A broadband HBT MMIC IQ modulator and millimeter-wave vector signal characterization

TU4B-4

A

Broadband

HBT

MMIC IQ Modulator and

Millimeter-wave Vector Signal Characterization

Hong-Yeh Chang*, Tian-Wei €hang*, Huei Wang*,

Yu-Chi Wang**, Pane-Chane Chao** and Chung-Hsu Chen**

*

Department

of Electrical Engineering and Graduate Institute

of

Communication Engineering,

National Taiwan University, Taipei, Taiwan

106,

ROC

**

WIN Semiconductors Corp., Hwaya Technology Park, Taoyuan, Taiwan 333, ROC

AbsfracI - A broadband 50-110 GHz HBT M M I C IQ

modulator and a vector signal measurement system for millimeter-wave applications are reported in this paper. For

the digital modulation measurement, a QPSK modulation format was used with a 2-Mhps data rate for the IQ modulation. Using the 50-110 GHz vector signal measurement system, this M M I C chip shows an error vector magnitude (EVM) o f within 12 % rms and a carrier rejection of more than 21 dB. The 110-GHz operation frequency of this M M I C i s the highest among all the reported HBT vector modulators. Also, this is the first time the E V M test was performed for an IQ modulator at V-hand and W-hand. This M m C is suitable for software defined radio (SDR), linearization techniques of power amplifier and related transmitter applications both in V-band and W-hand.

1. INTRODUCTION

Direct carrier modulator has the advantages of reducing the complexity of transmitters and thus their cost. For conventional designs, the IQ modulators consist of two BPSK modulators, a 90" phase shifter and an in-phase

power combiner. Most of BPSK modulators are

composed with a doubly-balanced mixer [I]-[3] due to superior spur performance, linearity and port to port

isolation. However, they usually need a high LO drive

power and require a built-in driver amplifier. Altemately,

hy using Si (CMOS, BIT 01 SiGe HBT) and GaAs (HBT

or HEMT) technologies, the IQ modulators based on the Gilbert cell architechue have been reported in recent years

[4]-[6]. Although they showed the performance of good

amplitude and phase match and high LO rejection, most of

them operated below IO GHz.

In this paper, the IQ modulator is based on the

quadrature modulation architecture, which contains two

sets of BPSK modulators, a 90" hybrid and a combiner.

We use a balanced reflection-type phase shifter [7] for the

BPSK modulator, which utilized a Lange coupler as a 90"

hybrid and two shunt cold-mode devices as switching devices. The cold-mode devices can be fabricated with HEMT, FET, or HBT technologies. Although the BPSK

modulators can be implemented in HEMT process [7]-[9],

0-7803-7695-1/03/$17.00 0 2003 E E E

99

2003 IEEE M l 7 - S Digest

however, they require negative biasing voltage tu perform modulation function. On the Other hand, the HBT based BPSK modulators have been reported in literatures [lo]-

[I I] with operating frequency below 50 GHz. Recently,

we have reported a HBT based BPSK modulator above 50

GHz successhlly [ 121. Using this BPSK modulator in our

IQ modulator design, this MMIC chip demonstrated a superior broadband performance, good amplitudelphase match and low insertion loss.

Vector signal measurements at 38 GHz can be found in

literatures [3], [13]. For a real digital modulation signal

coming from an IQ modulator, the amplitude and phase states are time variant. The amplitude and phase states of the dynamic signal cannot be measured via a network

analyzer or a scalar spectrum analyzer. Therefore, a

vector signal analyzer was used to detect the performance of the IQ modulator such as EVM, phase error, amplitude error, quadrature error, I-Q imbalance, DC offset and the

impairment of an imperfect LO source. In this paper, we

use a QPSK modulation format with a 2-Mbps data rate to test this IQ modutator MMIC between 50 GHz and 1 IO

GHz. Due to the equipment limitations, the test is

performed only with a data rate of 2-Mbps.

11. MMIC PROCESS AND CIRCUITDESIGN

The IQ modulator design is based on the

6"

1-w GaAs

HBT MMIC process on a 4-mil substrate provided by WIN Semiconductors. There are three metal layers, metal-insulator-metal (MIM) capacitors, via-holes, spiral

inductors and thin film resistors in this process. The

polyimide and SIN, are used for the isolated layer between metal and metal layers. The one emitter finger and emitter

size of 1 . 4 ~ 1 0 p2 HBT device (Q2B101) is selected for

the IQ modulator design. This device exhibits a peak unit

current gain frequencyfr of 70.5 GHz and peak maximum

oscillation frequencyf,, of 104 GHz at 1.5-V and 7-mA

.voltage of 9 V and maximum collector current of 11.2 mA

The photograph of the IQ modulator is shown in Fig.1

with a chip size of 2x2

"2.

