Take A Peek Inside Today's Spectrum Analyzers

Sept. 15, 2005
Depending on what kind of signals you're examining, a swept-tuned, vector, or real-time spectrum analyzer may be the optimum choice.

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.

Unlike the VSA, the RTSA converts the time-sampled data into the frequency domain in real time, prior to signal capture (Fig. 2c). This permits a pre-analysis of the signal spectrum before triggering a capture to memory. The instrument previews the signal and triggers only on spectral events of interest. A frequencymask trigger (FMT) enables the analyzer to detect and trigger on signals well below the largest signal level in the spectrum (Fig. 3). FMT technology goes beyond simple IF level triggering and provides a reliable way to look at intermittent RF signals embedded in complex spectrums.

Not long ago, the cost for such superfast real-time digital signal processing was prohibitive for most engineers. The computational speed needed to execute a 1024-point FFT in less than 12μs, before the next data frame is ready, requires a respectable amount of DSP horsepower. Advances in DSP technology and clever use of a combination of real-time FFTs for triggering with post-processing FFTs for measurements have enabled RTSA capability that fits even a modest budget.

Also, the modern single-box portable RTSA is much smaller than early models that occupied an entire equipment rack. Another unique RTSA feature is a full-bandwidth, streaming digital I-Q data-output port. The real-time spectrum analyzer is optimized for transient RF signals and has extensive time-correlated multidomain analysis capability.

Internally, the SA, VSA, and RTSA all feature special characteristics that enhance their performance for different applications. So which spectrum analyzer best fits each demanding application?

The swept-tuned spectrum analyzer is optimized for analog continuous signal measurements. The high dynamic range available on upper models is great for applications like testing linearity in a multicarrier power amplifier (PA).

The cost advantages of multicarrier PA designs have made them very popular in the cellular industry. But 3G multicarrier PAs require exceptional linearity to prevent unwanted intermodulation products from interfering with adjacent-channel signals. Testing these extraordinary amplifiers requires leading-edge analyzer dynamic range (Fig. 4). This prevents distortion within the spectrum analyzer itself, masking the amplifier's linearity characteristics during intermodulation and adjacent-channel power (ACP) measurements.

The swept-tuned analyzer also excels at integrated phase-noise measurements. Sensitive applications such as highcapacity point-to-point telecom radios have particularly difficult phase-noise requirements. The complex high-order modulations used by these radios demand an analyzer with superior phasenoise performance. High-end, swepttuned spectrum analyzers offer the best phase-noise performance. Most swept analyzers also come with optional phasenoise integration software not commonly found on other analyzers.

Like the swept-tuned spectrum analyzer, the VSA is optimized for some unique applications. For example, it dominates coherent two-channel measurements. At present, the VSA is the only analyzer that can perform cross-spectrum measurements between two channels. This capability allows for precise phase measurement and time correlation of pulses. The dual-channel VSA can make network-analyzer-like measurements with an arbitrary signal—even a noise spectrum. Crosschannel spectrum measurements are useful for phased arrays, radar, and interferometer applications.

Similar to the SA and VSA, the realtime spectrum analyzer is optimized for transient RF signals. The RTSA is preferred for signals that switch on and off rapidly or change frequency rapidly. The cost and integration advantages of the transmit/receive (T/R) switch used for time-division duplexing (TDD) have made transient RF signals like those used in WLAN, 3G, and many RFID devices quite popular. With its precapture spectral analysis and FMT, the RTSA is ideal for intermittent signal diagnostics.

The RTSA's FMT can reliably capture spectral emissions from an RFID tag that responds intermittently. Many existing proprietary anticollision schemes make it possible for an RFID tag interrogator to communicate with one tag without interference from other tags. Testing the tag's responses can be difficult in complex spectral environments. The RTSA's frequency mask trigger routinely captures asynchronous signals in dense RFID signal environments.

The RTSA's concentration on today's complex TDD modulations has given it a particularly rich multidomain analysis package. In RFID work, power-efficient modulations like frequency-shift keying (FSK) are often used. The RTSA tends to be better equipped for analysis of these signals, offering features such as built-in symbol decoding for FSK and amplitudeshift keying (ASK) modulations, while other analyzers offer only analog waveform measurements (Fig. 5).

