• 沒有找到結果。

Chapter 1 Introduction

1.1 Background and motivation

By the rapid development and large demand of wireless communications, fully integrated monolithic radio transceivers are the most significant considerations for communication applications. The recent rapid growth of the wireless communication market inspires many people to research the concerned region with strong passion. Of such many developments, enhanced operating frequency of CMOS technology encourages the designer to implement single-chip RF-to-baseband systems with it instead of bipolar or GaAs. One of the important design goals of portable wireless systems is low power consumption for long battery life.

CMOS technology satisfies the requirements of low power consumption, low cost, reduced size, and also a few gigahertzs operating frequency in wireless systems.

Historically, wireless communications have only used a narrow bandwidth and can hence have a relatively high power spectral density. Ultra-wideband (UWB) system is an emerging high-speed and low-power wireless communication approved by Federal Communication Commission (FCC) in 2002 for commercial applications in the frequency range from 3.1 to 10.6GHz. UWB performs excellently for short-range high-speed uses, such as automotive collision-detection systems, through-wall imaging systems, and high-speed indoor networking, and plays an increasingly important role in wireless personal area network (WPAN) applications. This technology will be potentially a necessity in our daily life, from wireless USB to wireless connection between DVD player and TV, and the expectable huge market attracts various industries.

The IEEE 802.15.3a task group is developing an UWB standard. For the conventional

UWB system, the pulses have a short time and very wide bandwidth. It is helpful to review some traditional wireless broadcast and communication applications and calculate their power spectral densities (PSDs) as shown in Table 1.1.1.

Table 1.1.1 Power spectral densities of some common wireless broadcast and communication systems

System UWB Radio Television 2G Cellular 802.11a

Transmission

Power (W) 1mW 20kW 100kW 10mW 1W

Bandwidth (Hz) 7.5GHz 75kHz 6MHz 8.33kHz 20MHz Power spectral

density

(W/MHz) 0.013 666,600 16,700 1.2 0.05

Classification ultra

wideband narrowband narrowband narrowband wideband

The IEEE 802.15.3a task group [1] currently discusses the standardization for UWB systems. Two possible approaches have emerged to exploit the allocated spectrum. One is the so-called “impulse radio” with code division multiple access (CDMA) modulation, based on the transmission of very short pulses, with pulse position or polarity modulation. This kind of receiver [2] is all digital circuit except LNA and a mixer, and time domain should be also considered to design especially for mixer because the carrierless signals possess wide frequency-band and using short pulse means discontinuous signal. Another is multi-band approach, with fourteen 528-MHz sub-bands, orthogonal frequency division multiplexing (OFDM) modulation, and frequency-hopping technique. This kind of receiver [3] can reject the wireless local area network (WLAN) signals and other causes of interference, and the division of the UWB frequency spectrum into sub-bands is illustrated in Fig. 1.1.1.

Fig. 1.1.1 Multiband spectrum allocation

A low noise amplifier (LNA) determines the performance of the receiver in the both modulation techniques. It is widely used in front-ends of narrowband communication systems.

For UWB applications these devices will play a slightly different role. In fact, the design of the UWB LNA is one of the biggest challenges, because it connects with the antenna and the pre-select filter, and the input matching should be 50Ω over the whole bandwidth.

Furthermore, we also focus on the design and implementation of LNA for low-power, low-voltage UWB system with bias voltage of only 1V. This work is designed and processed using TSMC 0.18µm mixed-signal/RF CMOS 1P6M technology, where the measured S11 <

-7.07dB and S22 < -12.5dB from 3.1 GHz to 10.6 GHz. The power gain (S21) is 10dB from 2.5 to 8.5 GHz, the -3dB bandwidth is 2-9 GHz. The minimum noise figure is 3.46dB while consuming 7.2 mW.

In the section of the frequency synthesizer, between the two modulation techniques, the multiband UWB has greater flexibility in coexisting with other international wireless systems and future government regulator, and could avoid transmitting in already occupied bands. The receiver of such a system should have high linearity and a wideband local oscillator (LO) capable of frequency hopping in less than 9ns. So, a direct frequency synthesizer structure with quadrature phases for UWB systems is presented. At first, an initial direct frequency synthesizer structure for UWB is designed with low phase noise performance. The circuit consists of a binary 8448MHz voltage controlled oscillator (VCO) and 2-stage frequency

dividers, and three LO bands (8448MHz, 4224MHz and 2112MHz) are produced individually.

The switched buffer as multiplexer with symmetrical independent architecture is used to select output frequency and lowers the phase noise. Fabricated in 0.18-μm CMOS technology, in three LO bands, this work achieves the phase noise of less than -121dBc/Hz@1MHz offset and the frequency tuning range of 10% while consuming 52.2mW from a 1.8-V supply.

Furthermore, according to the front design, a fast-hopping frequency synthesizer that generates more LO signals of twelve bands from 3 to 10 GHz is designed. The prototype is completed by combining a wideband quadrature voltage-controlled oscillator (QVCO) from 7.93 to 10.3 GHz, 2-stage dividers, switched buffer and only one quadrature single-sideband (SSB) mixer. Fabricated in 0.18-μm CMOS technology, this work achieves QVCO’s simulated phase noise less than -107dBc/Hz at 1 MHz offset, and the simulated output powers of twelve bands have better than 35 dB sideband rejection while consuming 60.76mW of the core circuit and 52.93mW of the buffer from a 1.8-V supply. The simulated switching time for hopping frequency is about 1ns.

相關文件