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1562 results in RF and microwave engineering

Page 29 of 79

  • 11 - Gravity waves and turbulence

  • Wayne K. Hocking, University of Western Ontario, Jürgen Röttger, Robert D. Palmer, University of Oklahoma, Toru Sato, Kyoto University, Japan, Phillip B. Chilson, University of Oklahoma
  • Book:
    Atmospheric Radar
    Published online:
    25 November 2016
    Print publication:
    17 November 2016, pp 596-671

  • Summary

    Introduction

    Wind motions in the atmosphere can cover a wide range of temporal and spatial scales. They may include variations on annual, seasonal, monthly, daily, hourly, and minute scales, down to scales of seconds. Spatially, motions may cover global scales down to synoptic (continental-sized), meso- (city-sized) and microscales (e.g., Ahrens, 1999, Figure 10.1).Windprofiler radars can study all of these scales. However, we cannot possibly cover all of them in this chapter. Larger scale motions (including planetary waves and tides) can be studied well with satellites and in-situ instruments carried by balloons and rockets, as well as numerical computer models. While profilers can also contribute here, it is at the smaller scales that windprofilers really make their best contributions. We will therefore concentrate in this chapter on synoptic, mesoscale and microscale motions, with strongest emphasis on the last two. The primary focus will be on height regions where MST radars have made a significant contribution, restricting discussion to the troposphere, lower stratosphere (below 25–30 km), and the upper mesosphere and lower thermosphere (60 to 100 km altitude). Other height regimes will be discussed primarily in their relation to these regions.

    In meteorology, atmospheric mesoscales motions refer to spatial scales between a few kilometers and one or two hundred kilometers, and temporal scales of the order of minutes to a few hours. In the troposphere, mesoscale events include thunderstorms, tornadoes, and various types of local circulations like land and sea breezes and valley breezes. Typical synoptic scale events include hurricanes, high and low pressure systems, and frontal systems.

    Some or all of these events may be quite familiar to many readers. In fact, these events are only really dominant in the lowest few kilometers of the atmosphere. MST atmospheric radars can be used to investigate these phenomena, and this has been done in the past (e.g. Strauch et al. (1984); Gage et al. (1991a); Webster and Lukas (1992); Teshiba et al. (2001) (and references therein); Röttger and Larsen (1990); Hooper andPavelin (2003), among others). Since this book has a special chapter on meteorology, these events will not be pursued here in any detail.

    When MST radars are used for studies to heights of ten kilometers and more, and even into the upper atmosphere, a different class of mesoscale/synoptic scale motion becomes apparent. This motion is often well organized and can propagate over large distances.

  • 12 - Meteorological phenomena in the lower atmosphere

  • Wayne K. Hocking, University of Western Ontario, Jürgen Röttger, Robert D. Palmer, University of Oklahoma, Toru Sato, Kyoto University, Japan, Phillip B. Chilson, University of Oklahoma
  • Book:
    Atmospheric Radar
    Published online:
    25 November 2016
    Print publication:
    17 November 2016, pp 672-730

  • Summary

    Introduction

    Some of the earliest applications of windprofiler radars were in regard to tropospheric and lower stratospheric studies. The radars developed at the Sunset site near Boulder, Colorado (Green et al., 1979) and in the Harz mountains in Germany (the SOUSY radar (Czechowsky et al., 1976)) were two of the earliest such instruments, and were certainly built with meteorological studies in mind. Some of these radars have already been described in Chapter 2, and the SOUSY radar was extensively discussed in Chapter 6.

    The most direct meteorological studies have been in regard to wind motions, but these radars have also been usefully employed in other areas, including studies of turbulence strengths and anisotropy, tropopause height measurements, gravity wave momentum fluxes, precipitation measurements, temperature profile determinations, and various others.

    It is impossible to cover all aspects of MST radar applications relating to the troposphere in just one chapter. For this reason, we will concentrate mainly on results, rather than on specific details about techniques. It will be assumed that the techniques have been sufficiently covered in earlier chapters.

    The early years of tropospheric studies have been especially well covered in several excellent reviews, including those by Röttger and Larsen (1990), Gage (1990), Larsenand Röttger (1982) and Balsley and Gage (1982). Some of the early parts of this chapter will involve a recap of the main results of those publications.

