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A Low Phase Noise VCO for Multi Band Wireless Transceivers

Shirin Bahramirad*, Jad G. Atallah**, Steffen Albrecht*

* Altran Technologies, The Netherlands, {bahramirad, albrecht}@altran-tech.nl

** Royal Institute of Technology, Sweden; atallah@imit.kth.se

Abstract—this paper presents a CMOS voltage controlled

oscillator for multi standard wireless transceivers. The VCO structure is based on All- PMOS LC oscillators. The frequency range extends from 1.7 GHz to 2.5 GH, and tuning between frequencies is done by means of capacitor banks and varactors.

Index Terms— All-PMOS LC oscillator, differential inductor, multi standard, phase noise, voltage controlled oscillator.

I. INTRODUCTION HE ever increasing use of wireless communication

during recent years has led to the demand for transceivers to be small in size, consume less power and have low cost in addition to low phase noise and high performance. Moreover, operating in multiple wireless standards is one of the basic requirements for new generation of transceivers, to be able to serve new standards as well as being compatible with existing ones. Despite problems caused by low resistivity silicon substrates in RF circuits [1], reasonable power consumption, low cost and relatively small size for higher data rate in CMOS technology as well as technology’s capability in implementing full system on one chip, make CMOS technology an interesting choice in radio frequency circuit design.

One of the key components in a wireless transceiver is its frequency synthesizer, which besides guaranteeing the accuracy of frequency; its phase noise, sidebands (spurs) and lock time will affect the performance of the transceiver. As a result, a high performance frequency synthesizer is required to cope with multi standard components. A typical synthesizer consists of a phase frequency detector, charge pump, loop filter, oscillator and a frequency divider. Among these, the oscillator is one of the components, which highly affect the phase noise performance of the PLL. Implementing a multi standard voltage controlled oscillator, as a built-in component in a frequency synthesizer will ease the necessary requirements for multi standard synthesizer. Hence, the design and implementation of a fully integrated VCO with high quality factor components and reasonable phase noise performance for multi standard receivers are the primary motivation behind this work.

The organization of this paper is as follow: in section II wireless standards and their frequency specifications, which are assumed for this work, are described. In addition, different VCO structures are being discussed and the reasons for the chosen architecture are argued. The proposed solution for this application and simulation results are summarized in section III. A brief conclusion and future work are presented in section IV.

II. DESIGN FRAMEWORK Among wireless standards, a set of five standards has been

chosen for this work in order to prove the idea, including GSM, WCDMA, IEEE 802.11b and Bluetooth [2]. The frequency requirements for uplink/downlink of these standards are summarized in Table I.

Table I. Wireless standards and their frequency ranges

Standard Mobile Transmit (MHz)

Mobile Receive (MHz)

E-GSM 900 1760-1830 1850-1920 DCS 1800 1710-1785 1805-1880 WCDMA II 1850-1910 1930-1990 WCDMA III 1710-1785 1805-1880 IEEE 802.11b 2412-2472 Bluetooth 2402-2480

Looking at the frequency of these standards, the VCO

should be tunable between 1.7 GHz and 2.5 GHz, to cover all the standards. Among all the above standards GSM has the most stringent requirement on phase noise, A -121 dBc/Hz at 600 KHz offset from center frequency, makes it the worst case requirement for phase noise for this work.

In the following, various types of oscillators and a comparison between their performances are presented. Then the reasons for why choosing All-PMOS LC oscillator will be stated.

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A. Ring Oscillator A ring oscillator is basically an odd number of cascaded

invertors in a feedback loop. Fig. 1 shows a three-stage differential ring oscillator.

+ -

- +

+ -

- +

+ -

- +

Figure 1. Ring oscillator

The circuit oscillates with frequency of MT

fd

osc 21

= ,

where Td is delay of each stage and M number of stages.

B. Relaxation Oscillator As presented in fig. 2, the relaxation oscillator, like ring

oscillators, is a resonatorless oscillator. The circuit oscillates simply because of the positive feedback around M1 and M2.

Vdd

R1 R2

M1 M2

IssIss

Figure 2. Relaxation oscillator

The major advantage of resonatorless oscillators is the lack

of large passive devices such as inductors and capacitors, which makes them small in size and suitable for integration. However, this is also the main disadvantage of these types of oscillators since there would be no filtering of noise at output signal. Therefore, ring oscillators and relaxation oscillators show a very poor phase noise performance, and as a result not suitable for our application.

