Proceedings of Asia-Pacific Microwave Conference 2007

Design of VCO for 2.4GHz Wireless Applications
Using Transmission Line Resonators
R. K. Pokharel, A. Imada, S. Sasaki, H. Kanaya, and K. Yoshida
Graduate School of Information Science and Electrical Engineering
Kyushu University
744 Motooka, Nishi-ku, Fukuoka, 819-0395 Japan
Email: pokharel@ed.kyushu-u.ac.jp
Abstract—A voltage-controlled oscillator (VCO) employing onchip coplanar waveguide (CPW) resonator is proposed for
2.4GHz wireless applications and comparative study between
the proposed VCO and a conventional VCO which is composedof LC-tank resonator (LC-VCO), is carried out. By comparing
the measured results on the fabricated chip on TSMC 0.35µm
BiCMOS technology, it has noted that the VCO employing onchip CPW resonator is smaller in size by about 30% andfrequency-tuning range is higher than LC-VCO. Phase noise iscomparable but power consumption is deteriorated.
Index terms: VCO, CPW resonator, LC-tank resonator, chipsize, frequency-tuning range.

I.

INTRODUCTION

As evidence from the recent and rapid growth ofwireless communication systems such as 3G cellular phones,
wireless LAN, RFID tags, ETC (ETC: Electronic-TollCollection) systems and so on, there are ever increasing
demands of low cost, high performance, small size, and highspeed microwave devices, circuits, components to meet theexisting and future demands of wireless systems [1]-[5].
Therefore, the design of high frequency analog circuits(RFIC components) has become important in LSI
technology. Although the performance of the CMOS chip
has a limitation in the high frequency and high-speed
applications, it has become a better option due to its low
cost compared to GaAs device [4], [5].
Spiral inductors are indispensable in designing RFIC
components such as amplifiers, mixers, voltage-controlled
oscillators (VCO), or impedance-matching circuits that
results in the miniaturization of RFIC components a difficult
task. Distributed elements made of transmission lines toreplace spiral inductors are particularly effective when theirsize becomes smaller, as the frequency in use increases.
Among the transmission lines, coplanar waveguide (CPW)

1-4244-0749-4/07/$20.00 @2007 IEEE.

is easy to fabricate by the LSI technology compared to its
counter part such as microstrip or strip lines because thesignal line and ground plane exist on the same plane [6] sothat there is no need of via holes.
The applications of the distributed elements made oftransmission lines were reported in the CMOS or BiCMOSLSI chip [7]-[9]. The CPW was exploited as an inductor andused to design a conventional-type matching circuit for anLNA [7] in microwave frequency band. However, theapplication of CPW as an inductor takes larger space than aconventional spiral inductor [7]. Some of the present authors
have also implemented CPW as a resonator to realize onchip matching circuits for LSI chip using quarter-wave
length resonator and impedance inverters [8], [9].
Furthermore, CPW has also been employed to design
VCO at various frequency bands where CPW acts as aninductor [10] and as a resonator [11]. They are particularity
effective and popular in millimeter-wave frequency band
and above [11]. The on-chip CPW resonator for VCO
design in microwave frequency band has not been reported
yet to our knowledge. It may be due to its difficulty toaccommodate the frequency-dependent resonator’s lengthfor on-chip fabrication.
In this paper, we implemented on-chip CPW quarter
wavelength resonator to design a VCO for 2.4GHz-band
wireless applications on TSMC 0.35 µm BiCMOStechnology. The CPW resonators are designed in meanderline structure so that it can easily exploit the vacant space ofthe chip. One of the advantages of the proposed VCO using
on-chip CPW resonator compared to a LC-VCO is the chiparea which is reduced by about 30%, and higher frequencytuning range (FTR).

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II.

DESIGN OF ON-CHIP CPW RESONATOR

0On-chip CPW resonator is designed and fabricated. To
adjust it inside chip, CPW resonator is designed in meanderline structure. Therefore, its losses are discussed in thissection at first. Fig. 1(a) shows the cross-section of CPW
lines and Fig. 1(b) shows the fabricated on-chip CPW linein meander structure. In order to avoid the loss in the Sisubstrate, we covered the lowest metal (metal-1) in all areaof the CPW structure, namely conductor backed CPW. The
conductance of the metal is 4.1x107 S/m. The signal width
and the interval between the slots of the CPW transmission
line are 5 µm and 15 µm, respectively.

|S11|, |S21| (d B)-5-15

S11-20Fig. 2 shows the comparison of the frequency responsesof the designed CPW line resonator where insertion loss(S21) of the EM-simulation result (Agilent Co., Momentum)
is almost in agreement with that of the experimental result.
However, it is noted that there is loss of about 2.5 dB at 2.4
GHz which has to be improved to achieve better
performance of the designed RFIC components using theseon-chip CPW lines and this is considered as one of thepriorities works in near future. Fig. 3 shows the air coplanarprobe system used in this paper for on-wafer measurement.
The input and output microwave characteristics aremeasured by using air coplanar probes (GSG 150; Cascade
Microtech Inc.) and computed controlled vector network
analyzer (HP-8722; HP).

