserting the metal plate horizontally above the spiral inductor, as shown in Fig. 2. This paper proposes the use of a vertical movable actuator for LC-VCO. The inductance can also be varied according to the distance between the spiral inductor and the metal plate by the vertical movement. The density of magnetic flux decreases at the higher position, so the counteractive flux is also decreased. Therefore, the inductance varies according to the height of the metal plate [8]. The vertical type of variable inductor can achieve wider variable range with short movement of the metal plate and smaller area than the horizontal one. 2.2 Measured Results of Fabricated Variable Inductor A symmetrical spiral inductor was fabricated using a 0.35 µm standard CMOS process with three aluminum metal layers. The spiral inductor has 20 µm line width, 4 µm line space, and outer diameter of 400 µm as shown in Fig. 1(b). The spiral (M3) and underpass (M2) is 0.95 µm thick aluminum. The metal plate has to be low resistivity material. In this work, copper is used as the metal plate. Vector network analyzer (Agilent 8720ES) was used for measurement of variable inductor. In this measurement, the metal plate was moved manually using the micromanipulator instead of the MEMS actuator. Figure 3 shows measured inductance and quality factor as a function of height of the metal plate h at 2.45 GHz. Before inserting the metal plate, inductance is 6.47 nH. When the metal plate is inserted above the spiral inductor, inductance decreases because of shielding magnetic flux. At h=100 µm, the inductance is 5.98 nH and quality factor is 3.87. As the height of metal plate h decreases, the inductance decreases continuously because of increase in the amount of shielded magnetic flux. At h=10 µm, the inductance becomes 3.87 nH and the quality factor becomes 2.98. The variable range of inductance has been found to be 35.4% from 10 µm to 100 µm of metal plate height. The proposed variable inductor uses a conventional spiral inductor, so the quality factor can be improved by the use of the advanced process technology such as the use of low resistance metal layers (Cu, etc.), thicker metal layers, thicker ILD, and so on. It is noted that quality factor is not degraded by

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(a) Inductance of measured variable inductor

Fig. 4 Conventional VCO consists of the spiral inductor, the varactor and the cross-coupled MOSFETs.

(b) Quality factor of measured variable inductor Fig. 3 Inductance of the fabricated variable inductor can be varied form 3.8 nH to 5.8 nH. That corresponds to the variable range of 35%.

the metal plate because the parasitic capacitance and the resistive loss is small [8]. The quality factor Q of the inductor is approximately given by Eq. (1). Q≈

ωL , R

(1)

where ω, L, R are the angular frequency, the inductance and the resistance of the inductor, respectively. In the Fig. 3, L decreases as the metal plate approaches. However, the degradation of Q is small although L decreases. It is caused by the decrease of resistive loss. The resistive loss of substrate depends on the magnetic flux in the substrate. The magnetic flux is also reduced by the metal plate. Therefore, the quality factor is not degraded so much as the decrease of L. This feature is useful for VCO since the quality factor of the inductor greatly affects the phase noise characteristics of LC-VCO. 3.

Wide Tuning Range LC-VCO

3.1 Proposed VCO Architecture Figure 4 shows a schematic of the conventional LC-VCO. Circuit topology is based on CMOS type LC-VCO, which consists of the on-chip spiral inductor, the varactor and the cross-coupled N- and P-MOSFETs as negative conductance. These spiral inductor and varactor form a LC resonator. The resonance frequency becomes oscillation frequency of LCVCO, and it is given by Eq. (2).

Fig. 5 Fabricated VCO consists of the variable inductor, the varactor diode and the cross-coupled MOSFETs. It’s based on the conventional CMOS-type VCO architecture.

fosc ∝ √

1 Ltank Ctank

,

(2)

where Ltank and Ctank are inductance and capacitance of LCtank including parasitic inductance and capacitance. In the conventional LC-VCO, oscillation frequency fosc is tuned by only the capacitance change of the varactor. The capacitance of LC-tank Ctank is given by Eq. (3). Ctank = Cvaractor + Cinductor + CMOSFET + ...

(3)

Effect of the parasitic capacitances becomes large on the interest frequency, so the VCO tuning range by the change of the varactor capacitance Cvaractor becomes small relatively. In the proposed LC-VCO, the circuit topology is based on that of conventional one. The variable inductor described in section 2 is used instead of the conventional spiral inductor, and the frequency of VCO can be tuned by both the varactor and the variable inductor. Therefore, the proposed VCO can achieve wider tuning range than that of conventional one. 3.2 Measured Results A symmetrical spiral inductor and LC-VCO were fabricated using a 0.35 µm standard CMOS process with 3.3 V supply voltage and three aluminum metal layers. Figure 5 shows

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Fig. 8 Measured phase noises characteristics at 1 MHz offset as a function of height of metal plate h.