C. LC Oscillators To gain a better phase noise, resonators have been added to

feedback network. A very simple resonator is an LC tank. The

oscillator tank resonates at frequencyLCc1

=ω .

In theory, the impedance of the inductor and capacitor are equal with opposite signs at resonance frequency,

cc jC

Ljω

ω 1= , making the impedance of the tank equal

to infinity at this frequency. However in practice both inductor and capacitor suffer from resistive components.

Qc and QL are the quality factors of capacitor and inductor,

and are defined as sRCω

1and

sRLω

. In general, the quality

factor of capacitors is much higher than that of inductors [3], hence the resulting parallel resistance of the tank, RP, is mainly determined by the inductor, RP ≈ RPL.

L C

Ls

Cs

Rs

LP Cp Rp

Figure 3. Impedance of LC tank

Lp ≈ Ls Cp ≈ Cs

RpL ≈ QL2.RsL RpC ≈ QC

2. RsC (1)

For QL» 1 For QC » 1 Based on the above model, the impedance of the tank can

be presented as:

PcpPc

CjRLj

Zω

ω++

= 111

(2)

Magnitude and phase of such an oscillator are displayed in fig. 4. As it is suggested in the figure, there is a significant filtering characteristic in the LC oscillator, which considerably reduces the phase noise.

fcf

(a)

+90

-90

(b)

f

Figure 4. (a) Magnitude and (b) phase response of an LC tank

At resonance, PC

PC CL

ωω 1

= and the voltage gain equals

+gmRP. It should be noted that for oscillation to start, Barkhausen’s criteria [3] has to be fulfilled.

There are different approaches to design an LC oscillator. The most common methods are Colpitts oscillator, Hartley oscillator, and Cross-Coupled oscillator.

Colpitts and Hartley, fig 5, use one transistor to provide sufficient gain and phase shift. Frequency of oscillation in

both topologies is eqeq

c CL1

=ω .

149

3

L1 Cp Rp

Iss

M1Vb

Vdd

L2

LP C1 Rp

Iss

C2M1Vb

Vdd

(a) (b) Figure 5. (a) Colpitts and (b) Hartley oscillators

Since Colpitts architecture uses only one inductor, and as a

result is smaller and more suitable for integrated circuits, it is more frequently used than Hartley oscillators.

Small signal analysis of Colpitts oscillator will result in following equation for gain

( )21

221

CCCCRg Pm

+= (3)

where gm is the gain from source of M1 and RP loss of the LC

tank. The minimum required gain occurs when 12

1 =CC

,

resulting the following expression to fulfill the Barkhausen’s criteria.

(4) 4≥Pm Rg

Another approach to design an LC oscillator is Cross-Coupled oscillator, fig.6, which is divided to three different types, PMOS-Only, NMOS-Only and Complementary Cross-Coupled.

Vdd

M1 M2

Itail

L1C

L2

Vo1 Vo2

(b)

Vdd

M1 M2

Itail

L1C

L2

Vo1 Vo2

(a)

Vdd

M1 M2

L

C

Vo2

M1 M2

Vo1

Itail

(c) Figure 6. Cross-Coupled oscillators

(a) NMOS-Only (b) PMOS-Only and (c) Complementary

It is presented in [3] that at resonance the total phase shift around the loop is zero because each stage provides a zero frequency dependant phase shift, resulting the following limitation on gain for oscillation to begin:

( ) 11 221 ≥→≥ PmPmPm RgRgRg (5)

To choose between Colpitts oscillator and Cross-Coupled

oscillator, it should be noted that Colpitts suffer from two drawbacks. First with regards to equations (4) and (5) it can be seen that with identical LC tank the gain has to be 4 times bigger in Colpitts, resulting either wider transistor size or larger biasing current and as a result larger power consumption. And secondly, Colpitts oscillators do not provide differential output. It should be mentioned here that maximum noise rejection is feasible in fully differential oscillator comparing to a single mode oscillator, and the level of SNR is higher in differential circuit, and as a result the fully differential model has been chosen for this project.

For this work, an All-PMOS oscillator has been chosen for

various reasons. Complementary Cross-Coupled has two more transistors comparing to PMOS Only, resulting in almost five times more parasitic capacitance and hence degradation in tuning range and introducing more noise sources. In addition, PMOS transistors have smaller 1/f noise due to lower mobility comparing to NMOS transistors, and they have less hot carrier effect [10]. Therefore PMOS VCO can achieve better phase noise performance than NMOS VCO. Moreover using PMOS transistors for current source provides excellent suppression of power supply noise. In summary PMOS VCO is more appropriate in terms of phase noise performance.

