Frequency measurements usually are made with swept spectrum analyzers. They make amplitude-versus-frequency measurements by sweeping a resolution bandwidth (RBW) filter over the frequencies of interest and recording the resultant amplitude at each frequency point. The RBW is the key measurement method for most RF measurements, where swept spectrum analyzers often provide an excellent dynamic range and highly accurate displays of a signal's static spectral components. However, the principal disadvantage of the swept spectrum analyzer is that it records amplitude data at only one frequency at a particular moment in time.
This is a weakness because new RF applications are emerging with RF signals that have complex time-related characteristics. Modern RF signals, especially those in the open-access Industrial, Scientific, and Medical (ISM) band, often employ spread spectrum techniques, such as Bluetooth and WiFi, that are more intermittent or "bursty" in nature. Such short duration wireless signals are significantly more variable in frequency than radio signals of the past. As a result, today's RF signals are more difficult to analyze with a traditional swept spectrum analyzer, which is limited in its digital modulation analysis and multidomain capabilities. Even the vector signal analyzer (VSA), which emerged to specifically address digitally modulated signals, has limited capabilities for analyzing transient signals over time in the frequency modulation domains.
Spectrum monitoring today often involves detecting elementary temporal events in the presence of nonstationary signals and uncorrelated noise. Put simply, transients, predictable and unpredictable frequency shifts, and complex modulation schemes are the norm in many of today's RF and wireless disciplines and applications. Common examples are spread-spectrum and RFID, which communicate via brief bursts of information. Although the traditional swept frequency spectrum analyzer and vector signal analyzer remain the instruments of choice for mainstream RF test and measurement applications, this chapter focuses on innovative real-time spectrum analysis (RTSA). We discuss RTSA because of the migration of today's RF applications to transient signal behavior. SI engineers now need to trigger and capture signals of interest simultaneously in the time and frequency domains.
Often SI engineers need to capture a continuous record of signal fluctuations, including transients and frequency deviations, and they need to analyze the signal for changes in frequency, amplitude, and modulation. Moreover, all these operations often need to be carried out over a lengthy period of time. For example, an SI engineer could wait a significant amount of time for a swept-spectrum analyzer to detect a transient event in a modern RF system. Even then there would be a limited means of determining when the event occurred or whether the engineer had actually missed the capture of one or more events.
The common thread that runs through many emerging RF applications is the time-varying nature of the wireless signal. This characteristic, coupled with the factors already discussed, calls for a new type of analysis. As a result, SI engineers and designers are using real-time spectrum analysis in increasing numbers. Although real-time spectrum analysis is not a new concept, and it is very similar in concept to the VSA, the relevance of RTSA to SI engineering is significant. Consequently, today's SI engineer is encouraged to consider both conventional frequency-domain information and RTSA. Moreover, given the trend in SI engineering toward RTSA as more systems require simultaneous time and frequency information to reveal underlying RF signal behavior, there is a reasoned argument for focusing on RTSA in this chapter.
Next: The Swept Spectrrum Analyzer
Geoff Lawday, David Ireland, Greg Edlund
About the Authors
Dr. Geoff Lawday is Tektronix Professor in Measurement at Buckinghamshire New University, England. He delivers courses in signal integrity engineering and high performance bus systems at the University Tektronix laboratory, and presents signal integrity seminars throughout Europe on behalf of Tektronix.
David Ireland, European and Asian design and manufacturing marketing manager for Tektronix, has more than 30 years of experience in test and measurement. He writes regularly on signal integrity for leading technical journals.
Greg Edlund Senior Engineer, IBM Global Engineering Solutions division, has participated in development and testing for ten high-performance computing platforms. He authored Timing Analysis and Simulation for Signal Integrity Engineers (Prentice Hall).
Title: Signal Integrity Engineer's Companion ISBN 0131860062, Prentice Hall, Chapter 10: The Wireless Signal.
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