(a)

S21-10-25

246Frequency (GHz)EM-Sim.
Exp.
810Figure 3. Frequency responses of the CPW meander line.

Metal 3 (b)Figure 4. Layout of CPW line resonator.

100µmSiO2 Metal 1

Si substrate

Figure 1. Cross-section of CPW (a) and chip photo of designed on-chip
CPW resonator (b).

III.

DESIGN OF VCO USING ON-CHIP CPW RESONATOR

In this section, we design a VCO using on-chip CPW
resonator instead of LC-tank resonator. Fig. 4 shows thelayout of on-chip CPW resonator designed andcharacterized by EM simulator. The signal line and slot sizeis 5 µm each, respectively. Bottom metal (Metal-1) is used
as ground planes covering all portion of CPW to reduce thelosses. Total length of the line is 6700 µm which is
supposed to be a quarter-wavelength resonator at 2.4 GHz.
Fig. 5 shows the schematics of the designed VCO. In thedesign, widely used CMOS cross-coupled structure [13][14]is used. In Fig. 5(a), LC-VCO is shown and that of using theCPW resonator is shown in Fig. 5(b). Co-simulations
method is used to simulate the passive components together
with active elements. Layouts of both VCO are shown inFig. 6. One of the advantages of using CPW resonators isthat it takes 30% less on-chip space than that of aconventional LC-VCO in same fabrication technology.

Figure 2. Air coplanar probe system (GSG 150; Cascade Microtech Inc.)

The comparison of simulated parameters between theVCOs using CPW resonator and LC resonator is shown inTable 1. From the table, it is noted that the VCO using CPW
resonator has advantages in terms of chip size andfrequency-turning range (FTR). On the other hand, it has the2552

adverse performance in terms of power consumption andphase noise.
IV.

Fig. 7 shows a chip photo of proposed VCOs integratedwith down conversion mixer. Our purpose was to fabricate
an integrated RF front-end containing LNA, mixer and VCO,
therefore we fabricated here an integrated down-conversion
mixers with the designed VCOs. Therefore, we could notcarry out the detail study of noise performance of thefabricated VCOs, so the motivation of this research is toconfirm the operation of the proposed VCO. In Fig. 7(a),chip photo of a LC-VCO is shown whereas chip photo ofVCO using on-chip CPW resonator is shown in Fig. 7(b).

LC resonator
CPW resonator(a)Figure 5.

FABRICATED CHIP AND EXPERIMENTAL RESULTS(b)Schematics of VCO using LC resonator (a) and proposed CPWresonator (b).

CPW resonator(a)

100[μｍ]100 [μｍ]
Spiral Inductor(a)(b)

Figure 6. Layouts of the designed VCO using LC resonator (a), and CPW
resonator (b).

TABLE I.
COMPARISON OF SIMULATED PARAMETERS BETWEEN TWO
VCO USING LC RESONATOR AND ON-CHIP CPW RESONATOR,RESPECTIVELY. [FTR=FREQUENCY-TUNING RANGE]LC resonator

CPW resonator

CenterFrequency [GHz]

2.456

2.789

FTR [%]

1024

Phase noise[dBc/Hz]Area2[m ]-126
2.2x10-118-71.8x10-7(b)Figure 7. Chip photographs of an integrated down conversion mixer withfabricated VCOs using LC resonator (a) and on-chip CPW resonator (b).

The integrated chip of VCO and mixer is measured. In
this paper, we present only a few of the measured results of
VCO to confirm its operation while mixer is in off-state. Aspectrum analyzer is used to measure the output frequencyof VCO. One of the examples of the output spectrum ofVCO is shown in Fig. 8. The variation of measured output
frequency so-called frequency-turning range (FTR) withrespect to the controlled voltage is shown in Fig. 9. The2553

measured results also verify the designed FTR shown inTable 1. In the measured results of the VCO using theproposed on-chip CPW resonator, the center frequency is
2.63GHz which is 0.23 GHz greater than the designed value.
One of the reasons of this discrepancy is the relative
permittivity of Si-substrate that assumed in the design ofCPW resonator which directly results in the length of CPWresonator.

proposed VCO employing on-chip CPW resonator iscomparable to that of using LC resonator.
ACKNOWLEDGMENT
This work was partly supported by a grant of Knowledge
Cluster Initiative implemented by Ministry of Education,
Culture, Sports, Science and Technology(MEXT). This work
was also partly supported by VLSI Design and Education
Center (VDEC), the University of Tokyo in collaboration
with CADENCE Corporation and Agilent Corporation.
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Figure 8. An example of VCO output measured by spectrum analyzer.

Frequency [ GHz ]VCO using on-chip CPW resonator

LC-VCO

Controlled Voltage [ V ]Figure 9. Measurement results of frequency-turning rage showing
comparison of two fabricated VCOs.

V.

CONCLUTIONS

A 2.4GHz-band VCO employing on-chip CPW
resonator instead of LC-tank resonator is proposed,
designed and fabricated on TSMC 0.35 µm CMOS
technology. The advantages of employing CPW resonator isthe larger frequency-tuning range, and it also saves about
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