A frequency synthesizer is one of the most critical building blocks in any integrated wireless transceiver system. Its design is getting more and more challenging as the demand for low-voltage low-power high-frequency wireless systems is continuously increased. At the same time, CMOS processes have advanced and been shown to be more and more attractive due to their potential in achieving systems with the highest integration level and the lowest cost. On the other hand, as the supply voltage is lowered, many existing design techniques for integrated frequency synthesizers are no longer applicable. However, it is still desirable to design RF frequency synthesizers at low supply voltages not only because of the device reliability due to the technology scaling but also because of the integration and compatibility with digital circuits.

There are currently only a few books available on integrated RFCMOS frequency synthesizers. The most comprehensive book on integrated CMOS frequency synthesizers available today is entitled Wireless CMOS Frequency Synthesizer Design by Craninckx and Steyaert (1998). More recently, another book entitled Multi-GHz Frequency Synthesis and Division by Rategh and Lee was also published in 2001. While the two books are still quite useful, they focus only on advanced design techniques of some selected building blocks, including voltage-controlled oscillators, dividers, and synthesizers, with emphasis only on a particular architecture. There exist many new synthesizer architectures and design techniques that are not covered in detail.

The advent of high speed computers has opened up a whole new range of possibilities for radio. If the RF signal can be adequately represented by a series of samples (at a rate that a computer can handle), standard operations such as mixing, filtering, signal synthesis and demodulation can all be handled as mathematical operations within the computer. Constructing systems that can handle the complex signal processing required by spread spectrum communications, radar and other more exotic RF systems is merely an exercise in computer programming. Since most of the processing will be done inside a computer, we have what is commonly termed software radio. Such radios can be extremely flexible and be instantaneously reconfigured to handle new forms of modulation and/or tasking. All we need is a suitable analogue-to-digital converter (ADC) to interface to the incoming analogue signal and a suitable digital-to-analogue converter (DAC) to produce the outgoing analogue signal. Obviously, any realistic implementation of software radio will involve many constraints, and issues such as sampling rate and quantisation error will need to be addressed. The following chapter introduces the basic ideas of digital RF techniques and their limitations.

The processing of digitised signals

We have already noted the utility of studying RF systems in terms of the complex signal exp(j2π ft). Since cos(θ) = ½ [exp(jθ) + exp(−jθ)], it is clear that the real signal s(t) = S cos(2π ft) will contain, in equal parts, contributions from frequencies f and − f.

The generation of a stable sinusoidal signal is a crucial function in most RF systems. A transmitter will amplify and suitably modulate such a signal in order to produce its required output. In the case of a receiver system, such a signal is fed into the mixer circuits for the purposes of frequency conversion and demodulation. A circuit that generates a repetitive waveform is known as an oscillator. Such circuits usually consist of an amplifier with positive feedback that causes any input, however small, to grow until limited by the non-linearities of the circuit. The feedback will need to be frequency selective in order to control the rate of waveform repetition. This frequency selection is often achieved using combinations of capacitors and inductors, but can also be achieved with resistor and capacitor combinations. In the present chapter, however, we will concentrate on feedback circuits based on capacitor/inductor combinations. We consider a variety of oscillator circuits that are suitable for RF purposes and investigate the conditions under which oscillation occurs. In addition, we consider the issue of oscillator noise since this can often pose a severe limitation upon system performance.

A particularly important class of oscillator is that for which the frequency can be controlled by a d.c. voltage. Such an oscillator is an important element in what is known as a phase locked loop. In such a system, there is a feedback loop that compares the oscillator output with a reference signal and generates a control voltage based upon their phase difference.

The following text evolved out of a series of courses on radio frequency (RF) engineering to undergraduates, postgraduates, government and industry. It was designed to meet the needs of such groups and, in particular, the needs of working engineers attempting to upgrade their skills. Thirty years ago, it appeared as if the fibre optics revolution would relegate wireless to a niche discipline, and universities accordingly downgraded their offerings in RF. In the past 10 years, however, there has been a renaissance in wireless and to a point where it is now a key technology. This has been made possible by the developments in very large-scale integration (VLSI) and CMOS technology in particular. In order to meet the manpower requirements of the wireless industry, there has been a need to upgrade the status of RF training in universities and to provide courses suitable for in-service training. The applications of wireless systems have changed greatly over the past 30 years, as has the available technology. In particular, there is a greater use of digital technologies, and antenna systems can often be of the array variety. The current text has been written with these changes in mind and there has been a culling of some traditional material that is of limited utility in the current age (graphical design methods for example). Material in the book has been carefully chosen to provide a basic training in RF and a springboard for more advanced study.