The MMIC consists of two

BPSK modulators, a Lange coupler and a Wilkinson

power combiner. The BPSK modulator [I21 is based on a

balanced reflection-type phase shifter, which features low insertion loss, broadband and good amplitude and phase balance. In Fig. 1 , l P and IN are the voltage control ports of in-phase BPSK modulator; while QP and QN are the voltage control ports of quadrature-phase BPSK

modulator. The vector summation of in-phase and

quadrature-phase BPSK modulators can be many phase

and amplitude states by adjusting the control voltages (IP,

IN, QP and QN). The transmission coefficient

S,,

of the

modulator cam be expressed as: [ W .

where r,and

TQ

are the HBT off-state reflection

coefficients of the in-phase and quadrature-phase BPSK

modulator respectively; while

r,

and are the HBT on-

state reflection coefiicients. For optimum design of the IQ modulator, the magnitude of the on-state and off-state reflection coefficients must be close to 1 with 180' out of phase.

Inpu

-Output

Fig. 1.

chip size of 2 x 2 mm2.

The'microphotograph of the HBT IQ modulator with a

The HBT Gummel Poon model provided by WIN Semiconductors [I41 is used for the circuit simulation.

The base bias of on-state is 4 V with 5 mA current

consumption; adversely, the base bias of off-state is 0 V. The transmission line, Lange coupler and Wilkinson power combiner are simulated using the full-wave EM-

simulator (Sonnet Software)

[IS].

For QPSK operation,

there are four phase states (0", 90", 180", 270" ) with the

same amplitude, which are represented as state (0, 0), state

(0, 1)

,

state (1, 0) and state ( I , I). The simulated

insertion losses are lower than IO dB, amplitude

imbalance is helow 2 dB, and phase imbalance is below

2.5' between 40 GHz and 100 GHz. The simulated input

and output return losses are better than IO dB between SO

GHz and 1 10 GHz.

111. MEASUREMENT RESULTS

The S-parameters measurements of the IQ modulator were performed via on-wafer probing. For IO-SO GHz, we utilize HP 8SlOC network analyzer with coaxial cable

connected to Cascade ACPSO probes. Above S O GHz, we

use HP V8S104A millimeter-wave S-parameters test-set

with WR-15 waveguide connected to Cascade ACP75

probes for 50-75 GHz and HP W8S104A millimeter wave

S-parameters test-set with WR-10 waveguide connected

GGB W-hand probes for 75-110 GHz.

The measured insertion losses of four states from IO

GHz to 110 GHz are plotted in Fig. 2. The insertion

losses of four states are below 12.5 dB, amplitude

imbalance is within 2.5 dB and phase imbalance is within

ISo between SO GHz to 85 GHz. The measured input

and output return losses are better than 8 dB between SO

GHz to 1 IO GHz.

I O 1 10 40 IO m I D 80 m lrm 110

Frequency (Gm)

Fig. 2.

GHz to 1 IO GHz for the HBT IQ modulator.

The measured insertion losses of four states from 10

The transfer function of the HBT input reflection

coefficient versus control voltage is a nonlinear curve. If apply the high level modulation format (n-QAM, n-PSK) to the IQ modulator directly, the amplitude and phase states will be distorted seriously. Therefore, we use the

calibration bias method [9] to extract exact

amplitude/phase states and minimize the amplitude imbalance and phase imbalance of the IQ modulator. The static constellation diagrams were obtained from CW- mode S-parameters measurement with a computer control

setup. The forward transmission coefficients at 94 GHz

control voltages of IP, IN, QP and QN have been swept

from 0 V to 4 V with step of 0.02 V. Based on forward

transmission coefficients, EVM calculation is used to extract the best amplitude and phase states. We can generate the static constellation diagrams of QPSK or higher order QAM modulations. We use 64-QAM as an example, to plot the extracted constellation diagram in Fig.

3(b), which features a minimum insertion loss of 14 dB,

amplitude imbalance of 1 dB and phase imbalance of I".

0

CW 94 GHz CW 94 GHz

(a) (b)

Fig. 3. The measured static constellation diagram of the HBT

IQ modulator at 94 GHz, (a) sweep I and Q control voltage with

a step of 0.02 V, (b) extracted 64-QAM.

The block diagram of the vector signal measurement

system is plotted in Fig. 4 for the millimeter-wave IQ

modulator. The H p 85105A millimeter-wave test set is

used to provide the carrier signal. The HP 11970-series

harmonic mixer and the HP 70000 spectrum analyzer are

used as a down-converter to convert millimeter-wave

signal to an IF frequency of 21.4 MHz. Finally, the 21.4-

MHz IF signal is fed into HP 89441A vector signal

analyzer for the analysis of the digital modulation quality.

