Low-Power and Wideband LC-VCO for WiMAX in CMOS Technology · Low-power and Wideband LC-VCO for WiMAX in CMOS Technology Mohammed Aqeeli, Zhirun Hu, Xianjun Huang, Abdullah Alburaikan,

Low-power and Wideband LC-VCO for WiMAX in CMOS Technology

Mohammed Aqeeli, Zhirun Hu, Xianjun Huang, Abdullah Alburaikan, Cahyo Muvianto School of Electrical and Electronic Engineering

The University of Manchester Manchester, UK

[email protected], [email protected], [email protected], Abdullah [email protected], [email protected]

Abstract—This work presents an ultralow phase-noise and wide turning-range voltage-controlled oscillator (VCO) for 5.72GHz WiMAX applications. The fully integrated VCO is designed and simulated using 130-nm CMOS technology. Instead of using the conventional diode-based varactor in the tank design, high-performance body-grounded NMOS transistors are employed as effective varactors. A controlled self-biasing current source is implemented to avoid higher power supply sensitivity and higher up-conversion of flicker noise. The proposed VCO-measured results demonstrate a worst case phase noise of -132.68dBc/Hz at 1MHz frequency offset with an excellent figure of merit (FOM), which is -201.6dBc/Hz under a power consumption of 2.21mW. The VCO shows a tuning range of approximately 37.59%.

Keywords—CMOS; CMOS processes; Body-grounded NMOS varactor; current source; (MOM) capacitor; figure of merit; low phase noise.

I. INTRODUCTION

The recent exponential growth and higher integration of wireless communication has attracted remarkable efforts to develop more channels in mobile communication applications. The IEEE.16 unlicensed band of Worldwide Interoperability for Microwave Access (WIMAX), which refers to interoperable operations of the IEEE 802.16 for wireless-networks standards approved by the Windex Forum, provides fixed and portable wireless broadband connectivity independent of a base station. Three spectrum bands are adapted for global deployment, one of which is the unlicensed band (5.725-5.850)GHz[1]. Nowadays, the demand for high-performance VCOs has increased, and consequently this demand has imposed stricter requirements on VCO phase noise. Phase noise can be triggered by a number of conditions, but it is mostly affected by VCO frequency stability, which is one of the most important parameters for the quality and performance of information transfer and in turn affects reliability in data communication[2]. P-N junction varactors[3], body-biased PMOS varactors[4] and switched capacitor arrays[5] are different approaches which are used widely to increase the bandwidth of VCOs design. Owing to the implemented techniques, the bandwidth and the phase noise of the VCO is substantially improved. However, PN junction present a narrower tuning range, large power dissipation is still unavoidable due to the stacking of body-biased PMOS transistors and additional noise appears because of the use of capacitor array switches.

High-performance LC-VCOs is a major challenge and require low-phase noise, low-loss and a wide tuning range varactors and sophisticated current biasing source. To achieve this object, the present work utilizes body-grounded NMOS devices as varactors to obtain high quality factor and better capacitance range and so better phase noise. In addition, a newly controlled current biasing source is designed to avoid higher power supply sensitivity and higher up-conversion of flicker noise. Furthermore, an improve phase noise property of VCO is achieved by implementing Metal-Oxide-Metal (MOM) capacitors in the tank of the VCO taking advantage of their high quality factor, high capacitance density, low parasitic capacitance, narrow spacing and thinner dielectric layers. This paper is organized as follows. Section II explains the VCO’s core design and the implementation process, while Section III presents the implementation and the measured results, followed by the conclusion in Section IV.

II. VCO DESIGN AND IMPLEMENTATION The schematic diagram and the layout of the proposed

VCO are shown in Fig. 1. The prime design considerations for the proposed VCO aim at improving tuning range and phase noise. For the tank circuit, body-grounded NMOS varactors are employed instead of PMOS or common PN-junction varactors. In order to implement a wide LC-VCO tuning range, it is important to decrease the number of parasitic capacitors. In this design, NMOS transistors are chosen due to the fact that parasitic capacitors in an NMOS pair are fewer than in a PMOS pair due to the smaller transistor size. Therefore, for the same value of gm, the W/L of an NMOS cross-couple is approximately one-third of a PMOS cross-couple. Also importantly, the proposed design utilizes a direct bias current source rather than the conventional mirrored bias current. The bias current of the VCO, which is an important parameter for phase noise optimization, is designed to handle the maximum current allowed by the specifications.