    Röttger and Larsen (1990) discussed the origins of VHF MST radar studies in the context of: (i) developments following the use of high-power X, S, and UHF band radars in the United States of America, as well as FMCW (frequency modulated continuous wave) techniques; coupled with (ii) the detection of tropospheric echo fading observed at Jicamarca (Peru) by Woodman and Guillen (1974); and (iii) the application of phasecoherent techniques. These events in turn led to the first dedicated VHF-ST radars being built at Sunset, near Boulder, Colorado, and in the Harz mountains of Germany (the SOUSY, sounding system radar). Phase coherent detection was especially important in the development of such systems, for without it, detection of useful tropospheric echoes with VHF systems would be nearly impossible.

  • 2 - The history of radar in atmospheric investigations

  • Wayne K. Hocking, University of Western Ontario, Jürgen Röttger, Robert D. Palmer, University of Oklahoma, Toru Sato, Kyoto University, Japan, Phillip B. Chilson, University of Oklahoma
  • Book:
    Atmospheric Radar
    Published online:
    25 November 2016
    Print publication:
    17 November 2016, pp 47-119

  • Summary

    Introduction

    The groundwork describing the atmospheric environment and the types of flows that radar can study in the Earth's atmosphere has been laid in the previous chapter. We now turn to a brief history of how radars came to be involved with studies of this type.

    While most of this book is about MST radar, it is important that MST radar be seen in a broader context. We therefore begin this section on the history of the development of MST radar by looking not at MST radar itself, but rather at the development of meteorological radar. As indicated earlier, the period following World War II saw various developments of radar. Two primary streams were (i) ionospheric studies for world-wide communication, and (ii) studies of contaminants in radar detection for military and civil applications. The first stream of development led to extensive studies of the upper atmosphere and ionosphere, and the second led to more intensive investigations of the troposphere. Only with the development of MST radar did the two streams once again really merge.

    Initially, there were two main aspects to radar detection – determination of range and, if possible, direction. Directional determination was achieved by using large antennas which concentrated the radar directionally, and range was generally found using timeof- flight delays.

    The atmospheric radar principle for range-detection is basically fairly straightforward. A short pulse of an electromagnetic wave of typically several microseconds duration is transmitted from the radar antenna, whereupon it eventually may strike a target. It is then scattered back from the target to the radar antenna. The receive signal is called an echo, by analogy with the sound heard when your voice echoes from a distant object. Multiple radar echoes can be detected if there are multiple targets.

    Echo samples are examined at consecutive time steps. Using early radars, this was done visually, whereas with more recent ones, digital sampling is used. Since the radar signal propagates with the speed of light c, the time t elapsed from the transmission of the pulse to the reception corresponds to a given range r = ct/2. We find, as an example, that echoes from backscattering targets at a range of, say 15 km, are received 100 microseconds after the pulse was transmitted. Echo samples are taken at a series of successive delays, called range gates.

  • 6 - Examples of specific atmospheric radar systems

  • Wayne K. Hocking, University of Western Ontario, Jürgen Röttger, Robert D. Palmer, University of Oklahoma, Toru Sato, Kyoto University, Japan, Phillip B. Chilson, University of Oklahoma
  • Book:
    Atmospheric Radar
    Published online:
    25 November 2016
    Print publication:
    17 November 2016, pp 337-380

  • Summary

    Introduction

    As discussed in Chapter 2, the MST technique began in part with Woodman and Guillen (1974) after discovery of atmospheric echoes from the troposphere using the Jicamarca incoherent scatter radar in Peru. Following this, recognizing the potential for meteorological applications, several groups set about building specialized radars for low altitude (less than 20 km) application, based broadly on the Jicamarca system. Primary groups who followed this course included a NOAA group in Boulder, Colorado, and a group at the Max Planck Institut für Aeronomie in Northern Germany. The NOAA Aeronomy group developed the so-called “Sunset radar” which was installed close to Boulder, and later a larger system at Poker Flat in Alaska. The Poker Flat system was in part also designed for mesospheric studies. The Max Planck group built a radar in the Harz Mountains, near Katlenburg-Lindau. These were the first VHF instruments designed specifically for meteorological studies.

    Later, similar radars were developed by other groups in the UK, Japan, Australia, and various other countries, eventually leading to large networks of such radars. The term “windprofiler” was adopted to describe such radars when used for tropospheric and lower stratospheric (meteorological) wind measurements. One notable development was the construction of the large MU (middle-upper) radar near Shigaraki in Japan. At the time this was a state-of-the-art instrument, and had many important developments incorporated into it.