Fig. 6 Chip micrograph of fabricated LC-VCO. Circuit area is 400 µm × 750 µm.

Table 1 Technology Supply voltage VDD VCO core current Power consumption Center frequency Tuning range Phase noise at 1 MHz offset

VCO summary. 0.35 µm standard CMOS process 3.3 V 8.75 ∼ 8.95 mA 28.8 ∼ 29.5 mW 2.63 GHz 2.14 GHz ∼ 3.13 GHz 38% −114.4 dBc/Hz (h=100 µm) −112.0 dBc/Hz (h= 20 µm)

Fig. 7 VCO can totally be tuned from 2.14 GHz to 3.13 GHz, which corresponds to the tuning range of 38%.

a schematic of the fabricated LC-VCO. Circuit topology is based on the CMOS type LC-VCO. The variable inductor shown in Fig. 3 is used instead of the conventional spiral inductor to extend VCO tuning range. Diode (p+ diffusion in n-well biased through n+ diffusion) is used as a varactor. Figure 6 shows a chip micrograph of fabricated VCO. Circuit area is 400 µm x 750 µm. Spectrum analyzer (Agilent 8563EC) was used for measurement. In this measurement, the metal plate was moved manually using the micromanipulator instead of the MEMS actuator. Figure 7 shows measured VCO tuning characteristics. At h=100 µm, VCO can be tuned from 2.14 GHz to 2.65 GHz with the change of varactor control voltage from 1.5 V to 3.3 V. As the metal plate height h decreases, an inductance decreases and VCO oscillates at higher frequency. At h=20 µm, VCO can be tuned from 2.83 GHz to 3.13 GHz. The fabricated VCO can totally be tuned from 2.14 GHz to 3.13 GHz. The tuning range has been found to be 38%, which is significantly wide for a single VCO tuning range. The variable inductor can provide such a wide tuning range and enables continuous frequency tuning. Figure 8 shows measured phase noises characteristics at 1 MHz offset as a function of height of metal plate h. The phase

Fig. 9 Proposed inductance-tunable LC-VCO can provide wide tuning range and tune frequency continuously.

noises at 1 MHz offset are −114.4 dBc/Hz at h=100 µm, and −112.0 dBc/Hz at h=20 µm. The VCO using the variable inductor has exhibited wide tuning range without critical degradation of phase noise. Table 1 summaries the measured results. 3.3 Discussion Figure 9 shows the feature of inductance-tunable VCO. There are three features of proposed inductance-tunable VCO architecture. 1. The wide tuning range can be achieved by the variable inductor since frequency can be tuned by both capacitance and inductance changes. 2. The variable inductor can be tuned widely and continuously, so the wide tuning range can be achieved with the small VCO sensitivity KVCO . The small KVCO re-

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sults in improvement of phase noise caused by the fluctuations of varactor control voltage. 3. The combination of the variable inductor and the switched capacitors can provide wider tuning range. Although these features are similar to VCO with switched capacitor tuning, there are differences between variable inductor tuning and switched capacitor tuning. VCO with the variable inductor has no degradation of the phase noise characteristics by the on-resistance of switched capacitors. VCO with the variable inductor can tune frequency continuously while tuning by switched capacitors is discrete. These features are quite useful for multiband/mode radio communication systems with reconfigurable RF circuit design. 4.

[9] V.M. Lubecke and J.-C. Chiao, “MEMS technologies for enabling high frequency communications circuit,” IEEE International Conf. on Telecommunications in Modern Satellite, Cable and Broadcasting Services, pp.1–8, 1999. [10] J.-C. Chiao, Y. Fu, D.C. Choudhury, and L.-Y. Lin, “MEMS millimeterwave components,” IEEE MTT-S International Microwave Symp., pp.463–466, 1999. [11] S. Gomi, Y. Yokoyama, H. Sugawara, H. Ito, K. Okada, H. Hoshino, H. Onodera, and K. Masu, “Variable RF inductor on Si CMOS chip,” International Conf. on Solid-State Devices and Materials, pp.398– 399, 2003. [12] H. Sugawara, Y. Yokoyama, S. Gomi, H. Ito, K. Okada, H. Hoshino, H. Onodera, and K. Masu, “Variable RF inductor on Si CMOS chip,” Jpn. J. Appl. Phys., vol.43, no.4B, pp.2293–2296, 2004. [13] W.C. Tang, M.G. Lim, and R.T. Howe, “Electrostatic comb drive levitation and control method,” J. Microelectromech. Syst., vol.1, no.4, pp.170–178, 1992.

Conclusion

This paper has proposed a novel wide tuning range LCVCO using a variable inductor. The variable inductor has the variable range from 3.73 nH to 5.82 nH by moving the metal plate vertically above the conventional spiral inductor. The fabricated LC-VCO can be tuned from 2.14 GHz to 3.13 GHz (tuning range of 38%) continuously without degradation of phase noise. The wide tuning range LC-VCO using the variable inductor is quite useful for multi-band/mode radio communication systems with the reconfigurable RF circuit design.

