A Low-Voltage Low-Power Low-Phase-Noise Wide-Tuning-Range 0.18-μm CMOS VCO with High-Performance FOMT of -196.3 dBc/Hz

To-Po Wang and Shih-Yu WangDepartment of Electronic Engineering and Graduate Institute of Computer and Communication Engineering,

National Taipei University of Technology, Taipei 10608, Taiwan, R.O.C. E-mail: tpwang@ntut.edu.tw

Abstract — This paper presents a low-voltage low-power low-phase-noise wide-tuning-range VCO in 0.18-µm CMOS. By using analog tuning varactors connected between the drain and source terminations of a cross-coupled pair, and combined with digital switching capacitor banks, the VCO tuning range can be effectively boosted. Moreover, inductors are utilized on top of a cross-coupled pair for enhancing the negative conductance (-Gm)and resonate the parasitic capacitors. This design methodology can easily overcome a VCO’s start-up condition. Based on the proposed architecture, the fabricated 0.18-�m CMOS VCO exhibits a measured 47.7% tuning range. Operating at 0.65-V low supply voltage, the VCO core consumes 2.99-mW dc power. At this bias condition, the measured phase noise is -115.6 dBc/Hz at 1-MHz offset from a 4.0-GHz oscillation frequency. Compared to recently published VCOs in C band, the proposed VCO can simultaneously achieve low supply voltage, low dc power dissipation, low phase noise, and wide tuning range, leading to a superior figure of merit considering the tuning range (FOMT = -196.3 dBc/Hz).

Index Terms —Negative conductance (-Gm), voltage-controlled oscillator (VCO).

I. INTRODUCTION

Because of demands on the increased high-data-rate wireless communication, the developments of radio-frequency integrated circuits (RFIC) are toward gigahertz. To implement RFIC, the technology of CMOS integrated circuit is one of the candidates due to the low supply voltage, low dc power, low cost, and high integration. With CMOS feature size advances to deep-submicron range, the CMOS voltage-controlled oscillators with operation frequencies in C band were reported[1]-[6]. In this paper, a low-voltage low-power low-phase-noise wide-tuning-range VCO with 0.65-V low supply voltage and 2.99-mW low core power is presented. At this bias condition, this VCO achieve measured phase noise of -115.6 dBc/Hz at 1-MHz offset from 4.0-GHz. The measured tuning range is 47.7%, and the figure of merit considering the tuning range (FOMT) is -196.3 dBc/Hz.

II. VCO DESIGN

A conventional LC-tank differential VCO is shown in Fig. 1(a). The conventional VCO consists of a cross-coupled pair (M1- M2) for negative conductance (-Gm) generation and a LCresonator for frequency determination. For extending VCO tuning range, the analog tuning varactor bank can be used. The VCO circuit topology in Fig, 1(a) is widely used in commercial applications. For operation in C band, the supply

(a)

(b) Fig. 1. Schematic of the (a) conventional differential cross-coupledVCO and (b) proposed low-voltage low-power low-phase-noisewide-tuning range CMOS VCO.

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voltage and dc power of the conventional VCO have to be increased for sustaining the difficult startup condition (e.g., supply voltages of previously published 0.18-�m CMOS VCOs are all higher than 1.4 V [1]-[4]). In addition, the output voltage swings are limited because of the degraded MOSFET transconductances (gm1 and gm2), leading to a poor VCO performance.

In order to overcome these difficulties, a fully-integrated low-voltage low-power low-phase-noise wide-tuning-range CMOS VCO is proposed in Fig. 1(b). Analog tuning varactors are connected between the drain and source terminations of a cross-coupled pair, and they combined with digital switching capacitor band for boosting the VCO tuning range. Moreover, inductors (LD3) are utilized on top of a cross-coupled pair to resonate the parasitic capacitors and enhance the negative conductance (-Gm).

