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4 Broadband RF Circuit for Versatile, Dependable Wireless Communications

4 Broadband RF Circuit for Versatile, Dependable Wireless Communications

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7 Connectivity in Wireless Telecommunications



275



millimeter-wave or even short-millimeter-wave (over 100 GHz) as well as microwave must be integrated. It is noted that low-power operation is always important to

improve battery lifetime, which is inevitable for mobile communication system.

Thus, in this section, two basic building blocks, a low-phase-noise oscillator and a

wideband amplifier, for silicon-integrated circuits applicable to millimeter wave are

discussed using CMOS process. Here, key technologies for an oscillator and an

amplifier are addressed. With regard to an oscillator, generally, phase noise

becomes degraded as frequency is increased. Nevertheless, phase noise below −90

dBc/Hz at 1 MHz offset is required for multi-symbol quadrature modulation. To

overcome this issue, we proposed a novel oscillator utilizing p-type MOSFETs,

which have not been used due to its performance inferior to n-type MOSFETs. With

regard to an amplifier, bandwidth more than 20 GHz is required for over 10 Gbps

communication. For this purpose, many-stage amplifiers with properly designed

matching networks are proposed. In the following sections, technologies for an

oscillator and an amplifier are discussed, which is important to realize high speed

and low power simultaneously in wireless integrated circuits.



7.4.2



Millimeter-Wave Oscillator Using P-Type Transistors



Since silicon integrated circuits have been widely used in home electronics such as

cellular phone, personal computer, television, etc., and still have a great potential of

low cost and low power in home electronics even in millimeter-wave band.

Therefore, we are studying millimeter-wave silicon integrated circuits due to their

potential nature of low cost and low power even though high frequency characteristics of silicon may generally be slightly inferior to those of compound semiconductor. Here, we have to consider availability of frequency bands for wireless

communication since radio wave is principally finite resource shared by many

different kinds of applications. To improve communication speed utilizing already

allocated frequency bands, multi-symbol quadrature modulation is a good choice

since they can offer more number of bits per hertz and thus communication

capacity. When multi-symbol quadrature modulation is used even in

millimeter-wave band, oscillators generating carrier signals should have

low-phase-noise characteristics. It is generally considered that n-type transistors

operate at high speed but generate large noise while p-type transistors operate at

low speed but generate low noise in silicon integrated circuits. From this reason,

n-type transistors were conventionally used for millimeter-wave applications.

However, not only n-type transistors but also p-type transistors are improved in

frequency characteristics due to reduction of channel length and mobility

improvement with strained silicon. Thus, we have studied a millimeter-wave

oscillator with p-type transistors in W-band (75–110 GHz) for applications such as

radars and high-speed communications, for p-type transistors generate less flicker

noise than n-type transistors and are expected to realize low-noise oscillators. We

have fabricated an 80 GHz oscillator comprising of p-type transistors with 65 nm



276



K. Tsubouchi et al.



Fig. 7.32 Chip

microphotograph of 80 GHz

oscillator with p-type

transistors



Fig. 7.33 Schematic of

80 GHz p-type transistor

oscillator



VDD



Vbuff

Vbias

OUT



25.3μm

1.1μm



n-MOS buffer

p-MOS core



Fig. 7.34 Phase-noise

characteristics of 80 GHz

p-type transistor oscillator



Fig. 7.35 Comparison

among 80 GHz p-type

transistor oscillator and other

oscillators ever published



7 Connectivity in Wireless Telecommunications



277



CMOS process [28]. Chip microphotograph of the fabricated oscillator with p-type

transistors is shown in Fig. 7.32 and its schematic is shown in Fig. 7.33. Measured

results of phase noise are shown in Fig. 7.34 and comparison of the figures of

merits (FOM) is shown in Fig. 7.35. In this study, we have successfully realized an

80 GHz oscillator with p-type transistors, which shows −92 dBc/Hz at 1 MHz

offset from carrier frequency. Finally, it is shown that the proposed oscillator

demonstrates the best FOM in W-band.



7.4.3



Short-Millimeter-Wave Wideband Silicon Amplifier



Silicon high-speed communication circuits with 60 GHz band becomes promising

candidate realizing more than one giga bit per second and will be implemented in

mass production in the near future thanks to advancement in manufacturing technology, In fact, as will be shown below, a wide frequency band in excess of

20 GHz will be available in the carrier frequency range above 100 GHz, which

would enable a throughput higher than 10 Gbit/s with low power consumption. Key

technology for realizing it is wideband silicon amplifiers operating at more than

100 GHz. The amplifiers must have flat frequency response in gain and group delay

over 20 GHz frequency band in order to amplify modulated signal without distortion. Therefore, we aim at realizing wideband amplifier with flat gain and

group-delay response operating at more than 100 GHz.

