A low phase noise 52-GHz push-push VCO in 0.18-/spl mu/m bulk CMOS technologies

RMO2B-4

A

Low Phase Noise 52-GHz Push-Push VCO

in

0.18-pm Bulk

CMOS Technologies

Yi-Hsien

Cho,

Ming-Da Tsai, Hong-Yeh Chang, Chia-Chi Chang, Huei Wang

Dept.

of Electrical Engineering and Graduate Institute of Communication Engineering,

National Taiwan University, Taipei, 106, Taiwan, R.O.C.

Abstract

-

A V-Band fully integrated complementary

push-push VCO is first presented in 0.1S-pn bulk CMOS

technologies. Thin-film microstrip (TFMS) lines are utilized

in the circuit to reduce the conductive substrate effect. In

order to lower the phase noise, complementary cross-coupled pairs are used to generate negative conductance. The

measured phase noise at 1-MHz offset is about -97 dB& at

52.6 GHz and is -104 dBc/Hz at 26.3 GHz. To the author’s best knowledge, this complementary CMOS VCO achieves

the lowest phase noise in comparison with other VCOs using standard bulk CMOS processes in V-Band.

Jndex Terms

-

CMOS, thin-film microstrip line, VCO.

I.

INTRODUCTION

The increasing demand of monolithic microwave integrated circuits (MMICs) is motivated by the rapid growth of high speed wireless communications, for example, wireless LANs and mobile phones. Voltage controlled oscillator (VCO) is an important building element in the implementation of modern wireless communication systems. For considerations of low cost and low power consumption, CMOS is the best choice for these applications. However, in microwave and millimeter-wave regime, integrated VCOs are generally realized by III-V compound devices or SiGe HBTs

[1]-[3].

CMOS VCOs found few applications due to its low maximum oscillation frequency (fm,}.

As CMOS technology progresses to deep submicron, it has become possible to realize millimeter-wave circuits in standard CMOS process, leading to IOW cost and high integration. So far, some V-Band VCOs have been reported in CMOS technology [4]-[5]. A 50-GHz VCO

was

presented which adopted a modified 0.25-pm CMOS process for high quality factor inductor and highfmv: of the device 141. It shows low phase noise of -99 dBcMz at 1-

MHz offset and the output power is -11 dBm. Another fully-integrated 51-GHz VCO fabricated with 0.12-pm CMOS process, demonstrated -30-dBm output power and -85 dBc/Hz phase noise at I-MHz offset [ 5 ] . Tn addition to adopting high

Lax

process technology, some other circuit structures were presented to achieve higher oscillation frequency. Push-push VCO is an attractive approach

0-7S03-8983-2/05/$20.00 0 2005 IEEE

which sums up the differential fundamental signals and takes the second harmonic as the output and is even able to work above devicefmu.

In this paper, we present the first complementary push- push VCO in V-Band. The VCO core is composed of NMOS and PMOS crass-coupled

pairs

to lower the phase noise. The second harmonic output signal is taken from

two fundamental differential buffers working in triode region for the nonlinearity which deliver more output power. Compared with the conventional topology, the circuit delivers 14 dB more

Yd

harmonic output power in our approach from simulations. The MMIC VCO achieves very low phase noise of -97dBc/Hz at 1-MHz offset with a tuning range of 0.5

GHz.

Table 1 summarizes the comparison among previously published MMW VCOs using silicon-based processes and this work. It can be observed that OUT chip demonstrated

the lowest phase noise and comparable chip

size,

power consumption and output power among the reported VCOs using standard bulk CMOS processes in V-Band.

11. CMOS CHARACTERIST~CS AND PROCESS The chip is implemented by TSMC’s

0.18-ym

commercial process C61-171, with a 2-pm AlCu top metal layer. The substrate conductivity is approximately 10 S/m. With optimized

CMOS

technology and deep n-well, this technology provides f, andf- better than 60 and 55

GHz,

respectively. MIM capacitors with 1

fF/pm’

are also provided in the process.

111. CIRCUIT DESlGN

Figure 1 shows the structure of the thin-film microstrip (TFMS) line which consists of the 2-pm AlCu top metal ( M 4 ) as the signal line and bottom metal (MI) as ground plane in the 0.l8-ptn CMOS process. The

TFMS

line has the advantage of the ground plane shielding to isolate the conductive substrate. The substrate thickness of

TFMS

line is about 6.52-ym, thus the coupling effect between transmission lines is negligibk. Therefore, the matching dements of transmission lines and inductors can be

132

TABLE 1

COMPARISON WITH PREVIOUSLY REPORTED VCOS IN V-BAND

Process

0.25-pm SiGe

BiCMOS

SiGe HBT

Frequency Phase noise @ I MHz Chip Size Poc Pout

(GHzt) offset ( d E c m t ) ("'1 (mw) ("1 60 -87 ' 0 . 4 ~ 0 . 7 5 73.8 -17 76 -9 I 0.4 xO.75 128 -7 50 -98 N/A N/A -5.6 -92

1

0.3x0.25

I

21

1

-6.8 0.09-pm CMOS 0.18-pm CMOS 0.25-pm CMOS I I I I I I 52.5 -86 0.8 x 1 41 -8 63 -86 0.45 x 0.7 118.8 -4 Modified

1

0,25-pmCMOS

1

50

1

-99

1

0 . 5 2 x 0 . 3 9

I

13

1

-11 Complementary cross- coupled and push-push

I I I I I

I

I This Work

1

0.12-pm CMOS

1

5 1

I

-85

I

0.5x0.9

I

1

I

-30

I

0.18-pmCMOS

I

52

I

-97

1

0.45 x 0.45 27.3 -16 replaced by meandering TFMS lines in a very small area

to reduce the circuit size. A full-wave EM simulation tool, Sonnet

[SI,

was used to estimate the impedance of the

TFMS

inductors over the frequency of interest.