    In this chapter, we will describe some of the details of three radars. One will be the German SOUSY radar in the Harz Mountains, one the MU radar, and the third a lowcost radar (CLOVAR) built in the 1990s in Canada. The objective is not so much to discuss the history of these radars (that was considered in some detail in Chapter 2), but to give more detail about technical developments through the course of evolution of the MST technique. The SOUSY radar was built at a time when personal computers were just starting to be developed, but were of very slow speed. Instruments like the Data General PDP-8 and NOVA mini-computers were just under development; the PDP-8 was developed around 1965, and the NOVA came into being in the late 1960s and early 1970s.

  • 10 - Extended and miscellaneous applications of atmospheric radars

  • Wayne K. Hocking, University of Western Ontario, Jürgen Röttger, Robert D. Palmer, University of Oklahoma, Toru Sato, Kyoto University, Japan, Phillip B. Chilson, University of Oklahoma
  • Book:
    Atmospheric Radar
    Published online:
    25 November 2016
    Print publication:
    17 November 2016, pp 549-595

  • Summary

    Introduction

    In this chapter, we discuss various extended, and in some cases unusual, applications of MST radar. These may be special cases of general MST techniques, or specific applications of the technique applied to special cases, or even quite unusual applications which are a substantial deviation from “normal” MST standard practices. If such a topic fits well in another chapter, it may appear there – if it is somewhat of an exception, or has a sightly unusual methodology, or is not really an operational technique, it may appear here. Polar mesosphere summer echoes are an example of an “extended” application. While the techniques used to study these unusual echoes are really the same as for other MST studies, the unusual physics associated with the scatterers that produce these echoes makes them of particular interest. Lightning study is an example of a slightly “miscellaneous” application, in that the techniques are a little unusual (high PRFs, and the events are very short lived). Meteor study is an example of a slightly non-standard application that has grown into a substantial field all of its own. Differential absorption is a technique developed early in the days of radar in the 1960s and 1970s which has had a rebirth in the last decades, and deserves a brief mention here. Precipitation study with MST radars is a relatively mature field, but is still a secondary application, so is also included here.

    Each of these fields has a significant role in its own right, but extended discussion of them would simply take up too much space, and would spread the intended application of this book beyond its original goals. Hence the topics are summarized briefly in this chapter – maybe too briefly for some, but we have tried to give sufficient references that interested readers may expand their knowledge through these references.

    This book is intended to concentrate on experimental and analysis techniques, and the underlying processes (both geophysical and technical) that guide the experiments and their design. Examples of the latter include the basic theory of turbulence, and the theory behind gravity waves (see the next chapter, and also some small discussion in Chapter 2).

  • 1 - An overview of the atmosphere

  • Wayne K. Hocking, University of Western Ontario, Jürgen Röttger, Robert D. Palmer, University of Oklahoma, Toru Sato, Kyoto University, Japan, Phillip B. Chilson, University of Oklahoma
  • Book:
    Atmospheric Radar
    Published online:
    25 November 2016
    Print publication:
    17 November 2016, pp 1-46

  • Summary

    Introduction

    Many instruments have been used to study the atmosphere, both by in-situ and remote methods. From anemometers to satellites, chemical sensors to balloons and rockets, the array of tools is broad. Since the early 1900s, a key instrument for such studies has been radar. RADAR stands for Radio Detection And Ranging. Radars operating in a variety of frequency bands, from wavelengths of kilometers to wavelengths of millimeters, have all found application. They have been used to study the upper ionosphere and the neutral atmosphere, right down to ground level.