Two HP 33120A arbitrary waveform generators are used

as the baseband IQ signal sources, which are controlled

with a computer and

HP

VEE software. The IQ sources

are fed into a single-to-differential circuit network to

produce differentia1 IQ sources (IP, IN, QP and QN). For

channel power measurement, the

HF'

70000 spectrum

analyzer is replaced with the HP 8565EC spectrum

analyzer with built-in channel power measurement

software. The overall measurement system features

maximum operation frequency of 110 GHz, maximum

symbol rate of 2-Mps, maximum analysis bandwidth of 3

MHz and supports multiple digital modulation formats.

The measured output spectrum of QPSK modulation is

plotted in Fig. 5 and the output power is about -21.3 dBm

with a channel bandwidth of 2 MHz. The measured

QPSK modulation quality results at 94 GHz are plotted in

Fig. 6, including I-Q vector diagram, eye diagram, error

versus time plot and performance summaty. The LO

power is about -8 dBm and the amplitude of the baseband

IQ signal is 2-V,. The baseband IQ is coded with a

QPSK modulation format, a 2-Mhps data rate, pseudo random noise code.and the mapping table used with the extracted results fiom static constellation diagram measurement. For minimum the spectrum spread, we use

a root raised cosine (RRC) filter with a 0.5-avalue to

filter the baseband IQ signal. The measured EVM is

below 9.7 %, the amplitude error is below 7.3 % and the

phase error is below 3.7" in root-mean-square (RMS)

format.

The EVM degradation is due to the quadrature error and IQ imbalance within the IQ modulator. Additionally, the

imperfect LO/IQ sources in the test setup will also

degrade the EVM performance. For the W-band

applications, the phase noise and amplitude noise of the

LO source may result in 3-5 % EVM degradation, which

was analyzed with a THRU pad. The EVM analysis of

CW-mode LO source is like an all-0 data modulation

signal. On the other hand, the EVM may he degraded 3-4

% from the IQ sources, which was measured with a vector

signal analyzer. This EVM degradation from IQ sources is due to high data rate operation of the arbitrary waveform generators and the nonlinearity of the single-to- differential circuit network. The performance summary of the IQ modulator is summarized in Table I. As can he observed, this MMIC demonstrated wider bandwidth, low EVM degradation and good LO rejection. The success of

this chip development provides many potential

applications in millimeter-wave communication systems.

Fig.4. The block diagram of the vector signal

measurement system for the millimeter-wave IQ modulator.

IV. CONCLUSION

A broadband HBT IQ modulator and a vector signal

measurement system for millimeter-wave IQ modulator have been implemented and proposed in this paper. This IQ modulator MMIC is suitable for broadband digital modulated applications due to its wide bandwidth, low

insertion loss, low amplitudeiphase imbalance and good

inpuuoutput return loss. Also, this MMIC has been tested

and verified under the vector signal measurement

successfully. The vector signal measurement system

presented in this paper provides a dynamic signal test

bench for the IQ modulator. For the digital modulation

signals, the dynamic signal measurement is more Department of Education of Republic of China (ME-89-E-

meaningid than the static signal measurement. Therefore, FA06-24-6). The MMIC chip is fabricated by WIN

we can figure out the characteristic of the MMlC circuit Semiconductors. The authors would like to thank MI. G.

and the impairment coming from the imperfect signal G. Boll of GGB Inc. for his providing the W-hand probes

source. for chip on-wafer testing.

C L 4 0 9 d B UAUG 100

R L 0 d B m I 0 d W

CENTER 9 4 0 0 0 0 0 G H x SPRN 10 00MHz

RBW l 0 0 k H i UBW 100kHr SUP 5 0 0 m r

Fig. 5. The measured output spectrum of the HBT IQ

modulator is applied QPSK modulation format and the output

power is -21.3 dBm with a channel bandwidth of 2 MHz.

I -a aa / d i *

Fig. 6. The measured QPSK modulation quality results at 94

GHz, I-Q vector diagram, eye diagram, error vector versus time plot and performance summary.

Table I Performance Summary of the HBT IQ modulator

I10 GHz

Frcqucncy I50 GHa

I

60 GHa

I

77 GHz

I

94 GHz

I

Insertion

I

9 d B

I

8dB

1

9 d B

I

14dB

I

18dB

I

I

I

I

I

*With calibration bias: #QPSK modulation with 2-Mps data rate

ACKNOWLEDGEMENT

This work is supported in part by National Science Council (NSC 89-2213-E-002-178 and NSC 90-2219-E-

002-007) and Research Excellence Program funded by

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