Spectrum analyzers respond to digital modulation
Digital RF techniques such as W-CDMA and WiMAX require measurements that classic swept-spectrum analyzers can’t handle.
By Paul G. Schreier, Contributing Technical Editor -- Test & Measurement World, 6/1/2007
Engineers need test tools that can help them implement advanced protocols in the lab and the field. Fortunately, the necessary test instruments—whether you call them signal analyzers or spectrum analyzers—are increasing their capabilities to keep pace with these developments.
“Innovation in radio technology is exploding,” explained Elaine May, director of marketing for the real-time spectrum analyzer product line at Tektronix. “What radios previously did in the analog domain, they’re now doing in the digital domain. This adoption of digital technology also means that they now benefit from the instrument version of Moore’s Law: In the past, more performance meant a higher price, but today’s instruments are not only getting more powerful, they’re getting less expensive.”
An important trend, agreed almost all manufacturers of spectrum analyzers, is the move toward wider frequency ranges and wider capture bandwidths to accommodate new modulation schemes. Explained Mark Elo, director of business development for RF products at Keithley Instruments, “Ten years ago, GSM was popular and had a 300-kHz bandwidth. Then came W-CDMA with 5 MHz. Now, we’ve got WiMAX and multiple-input multiple-output [MIMO] systems with bandwidths as large as 40 MHz.”
Instruments for modern requirementsFor these more complex communications schemes, traditional swept-tuned spectrum analyzers can’t demodulate signals. Based on a classic architecture from the 1940s, the analyzers use either a local oscillator or a digitally synthesized frequency source to sweep or step through a range of frequencies and then use filters to measure the signal power in particular segments. The results appear as a plot of amplitude vs. frequency.
Thus, in simplified terms, these analyzers are superheterodyne receivers with a display, but they still have an important role. Bryan Harber, product manager for microwave and spectrum analyzers at Aeroflex, commented, “The average cell-site guy isn’t interested in demodulation. He wants a spectrum mask to see if a signal is within bounds.” Added Herbert Schmitt, product manager for spectrum and network analyzers at Rohde & Schwarz, “The swept architecture has a brilliant future. There are tons of applications where users look for spurious signals, harmonics, and phase noise.”
But because of the measurement tasks for which they were designed, these classic analyzers have a small capture bandwidth and can easily miss simultaneous signals, a rapid change, or a momentary pulse that falls outside that bandwidth. This is where a wide capture bandwidth, also called the resolution bandwidth (RBW), is useful along with demodulation information.
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The DPX spectrum display on the Tektronix RSA6100A spectrum analyzer offers a live color view of signal transients in the frequency domain. Courtesy of Tektronix. |
A signal analyzer’s front end looks like that of a swept analyzer, except it digitizes the signal for digital postprocessing. Signal analyzers also have a large RBW to capture a wideband signal and any digital modulation effects at one instant in time. Most models use an internal CPU to run built-in routines or optional firmware “personality packs” that demodulate a specific modulation scheme (such as GSM, 3GPP, TD-SCDMA, and WLAN 802.11a,b,g) and present the results in a meaningful way, such as in a constellation plot or a error-vector magnitude (EVM) plot.
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Agilent’s MXA signal analyzer Model N9020A comes with built-in software for advanced modulation analysis and troubleshooting.
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Depending on the signal under examination, a VSA’s combination of RBW and sweep speed might capture everything of interest. But some newer modulation schemes might have key details that slip between a sweep. Addressing this demand, the real-time spectrum analyzer (RTSA) continuously examines digitized data and transforms it into the frequency or modulation domain.
An early innovator in the RTSA field was Tektronix, although other vendors indicate they are working on similar capabilities. The Tektronix DPX spectrum-processing engine performs 48,000 frequency transforms/s and updates its display 33 times/s to show frequency-domain transients as brief as 24 μs.* Depending on the acquisition bandwidth (from 100 Hz to 100 MHz), the instrument’s memory can store hours of data.