A. Body-grounded NMOS varactor in 130nm CMOS Implementing (VCOs) in standard complementary metal

oxide semiconductor (CMOS) technology is a major challenge for the design of radio-frequency (RF) transceiver integrated circuits (ICs). MOS devices in VCOs can produce wider tuning range, better Q and lower phase noise than diode varactors.

2014 UKSim-AMSS 16th International Conference on Computer Modelling and Simulation

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DOI 10.1109/UKSim.2014.79

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(a)

(b)

Figure1 (a) Schematic diagram (b) Layout of the designed 5.2 GHz 130nm CMOS VCO.

B. Body-grounded NMOS varactor in 130nm CMOS Implementing (VCOs) in standard complementary metal

oxide semiconductor (CMOS) technology is a major challenge for the design of radio-frequency (RF) transceiver integrated circuits (ICs). MOS devices in VCOs can produce wider tuning range, better Q and lower phase noise than diode varactors. The two most important types used for VCOs application are the PMOS and NMOS varactors.�A PMOS varactor is mediated between the source of the PMOS transistor and a power supply, while an NMOS varactor is mediated between the source of the NMOS transistor and ground.

An improved four parallel NMOS varactor for the proposed LC-tank VCO applications is implemented in this work by connecting the NMOS varactor fourth terminal BG to a ground-isolated common mode point of the negative transconductance (gm) devices used to pump the VCO tank.

This new varactor mode is referred to as body-grounded NMOS varactor. Since the tuning voltage and the common mode point are both Vdd referred, supply noise is not coupled into the tank by the varactor back gate capacitance and the VCO has excellent supply. The current biasing source, maintain a constant large signal tank swing across NMOS varactors, thereby producing a constant common mode point from a constant current into a constant tank resonate impedance[6]. In the proposed body-grounded varactor, four pairs of symmetrical NMOS varactors are formed with 64 gate fingers. If the voltage VGS<VTH, an inversion zone is established. The minimum value Cmin is reached when the voltage difference between electrodes is equal to the threshold voltage VTH. The maximum capacitance value per unit of area, Cmax is equal to �/tox which corresponds to a high accumulation value under the gate oxide. The tuning range is defined by the ratio between Cmin and Cmax. According to the Cadence virtuoso analogue simulation results, each pair of varactors alters capacitance from 0.25 to 0.70 pF as tuning voltage varies from 0 to 2.5V.

C. Self-biasing current source A new controlled current source is designed specifically

for the proposed VCO to achieve optimal phase noise and well-balanced differential outputs but with less power consumption. The tail current reference source is designed to generate current independently of the supply voltage because of the high impedance element which has the ability to overcome any possible variations. The current mirror is formed by PMOS transistors (M3, M6) developing current in the two branches of the circuit. The start-up circuit is used to inject current and to move the current source from the zero condition to the point of operation. By adjusting resistor R1, the circuit will generate the suitable current for the VCO. This current source is optimized properly to reduce swing and as a result decrease phase noise even more, in which case the circuit consumes less than 150 �A. To sustain the oscillation, the higher the gm must be,

L

RCgm ≥ (1)

The minimum biasing current, Ibias=2ID is inversely proportional to the transconducance efficiency and the inductance value and its quality therefore: ��������������������������������(2)�

Taking into consideration all of the parasitic mostly associated with the tank of the VCO’s, the frequency of oscillation can be derived as:

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2

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LCCCR

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gdgs

++−

++=ω

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( )( )

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gs gdmL

D gs gd

I mAL R C C Cg Q

I C C C

≥ ≈− + +� �

� �� � + + �

In this work we can calculate the minimum required biasing current from (4). From this equation it can be seen clearly that transistor capacitance C requires a decrease in the C value compared with the ideal case for a given resonant frequency[2]. This equation establishes the importance of MOSFET size, and the result is that the bigger it is, the higher the gm must be to achieve oscillation.