Fig. 2 shows the simplified equivalent half-circuit model of the proposed VCO is depicted in Fig. 1(b), where RD1 and RSrepresent the losses of the on-chip inductors LD1 and LS,respectively. The LS is a short metal sheet in this work. To derive the operation frequency and investigate the tuning mechanism, the transfer function between Vo and Vi can be derived as

012

23

34

45

5

012

2

asasasasasabsbsb

VV

i

o

�������

�� (1)

Where

� � � �� �

� �� �� �

25 1 bank_tot var1,2

4 1 bank_tot 2 var1,2 bank_tot var1,2 1 1

3 1 bank_tot 2

bank_tot 1 1 2 var1,2

1 var1,2 var1,2 bank_tot 1

2 bank_tot

1

1

S D

S D S m S S S D D S

S D m S

S D D S S m S

S D S S D S

a L L C C

a L L C L g R C L C C L R L R

a L L C g R

C L R L R L g R C

L L C L C C R R L

a C

�

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� � �

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� �� �� �

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� �

2 1 1 1 var1,2

2 var1,2 bank_tot 1

1 var1,2 var1,2

1 2 bank_tot 1 1 var1,2

2 var1,2

0 2

2 2 1

1 2 1 1

0 2 1

1

1

m S S D D S D S

S m S S D S

S D S S

m S S D S D S

S S m S

S m S

m D S

m D S S D

m D S

g R L R L R R L C

L g R C C R R L

R L C R L C

a g R C R R L R R C

R L g R C

a R g Rb g L Lb g L R L Rb g R R

� � �

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Oscillating will occur if the circuit loop gain is unity, corresponding to a voltage gain Vo/Vi = –1 with the oscillation frequency at �o. With a proper arrangement, the oscillation frequency (fo) can be approximated by

� �1 bank_tot 1 var1,2

1 12 2

oo

D D S

fL C L L C

�

� � � �

�����(2)�

Based on (2), the oscillation frequency (fo) of the VCO can be adjusted by using the control voltage (Vctrl, Va, and Vb). As

Fig. 2. Simplified half-circuit model of the proposed VCO.

Fig. 3. Microphotograph of the fabricated VCO.

Fig. 4. Measured oscillation frequency of the fabricated VCO.

Fig. 5. Measured output power of the fabricated VCO.

978-1-4673-2141-9/13/$31.00 ©2013 IEEE

a result, the tuning range of the VCO around frequency fo can be written as

� �1 bank_tot 1 var1,2

1 12 D D S

fL C L L C

� � � � � �

(3)

From (3), it is observed that the tuning range of this VCO is controlled by digital switch ( Cbank_tot) and analog tuning varactor ( Cvar1,2). Compared to conventional VCO shown in Fig. 1(a), this work achieves wider tuning range due to the digital switch capacitors and analog tuning varactors.

III. EXPERIMENTAL RESULTS

Fig. 3 shows the microphotograph of the fabricated VCO with a chip size of 0.8 � 1.035 mm2 including the testing pads. To evaluate the high-frequency performance, the output spectrum and phase noise were characterized by a 26.5-GHz spectrum analyzer. On-wafer probing was used to test the VCO performance. The losses of the measurement setups were de-embedded and calibrated in the experimental results.

While operating at the supply voltage (VDD) of 0.65 V, the VCO core consumes 2.99 mW. Besides, the supply voltage (VBUF) of the buffers is 0.65 V, and the two buffers consume total 2.34 mW. Fig. 4 and Fig. 5 show the measured tuning characteristics, and the VCO bands are adjusted by Va and Vb.When sweeping the controlled voltage (Vctrl) from 0 V to 1.8 V, the overall operation frequency is from 3.24 GHz to 5.27 GHz, exhibiting a tuning range of 47.7 %. At this bias condition, the measured phase noise is -115.6 dBc/Hz at 1-MHz offset from the 3.958-GHz carrier, as shown in Fig. 6.