A key feature of the Tek RTSA is the frequency mask trigger (FMT), which can trigger the display if a certain amount of power appears at a certain frequency. Another special feature is the use of color to indicate how often a frequency event has happened during a scan.
Evaluating analyzer specsWhen shopping for a spectrum analyzer, many engineers first look at the frequency coverage. You’ll find various general-purpose units that offer a low end in the kilohertz range and that extend to at least 2.5 GHz.
But given the emergence of broadband modulation, you also should consider RBW. RBW indicates the size of the window that sweeps across the frequency band. It must be wide enough to encompass the signal and its modulation but small enough to resolve narrow effects such as a low-level signal sitting near a carrier. With a given RBW, though, a fast sweep might pass by an unknown digital event before the detection system can respond to it; sweeping too slowly wastes operator time. To accommodate modern modulation techniques, RBW now sometimes exceeds 100 MHz.
A tradeoff with a wide RBW is dynamic range. Here, though, you may find it difficult to compare instrument specs, because dynamic range depends on the input signal level, frequency range, and offset from the carrier. Aeroflex’s Harber said, “The definition of dynamic range is cranky and there’s lots of 'specsmanship.’ It measures the ability to see high and low levels without distortion, and with those last two words you must be very careful.”
Keithley’s Elo elaborated, “Dynamic range has many elements: third-order intercept [TOI], displayed average noise level [DANL], and phase-noise performance. 3GPP includes a standard for adjacent channel performance [ACP] that encompasses those three, and it’s a good quick indicator of performance.”
Engineers need to be careful when working with add-on mixers that extend an instrument’s dynamic range, sometimes beyond 300 GHz, because mixers can lead to a reduction of 30 to 40 dB due to conversion losses.
Other specs to review, said Sai Yang, product manager for high-performance spectrum analyzers at Agilent Technologies, include an instrument’s sensitivity for measuring low-level signals, the amplitude accuracy and the flatness of the frequency response, and the third-order intercept (TOI), which helps users determine whether distortion products are being generated internally.
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The Model 9201 from Willtek includes a high-contrast display readable in direct sunshine. |
Many units now integrate a full PC, providing the benefits of mass storage and connectivity. The Anritsu 2781B even includes Matlab software so you can perform nonstandard analysis on captured data.
A hard disk or CD-ROM is convenient for storing measurements for later analysis. Almost all desktop and portable analyzers have various interfaces including USB, GPIB, or Ethernet. A few units, such as those from Agilent, Keithley, and Rhode & Schwarz, are also starting to incorporate LAN eXtensions for Instrumentation (LXI) capability, which is an alternative to GPIB that uses standard hardware. Further, with LXI, users can operate an instrument over the Web similar to how they use the front-panel interfaces.
Although Agilent helped initiate the LXI Consortium and has more than 60 instruments that support the standard, in the spectrum analysis area only its MXA midrange analyzer is LXI compliant. (The company’s high-end PSA does not yet have LXI, because the unit predates the release of the LXI standard in late 2005.) Anritsu is also on the Consortium but has focused engineering resources on other features and has yet to implement LXI connectivity, but product manager Steve Thomas said it’s on the way.
More features in less spaceNot surprisingly, benchtop units are becoming more powerful and more affordable, but handheld units are also improving greatly. Tom Riedl, Willtek’s product manager for handheld spectrum analyzers, said, “Handhelds are coming up to near-real-time performance in terms of sweep times and display rates. They are incorporating high computational power with low power consumption and frequency ranges that rival those of desktop units. Their displays are getting better in size and contrast—after all, a great frequency range is no good if you can’t use it, and displays such as ours allow viewing in direct sunlight.”
In general, advances in all classes of spectrum analyzers will track new communications methods and standards. Said Tektronix’s May, “In two years, we’ll see different triggering capabilities and higher bandwidth. Designers will be able to see problems they’ve never seen before.” Agilent’s Yang added, “We’ve got the horsepower in the box; now, we have to get software to handle new formats.”
*These figures were reported incorrectly in the original version of this article. Updated July 11, 2007.
