An ultra low phase noise W-band GaAs-based PHEMT MMIC CPW VCO

An Ultra Low Phase Noise W-Band GaAs-Based

PHEMT

MMIC CPW VCO

Ping-Yu Chen, Zou-Min Tsai, Shey-Shi Lu, and Huei Wang

Dept. of Electrical Engineering and Graduate Institute of Communication Engineering

National Taiwan University, Taipei, Taiwan, ROC

Phone: +886-2-23635251 ext.3 17 Fax:+886-2-23638247 E-mail: [email protected]

Abstracr

-

A W-band voltage control oscillator (VCO)

using 0.1- m AICaAs/InGaAs/GaAs PHEMT MMIC

technology with ultra low phase noise is presented. This

VCO demobslrated an operation frequency centered at 97

GHz with a tuning range of 2 GHz and an output power of 1 mW. The measured single side-band phase noise is -88

dBc/Hz at lMHz offset. To the hest of our knowledge, this phase noise performance is not only the best among the previously reported results for HEMT MMIC VCO at this frequency, hut also rivals those for most VCOs using HBTs.

I. INTRODUCTION

Oscillators are important components in communication systems. In the past, most high frequency oscillators are

based on IMPATT and Gunn diodes. Though they

demonstrated good power performance, the special processes are not compatible with other MMIC components. With the MMIC technologies advanced in the recent years, millimeter-wave MMIC oscillators operated at W-band frequencies implemented based on HEMT and HBT devices were reported [1]-[7]. HEMT devices have the advantages of higher gain for high frequency operation and therefore quite a few MMIC VCO were reported [1]-[SI. On the other hand, HBTs has inherent low device lif noise characteristics, oscillators based on InP-based HBT to achieve low phase noise performance [6]-[7].

In this paper, a low phase noise W-hand MMIC HEMT VCO is presented. This VCO demonstrated an operation frequency centered at

97 GHz with 2-GHz

tuning range and 1-mW output power. The most significant feature of this VCO is that the measured phase noise is -88 dBc/Hz at 1-MHz offset. Table 1 summarizes the previously reponed performances and features of MMIC VCOs in W-band. It can be observed that the phase noise performance reported in this paper is not only the best among the previously reported W-band HEMT VCOs, but

also rivals those of most HBT-based VCOs except the one in [XI, which reported a superior phase noise performance of -95 dBc/Hz for 1-MHz offset at 104 GHz.

11. DEVICE CHARACTERISTICS AND MMIC PROCESS The HEMT device used in this design is TRW 0.1-pm high linearity AIGaAsIInGaAdGaAs PHEMT MMIC process. The HEMT device has a typical unit current gain cutoff frequency

(fT)

of higher than 100 GHz and maximum oscillation frequency (f") of greater than 250 GHz at 2-V drain bias, with a peak dc transconductance (C,) of 600 mSimm. The gate-drain breakdown voltage is 6 V, and the drain current at peak transconductance

(Ibpk) at 2-V drain-source voltage is 600 mA/mm. This MMIC process also includes thin-film resistors, MIM capacitors, spiral inductors, and air-bridges. The wafer is thinned to 4-mil for the gold plating of the backside and reactive ion etching via holes are supported.

111. VCO DESIGN

The W-band oscillator circuit schematic diagram and photograph are shown in Fig. 1. The chip size is 1.5 nnn

x 1 mm. The HEMT device used in this design is TRW 0.1-pm high linearity AlGaAsiInGaAs/GaAs PHEMT MMIC process. A HEMT device with four-finger S O - p m

gatewidth is employed and grounded coplanar waveguide (GCPW) is used as the transmission medium in this design. Two transmission lines connected to two source terminals and one transmission line to gate are dedicated to decide the operation frequency and form the positive feedback. Via holes are placed on the ground metals to

suppress the undesired parallel plate modes of the GCPWs. In addition, airbridges are used at each discontinuity to suppress the undesired slot mode excitation. All the passive structures were characterized by a full-wave EM analysis to eliminate the uncertainty of passive element models.

IV.

MEASURED RESULTS

This VCO was measured via on-wafer probing and the output signal was monitored through the spectrum analyzer by using a sub-harmonic mixer to down-convert the W-band RF signal to IF band. By tuning the gate

voltage from -0.3 to 0.4 V, the frequency and the power of the output signals were read from the spectrum analyzer. Fig. 2 shows the oscillation frequency and output power as functions of gate voltage for V, = 4 V.

