Description
Preface
This is an exciting time for the designers and users of communications receivers. The promise of software-defined radio (SDR) technologies has been fulfilled in a broad array of products. When the third edition of this book was published more than a decade ago, SDR was a well-developed technology, but one that was not widely fielded. Today, the situation is drastically different, with SDR at the core of modern communications systems. The advancements in SDR, driven by improvements in components and techniques, have led to a new edition of Communications Receivers that reflects the many exciting changes that have occurred over the last 10 years.
The authors of the third edition, Dr. Ulrich L. Rohde and Jerry C. Whitaker, are pleased to welcome a new coauthor, Hans Zahnd, an RF engineer by trade, who brings a wealth of experience with SDR systems to the fourth edition.
The many benefits of SDR-based systems are covered in detail in the following pages, along with key analog technologies that are still critically important for high-performance communications systems. SDR, like any technology, has certain limits, driven by the limitations of the state of component development, notably analog-to-digital converters (ADCs) and digital signal processing (DSP) devices. While the performance of these devices continues to advance, they are not limitless in their capabilities. Likewise, the operating environment of a communications receiver may differ widely, depending on the application and location. Interfering signals, either natural or intentional, must be dealt with. These real-world operating constraints mean that for many applications, analog components still play an important role. While front-end preselectors, filters, and other analog devices continue to be used in high-end applications, some traditional receiver stages are hardly recognizable compared to their analog predecessors. Nowhere is this more apparent than RF amplifiers, where “gain boxes” dominate, and demodulation, where DSP performs multiple functions that go far beyond just recovering the aural message.
The ultimate manifestation of SDR is direct digital conversion (DDC), which involves digital downconversion, decimation of the channel rate, baseband I/Q generation, channel filtering, and offset cancellation. Until recently, commercially available ASICs (application-specific integrated circuits) have usually been applied, followed by DSP for demodulation, clock and carrier synchronization, decryption, audio processing, spectrum analysis, etc. The rapid advancement of FPGAs (field-programmable gate
arrays) now allows designers to implement several receivers on the same chip. This trend is moving toward SoC (silicon on chip) devices, combining a large amount of very fast logic elements with powerful signal processing capabilities on the same device. This trend is extraordinarily important as it fundamentally changes the scope of what is possible in a communications receiver.
Another by-product of SDR and DSP can be found in transceivers. The concept of the transmitter and receiver in the same physical box is nothing new, of course. Today, the difference is the level of integration between the transmit and receive functions. A decade ago, technologies for reception and those for transmission were largely different disciplines. One operated at microvolts, and the other at tens of watts (and above). Although the two extreme ends of the transceiver—the receiver front end and the transmitter power amplifier—remain distinctly different, the stages in between are beginning to merge around SDR and DSP technologies. It is for this reason that, in a departure from previous editions, we have included a chapter specifically discussing transceiver systems.
Communications Receivers, fourth edition, includes 11 chapters and an appendix:
• Chapter 1, Basic Radio Considerations
• Chapter 2, Radio Receiver Characteristics
• Chapter 3, Receiver System Planning
• Chapter 4, Receiver Implementation Considerations
• Chapter 5, Software-Defined Radio Principles and Technologies
• Chapter 6, Transceiver SDR Considerations
• Chapter 7, Antennas and Antenna Systems
• Chapter 8, Mixers
• Chapter 9, Frequency Sources and Control
• Chapter 10, Ancillary Receiver Circuits
• Chapter 11, Performance Measurement
• Appendix: Example Receiver Implementation
With the dramatic change from all-analog designs to all-digital or hybrid (analog/digital) systems, the importance of covering certain analog technologies has diminished. In the fourth edition, we have tried to strike the right balance between removing material from the previous edition that is no longer needed and providing the reader with a solid examination of fundamental principles and technologies. A printed book has a certain practical size, and so some tough decisions have been made with regard to “legacy” technologies. Page constraints have also made it necessary to treat some areas in less detail than we would prefer. However, throughout the book we have tried to provide references where more information can be found.
We would like to thank Prof. Dr.-Ing. Martin Buchholz, University of Applied Science, Saarbrücken (Hochschule fuer Technik und Wirtschaft des Saarlandes), for significant mathematical contributions to this book. In addition, we want to thank other friends and colleagues, and many radio amateurs, all of whom provided valuable advice and input, notably, Dr.-Ing. habil. Ajay Kumar Poddar (AC2KG). We also wish to acknowledge the considerable support of Rohde & Schwarz GmbH & Co., Munich, which made a number of resources available, including (but not limited to) various application notes and white papers on core receiver technologies and system testing.
This book has a long history, dating back to the 1980s. As such, it enjoys a longevity that is unmatched in the field. The authors take this legacy very seriously. With each edition, we have tried to chronicle and explain the latest technologies that comprise the discipline of communications receivers. At the risk of broad generalizations, the first edition focused on implementations based on discrete semiconductors. In the second edition, the book expanded to include implementations based on integrated circuits (ICs). In the third edition, digital technologies became available and practical. Now, in the fourth edition, SDR is the driving force behind receiver development.
It is our sincere hope that Communications Receivers, fourth edition, will serve as a valuable reference for years to come.
Ulrich L. Rohde
Jerry C. Whitaker