    In this book, we will concentrate on a class of radar generally referred to as MST radar. In this description, M stands for Mesosphere, S for Stratosphere, and T for Troposphere, where these three “spheres” refer to different height-regimes of the atmosphere which collectively cover the region from ground level up to about 90 km altitude. More exact definitions will be given shortly. For now, consider the troposphere as the region from the ground to 12 km altitude, the stratosphere as the region from 12 to 50 km altitude, and the mesosphere the region from 50 to 90 km altitude. Under the narrowest definition, the term MST radar was originally used primarily to refer to radars operating in the VHF (very high frequency) band, with special emphasis on frequencies around 50 MHz, which could probe (at least in part) all three regions. More generally it has come to refer to any radars that can be used for studies of any of these three regions of the atmosphere. These radars include MF (medium frequency), HF (high frequency), VHF, and UHF (ultra-high frequency). They also include so-called meteor radars. Generally, precipitation radars (referred to as “Doppler radars” by the meteorological community) are not considered to be MST radars, although we will discuss them a little in this book. (As an aside, we will generally refer to these radars as precipitation radars in this book. The phrase “Doppler radar” is not a good one to describe these radars, since they are most certainly not the only Doppler radars! The term “Doppler radar” arises from the fact that these radars can measure the Doppler frequency-shift of reflected signals.

  • 4 - Fundamental concepts of radar remote sensing

  • Wayne K. Hocking, University of Western Ontario, Jürgen Röttger, Robert D. Palmer, University of Oklahoma, Toru Sato, Kyoto University, Japan, Phillip B. Chilson, University of Oklahoma
  • Book:
    Atmospheric Radar
    Published online:
    25 November 2016
    Print publication:
    17 November 2016, pp 217-267

  • Summary

    Introduction

    Fundamentally, atmospheric radars are designed to transmit an electromagnetic (EM) wave and to observe the effects that the atmosphere has on the scattered wave. These interactions may take the form of bending of the radiowave path, or reflection and scattering. In the simplest case, a transmit antenna and a receive antenna are required, which may be located at separates sites. More complex systems might involve multiple receivers and even multiple transmitters. Most commonly in MST atmospheric work, the transmitter and receiver are co-located; in these cases, refraction of the ray paths is not generally significant. Reflection and scattering are the primary phenomena that need to be considered in MST studies.

    The radar targets in MST studies

    Atmospheric reflection and scattering occur due to the interaction of the EM wave with changes in the refractive index. As discussed in Chapter 3, these refractive-index changes may be caused by a variety of phenomena. We will quickly revisit some of these processes here, because they help us to understand the different modes of radar analysis that we will discuss. Of course, aircraft and missiles are perhaps the most obvious examples of targets that spring to mind when we talk of radar, but these are not the primary targets when it comes to atmospheric studies. One simple example that is relevant is water droplets embedded in the air. In this case, the refractive index inside the water droplets is very different to that of the surrounding air, so each water droplet may scatter a small amount of incident radiation. In this case, scatter from a large number of water droplets is required before a detectable scattered signal can be produced. Insects and birds contain water, so they too can act as radiowave scatters. Indeed some radars use insects as tracers of atmospheric motions. Another example is the ionized trail of plasma left behind when a meteoroid enters the atmosphere. Meteoroids are generally small grains of dust (with diameters from micrometers to centimeters, though larger ones can occur) which enter the atmosphere at high speed (typically 10 to 70 km/s), creating large levels of frictional heating and thereby ionizing the air around them. As a result, a long trail of plasma (typically a few km in length) exists behind the meteoroid, and this so-called “meteor trail” can reflect radiowaves.

  • 5 - Configuration of atmospheric radars – antennas, beam patterns, electronics, and calibration

  • Wayne K. Hocking, University of Western Ontario, Jürgen Röttger, Robert D. Palmer, University of Oklahoma, Toru Sato, Kyoto University, Japan, Phillip B. Chilson, University of Oklahoma
  • Book:
    Atmospheric Radar
    Published online:
    25 November 2016
    Print publication:
    17 November 2016, pp 268-336

  • Summary

    Introduction

    In earlier chapters, we have discussed radars in a general sense, and dealt with some of the techniques available to optimize signal detection. We have discussed the conceptual difference between CW and pulsed systems, and concepts like range resolution and sampling strategies. In this chapter, we will take a closer look at the electronics and engineering that is required to develop a radar, and the associated hardware. Key topics will include antennas, transmitters, receivers, and controllers. Some topics from the previous chapter may be repeated, but generally in greater detail.