D. Center taped inductor The proposed inductors turned out to be near optimum

for the oscillator design goal of reducing the internal series resistor. Relatively small L and high Q inductor values are key to a high-performance, low-power and low-noise oscillator. In this work, the designed inductor is a symmetrical center-tapped inductor, taken into consideration by obtaining the maximum Q factor at 5.72 GHz[2]. It was simulated at different frequencies, as shown in Fig. 2, and the measured quality factor Q was 18.10 for 359 pH inductors, while the area was 15190 �m2. For the planner inductor, value L can be given approximately by[6]:

���

����

����

����

� ++

= 2.0)(

ln..2

0

wtnllL

πμ ��������������(5)�

The total length of the winding (l) is:

( ) )()1(914 swNNrnl ++++= (6) ��������where s is winding spacing, t is the thickness of the material, n is the winding count, w is the winding width, N= integer (n), � is the conductivity of the interconnect, � is skin depth and 0μ is magnetic permeability.

Figure 2. Inductor quality and SRF versus L values in 130nm CMOS process technology

E. MOM capacitors VCOs depend only on technology-dependent factors, in

other words on the Q of inductor(s) and capacitors[2]. Integrated inductance-capacitances (LCs) are standard components in LC tank VCO circuits and may become the most important part of a high-performance VCO circuit design. VCOs use various types of integrated capacitors utilizing MOS, p-n junctions, MIM (metal-insulator-metal), poly-to-poly, MOM and other structures. MOM capacitors are one of the most widely used, due to their high capacitance density, low parasitic capacitance, symmetrical plate design, superior RF characteristics and no additional masks or process steps – and thus low cost.

In symmetric-type MOM structures, the architecture consists of two ports, and the number of fingers per layer is limited to even numbers, in order to maintain symmetry. There are six metal layers, finger length (Lf) is 12 and the area is 424.36�m2. Fig.3 illustrates simulated frequency dependent quality factor of MOM capacitor designed with a 130nm CMOS process technology[2].

0 2 4 6 8 10

10

20

30

40

50

60

70

80

Qua

lity

fact

or

Frequency(GHz)

Frequency - Quality factor

Figure 3. Characteristics of a MOM capacitor designed with 130nm CMOS process technology.

III. SIMULATION RESULTS The demonstrated VCO generates stable periodic signals

with a harmonic index and measured output power of 5.73dBm at the resonance frequency, as shown in Fig. 4.

0 2 4 6 8 10 12 14 16 18 20 22

-100

-80

-60

-40

-20

0

20

V/Vout:pss dB2010.40286GHz-37.39623dB

Har

mon

ics(

dB)

Frequency(GHz)

V/Vout:pss dB205.7201GHz5.73823dB

Figure 4. Simulated power output plot of a 5.72 GHz VCO

(4)

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By implementing body-grounded NMOS varactors, the VCO can significantly increase the tuning range from (4.62-5.68)GHz to (4.63-6.78)GHz. This provides up to 19.23% better performance than the conventional NMOS varactors and approximately 12% better than the body-biased PMOS varactors as shown in Fig.5, with tuning voltage varying from 0 to 2.5 V. The solid and the dotted lines represent simulation results using NMOS varactors with and without body-grounding.

0.0 0.5 1.0 1.5 2.0 2.54.0

4.5

5.0

5.5

6.0

6.5

7.0

Freq

uenc

y(G

Hz)

Tuning voltage(V)

Simulated tuning range(body-grounded NMOS) (body- biased NMOS) (body-biased PMOS)

Figure 5. Output frequency of the designed VCO

Generally, random FM walk noise is an enduring component in oscillator phase noise [6]. This component seems to be unnoticeable because the component, which is up-converted from the flicker noise in active devices and current sources, significantly outweighs random FM walk noise. However, since the proposed circuit design can reduce flicker noise in MOSFETs, FM flicker noise is kept to a minimum. The phase noise of an LC-VCO is described, according to Leeson, as [7]

