Part I
Part II
Part III
The Vector Signal Analyzer
Traditional swept spectrum analysis enables scalar measurements to provide information about the magnitude of each frequency component in an input signal. Analyzing signals that carry digital modulation requires a vector measurement to provide both magnitude and phase information about the input signal. The vector signal analyzer is a tool specifically designed for digital modulation analysis. Figure 10.6 shows a simplified VSA block diagram, where the VSA is optimized for modulation measurements.
Like the Real-Time Spectrum Analyzer (RTSA) described in the next section, a VSA digitizes all the RF energy within the instrument's passband to extract the magnitude and phase information required for digital modulation measurements. However, most, but not all, VSAs are designed to take snapshots of the input signal at arbitrary points in time. This makes it difficult or impossible to store a long record of successive acquisitions for a cumulative history of how a signal behaves over time. Like a swept SA, the triggering capabilities typically are limited to an intermediate frequency (IF) level trigger and an external trigger.
Within the VSA, an ADC digitizes the wideband IF signal, and then the down-conversion, filtering, and detection are performed numerically. The transformation from the time domain to the frequency domain is processed by Fast Fourier Transform (FFT) algorithms. The linearity and dynamic range of the ADC are critical to the instrument's performance. Equally important, there must be sufficient DSP power to enable fast measurements. Nonetheless, some VSAs have significant measurement capabilities and can provide a detailed analysis of RF signal behavior.
The VSA typically measures modulation parameters such as error vector magnitude (EVM) and provides other displays, such as a constellation diagram. A stand-alone VSA is often used to supplement the capabilities of a traditional swept SA. In addition, many modern instruments have architectures that can perform both swept SA and VSA functions. These instruments provide frequency and modulation domain measurements within one instrument; however, both measurements are not always correlated in time.
OVERVIEW OF THE REAL-TIME SPECTRUM ANALYZER
The RTSA is designed to capture and analyze RF signals that have transient and dynamic characteristics. The fundamental concept of real-time spectrum analysis is to trigger on an RF signal event, seamlessly capture the digitized data into memory, and then provide a built-in analysis of the data in multiple domains. Figure 10.7 is a simplified block diagram of the RTSA architecture. The RF front end can be tuned across the instrument's entire frequency range, and it down-converts the RF input signal to a fixed IF that is related to the RTSA's maximum real-time bandwidth. The signal is then filtered, digitized by the ADC, and passed
to the DSP engine that manages the instrument's triggering, memory, and analysis functions.
While elements of the RTSA block diagram and acquisition process are similar to those of the VSA architecture, the RTSA is optimized to deliver real-time triggering, seamless signal capture, and time-correlated multidomain analysis. An important consideration for the RTSA is the need to incorporate advanced ADC technology to enable a conversion with high dynamic range and low noise for the measurement to be comparable to a swept SA. For measurement spans that are less than or equal to the instrument's real-time passband, the RTSA architecture lets you seamlessly capture the input signal with no gaps in time by digitizing the RF signal and storing the time-contiguous samples in memory. RTSA has several advantages over the acquisition process of a swept spectrum analyzer for measuring modern "bursty" wireless signals. This makes it an exciting modern instrument to discover and is why RTSA is the focus of this chapter. Nonetheless, the SI engineer must bear in mind that the classic RBW spectrum analyzer remains the instrument of choice for a significant number of RF measurements.
Next: Fast Fourier Transform Analysis
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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