    One thing that all radars have in common is a need for a transmit antenna and a receive antenna. These may or may not be located at separate sites. The transmitter transmits radiowaves through a transmitter antenna into the air, and receives echoes from a target, or from multiple targets, with the receiver antenna. When the transmitter and receiver are co-located, the radar is referred to as a “monostatic radar,” while the term “bistatic radar” refers to the case that the transmitter and receiver are physically separated. If two or more receivers which detect echoes from a common target are located at different places, the system is called a “multistatic radar.” The degree of separation can be an important factor as well – if the transmitter and receiver are within maybe a few wavelengths of each other, they may be referred to as either monostatic or bistatic, depending on the application, even though, in the strictest sense, they are bistatic/multistatic. An example is the so-called “spaced antenna method” for measuring winds, in which case there are multiple receiver antennas but the theoretical development is often done in a quasi-monostatic sense. Generally, if the separation between the transmitting and the receiving antennas can be neglected compared with the distance to the target, the system is considered as monostatic, although even then the meaning of “small” and “large” distances depends on the objectives of the experiment. Experiments requiring detailed phase information between receivers may need to be considered multistatic, whereas if no phase information is needed the same configuration might be considered as monostatic, for example.

  • 12 - Antenna Integration

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 363-396

  • Summary

    INTRODUCTION

    Chapter 2 provided an introduction to antenna fundamentals in which antennas were described as transducers of guided electrical signals to unguided electromagnetic waves, and vice versa. In Chapter 4, receive antennas were viewed as receptors of environmental noise, resulting in sensitivity-limiting electrical noise characterized in terms of antenna temperature. Intervening chapters have introduced methodology for analysis and design of radios which accounts for effective radiated power and sensitivity (Chapter 7), impedance matching (Chapter 9), and noise within a receiver (Chapter 11). We are now in a position to confront some antenna issues that were bypassed in Chapter 2, namely: How to most effectively integrate an antenna with radio electronics so as to achieve system-level design goals. In this chapter we address the topic of antenna integration so as to close this gap in our review of radio electronics, before moving on to the topics of receiver and transmitter design (Chapters 15 and 17, respectively).

    The organization of this chapter is as follows. In Section 12.2 we consider the manner in which an antenna contributes to the SNR achieved by a receiver, thereby limiting sensitivity. In Section 12.3 we consider VSWR and efficiency of power transfer in transmit operation. These considerations are closely related to the issue of impedance matching between antenna and transceiver, which is addressed in Section 12.4. The trend toward increasingly small antennas is considered, first in terms of the impedance-matching challenge, and then (in Section 12.5) in terms of practical limits on an antenna size. Section 12.6 gives a brief introduction to antenna tuners, a resurgent technology that facilitates improvements in the performance of radios that must cover large tuning ranges using antennas which are narrowband or electrically-small. This chapter concludes with an introduction to baluns (Section 12.7), which are needed to accomplish the common tasks of interfacing single-ended antennas to differential transceivers, and differential antennas to single-ended transceivers.

    RECEIVE PERFORMANCE

    Let us now quantify the role an antenna plays in the sensitivity of a radio receiver. This issue was previously addressed in Sections 7.4 and 7.5, and a review of those sections is recommended before engaging with this section. We begin by elaborating on the concepts presented in those sections and consider the implications for the design of antennas and the transmission lines and matching networks used to interface them to receivers.

  • 9 - Impedance Matching

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 259-284

  • Summary

    INTRODUCTION

    In Chapter 8 we noted the central role two-port theory plays in RF circuit design, and especially the role of reflection coefficients in determining the stability and gain of two-ports. Reflection coefficients were introduced as surrogates for source and load impedance (recall Equations (8.32) and (8.37)), facilitating the use of scattering parameters. In this chapter we consider impedance matching, which is the process of converting impedances (equivalently, embedded reflection coefficients) from a given value to some other value.

    Applications for impedance matching include (1) maximizing power transfer between twoports, (2) eliminating reflections (not the same thing!), and (3) converting from a given impedance to another impedance necessary for setting the stability and/or gain of a two-port. In each of these cases, the problem is essentially as shown in Figure 9.1: We wish to design a passive two-port that converts the output impedance of the source, ZS = RS + jXS , into the desired input impedance for the load, ZL = RL + jXL . Usually we would like to achieve this with a TPG as close to 1 as possible, and often there is a frequency span over which a minimum TPG which must be achieved. At the design frequency, the nominal embedded input impedance Zin of the matching two-port is either for maximum power transfer or ZS for reflectionless matching. Similarly, the nominal embedded output impedance Zout of the matching two-port is either or ZL at the design frequency.