The tuning range is about 2 GHz centered at 97 GHz and the average output is -1 dBm. The phase noise is directly estimated from the spectrum analyzer as shown in the spectrum plot of Fig. 3, which indicates the phase noise to be -88 dBc/Hz at 1-MHz offset. This is not only the best phase noise result among the previously reported W-band HEMT VCOs [l]-[S]. but also rivals those of the HBT- basedVCOs [ 6 ] - [ 7 ] .

(b)

Fig. 1. The (a) circuit schematic diagam. (b) chip photograph (chip size 1.5 mm X 1 mm), of the W-band MMIC CPW v c o .

.01 . o , 4 2 4 1 D O 0 1 0 1 0 3 0 4 0 % v, c / )

Fig. 2. The measured oscillation frequency and output power as functions of gate voltage for V, = 4 V.

CL 39 E d B UAUG 25 nMKR - 8 8 I 8 d W H z RL 9 . 8 d B m I 0 d B / I . 0 0 M H z 0 S CENTER 96 6 8 4 4 0 G H z SPRN I 0 00MHz RBW I 0 B k H z UBW l 0 0 k H z SWP 5 0 . 0 m r

Fig. 3 . The measured specmm plot of the W-band MMIC

CPW VCO. The VCO oscillates at 96.7 GHz with phase noise

of -88 dBciHz at I-MHz offset.

v.

DISCUSSIONS

Although ibis W-hand GCPW VCO utilized the same HEMT device (0.1-pm GaAs-based PHEMT MMIC process provided by TRW) and a similar circuit architecture

as

the W-band microship-line VCO reported in [I], the phase noise performance i s significantly different from each other. The reason is described briefly

as the following.

The phase noise of an oscillator is related to two noise sources, which are the up-converted low-frequency noise and the multiplied noise near by the carrier frequency [Y].

The up-converted low-frequency noise is due to the nonlinearity of the transistor that can be treated as a mixer. The power incident into the gate , and the feedback

power out of the matching circuit P, can be analyzed via

the harmonic balance (HB) analysis by defining a feedback loop of the gate and drain of the PHGMT device. It is observed that there is a trade off between output power and phase noise. On the other hand, the multiplied noise near by the carrier frequency is related to the open loop quality factor Q, which is a function of the angular frequency derivatives of the magnitude and phase of the open loop gain [9]. One can conclude that the phase noise will be improved as Q increases.

Comparing the microstrip-line VCO repartcd in [ l ] with this CPW VCO the Q-factors are similar in both designs and therefore the up-converted low-frequency noise power will dominate the phase noise performance. Since the small signal open loop gain of the microstriphe VCO is higher than that of this CPW VCO, the microstrip-line VCO operated at a higher saturation condition and cause higher f: with a higher near carrier noise power. Due to a lower

e.,

the CPW VCO exhibits a better phase noise performance but lower output power.

VI. CQNCLUSKX

In this paper, an ultra low phase noise W-band MMIC CPW VCO using GaAs-based HEMT is presented. The measured tuning range of this VCO is 2 GHz with an average output power of -1 dBm over its tuning range. A

phase noise of-88 dBc/Hz at 1-MHz offset was achieved, which is the best result among the previously published VCOs in this frequency regime.

ACKNOWLEDGEMENT

This work is supported in part by National Science Council (NSC 89-2213-E-002.178, NSC 89-2219-E-002- 042), and Research Excellence Program funded by Department of Education (ME 89-E-FA06-2-4) of Republic of China. The chip is fabricated by TRW foundry service through CIC of Taiwan. The authors would like to thank Mr. G . G. Boll of GGB Corporation for his providing the W-band RF probes. Thanks also go to Hong-Yeh Chang and Kun-You Lin for their help on the chip testing.

THE PREVIOUSLY I. 0.1 pm 0.15 pm PHEMT I M H z offset ihase noise -68.2 -67 (dBclHz) TABLE I

PORTED PERFORMANCE AND FE)

et al., [31 et al., [5]

I

I

O . l m

0.15 pm 0.12 pm AllnAsIGaln

PHEMT

I

HEMT

I

AsllnP

I

HEMT

Negative

resistance

94

I

77

I

tune)

URES OF W-BAND MMIC

K.

W. ! y . i a e y ens! Uch;;? K.

I

This work

1

(Obayashi et al., [7] et al., [6] InGaPilnGa 0.1 pm

I I

AsHBT

I

PHEMT

1

108

I

94

I

104

I

97

1

2.73 GHr

tune)

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