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Fig. 6 shows the phase noise measurement results at 5.72

GHz, for which the SSB phase noise measured at 1 MHz offset from the carrier frequency were -132.68dBc/Hz. By using a 1.2 V supply voltage, the total power consumption of

the VCO core is 2.21 mW. To compare the performance of previously published oscillators and the FOM, we used the model adopted by Ham and Hajimiri[8], which normalizes the measured phase noise with respect to center frequency and power consumption. It is defined by equation (8):

{ }���

���+

���

���

Δ−Δ=

mWPd

fffLFOM

1log10log20 0 (8)

Fig.7 depicts the curves of the oscillation frequency and FOM versus the controlled voltage from the simulation.

-160

-140

-120

-100

-80

-60

-40

-20

0

1.0 10.0 100.0 10K 100K 0.1M 1.0MPh

ase

nois

e(dB

c/H

z)Reative frequency (Hz)

Phase noise: -132.6805dBc/Hz

Figure 6. Simulated phase noise at 1 MHz offset.

Figure 7. Tuning characteristics and FOM of the VCO.

TABLE I. PERFORMANCE COMPARISON OF CMOS VCOS

Process F (GHz) P (mW) PN(dBc/Hz) Offset TR(GHz) FOM(dBc/Hz) Ref.

0.09µmCMOS 5.63 14.00 -108.50 1.0 4.50-7.10 -172.00 [9] 0.18µmCMOS 5.20 9.70 -113.70 1.0 4.39-5.26 -180.00 [10] 0.18µmCMOS 6.00 12.50 -115,50 1.0 5.72-6.02 -179.80 [11] 0.18µmCMOS 5.80 10.08 -117.00 1.0 5.27-6.41 -184.00 [12] 0.18µmCMOS 5.10 9.70 -122.40 1.0 4.80-5.40 -152.00 [13] 0.25µmCMOS 4.77 18.18 -122.10 1.0 3.60-4.77 -184.90 [14] 0.18µmCMOS 6.98 3.40 -108.10 1.0 6.54-6.98 -180.00 [15] 0.09µmCMOS 3.95 6.60 -147.00 10.0 3.40-4.50 -191.00 [16] 0.13µmCMOS 5.72 2.21 -132.68 1.0 4.63-6.78 -201.60 This work

F: Frequency, P: power, PN: Phase noise, TR: Tuning range, FOM: Figure of Merit.

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Table I summarizes measured performance, where the figure of merits with tuning range is used for comparison between this work and other state-of-the-art CMOS applications. Phase noise automatically decreases, while power consumption is reduced; thus, there is a trade-off between power consumption and phase noise[6].

IV. CONCLUSIONS A general design methodology for low-power and

wideband LC-VCO with body-grounded NMOS varactors is proposed to reduce the flicker noise, and meanwhile increase the tuning range. Moreover, the self-biasing current source circuit avoids two major disadvantages; higher power supply sensitivity and higher up-conversion of flicker noise. It is shown that optimization will yield a trade-off between phase noise and power consumption. As proof of the concept, the overall measured worst-case phase noise was -132.68dBc/Hz at a 1 MHz frequency offset, approximately over the whole working band. As a result, this CMOS VCO achieves a best FOM of -201.60dB. The VCO shows an approximate 37.59% tuning range. The VCO is tuned from (4.63-6.78) GHz with a tuning voltage varying from 0 to 2.50 V and a power dissipation of only 2.21 mW. Importantly, the proposed NMOS VCO design demonstrates very low phase noise and approximately fixed values at all tuning range due to the high quality LC-tank components and the direct bias current source

ACKNOWLEDGMENT

The authors would like to thank Prof. Zhipeng Wu and Prof. Ali Rezazadeh, for all their help and support.

REFERENCES

[1] H. Labiod, Wi-Fi, Bluetooth, Zigbee and WiMAX / by H. Labiod, H. Afifi, C. De Santis. Dordrecht: Dordrecht: Springer, 2007.