    The organization of this chapter is as follows. In Section 9.2 we consider some rudimentary approaches to impedance matching, including resistive matching, transformer matching, and reactance canceling. Section 9.3 introduces narrowband matching using two discrete reactances in an “L” configuration, which is useful both as a standalone technique as well as a building block for more sophisticated techniques. Section 9.4 considers the issue of bandwidth, and introduces the concept of quality factor to provide some insight into how the bandwidth of matching circuits is determined and can be manipulated. In Section 9.5 we combine the concepts of previous sections to develop impedance matching circuits having bandwidth which can be wider or narrower than that obtained by using discrete two-reactance matching. Section 9.6 explains how to obtain differential versions of the single-ended circuits developed in earlier sections.

  • 16 - Frequency Synthesis

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 486-515

  • Summary

    INTRODUCTION

    Frequency synthesis is the generation of a periodic waveform having a specified frequency. Here we concern ourselves primarily with the generation of sinusoids, which are commonly required as LO signals in frequency conversion, and as carriers in analog modulations such as AM and FM. In this chapter we consider the synthesis of sinusoids using feedback oscillators (Sections 16.2–16.6), phase-locked loop synthesizers (Section 16.8), and direct digital synthesis (Section 16.9). The problem of phase noise and its effect on radio system performance is introduced in Section 16.4. Also, Section 16.10 summarizes additional considerations that arise in IC implementation of frequency synthesis devices.

    LC FEEDBACK OSCILLATORS

    In Section 8.5.1, it was noted that two-ports that are active and include internal feedback are prone to instability, and that one particular consequence of instability could be oscillation. In that context, oscillation was undesirable; now, however, let us consider how we might exploit this type of mechanism for frequency synthesis.

    The LC Resonator

    Let us begin with a well-known passive circuit that exhibits oscillatory behavior: The LC resonator, which is an inductor (the “L”) and a capacitor (the “C”) in parallel as shown in Figure 16.1. One first establishes a steady-state condition (Figure 16.1(a)) by applying a DC voltage source set to some non-zero value v 0> 0, which charges the capacitor to a voltage of v 0. At this point no current flows through the capacitor; subsequently, the voltage across the inductor is zero.

    Oscillation begins when the applied voltage is removed, as indicated in Figure 16.1(b). No longer constrained by the source voltage, the capacitor begins to equalize the charge between its plates by sending current through the inductor. The resulting current through the inductor creates a magnetic field which grows stronger the longer the current flows. The voltage v across the capacitor decreases because the net difference in charge across the plates is diminishing.

    Once the charge is evenly distributed between the plates of the capacitor (i.e., the capacitor is fully discharged), v = 0 (Figure 16.2(a)). However, the magnetic field that has built up in the inductor keeps the current flowing, and so the redistribution of charge across the capacitor plates continues. A short time later we have the situation shown in Figure 16.2(b): The voltage across the capacitor is negative and decreasing, the flow of current is decreasing, and the magnetic field of the inductor is decreasing.

  • 10 - Amplifiers

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 285-337

  • Summary

    INTRODUCTION

    The primary purpose of most amplifiers used in radio frequency systems is to increase signal magnitude. This is necessary in receivers because propagation path loss results in a signal arriving with voltage or current which is far too small to be digitized or directly demodulated. Amplification appears in transmitters when it is convenient to synthesize a signal at low power and amplify it, as opposed to synthesizing signals directly at the intended transmit power; the latter is difficult for all but very simple analog modulations.

    This chapter is organized as follows. Section 10.2 introduces the transistor, its use as a gain device, and provides a brief summary of transistor types and device technologies. The design of narrowband single-transistor amplifiers is introduced in four parts: biasing (Section 10.3), small-signal design to meet gain requirements (Section 10.4), designing for noise figure (Section 10.5), and designing for voltage standing wave ratio (Section 10.6). Section 10.7 presents a design example intended to demonstrate the principles of amplifier design as laid out in the previous sections. Section 10.8 introduces the concepts of multistage, differential, and broadband transistor amplifiers. Section 10.9 describes the implementation of transistor amplifiers as integrated circuits.

    TRANSISTORS AS AMPLIFIERS

    The fundamental building block of modern radio frequency amplifiers is the transistor. A transistor is a three-terminal semiconductor device in which the current between two terminals can be manipulated using the voltage or current at a third terminal. The application to RF design is most easily explained in the context of a particular device – for our purposes, the silicon (Si) bipolar junction transistor (BJT) is a good place to start.