We have designed multi-stage amplifier operating in D band (110–170 GHz)

which can be allocated for applications for wireless communication [29, 30].

Although frequency response of the MOSFETs has been improved due to miniaturization of transistor size, transistor gain per stage is still less than 5 dB in a typical

case which is insufficient for the amplifier gain. Thus, a many-stage amplifier is

inevitable for realizing reasonable gain. Here, matching networks connect each gain

stage for impedance transformation to transfer input power to output efficiently.

Matching network has own frequency characteristics determined by circuit parameters. By allocating frequency response properly in the matching networks inserted

between gain stages, wideband amplifier with flat gain and group-delay frequency

response can be realized. By applying this design methodology, we have designed

two types of amplifiers operating at 120 and 140 GHz bands. The amplifiers have

been fabricated with 65 nm CMOS process. Circuit schematic and chip microphotograph are shown in Figs. 7.36 and 7.37, respectively. Measured frequency

responses are shown in Fig. 7.38. Both amplifiers for 120 and 140 GHz bands show

wideband characteristics more than 20 GHz, and the desired flat frequency responses

in gain and group delay have been successfully realized.



278



K. Tsubouchi et al.



Fig. 7.36 Schematics of short-millimeter-wave amplifiers with silicon integrated circuits



Fig. 7.37 Chip

microphotograph of

short-millimeter-wave

amplifiers



Fig. 7.38 Frequency

responses of

short-millimeter-wave

amplifiers



7 Connectivity in Wireless Telecommunications



7.4.4



279



Discussions and Further Investigations



In this section, two basic building blocks, a p-type oscillator and many-stage

amplifiers, on millimeter-wave silicon integrated circuits are introduced for realizing high-speed communication. One is 80-GHz oscillator with p-type transistors,

which is firstly applied in millimeter-wave operation. The fabricated p-type 80-GHz

oscillator realizes −92 dBc/Hz at 1 MHz offset, which is useful for 16- or

64-quadrature amplitude modulations (QAM) for higher speed communication. The

other one is short-millimeter-wave silicon amplifiers with frequency band over

20 GHz. It is shown that silicon integrated circuits, generally inferior to compound

semiconductor circuits, can still operate in short-millimeter-wave band beyond

100 GHz. Furthermore, it is shown that flat frequency responses in gain and group

delay can be realized by designing matching networks properly. It will be useful for

realizing high-speed communication more than 10 Gbit/s using wide frequency

band more than 20 GHz. The techniques introduced in this section will contribute

to super broadband RF circuits for versatile, dependable wireless communications.

Since we opened a door to future super broadband wireless system, the complete

wireless hardware and system should be studied in the next step.



7.5



All-Si-CMOS Front-End ICs for Multiband

Micro-/Millimeter-Wave Communications



Ryuji Inagaki, Mitsubishi Electric Corporation

Masami Tsuru, Mitsubishi Electric Corporation

Eiji Taniguchi, Mitsubishi Electric Corporation

Hiroshi Fukumoto, Mitsubishi Electric Corporation



7.5.1



Techniques for the Multiband Front-End IC



Recently, various different types of wireless communication systems, such as

Mobile Broadband Wireless Access (MBWA), Wireless Local Area Network

(WLAN), and Wireless Personal Area Network (WPAN), have been used. If a

system is down, a user subscribing only to that specific system is totally cut out of

the communication network, but it will be inconvenient to have many mobile

devices for each communication system. One of methods to solve this problem is to

realize a multiple wireless system, such as described as the Dependable air [12, 13].

Figure 7.39 shows the block diagram of a mobile device for multiple wireless

systems. The main circuits constituting the mobile device are Radio Frequency (RF)

Transmitter/Receiver Module, Digital–Analog (D/A)/Analog–Digital (A/D) converter [31], Transmitter/Receiver Baseband Module [32], etc. The mobile device



280



K. Tsubouchi et al.



Fig. 7.39 Block diagram of a mobile device for multiple wireless systems



provides dependable connection because it automatically switches to another

available system even if one system is down. The RF Transmitter/Receiver Module

takes the role of converting a baseband signal to RF signals, and vice versa.

A family of multiband transmitter/receiver front-end Integrated Circuits (ICs) that

can be used for both microwave- and millimeter-wave communications is necessary

for the RF Module.