The RTSA technology supported WLAN signals very early. Consequently, it offers a well-developed set of WLAN measurements. Advanced features like autodetection of complementary code keying (CCK) or orthogonal frequency-division multiplexing (OFDM) modulations are found only on the RTSA. Place the marker on a signal burst, and the RTSA figures out the appropriate WLAN demodulation settings (Fig. 6).

Real-time analyzer software is designed for time-correlated multidomain analysis. This means a marker on a power-versus-time display can be made to time-correlate precisely with markers on the spectrogram and constellation displays. Timecorrelated markers improve diagnostic reliability by providing positive correlation between anomalies in different displays.

Phase-hits captured with the FMT can be positively time-correlated to symbol errors. The RTSA has made useful contributions in diagnosing challenging microphonic problems with its time-correlated markers and FMT. The RTSA technology also has found many successful applications with continuous signals that rapidly vary in frequency. The FMT easily captures phase-locked-loop synthesizer tuning transients.

Also, the RTSA offers captured and real-time I-Q data export capability. Real-time I-Q export is of particular interest to software-demodulator developers. Using a special port, today's RTSA can deliver I-Q data pairs to an external FPGA at bandwidths up to 36 MHz. So, software developers can construct and debug demodulators with reliable RF hardware, greatly speeding up the development process.

The RTSA is especially useful as a front-end digitizer for surveillance operations or as a test receiver for modulation research. Another special feature of the RTSA, from its surveillance roots, is a removable hard-drive for use in secure areas.

BASIC MEASUREMENTS Each of the three spectrum-analyzer types has unique abilities. Yet a great deal of capability overlap exists between them for basic measurements. Swept analyzers, VSAs, and RTSAs all compete directly with less demanding basic spectral measurements. VSAs and RTSAs have basic SA modes that emulate a traditional swept-tuned spectrum analyzer. Swept analyzers have made progress in offering high-performance vector signal analysis.

So, what considerations matter in picking the instrument most likely to get the best basic measurement data on the bench? When the special instrument capabilities discussed earlier aren't required, the goal is usually to provide the quickest diagnostic time-to-insight for the engineer. The faster a problem can be recognized, the shorter the engineering time required.

Human factors such as portability, size, weight, logical menu navigation, and setup time tend to be important for rapid time-to-insight with basic measurements. The industry has found that display versatility is a key ingredient in rapid and reliable troubleshooting. Multidomain instruments like the RTSA or VSA with spectrum, time-domain, modulation-domain, code-domain, and spectrogram views are more apt to provide conclusive diagnostics than older single-display swept analyzers.

The swept SA industry has raced to provide insight-enhancing displays on new models that go beyond the basic spectral information. Implementations range from clean lightweight portables to bulky multibox solutions. When the choice isn't so easy, designers might wish to consider the specialization of each analyzer.

INVENTORY MANAGING A good rule of thumb when selecting an analyzer that gets the best diagnostic insight for basic measurements is to consider the type of signal being measured.

The swept SA usually can be an effective tool for spectral measurements on pure analog signals, like an LO or analog modulation. The VSA may be useful in continuous digital modulations or twochannel measurements. And, the RTSA is usually preferable in intermittent or transient RF-signal applications. In recent years, virtually all of the most popular new wireless systems have taken on a transient nature, making the highest-productivity instrument the RTSA.

Unfortunately, many established RF companies find their spectrum-analyzer inventory loaded with older swept-tuned analyzers. In many cases, the older SAs' limited capability with today's popular transient RF signals warrants the addition of some RTSAs to rebalance the test equipment pool. Frequently, companies that want to participate in the explosive growth of WLAN, RFID, 3G, and other TDD systems find it necessary to retool their test resources to remain competitive. Each of the three basic types of spectrum analyzers was created and optimized to solve different problems. To get the most out of any spectrum analyzer, consider what type of signal will be analyzed. Understanding the key differences between today's spectrum analyzers lets designers match the analyzer type to signal characteristics, improving diagnostic efficiency.

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