Fig. 1. The structures of the TFMS on bulk CMOS process.

The conventional cross-coupled push-push VCO circuit structure is shown in Fig. 2 where it adopts NMOS cross- coupled pair to generate negative conductance and the output signal is delivered from the middle of the inductor of the LC tank where the differential fundamental signals cancel out each other and the second harmonic signals combine i n phase [9]-[10].

Complementary cross-coupled pairs show lower phase noise because it has larger voltage swing within the LC tank and it shows more symmetric effective impulse sensitivity function [ 111. Unfortunately, the

f,,

of PMOS is usually much lower than NMOS so that it can not be

Topalogy Push-push Cross-coupled 1141 Cross-coupled Cross-coupled Single-ended 1131 Cross-coupled and DUh-DUh

used in millimeter-wave regime. But for push-push VCO, we can still use PMOS to make the VCO oscillate at fundamental frequency and deliver the 2* harmonic signal to achieve low phase noise in high frequency. Figure 3 shows the circuit schematic of our design. The LC tank is composed of the TFMS differential inductors and NMOS connected as varactors. We omit the current source to lower the phase noise such that the noise from current source can be eliminated and tank voltage swing can be increased [12]. In addition, the negative conductance is generated by complementary cross-coupled pairs consisting of M,-M, to lower the phase noise. In this design, if we remove the PMOS cross-coupled pair

(M,

and M,) and maintain the DC current, the phase noise degrades by about 3 dB from the simulation results.

The circuit can deliver both fundamental and second harmonic signals simultaneously. The fundamental 26- GHz output signals are delivered through common-source buffers. Unlike the conventional push-push topology where the 2"d harmonic signal is extracted from the virtual short point inside the resonant tank, the 2" harmonic signal is first amplified through buffers

(M,

and

M,)

and taken from the middle of TFMS inductors (L, and L,). To acquire more output power, M, and M, are biased in triode region, and therefore, with the increased device nonlinearity, the Znd harmonic power can be increased. From the simulations, it delivers 14 dB more 2nd harmonic output power in our approach than the conventional one in which the output is taken from the middle of the inductor of the LC tank, that is, from point A shown in Fig. 3.

fo!

+

Fig. 2. Schematic of conventional NMOS-only cross-coupled

PUSh-pUSh VCO.

Fig. 3. Schematic ofproposed VCO.

Iv. MEASUREMENT RESULTS

This chip was tested via on-wafer probing and was biased by aluminum bond wires. The VCO core draws 13 mA at 2.1 V and the buffer amplifiers draw 4 mA at 0.3 V. The measured output power is about -16 dBm and the oscillation frequency

is

from

52.6

to 52

GHz

with control

voltage (VJ from 0.8 to 2.1 V. When we increase the VCO bias voltage up to 3.6 V with 34 mA and the buffers draw 19 mA at 0.7 V, the output power goes up to

-5

dBm. And the oscillation frequency is from 52.5 to 52 GHz with control voltage ( V J from 1 to 3 V. Figure 4 shows the output power and oscillation frequency versus control voltage in both bias conditions. The measured phase noise is -97 dBc/Hz at 1-MHz offset at 53 GHz and -104 dBcMz at 26.5 GHz as shown in Fig. 5 and Fig. 6 respectively. The measured fundamental power from the 2nd harmonic output is -58.6 dBm shown in Fig. 7 with

20

dB

cable and transition loss, that is, the fundamental rejection is about 30 dB. Figure 8 shows the chip photo and the size is 0.45 x 0.45 mm2 including all test pads.

’1 - 0 u t p u f P o w e r ~ V d d ~ I . l V l +Frequeory(Vdd=Z.lVj 40utputPower( Vdd=3.6V) -Freqwncy( V d d E 3 . W ) 0- -5

-

-

-la;

+

1.5 2.0 2.5 3.0 3.5 0.5 1 .o Vctr (v)

Fig.

4.

Measured output power versus control voltage.

V. CONCLUSION

This paper presents the design and measurement of a 52 GHz complementary push-push VCO using a standard CMOS 0.18-ym process. The chip shows very low phase noise of -97 dBc/Hz at 1-MHz offset at 52 GHz with 0.5-

GHz tuning range. The output power is about -16 dBm while dissipating 27.3-mW dc power. This chip shows a

miniature size of 0.45 x

0.45

mm2. To

the best

of our

knowledge, the VCO achieves the lowest phase noise in V-Band in comparison with other VCOs in standard CMOS process.

ACKNOWLEDGEMENT

This work is supported in part by National Science Council of Taiwan, R.0.C (Projects NSC 93-22 19-E-002- and

NSC 93-2752-E-002-002-PAE). The chip was

fabricated by TSMC through the Chip Implementation Center (CIC) of Taiwan,

R.O.C.

016, NSC 93-2219-E-002-024, NSC 93-2213-E-002-033,

A Mkrl 1.08 MHr

Ref B dBm Ext MIX Noise -96 96 dB/Hz

Res BN 91 kHz *WBN 3 kHz Sweep 28 6 ms (601 pts)

Fig. 5 Measured phase noise at 1 MHz offset

Center 26.366 7 GHr

#Res BW i B B kHz

Span 508 HHz- iUBW 3 kHz

Fig. 6 . Measured fundamental power from 2”* harmonic output.

Sweep 1.3 s (601 pts)

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Fig. 7. Chip photo of the push-push VCO with a chip size of

0.45 x 0.45 mm’.