In propagating through free space, radio waves will suffer a reduction in amplitude as they spread outwards from the source. When a transmission must reach a large number of geographically dispersed receivers, such as in broadcast radio, there is little that can be done about this loss. If it is only required to reach one receiver, however, it is desirable to transmit all the energy to this one device. A structure for achieving this is known as a transmission line. Such structures will normally have a small uniform cross-section and can be constructed so that the loss along the line is extremely small. Transmission lines allow efficient and unobtrusive transmission over long distances. Two of the most common varieties of transmission line are the coaxial cable and the twin parallel wire. This chapter considers a simple lumped circuit model of such transmission lines and describes some important applications of these structures. The study of transmission lines leads very naturally to the concept of the reflection coefficient, a concept that provides an alternative description of impedances. Reflection coefficients generalise to the concept of scattering matrices which themselves provide an alternative means of describing multiport networks. The present chapter introduces the basic idea of the scattering matrix and shows howit can be applied to the design of small signal amplifiers at high frequencies.

The transmission line model

Figure 6.1 shows the construction of two important transmission lines, the coaxial cable and twin parallel wire.

Broadly speaking, radio frequency (RF) technology, or wireless as it is sometimes known, is the exploitation of electromagnetic wave phenomena in that part of the spectrum between 3 Hz and 300 GHz. It is arguably one of the most important technologies in modern society. The possibility of electromagnetic waves was first postulated by James Maxwell in 1864 and their existence was verified by Heinrich Hertz in 1887. By 1895, Guglielmo Marconi had demonstrated radio as an effective communications technology. With the development of the thermionic valve at the end of the nineteenth century, radio technology developed into a mass communication and entertainment medium. The first half of the twentieth century saw developments such as radar and television, which further extended the scope of this technology. In the second half of the twentieth century, major breakthroughs came with the development of semiconductor devices and integrated circuits. These advances made possible the extremely compact and portable communications devices that resulted in the mobile communications revolution. The size of the electronics continues to fall and, as a consequence, whole new areas have opened up. In particular, spread spectrum communications at gigahertz frequencies are increasingly used to replace cabling and other systems that provide local connectivity.

The purpose of this text is to introduce the important ideas and techniques of radio technology. It is assumed that the reader has a basic grounding in electromagnetic theory and electronics.

Active devices are important elements in RF systems where they perform functions such as amplification, mixing and rectification. For mixing and rectification, the non-linear properties of the device are of paramount importance. In the case of amplification, however, the non-linear properties can have a damaging effect upon performance. This chapter concentrates on small signal amplifiers for which it is possible to select conditions such that there is, effectively, linear amplification. Amplifiers based on both bipolar junction and field effect transistors are considered and the chapter includes some revision concerning their characteristics and biasing. The high frequency performance of transistor amplifiers is limited by what is known as the Miller effect and a large part of the chapter is devoted to techniques for overcoming this phenomenon.

The semiconductor diode

The semiconductor diode is a device that allows a current flow, but for which the magnitude of the flow depends in a non-linear fashion upon the applied voltage. Many varieties of diode are manufactured by forming junctions out of p- and n-type semiconductors. There are, however, several important diode varieties that have other types of junction (the semiconductor to metal junction of a Schottky diode for example). A p-type semiconductor can be formed by introducing a small amount of indium into silicon. This creates a structure that allows electrons to flow at energy levels slightly above those of the bound electrons. An n-type semiconductor can be formed by introducing a small amount of arsenic into the silicon.

Semiconductor devices will always have some form of non-linearity in their characteristics and this can be both an advantage and a disadvantage. For amplifiers, non-linearity is clearly a disadvantage in that the amplified signal will not be a faithful reproduction of the input signal. Mixers, however, provide an example of the advantages of non-linear behaviour in semiconductor devices. An ideal mixer is a device for which the output is the product of two input signals. For purely sinusoidal inputs, this will imply outputs at the sum and difference frequencies. Mixers are essential for operations such as frequency translation, modulation and detection. As a consequence, they are an important building block for both transmitters and receivers. The current chapter considers the operation of a variety of mixers and their application to modulation and demodulation. In addition, the chapter considers some modulation and demodulation processes that do not involve mixing.

Diode mixers

Diode mixers are important because of their low noise characteristics. Whilst single diode mixers are not normally used at frequencies below 1 GHz, their analysis provides some useful insight into the operation of mixers in general. It should be noted that diode capacitance can have a detrimental effect upon mixer performance and low capacitance devices, such as the Schottky barrier diode, will normally be required for operation at higher frequencies.

The circuit in Figure 4.1 shows a single diode mixer that converts an RF input signal at frequency ωRF into an intermediate frequency (IF) signal at frequency ωIF = |ωRF − ωLO| by mixing it with a local oscillator (LO) signal at frequency ωLO.