This section presents the result of the development of the multiband receiver

front-end ICs. In order to realize the multiband receiver front-end IC, small size,

switching function, and low-noise performance are required. The assembly of the

millimeter-wave ICs is also important. For miniaturization, it is effective to use

sub-micron process on Si and to share a part of some receiver front-ends. Each

received signal is switched in the shared circuit. However, the power level of each

received signal differs between the systems. Therefore, it is needed to adjust the

power level using a variable gain amplifier (VGA), because the received signal is

strained by saturation of a mixer of shared circuit and the signal quality is degraded.

In order to achieve a low-noise performance, it is important to develop a high gain

low-noise amplifier (LNA) because it reduces influence of the subsequent circuit

such as the mixer. The LNA has high output power, requiring high saturation

performance for the mixer. The design of the assembly including the flip-bonded

millimeter-wave ICs on a substrate requires careful waveguide engineering to avoid

spurious propagation modes.

The outline of the section is as follows. Section 7.5.2 describes the configuration

of the multiband receiver front-end IC for 5 GHz band and 60 GHz band systems,

such as WLAN and WPAN. Section 7.5.3 presents the circuit components for the

multi-band receiver front-end IC: a high gain LNA, a high saturation transistor pair

type even harmonic mixer (HMIX) for low-noise performance and the 5 GHz band

Intermediate Frequency (IF)-VGA to change the gain. Section 7.5.4 presents suppression of spurious modes in flip-chip assembly. Section 7.5.5 shows measurement results of the fabricated multiband (5 GHz/60 GHz) receiver front-end IC.



7 Connectivity in Wireless Telecommunications



7.5.2



281



Configuration of the Multiband Receiver

Front-End IC



Figure 7.40 shows a configuration of a 5 GHz/60 GHz receiver front-end IC [33].

The architecture of the receiver front-end IC is a direct conversion configuration at

5 GHz band and a superheterodyne configuration at 60 GHz band. The 5 GHz

front-end and the 60 GHz front-end share an IF-VGA and a quadrature mixer for

reduction of the area of IC.

The 5 GHz front-end consists of a 5 GHz band LNA and a 5 GHz band common circuit. The 60 GHz front-end consists of the 60 GHz RF front-end and the

5 GHz band common circuit. The 5 GHz band common circuit consists of a switch,

the IF-VGA, the quadrature mixer, a baseband amplifier (AMP), and a 10 GHz

band frequency divider. The 60 GHz RF front-end consists of a 60 GHz band LNA,

a balun, a 60 GHz band HMIX, and a 5 GHz band IF-AMP.

The 60 GHz RF front-end converts 60–5 GHz as IF signal. The IF signal is input

into the quadrature mixer through the switch and the IF-VGA. The power level of

each IF signal differs between the systems. Is needed to adjust the power level using

the VGA because the received signal is strained by saturation of the quadrature

mixer and the signal quality has been degraded.



5GHz RFIC

band



5GHz front-end

5GHz band common circuit



5GHz band LNA

60GHz

band



A/D

quadrature mixer

60GHz band

HMIX

Balun



60GHz

band

LNA

60GHz RF

front-end



Q

5GHz Switch IF-VGA

band

10GHz band

IF-AMP

Frequency

divider

60GHz front-end



LO

input



(30GHz band)



I



LO

input



(10GHz band)



Fig. 7.40 Configuration of the 5 GHz/60 GHz receiver front-end IC



DIV



Base

band

AMP



282



7.5.3



K. Tsubouchi et al.



Circuit Components for the Multiband Receiver

Front-End IC



60 GHz band LNA

A high gain LNA contributes to achieve a low-noise performance because it

reduces influence of the subsequent stage in the circuit, such as the mixer. A cascode configuration [34, 35] is used for the high gain LNA. Figure 7.41 shows the

configuration of an inductive matched cascode LNA (four-stage amplifier) [36, 37].

In high frequency band, capacitances of the field-effect transistors (FETs) reduce

the gain of a cascode amplifier. To cancel out the capacitances of FETs [38], an

inductor formed by the microstrip lines (MSLIS) is connected in parallel to the

interstage (node A1–4) of the cascode LNA. The optimum inductance of MSLIS was

determined analytically in terms of the interstage matching of the cascode amplifier.