Yoshiaki Yoshihara received the B.E. degree in Electronical and Electronics Engineering from Tokyo Institute of Technology, Tokyo, Japan, in 2003. Since 2003, he has been a master candidate in Department of Advanced Applied Electronics, Tokyo Institute of Technology, Yokohama, Japan. His research interests include RF circuit design. He is a member of IEEE and the Japan Society of Applied Physics (JSAP).

Acknowledgments This work was partially supported by MEXT.KAKENHI, JSPS.KAKENHI, CMP-TIMA, and VDEC in collaboration with Cadence Design Systems, Inc. References [1] Y. Neuvo, “Cellular phones as embedded systems,” IEEE International Solid-State Circuits Conf., pp.32–37, 2004. [2] Y. Yoshihara, H. Sugawara, H. Ito, K. Okada, and K. Masu, “Reconfigurable RF circuit design for multi-band wireless chip,” IEEE Asia-Pacific Conf. on Advanced System Integrated Circuits, pp.418–419, 2004. [3] K. Okada, Y. Yoshihara, H. Sugawara, and K. Masu, “A dynamic reconfigurable RF circuit architecture,” to be presented at Asia South Pacific Design Automation Conf., 2005. [4] Y. Yoshihara, H. Sugawara, H. Ito, K. Okada, and K. Masu, “Inductance-tuned LC-VCO for reconfigurable RF circuit design,” IEICE Electronics Express, vol.1, no.7, pp.156–159, 2004. [5] A. Kral, F. Behbahani, and A.A. Abidi, “RF-CMOS oscillators with switched tuning,” IEEE Custom Integrated Circuits Conf., pp.555– 558, 1998. [6] H. Kutuk and S.M. Kang, “A field-programmable analog array (FPAA) using switched-capacitor techniques,” IEEE International Symposium on Circuits and Systems, vol.4, pp.41–44, 1996. [7] H. Kutuk and S.M. Kang, “A switched capacitor approach to fieldprogrammable analog array (FPAA) design,” Analog Integr. Circuits Signal Process., vol.17, pp.51–65, 1998. [8] H. Sugawara, Y. Yoshihara, H. Ito, K. Okada, and K. Masu, “Widerange RF variable inductor on Si CMOS chip with MEMS actuator,” IEEE European Microwave Conf., pp.701–704, 2004.

Hirotaka Sugawara received the B.E. degree in Electronical and Electronics Engineering from Tokyo Institute of Technology, Tokyo, Japan, in 2004. Since 2004, he has been a master candidate in Department of Advanced Applied Electronics, Tokyo Institute of Technology, Yokohama, Japan. His research interests include RF devices and circuit design. He is a member of IEEE and the Japan Society of Applied Physics (JSAP).

Hiroyuki Ito received the B.E. degree in Electronics and Mechanical Engineering from Chiba University, Chiba, Japan, in 2002, and M.E. degree in Department of Advanced Applied Electronics, Tokyo Institute of Technology, Yokohama, Japan, in 2004. Since 2004, he has been a doctor candidate in Tokyo Institute of Technology, and a Research Fellow of the Japan Society for the Promotion of Science. His research interests include RF circuit design. He is a member of IEEE, the Japan Society of Applied Physics (JSAP) and the Japan Institute of Electronics Packaging (JIEP).

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Kenichi Okada received the B.E., M.E. and Ph.D. degrees in Communications and Computer Engineering from Kyoto University, Kyoto, Japan, in 1998, 2000, and 2003, respectively. From 2000 to 2003, he was a Research Fellow of the Japan Society for the Promotion of Science. Since 2003, he has been an Assistant Professor in Tokyo Institute of Technology. His research interests include computer-aideddesign for statistical analysis and RF circuit design. He is a member of IEEE, the Information Processing Society of Japan (IPSJ), and the Japan Society of Applied Physics (JSAP).

Kazuya Masu received the B.E., M.E. and Ph.D. degrees in Electronics Engineering from Tokyo Institute of Technology, Tokyo, Japan, in 1977, 1979 and 1982, respectively. He was with the Research Institute of Electrical Communication, Tohoku University, Sendai, Japan since 1982. Since 2000, he has been with Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Japan and he is currently a professor. He was a visiting Professor in Georgia Institute of Technology in 2002. His current interests are signal integrity and GHz signal propagation in multilevel interconnect of Si ULSI, reconfigurable RF circuit technology, performance evaluation and prediction based on interconnect wire length distribution, and BEOL process technology. He is a member of the IEEE, the Japan Society of Applied Physics (JSAP), the Institute of Electrical Engineers of Japan, and the Electrochemical Society.