Antennas are the means by which electromagnetic wave energy is fed into, and extracted from, the propagation medium. They are a key element in RF systems and their design and analysis constitutes a very important area of RF engineering. The problems of antenna design are many and varied. Modern spread spectrum systems will require antennas that are capable of operating over a wide range of frequencies. Mobile communications have a requirement for small efficient antennas that radiate over a wide arc. Radar, on the other hand, requires antennas that illuminate only a narrow arc, but can be steered over a wide region. In many systems, the requirements turn out to be conflicting and it is important for the designer to understand the practical constraints in order to achieve the best compromise solution. The present chapter seeks to introduce the most fundamental concepts of antenna engineering and to describe some important varieties of antenna.

Dipole antennas

Broadly speaking, an antenna is a device that transforms wave propagation down a transmission line (a physically narrow channel) into wave propagation through free space (a physically wide channel) and vice versa. If we consider a parallel wire transmission line, we could conceive of opening it out at its end to better couple the waves into free space. The opened out section would then correspond to a dipole antenna. In its unopened state, the transmission line will reflect back to the source almost all of the energy that reaches its end.

RF signals will often need to be transmitted with considerable power if they are to survive propagation with adequate signal level. As a consequence, we will need to consider amplifiers that can operate at large signal levels. Up to this point, we have concentrated on small signal amplifiers for which efficiency and linearity have not been a major problem. These aspects, however, require careful consideration in the case of RF amplifiers operating at large signal levels. Small signal amplifiers are typically of the class A variety and highly linear. Whilst class A amplifiers are sometimes used at high power levels, they do not represent an efficient use of the d.c. energy that is supplied to the amplifier. Class B, AB, C and E amplifiers are far more efficient, but have the disadvantage that they are highly non-linear and hence create considerable harmonics. These harmonics can be troublesome and require specialised techniques, or filtering, for them to be brought down to an acceptable level. The following chapter considers power amplifiers in the class range from A to E. It concentrates on BJT amplifiers, but the same principles can be applied to FET amplifiers.

Class A

Class A amplifiers attempt to operate over that part of the transistor characteristic for which there is linear translation of the input signal to the output. For a BJT, a typical configuration is shown in Figure 7.1.

Frequency selective circuits are extremely important elements of an RF system. They often consist of a combination of inductors and capacitors that achieves maximum power transfer at a particular frequency or range of frequencies. Since there will be maximum power transfer to a load when its source has conjugate impedance, such combinations will often be designed to achieve an impedance match at the frequencies to be selected. Frequency selective circuits need not be restricted to combinations of inductors and capacitors, but can also consist of lengths of transmission line or electromechanical devices such as quartz crystals. In the current chapter, we will concentrate on combinations of inductors and capacitors that have maximum transfer at a particular frequency, leaving broadband and transmission line circuits to later chapters.

Fundamental to selective circuits is the concept of resonance. That is, if a circuit is driven by an oscillatory stimulus, there will be a frequency, or frequencies, at which the circuit response peaks. We start the chapter by investigating this concept.

Series resonant circuits

For an inductor and capacitor in series, there will be a frequency (the resonant frequency) at which the reactance of the capacitor will cancel that of the inductor (i.e., the combination will behave as a short circuit at this frequency). Below the resonant frequency, the combination will have a capacitive reactance whose magnitude increases with decreasing frequency. Above the resonant frequency, the combination will have an inductive reactance that increases with frequency.

INTRODUCTION

The subject of CMOS RF integrated circuit design resides at the convergence of two very different engineering traditions. The design of microwave circuits and systems has its origins in an era where devices and interconnect were usually too large to allow a lumped description. Furthermore, the lack of suitably detailed models and compatible computational tools forced engineers to treat systems as two-port “black boxes” with frequency-domain graphical methods. The IC design community, on the other hand, has relied on the development of detailed device models for use with simulation tools that allow both frequency- and time-domain analysis. As a consequence, engineers who work with traditional RF design techniques and those schooled in conventional IC design often find it difficult to converse. Clearly, a synthesis of these two traditions is required.

Analog IC designers accustomed to working with lower-frequency circuits tend to have, at best, only a passing familiarity with two staples of traditional RF design: Smith charts and S-parameters (“scattering” parameters). Although Smith charts today are less relevant as a computational aid than they once were, RF instrumentation continues to present data in Smith-chart form. Furthermore, these data are often S-parameter characterizations of two-ports, so it is important, even in the “modern” era, to know something about Smith charts and S-parameters. This chapter thus provides a brief derivation of the Smith chart, along with an explanation of why S-parameters won out over other parameter sets (e.g., impedance or admittance) to describe microwave two-ports.