    Bipolar Transistors

    A BJT is depicted in Figure 10.1. The three terminals of a BJT are known as the base, the collector, and the emitter. The associated terminal currents and voltages are as defined in Figure 10.1. The Si BJT comes in two varieties, known as “NPN” and “PNP,” referring to the geometry of semiconductor materials within the device. The NPN variant is somewhat more common in RF applications and is specifically considered here (and shown in Figure 10.1); the PNP variant is relatively easily understood once the NPN variant is mastered.

  • 6 - Digital Modulation

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp 139-202

  • Summary

    INTRODUCTION

    In Section 5.1 we defined modulation as the representation of information in a form suitable for transmission. Digital modulation might be defined as the representation of information as a sequence of symbols drawn from a finite alphabet of possible symbols. This definition is not completely satisfactory, since it would seem to include a variety of techniques that many practitioners would consider to be analog modulations; these include continuous wave (CW) signaling (the use of an interruptible carrier to send Morse code). In Section 5.1 we settled on the notion that digital modulation also requires the information to be transmitted to be represented in digital (not merely discrete-time) form at some point in the modulation process, whereas this is not required in analog modulation.

    Overview of a Digital Communications Link and Organization of this Chapter

    With this definition in mind, Figure 6.1 shows an overview of a digital communications link. Here is the usual signal flow.

  • • The starting point of the link is source coding, which is the processing of data and/or digitized analog message signals – e.g., voice – in order to render the data in a form better-suited for digital modulation. A common goal in source coding is compression; that is, reduction of data rate so as to reduce the required bandwidth. Source coding is addressed in Section 6.2.

  • • Data are often encrypted for security. The principles of encryption are not specific to radio communications and so are not addressed further in this book.

  • • Channel coding is the processing of data with the goal of making them more robust to propagation channel impairments, including noise, multipath, and interference. Channel coding is addressed in Sections 6.13 (error control coding) and 6.14 (interleaving).

  • • At this point the data are processed by what is traditionally considered “modulation.” The result is an analog signal representing a discrete-time finite-alphabet message, and which is suitable for frequency conversion and amplification. It is this form of the signal that is ultimately transmitted to the other end of the link. Modulation is addressed in Sections 6.3, 6.4, and portions of Sections 6.10–6.12, 6.18 and 6.19.

  • Preface

  • Steven W. Ellingson, Virginia Polytechnic Institute and State University
  • Book:
    Radio Systems Engineering
    Published online:
    28 May 2018
    Print publication:
    06 October 2016, pp xxxix-xlii

  • Summary

    The topic of this book is the analysis and design of radio systems and their constituent subsytems and devices. Chapters 1–6 cover the basics of radio: antennas, propagation, noise, and modulation. Chapters 8–18 cover radio hardware: circuits, device technology where relevant, and the basics of digital signal processing. The intervening Chapter 7 (“Radio Link Analysis”) serves as the bridge between these two parts of the book by demonstrating how system-level characteristics lead to particular hardware characteristics, and vice versa. Appendix B serves as a summary of commonly-encountered radio systems in this context. Appendix A addresses modeling of path loss in terrestrial channels, building on the fundamentals presented in Chapter 3 (“Propagation”).

    This book presumes the reader is interested primarily in communications applications, however, most of the material is relevant to other applications of radio including navigation, radar, radio astronomy, geophysical remote sensing, and radio science.

    The use of the term “systems engineering” in the title of this book bears some explanation. In the parlance of communications engineers, the scope of this book is limited to considerations at the physical layer and below, with little coverage of issues at higher layers of the protocol stack. Further, some readers may judge that only Chapters 1, 7, and Appendix B (or maybe just Chapter 7) address systems engineering, in the sense of complete “end-to-end” system analysis and design. Nevertheless, the title “Radio Systems Engineering” seems appropriate because each topic – especially those in Chapters 8–18 – is presented in the context of system-level design, with extensive cross-referencing to associated topics in the first half of the book and to Chapter 7.

    Intended uses of this book This book is has been written with three possible uses in mind.

  • • Primary textbook in senior- and beginning graduate-level courses in radio systems engineering. This book can serve as a single textbook for a two–three semester course sequence. Associated courses typically have titles such as “radio communications systems,” “wireless engineering,” “wireless systems,” etc.

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