7/31/2019 05118208_2009_lu_vco

1/4

A Wide Frequency Tuning Range Active-Inductor

Voltage-Controlled Oscillator for Ultra Wideband

Applications

Dominic DiClemente and Fei Yuan

Department of Electrical and Computer Engineering

Ryerson University, Toronto, Ontario, Canada

Email:fyuan@ee.ryerson.ca

AbstractThis paper presents a new active inductor LC-tank voltage-controlled oscillator with an ultra wide frequencytuning range. The large frequency tuning range is obtained byvarying the inductance of the active inductor. Two inductancetuning mechanisms, namely the wide-band tuning mechanismfor coarse frequency adjustment over a large frequency range

for band selection and the primary tuning mechanism for thefine frequency tuning in frequency synthesis, are introduced. Theproposed oscillator was designed and implemented in TSMC-0.18m 1.8V 6-metal 1-poly CMOS technology. The oscillatoroccupies a small active area of 85 50 m2. The wide-bandtuning mechanism provides a frequency tuning range from 0.2GHz to 6.5 GHz while the primary tuning mechanism providesa frequency range from 1.4 GHz to 1.7 GHz when the centerfrequency of the oscillator is set to 1.6 GHz. The phase noisewith the VCO tuned to 1.6 GHz is -118.5 dBc/Hz at 1 MHzfrequency offset.

I. INTRODUCTION

The ever increasing growth of the wireless communica-

tion market has lead to multiple wireless standards. These

multiple standards can co-exist within the same frequencyband or span across a large frequency spectrum. It is highly

desirable to reuse existing hardware for multiple standards.

To accomplish this a Voltage-Controlled Oscillator (VCO)

with a large frequency tuning range is essential. The inher-

ent frequency selective nature of LC-tank oscillators allows

these oscillators to exhibit better phase noise performance

over ring oscillators. Several mechanisms exist to tune the

oscillation frequency of LC-tank oscillators with varactors the

most widely used. The tuning range of the capacitance of

a varactor is limited by the ratio of the capacitance of the

varactor to the total capacitance at the node to which the

varactor is connected. The typical frequency tuning range of

varactor LC-tank VCOs is less than 30%. For a single VCOto be used for multiple bands a frequency tuning range of

over 70% is often required. Although the frequency tuningrange of LC-tank oscillators can be increased by employing

digitally switched capacitor arrays, this is at the cost of

increased hardware complexity and silicon consumption. This

paper proposes an active inductor LC-tank voltage-controlled

oscillator with an ultra wide frequency tuning range. The large

frequency tuning range is obtained by varying the inductance

of the active inductor. Two inductance tuning mechanisms,

namely the wide-band tuning mechanism for coarse frequency

adjustment over a large frequency range for band selection and

the primary tuning mechanism for the fine frequency tuning

in frequency synthesis, are introduced. The absence of spiral

inductors significantly reduces the silicon consumption of the

oscillator and the tunability of the quality factor of the activeinductor using two negative inductors results in good phase

noise performance. This paper is organized as follows. Section

II provides the details of the active inductor, the proposed LC-

tank VCO and its two proposed frequency tuning mechanisms.

Section III provides the simulation results of the proposed

VCO, the layout and the micro photo of the fabricated VCO.

The paper is concluded in Section IV.

I I . PROPOSED ULTRA WID E-BAND VCO

A. Gyrator-C Floating CMOS Active Inductors

Active inductors synthesized using active devices offer the

advantages of a tunable inductance, and a small silicon area

over their spiral counterparts. Two back-to-back connectedtransconductors, known as gyrators, with one port terminated

with a capacitive load, as shown in Fig.1, exhibit an inductive

characteristic at the other port. It can be shown that when

the input impedance of the gyrators is infinite, the admittance

looking into port 1 of the gyrator is given by

Y(s) = sC2 + Go2 +1

s

C1gm1gm2

+ Go1

gm1gm2

. (1)

Eq.(1) can be represented equivalently by the RLC network

shown in Fig.1 with

Rp =1

Go2, Cp = C2,

Rs = Go1gm1gm2 , L =C1

gm1gm2. (2)

It is seen that a large inductance can be obtained by lowering

gm1 and gm2. This, however, has a negative impact on the

parasitic series resistance Rs.

To find out the effective frequency range over which the

gyrator is inductive, we examine the impedance of the inductor

Z(s) = Rs

CpL

s LRs

+ 1

s2 + s

1

RpCp+ Rs

L

+

Rp+RsRpCpL

. (3)

978-1-4244-3828-0/09/$25.00 2009 IEEE 2097

7/31/2019 05118208_2009_lu_vco

2/4

Y(s)

Go2 Go1 C 1C2

g (V - V )m1 + -

g (V - V )m2 + -

CR

R

L

p p

s

Y(s)

Fig. 1. Configuration of floating gyrator-C active inductors. Go1 and C1

are the output conductance of gyrator 1 and the input capacitance of gyrator2, respectively. The same applies to Go2 and C2.