Table I summarizes the performance of this VCO and compared to the state-of-the-art C-band VCOs in the literature. It is observed that this VCO can achieve the measured low supply voltage (VDD=0.65V), low dc power consumption (PDC=2.99 mW), low phase noise, and widest tuning range of 47.7%. In order to characterize the VCO performance, the

widely used figure-of-merit including tuning range (FOMT)[5]-[7] is adopted in this work. The FOMT is written as

� � )1

(log10)10

(log20 1010 mWPRangeTuning

fffLFOM DCo

T ��

��� (4)

where L(�f) is the VCO phase noise, �f is the offset frequency, fo is the carrier frequency, and PDC is the dc power consumption. From table I, it is observed that this low-voltage low-power low-phase-noise wide-tuning-range VCO exhibits the superior FOMT of -196.3 dBc/Hz.

IV. CONCLUSION

The proposed low-voltage low-power low-phase-noise wide-tuning-range VCO is demonstrated in a 0.18-�m CMOS process. Using varactors connected between drain and source termination of the cross-coupled pair and combined with digital switching capacitor bank, an extended tuning range is achieved. Moreover, inductors are utilized on top of a cross-coupled pair to enhance the negative conductance (-Gm) and resonate the parasitic capacitance. This leads to an increased voltage swings across the cross-coupled pair, and it thus achieves low phase noise. According to the measured results, this work demonstrates impressed VCO performance in terms of tuning range, phase noise, supply voltage, dc power dissipation, and figure of merit considering the tuning range (FOMT).

ACKNOWLEDGEMENT

The authors would like to thank the National Chip Implementation Center (CIC), Hsinchu, Taiwan, for chip fabrication, and the National Nano Device Laboratories (NDL), Hsinchu, Taiwan, for measurement support. This work is supported in part by the National Science Council under Contract NSC 100-2221-E-027-091, NSC 101-2119-M-027-003, and NSC 102-2623-E-027-004-NU.

Fig. 6. Measured phase noise of the fabricated VCO.

TABLE IPERFORMANCE SUMMARY AND COMPARED TO THE

STATE-OF-THE-ART C-BAND VCOS

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REFERENCES

[1] Y. J. Moon, Y. S. Roh, C. Y. Jeong, and C. Yoo, “A 4.39-5.26 GHz LC-tank CMOS voltage-controlled oscillator with small VCO-gain variation,” IEEE Microw. and Wireless Compon. Lett., vol. 19, no. 8, pp. 524-526, Aug. 2009.

[2] J. A. Hou and Y. H. Wang, “A 5 GHz differential Colpitts CMOS VCO using the bottom PMOS cross-coupled current source,” IEEE Microw. and Wireless Compon. Lett., vol. 19, no. 6, pp. 401-403, June 2009.

[3] T. N. Nguyen and J. W. Lee, “Low phase noise differential vackar VCO in 0.18 �m CMOS technology,” IEEE Microw. and Wireless Compon. Lett., vol. 20, no. 2, pp. 88-90, Feb. 2010.

[4] M. T. Hsu and P. H. Chen, “5GHz low power CMOS LC VCO for IEEE 802.11a application,” in Asia Pacific Microw. Conf. Tech. Dig., 2011, pp. 263-266.

[5] E. S. A. Kytonaki and Y. Papananos, “A low-voltage differentially tuned current-adjust 5.5-GHz quadrature VCO in 65-nm CMOS technology,” IEEE Trans. Circuits Syst. II, Exp. Briefs, vol. 58, no. 5, pp. 254-258, May 2011.

[6] T. W. Brown, F. Farhabakhshian, A. G. Roy, T, S, Fiez, and K Mayaram, “A 475 mV, 4.9 GHz enhanced swing differential Colpitts VCO with phase noise of -136 dBc/Hz at a 3 MHz offset frequency,” IEEE J. Solid-State Circuits, vol. 46, no. 8, pp. 1782–1795, Aug. 2011.

[7] T. P. Wang, “A fully integrated W-band push-push CMOS VCO with low phase noise and wide tuning range,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 58, no. 7, pp. 1307-1319, July 2011.

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