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Fundamentals of Real-Time Spectrum Analysis

Engineers and scientists have been looking for innovative uses for RF technology ever since the 1860s, when James Clerk Maxwell mathematically predicted the existence of electromagnetic waves capable of transporting energy across empty space. Following Heinrich Hertz’s physical demonstration of “radio waves” in 1886, Nikola Tesla, Guglielmo Marconi, and others pioneered ways of manipulating these waves to enable long distance communications. At the turn of the century, the radio had become the first practical application of RF signals. Over the next three decades, several research projects were launched to investigate methods of transmitting and receiving signals to detect and locate objects at great distances. By the onset of World War II, radio detection and ranging (also known as radar) had become another prevalent RF application.

Due in large part to sustained growth in the military and communications sectors, technological innovation in RF accelerated steadily throughout the remainder of the 20th century and continues to do so today. To resist interference, avoid detection, and improve capacity, modern radar systems and commercial communications networks have become extremely complex, and both typically employ sophisticated combinations of RF techniques such as complex and adaptive modulation, bursting and frequency hopping. Designing these types of advanced RF equipment and successfully integrating them into working systems are extremely complicated tasks.

At the same time, the increasingly widespread success of cellular technology and wireless data networks combined with the advancing state of semiconductor technology and packaging has caused the cost of basic RF components to drop significantly over time. This has enabled manufacturers outside of the traditional military and communications realms to embed relatively simple RF devices into all sorts of commodity products. RF transmitters have become so pervasive that they can be found in almost any imaginable location: consumer electronics in homes, medical devices in hospitals, industrial control systems in factories, and even tracking devices implanted underneath the skin of livestock, pets, and people.

As RF signals have become ubiquitous in the modern world, so too have problems with interference between the devices that generate them. Products such as mobile phones that operate in licensed spectrum must be designed not to transmit RF power into adjacent frequency channels and cause interference. This is especially challenging for complex multi-standard devices that switch between different modes of transmission and maintain simultaneous links to different network elements. Devices that operate in unlicensed frequency bands must be designed to function properly in the presence of interfering signals, and are legally required to transmit in short bursts at low power levels. These new digital RF technologies that involve the combination of computers and RF include wireless LANs, cellular phones, digital TV, RFID and others. These, combined with new advances in Software Defined Radio (SDR) and Cognitive Radio (CR) provide a new path forward and will fundamentally change spectrum allocation methodologies resulting in increased efficiency in the way that the RF spectrum, one of the scarcest commodities, is utilized.

To overcome these evolving challenges, it is crucial for today’s engineers and scientists to be able to reliably detect and characterize RF signals that change over time, something not easily done with traditional measurement tools. To address these problems, Tektronix has designed the RealTime Spectrum Analyzer (RSA), an instrument that can discover elusive effects in RF signals, trigger on those effects, seamlessly capture them into memory, and analyze them in the frequency, time, modulation, statistical and code domains.

This document describes how the RSA works and provides a basic understanding of how it can be used to solve many measurement problems associated with modern RF signals.

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