Figure 7.42 shows the simulation results of NFmin as a function of the gate width

(Wg) of the FET. NFmin is the minimum of noise figure (NF) that can be obtained

by input matching. The reduction of NF is enabled by reducing the parasitic

resistance and capacitance on the gate of the FET. The simulated NFmin of the LNA

VDD



RF

output



RF

input



MSLIS



A1



MSLIS



A2



MSLIS



A3



MSLIS



A4



VGG



Fig. 7.41 Configuration of an inductive matched cascode LNA (4-stages amplifier)



Fig. 7.42 Simulation results

of NFmin plotted against the

gate width Wg of the FET as

the variable



7 Connectivity in Wireless Telecommunications

10



inductive matched

cascode LNA



8



gain (dB)



Fig. 7.43 Simulation results

of the gain of a cascode LNA

(1-stage amplifier) with

inductive matching (solid

line) and without inductive

matching (broken line)



283



6



1dB



4



cascode LNA without

the inductive matchded



2

0

50



55



60



65



frequency (GHz)



Fig. 7.44 Photograph of the

fabricated 60 GHz band LNA

(0.6 mm × 1.5 mm)



(1-stage amplifier) has an optimum at the gate width Wg of 48 μm. Figure 7.43

shows the simulation results of gain of the cascode LNA (1-stage amplifier) with

and without inductive matching. The Wg of the FET in each LNA is 48 μm. The

gain of the inductive matched cascode LNA is 1 dB higher than without inductive

matching.

Figure 7.44 shows a photograph of the fabricated 60 GHz band LNA in 90 nm

Complementary MOS (CMOS) technology. Figure 7.45 shows the measurement

results of the fabricated 60 GHz band LNA. The bias conditions are VDD = 1.2 V

and IDD = 5.6 mA at each stage. The fabricated LNA achieved a gain of 30.8 dB

with an NF of 5.8 dB at 60 GHz. Output 1 dB Compression Point (OP1dB) was

−2.4 dBm.

60 GHz band HMIX

Figure 7.46 shows the configuration of the 60 GHz band HMIX [39]. The HMIX

has two transistor pairs. Each transistor pair consists of two FETs which have a

common drain and a common source.

The load TL1 at the common source consists of a transmission line of which the

length is a quarter of the RF wavelength. The input RF signal amplitude at the

common source is kept large because of the high input impedance. In addition,

the HMIX realizes high saturation characteristics for the RF input power because

the transistor which is an active device is not used for the input circuits. The output

load TL2 at the common drain consists of a transmission line whose length is a

quarter wavelength of the 2nd harmonic (2LO) of the Local Oscillator (LO).



284



K. Tsubouchi et al.



Fig. 7.45 Measurement

results of the fabricated

60 GHz band LNA



40



gain (dB)



30

20

10

0

-10

-20

50



55



60



65



70



frequency (GHz)



(a) gain



(b) noise figure



Fig. 7.46 Configuration of

the 60 GHz band HMIX



VDD

IF

IF

TL2

λ/4@2fLO



Cp

Cp

Transistor pair



VREF



TL2

λ/4@2fLO



VREF2

LO

LO

2LO canceler

TL1

λ/4@fRF



TL1

λ/4@fRF



RF

RF



7 Connectivity in Wireless Telecommunications



285



-10



-5



-15



-10



-20



-15



-25



-20



-30



-25



-35

-20



-15



-10



IP1dB



-5



-30



conversion gain (dB)



Fig. 7.48 Measurement

results of the fabricated

HMIX



IF output (dBm)



Fig. 7.47 Photograph of the

fabricated HMIX

(0.8 mm × 0.8 mm)



0



RF power (dBm)



The capacitance Cp is chosen to become a short circuit in the millimeter-wave band

and an open circuit in the IF band. The leakage of the 2LO to IF band output circuit

is reduced due to the high output impedance for the 2LO. In addition, the 2LO

canceler circuit connected to the common drain suppresses the 2LO that occurs on a

grand terminal and a power supply terminal for the influence of wire bonding.

Figure 7.47 shows a photograph of the fabricated HMIX in 90 nm CMOS

technology. Figure 7.48 shows the measurement results of the fabricated HMIX.

Supply voltage is VDD = 1.2 V and the consumption power is 1.2 mW. RF is

60 GHz, IF is 5 GHz, and LO frequency is 27.5 GHz. The fabricated HMIX

realized conversion gain of −12.7 dB, NF of 29.6 dB, and Input 1 dB Compression

Point (IP1dB) of −5 dBm at the LO power of 2 dBm.

5 GHz band IF-VGA

Figure 7.49 shows a configuration of the 5 GHz band IF-VGA. The IF-VGA

comprises two-stage amplifiers [40]. The 1st VGA is a stacked differential circuit

with a resistive output load. The 2nd VGA is also a stacked differential circuit with

a reactive output load. 5 GHz band frequency characteristics of the IF-VGA are

optimized by properly tuning the resistive and reactive loads. The current through



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