When complex conjugate poles are encountered, the

impedance has its resonant frequency o

1

LCp=

t1t2,

where RpRs was utilized and t1,2 = gm1,2C1,2 is the cut-offfrequency of the gyrators. Observe that Z(s) has a zero atfrequency z =

RsL

= Go1C1

. The Bode plots of Z(j ) aresketched in Fig.1. It is evident that the gyrator is resistive

when < z, inductive when z < < o, and capacitive

when > o.

90

-90

0

|Z(j )|w

Z(j )w

w

w

45 deg./dec.

-90 deg./dec.

Inductive CapacitiveResistive

20 dB/dec.-20 dB/dec.

w wz o

w wz o

(dB)

(Degree)

R R

R +R

s p

ps

Fig. 2. Bode plots of gyrator-C active inductors.

B. Proposed Ultra Wide-band VCO

The proposed ultra wide-band VCO is a fully differential

active inductor LC-tank oscillator utilizing the active inductor

proposed by Lu et al [5]. Lu active inductor is a differentially

configured gyrator-C active inductor. The input gyrator is a

pair of pseudo-differential common gate amplifiers composed

ofM1,3,4,6 and the output gyrator is a pair of source followersconsisting of M2,5. A negative resistor network is formed by

the cross-coupled pair M11,12 to compensate for the resistive

loss of the active inductor. Transistors M1,4 are biased in

the triode and behave as a pair of voltage-controlled resistors

whose resistance is controlled by Vb1. It can be shown that the

differential input impedance of the active inductor (looking

into nodes 1 and 2) is given by

Zin =Vin

iin=

2[s(Cgs3 + Cgs2) gm3 + gds1]gds1[gm3 + gm2 + s(Cgs3 + Cgs2)]

(4)

Vb1M1 M4

M3

M11 M12

M2

M6

M8M7

M9 M10

M5

21

Vb2

Fig. 3. Schematic of proposed VCO.

The inductance of the active inductor is given by

Leq =2(Cgs3 + Cgs2)

gds1(2gm3 + gm2 gds1)(5)

It is evident that the inductance is dependent of gm2, gm3,

and gds1. Although the inductance of the active inductor can

be tuned by varying these parameters, the most convenient

means is to vary Vb1, which will in turn tune gds1,4.

C. Frequency Tuning Mechanisms

The schematic of the proposed ultra wide-band VCO is

shown in Fig.3. A negative resistor formed by M11,12 is

employed to cancel out the resistive loss of the LC tank formedby Lu active inductor and the capacitance of the devices. Two

frequency tuning mechanisms are utilized. The first tuning

mechanism, called the wide-band tuning mechanism, tunes the

oscillation frequency of the oscillator by varying Vb1, which in

turn tunes gds1,4. The frequency tuning range obtained from

varying Vb1 is shown in Fig.4. It is seen that Vb1 provides

the frequency range from 0.2 GHz to 6.5 GHz. This large

frequency tuning range allows both band selection and the

effective compensation for PVT variations.

The frequency sensitivity of the wide-band tuning mecha-

nism, however, is not suitable for closed-loop locking of fre-

quency synthesis. To obtain the tuning sensitivity that is more

appropriate for closed-loop frequency locking, the second fre-

quency tuning mechanism called the primary frequency tuning

mechanism that provides a fine frequency tuning sensitivity is

needed. The primary frequency tuning element is composed

ofM7,8,9,10. Vb2 controls the inductance of the active inductor

by adjusting the current drawn by M7,8,9,10 subsequently the

transconductances of M2,3,5,6.

Leq =2(Cgs3 + Cgs2)

gds1(3

IDS3 + 2

IDS2 gds1)(6)

2098

7/31/2019 05118208_2009_lu_vco

3/4

0.5 0.7 0.9 1.1

1

3

5

7

Tuning Voltage (V)

Fre

quency(GHz)

Fig. 4. Dependence of the oscillation frequency of VCO on Vb1.

where 2 and 3 are the transconductance parameters of M2

and M3 given by 2,3 =

2nCoxW

L2,3. Fig.5 shows the

dependence of oscillation frequency of VCO on Vb2.

0.9 1.1 1.3 1.5

1.5

1.7

1.9

2.1

Control Voltage (V)

Frequency(GHz)

10um

2um

500nm

Fig. 5. Dependence of the oscillation frequency of VCO on Vb2.