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III. DESIGN AND SIMULATION RESULTS Now that the structure of oscillator has been chosen, the

components should be selected so that the circuit is capable of oscillating at six different frequency bands in the range of 1.7 GHz to 2.5 GHz. The proposed circuit is presented in fig. 8.

The base frequency is chosen to be 2.5 GHz, so the fix capacitor and inductor are resonating at this frequency. However, the total tank capacitance is formed by combination of the fixed capacitor, capacitors banks, varactors and parasitic capacitances. Capacitor banks are added to switch between different frequency bands and varactors are used for fine-tuning within each frequency. With capacitor C1 added to LC tank, center frequency changes to fc = 2.34 GHz. Addition of each capacitor bank from C2 to C5 will result in a change of center frequency by 160 MHz.

It should be noted that the circuit is fully differential and must remain symmetrical. For this reason, each level consists of two capacitors. The values of capacitors are summarized in Table II.

Table II. The value of switched capacitors

Frequency [GHz] 2.5 2.34 2.18 2.02 1.86 1.7

Capacitor C1–C5 [fF] 150 487.9 566.4 659.95 834.03 949.8

Inductors are the most area-consuming components on the

chip and the overall phase noise of the circuit depends primarily on the quality factor of the inductors, as the quality factor in capacitors is much higher than in inductor’s. Therefore, the resulting parallel resistance of the tank, RP, is mainly determined by the inductor. It is more desirable to use a differential inductor instead of two single ended because a differential inductor will consume almost half the area of two single ended inductors with considerably higher Q, due to the reduction in the substrate loss. For this work, a differential inductor has been designed using Momentum simulator of Agilent ADS, shown in fig. 7, with value equal to 4.2 nH and a simulated Q value of 12 at frequency of 2.5 GHz.

Figure 7. Differential Inductor

Varactors are used to tune the frequency inside the selected

band, and connected differentially to the circuit.

The overall specifications for the VCO are summarized in Table III.

Table III. VCO specification

Frequency of Oscillation 1700MHz ~ 2500MHz

Min. Sensitivity (KVCO) 35 MHz/Volt

Control Voltage (Vtune) 1Volt (0.4 ~ 1.4)

Phase Noise -121dBc/Hz at 600 KHz Offset

Current consumption 13 mA The circuit is designed in UMC 0.18μm CMOS process

using Agilent ADS and Cadence. The circuit shown in fig. 8 includes the oscillator core along with biasing circuit, limiter and output buffers.

The steady state output of the oscillator at 2.5GHz is shown in fig 8. The output is seen from the RL, Vx and Vy in the circuit.

Fig. 10 shows phase noise values at center frequency of 2.5 GHz. Phase noise performance of the oscillator for all frequency bands at 600 KHz offset frequency are summarized in Table V. Figure 11 and Table VI represent the frequency tuning over the control voltage, and the tuning range, VCO sensitivity, and KVCO for each frequency band respectively. A comparison between recently published LC oscillators is shown in Table IV.

IV. CONCLUSION This paper presents a 1.8V fully integrated multi standard

low noise oscillator designed in 0.18μm CMOS technology. The VCO operates between 1.7 GHz and 2.5 GHz by means of array of switched capacitors. Varactors have been used for fine-tuning around each frequency of oscillation. The circuit consumes supply current of 13mA. The oscillator shows phase noise performance of –127.1/-119.1 dBc/Hz at 600KHz offset from 1.7 GHz and 2.5 GHz respectively.

Ongoing work by the authors targets VCO operation over the entire band from 1.7 GHz to 2.5 GHz, covering all currently existing gaps in the operation range.

REFERENCES [1] Lawrence E. Larson, “Integrated Circuit Technology Options for RFIC’s

Present Status and Future Directions”, IEEE Journal of Solid State Circuits, vol. 33, no. 3, pp. 387-399, March 1998.

[2] J. G: Atallah, M. Ismail, “A CMOS Frequency Synthesizer for Multi-Standard Wireless Devices.” Proceedings of the 46th IEEE International Midwest Symposium on Circuits and Systems, 2003, vol. 3, 27 – 30 Dec. 2003.

[3] Behzad Razavi, RF Microelectronics, Prentice Hall, Inc. 1998. [4] Jesper Bank, “A Harmonic-Oscillator Design Methodology Based on

Describing Functions,” Doctoral Thesis, Chalmers University of Technology, Sweden 2006.

