Like most modern radio communications devices, wireless local-area networks (WLANs), RFID tags, and 3G cellular systems rely on complex modulations that require specialized signal-analysis capabilities. Consequently, testing today's complex RF signals has become increasingly challenging. The test and measurement industry has responded with a bewildering array of swept spectrum analyzers, vector signal analyzers, and real-time spectrum analyzers.
RF spectrum analyzers have been popular since the 1960s. A considerable evolution in technology and capability has taken place inside the spectrum analyzer over the years. Today, three basic types of spectrum analyzers exist: the swept-tuned spectrum analyzer (SA), the vector signal analyzer (VSA), and the real-time spectrum analyzer (RTSA).
Though these measurement instruments have overlapping capabilities, there are some important differences that distinguish them. Understanding the differences is essential for their proper application to complex WLAN, RFID, and 3G diagnostic problems.
EVOLUTION OF SIGNAL ANALYSIS The first RF spectrum analyzers were swept-tuned instruments, ideal for analyzing the simple analog continuous signals that made up the RF industry until the late 1970s (Fig. 1a). In the late 1970s, semiconductor speed and integration increased enough to make digital modulations practical. New signal-analysis tools were needed to provide diagnostic information for more complex quadrature-amplitude-modulated (QAM) signals. The test and measurement industry responded with the constellation analyzer. In the early days, the constellation analyzer was nothing more than an oscilloscope with special wideband channels and a Z-axis beam brightness control to allow for mapping of data symbols on the cathode ray tube's X and Y display.
The constellation analyzer was a baseband time-domain measurement device. As semiconductors improved, it became possible to digitally sample the baseband QAM signals for a flicker-free constellation display. In a revolutionary step, the digitally sampled constellation analyzer was packaged with an RF downconverter to create the modern VSA. The VSA also could post-process digitally time-sampled data into the frequency domain using the fast Fourier transform (FFT), creating a second type of spectrum analyzer (Fig. 1b).
About the same time that the VSA was under development, another breakthrough in spectrum analysis occurred for a much different reason. The intelligence community had concerns about intermittent signal bursts that might go undetected. The swept-spectrum analyzer and VSA, originally developed for continuous signals, only examined the RF spectrum part-time. The unanalyzed, blanked-out periods between sweeps or recordings presented a serious concern for national security.
The need arose for a spectrum analyzer that could analyze the RF spectrum continuously. This required the ability to process time-domain data into the frequencydomain in real time, something that neither the swept-tuned spectrum analyzer nor the VSA was designed for. The need to capture and analyze the intermittent signal led to the development of the real-time spectrum analyzer (Fig. 1c).
Different analysis applications—analog continuous signals, continuous digital modulations, and intermittent digital modulation bursts—spawned the three basic types of spectrum analyzers available today. Each has continued to evolve, optimized for different applications.
HOW EACH ANALYZER WORKS At the swept-tuned SA's input, a variable attenuator adjusts the signal level and filters it with a preselector (Fig. 2a). The narrowband tunable preselector, usually made with yttrium-iron-garnet (YIG) resonators, eliminates unwanted signals, preventing the creation of spurious products in the first mixer. At frequencies below a few gigahertz, a low-pass filter is usually switched in for preselection.
A sweep generator then tunes the local oscillator (LO) across the analysis frequency span (X-axis) downconverting the signal into the resolution-bandwidth (RBW) filter. Then the signal amplitude is envelope-detected, video-filtered, and displayed on the Y-axis.
In early spectrum analyzers, the sweep generator outputs and vertical-axis drive actually drove the cathode ray tube's Xand Y-axes as depicted. Modern units digitize these voltages to drive LCDs. Some of the latest analyzers digitize the entire IF, implementing this digitally. Most modern SAs also have digitally synthesized LOs that are step-tuned across the frequency span instead of a continuous analog sweep. The swept-tuned SA is optimized for dynamic range, LO phase noise performance, and frequency coverage. Swept-tuned SAs cover a range from a few hertz to hundreds of gigahertz with external mixing.
The VSA's block diagram, like that of the SA, has a variable attenuator at its input. Because signal phase information is preserved in the VSA, the YIG pre-selector is replaced with a more phase-stable, wideband fixedfrequency band-pass filter (Fig. 2b).
Next, the signal is downconverted using a stationary LO. The VSA's analog-to-digital converter (ADC) digitizes the signal and stores it to memory. The recorded data is transferred to a microprocessor for postcapture analysis. The microprocessor can execute an FFT to view the frequency spectrum or digitally demodulate the signal for evaluation.
VSAs are frequently limited in spectrumanalysis abilities. SAs and VSAs both can trigger on the power present in their IF bandwidth but contain no frequencyselective triggers. This can render them incapable of capturing transient signals. Some hybrid SAs/VSAs let users choose between the dynamic range of an SA or the modulation analysis of the VSA.
The RTSA's front-end downconversion and digitization are similar to that of the VSA. The major difference between the VSA and RTSA occurs in the DSP processing after the signal is digitized. The RTSA incorporates some real-time signal-processing components that aren't found in the VSA.