The additional bias current from M7,8 will not flow through

the negative resistance network ofM11,12. This will reduce the

quality factor of the oscillator subsequently increase the phase

noise. To reduce the phase noise performance dependency

on the additional bias current a second negative resistancenetwork M9,10 is added.

D. Quality Factor

The quality factor of the active inductor ultimately sets the

overall quality factor of the VCO. The quality factor of the

active inductor can be obtained from the ratio of the imaginary

part of the impedance of the active inductor to the real part

of the impedance.

0.7 0.9 1.1 1.3 1.5

120

110

100

Tuning Voltage (V)

Phas

e

Noise

(dBc/Hz)

Compensated

Uncompensated

Fig. 6. Effect of the negative resistance networkM9,10 on the phase noiseof VCO.

Q = (Cgs3 + Cgs2)(2gm3 + gm2 gds1)(gm3 + gm2)(gds1 gm3) + 2(Cgs3 + Cgs2)2

(7)

It becomes evident that the quality factor of the active

inductor is dependent of (i) the biasing condition of the active

inductor and (ii) the frequency tuning voltages Vb1,b2. Fig.7

shows the dependence of the phase noise of the oscillator on

Vb2

0.6 1 1.4

120

110

100

Control Voltage (Volts)

Phase

Noise

(dBc/Hz)

2um

10um

500nm

Fig. 7. Dependence of the phase noise of VCO on Vb2.

III . SIMULATION RESULTS

The proposed VCO was designed and implemented in

TSMC-0.18m 1.8V 6-metal 1-poly CMOS technology. The

layout of the VCO is shown in Fig.8. The output is buffered

with an open drain PMOS matched to 50 for wafer probemeasurements. The oscillator was analyzed using SpectreRF

from Cadence Design Systems with BSIM3V3 device models.

The output of the VCO is shown in Fig.9 with a nearly full

rail to rail swing.

2099

7/31/2019 05118208_2009_lu_vco

4/4

Fig. 8. Layout of VCO.

0.2 0.60

0.4

0.8

1.2

1.6

Time (ns)

Voltage

(V)

Fig. 9. Waveform of the output voltage of VCO.

The simulated phase noise of the VCO is shown in Fig.10

with the VCO tuned to 1.6 GHz. It is seen that the phase

noise is -118.5 dBc/Hz at 1 MHz frequency offset. The power

consumption of the VCO is 45 mW. The micro-photo of the

fabricated VCO is shown in Fig.11.

IV. CONCLUSIONS

A new active inductor LC-tank voltage-controlled oscillator

with an ultra wide frequency tuning range has been presented.It has been shown that the large frequency tuning range of

the VCO is obtained by varying the inductance of the active

inductor. Two inductance tuning mechanisms, namely the

wide-band tuning mechanism for coarse frequency adjustment

over a large frequency range for band selection and the primary

tuning mechanism for the fine frequency tuning in frequency

synthesis, have been proposed. The oscillator occupies a

small active area of 85 50 m2 due to the absence ofspiral inductors. The wide-band tuning mechanism provides

100K 1M 10M

122

118

114

110

Frequency (Hz)

Phase

No

ise(dBc/Hz)

Fig. 10. Simulated phase noise of VCO.

Fig. 11. Microphoto of VCO.

a frequency tuning range from 0.2 GHz to 6.5 GHz while the

primary tuning mechanism provides a frequency range from

1.4 GHz to 1.7 GHz when the center frequency of the oscillator

is set to 1.6 GHz. The phase noise with the VCO tuned to 1.6

GHz is -118.5 dBc/Hz at 1 MHz frequency offset.

REFERENCES

[1] A. Thanachayanont and A. Payne, CMOS floating active inductor and

its applications to band-pass filter and oscillator design, IEE Proc. PartG. - Circuits, Devices, and Systems, Vol. 147, No. 1, pp.42-48, Feb. 2000.

[2] Y. Wu, M. Ismail, and H. Olsson, CMOS VHF/RF CCO based on activeinductors, IEE Electronics Letters, Vol. 37, No. 8, pp.472-473, Apr. 2001.

[3] M. Grozing, A. Pascht, and M. Berroth, A 2.5 V CMOS differentialactive inductor with tunable L and Q for frequencies up to 5 GHz, inProc. IEEE Radio Freq. Integrated Circuits Symp., pp. 271-274, 2001.

[4] F. Mahmoudi and C. Salama, 8 GHz tunable CMOS quadrature generatorusing differential active inductors, in Proc. IEEE Intl Symp. CircuitsSyst., vol.3, pp.2112-2115, May 2005.

[5] L. Lu, H. Hsieh, and Y. Liao, A Wide Tuning-Range CMOS VCO Witha Differential Tunable Active Inductor,IEEE Trans. Microwave Theoryand Techniques,vol.3, pp.3462-3468, Sept. 2006.

2100