[5] Behzad Razavi, “A Study of Phase Noise in CMOS Oscillators,” IEEE Journal of Solid State Circuits, vol. 31, no. 3, March 1996.

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[6] M. Danesh, J. R. Long, “Differentially Driven Symmetric Microstrip Inductors,” IEEE Transaction on Microwave Theory and Techniques, vol. 50, no. 1, January 2002.

Table IV. Comparison with recently published VCOs

[8] [9] [11] [7] T. H. Lee, “The Design of CMOS Radio Frequency Integrated Circuit”,

Cambridge University Press, 1998. Supply 2.6V 1.8V 1.8V Process 0.25μm 0.18μm 0.18μm [8] Z. Tang, J. He, H. Min, “A Low Phase Noise 1-GHz LC VCO

Differentially Tuned by Switched Step Capacitors”, IEEE Asian Solid State Circuits Conference, 2005.

Frequency 1GHz 2/5.8 GHz 2.97-4.45GHz

Phase noise

-130 dBc/Hz at

1MHz

-112/-107 dBc/HZ at

1MHz

-124.2 dBc/Hz at 1MHz

[9] M. Yeh, W. Liou, T. H. Chen, Y. C. Lin, J. Ho, “A Low Power 2/5.8 GHz CMOS LC VCO for Multi band Wireless Communication Applications”, International Conference on Communication, Circuits and Systems, 2006.

Power 9mW 11.7/9.3mW 7mW [10] J. Y. Chen, “CMOS Devices and Technology for VLSI”, Englewood

Cliffs, NJ: Prentice Hall, 1990. [11] B. Q. Diep, C. S. Park, “All PMOS Wideband VCO for Multi band

Multi Standard Radios”, The 9th International Conference on Advanced Communication Technology, 2007.

Vtune

Vx

RL=50OVdd

Vy

RL=50O

Vdd

75/0.18

40/0.18

25/0.18

100O

280/0.58280/0.58280/0.5828/0.58

280/0.58

95/0.1895/0.18

15O

100O

25/0.18

40/0.18

75/0.18 75/0.18

371.17O

V=1.1V

1KO

1KO

Vdd

1KO

Vdd

1KO

V=1.1V

C=150fF

C1 C1

C2 C2

C3 C3

C4 C4

C5 C5

LL

Figure 8. Fully differential All-PMOS VCO

Out

putV

oltag

e[V

]

0.9

1.0

1.1

1.2

Time[nsec]

100 200 300 400

Vx

Vy

Phas

e N

oise

[d

Bc/H

z]

Offset Frequency

10k1k 100k 1M 10M 100M

[Hz]

-50

-70

-90

-110

-130

-150

-170

Figure 9. Output voltage at 2.5 GHz oscillation frequency Figure 10. Phase noise performance at 2.5 GHz

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Table V. Phase noise analysis of the VCO at each frequency at 600KHz offset

Oscillation Frequency 2.5GHz 2.34GHz 2.18GHz 2.02GHz 1.86GHz 1.7GHz

Phase Noise [dBc/Hz] -119.1 -120.7 -122.4 -124.2 -125.8 -127.1

Frequency of Oscillation @output

1,6

1,8

2

2,2

2,4

2,6

0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6

Vtune [Volts]

Freq

[GHz

]

Figure 11. Fine-tuning with varactors

Table VI. The tuning range and VCO sensitivity in each frequency band

fosc [GHz] Vtune [V]

2.5 2.34 2.18 2.02 1.86 1.7

0.4 2.432 2.285 2.131 1.986 1.829 1.686 0.5 2.446 2.295 2.143 1.995 1.836 1.692 0.6 2.463 2.303 2.155 2.002 1.843 1.697 0.7 2.474 2.336 2.163 2.01 1.849 1.702 0.8 2.487 2.34 2.174 2.017 1.855 1.706 0.9 2.501 2.34 2.18 2.024 1.86 1.71 1.0 2.514 2.351 2.19 2.03 1.865 1.714 1.1 2.525 2.364 2.194 2.035 1.868 1.717 1.2 2.534 2.369 2.202 2.039 1.871 1.721 1.3 2.541 2.373 2.206 2.043 1.874 1.721 1.4 2.548 2.379 2.212 2.047 1.877 1.723

KVCO [MHz/V] 116 94 81 61 48 37

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