[2] M. Aqeeli, Zhirun Hu, Cahyo Muvianto, “A 6.72GHz Low-Phase-Noise Voltage Controlled Oscillator Adopting Metal-Oxide-Metal Capacitors Using 130nm CMOS Technology” the 2013 Fifth International Conference on Computational Intelligence, Communication Systems and Networks, pp.101 – 106, 2013.

[3] M. Dousti, F. Temcamani, J. Gautierassoud, “A Fully Differential Low Phase Noise and Extra Linear VCO Design in SiGe BiCMOS Technology,” 3rd International Conference on Information and Communication Technologies, 2008.

[4] D. Cordeau, J-M. Paillot, L.A. Dascalescu, “5-GHz fully integrated full PMOS low-phase-noise LC VCO,” IEEE Journal of Solid-State Circuits, vol. 40, pp. 2087 – 2091, 2005.

[5] X. Tang, F. Huang, M. Shao, Y. Zhang “A wideband 0.13�m CMOS LC-VCO for IMT-Advanced and UWB applications,” IEEE MTT-S International Microwave Workshop Series on Millimeter Wave Wireless Technology and Applications (IMWS), 2012.

[6] Marc Tieboud, “Low power VCO design in CMOS,” Berlin, Heidelberg, Netherlands: Springer, 2006.

[7] D. B. Lesson, “A simple model of feedback oscillator noise spectrum,” Proceedings of the IEEE, vol. 54, no. 2, Feb. 1966.

[8] D. Ham and A. Hajimiri, “Concepts and methods in optimization of integrated LC VCOs,” IEEE Journal of Solid-State Circuits, vol. 36, no. 6, pp. 896–909, 2001.

[9] B. Soltanian, H. Ainspan, R. Woogeun, D. Friedman, and P. Kinget, “An Ultra Compact Differentially Tuned 6 GHz CMOS LC VCO with Dynamic Common-Mode Feedback,” IEEE Circuits Conference in Custom Integrated, CICC ’06, pp. 671-674, 2006.

[10] M. Young-Jin, R. Yong-Seong, J. Chan-Young, and Y. Changsik, “A 4. 39&5.26 GHz LC tank CMOS Voltage-Controlled Oscillator With Small VCO-Gain Variation,” Microwave and Wireless Components Letters, IEEE, vol. 19, pp. 524-526, 2009.

[11] J. Lin, C. Yeung Bun, and Y. Wooi-Gan, “A 5.8-GHz VCO with Precision Gain Control,” (RFIC) IEEE Symposium in Radio Frequency Integrated Circuits (RFIC), pp. 701-704, 2007.

[12] G. Chao, H. Jun, Z. Siheng, S. Houjun, and L. Xin, “A 5-GHz low-phase-noise CMOS LC-VCO for China ETC applications,” in Microwave Technology & Computational Electromagnetics (ICMTCE), 2011 IEEE International Conference on, pp. 267-269, 2011.

[13] P. Ruippo, T. A. Lehtonen, and N. T. Tchamov, “An UMTS and GSM Low Phase Noise Inductively Tuned LC VCO,” Microwave and Wireless Components Letters, IEEE, vol. 20, pp. 163-165, 2010.

[14] B. Catli and M. M. Hella, “A 1.94 to 2.55 GHz, 3.6 to 4.77 GHz Tunable CMOS VCO Based on Double-Tuned, Double-Driven Coupled Resonators,” IEEE Journal of Solid-State Circuits, vol. 44, pp. 2463-2477, 2009.

[15] L. Jianing, Z. Jun, G. Yuanlin, and L. Huihua, “A ultra-low-voltage 6.9 GHz CMOS quadrature VCO with superharmonic and back-gate coupling,” The International Workshop in Microwave and Millimeter Wave Circuits and System Technology (MMWCST), 2012.

[16] L. Fanori and P. Andreani, “Highly Efficient Class-C CMOS VCOs, Including a Comparison With Class-B VCOs,” IEEE Journal of Solid-State Circuits, vol. 48, pp. 1730-1740, 2013.

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