Electronic Design

RF Instruments Must Cope With Massive Market

Test and measurement's greatest challenges lie in the RF spectrum. RF transmissions come from myriad sources, from cell phones to wireless local-area networks to RF identification. All of it permeates the shrinking communications frequency spectrum. And it's going to get worse, as the numbers of cell phones and RFID tags skyrocket.

Designers once characterized the clock frequencies of high-speed logic devices running on pc-board planes as microwave frequencies. But RF systems now operate at gigabit clock rates. These systems' signals, which need to be detected, measured, and verified, are very difficult to accurately analyze and quantify.

Also, designers must measure the electromagnetic interference (EMI) of today's multitasking cell phones. Electromagnetic compatibility (EMC) is a key design issue. The higher-speed applications equipment will be required to support multiple communications standards like 3GPP and 3GPP2. To that end, evolving RF test techniques should meet digital video broadcast-handheld (DVB-H) and digital video broadcast-terminal (DVB-T) standards.

Design engineers are more aware than ever about EMC, spurred on by regulatory requirements in the U.S. and elsewhere. T&M companies often develop their own compliance modeling tools to ensure that their products will be accepted in the market.

T&M instrument manufacturers work hard to satisfy modern RF testing needs with sophisticated test gear, both hardware and software. But the RF arena suffers from a dearth of up-to-the-task T&M equipment. T&M equipment manufacturers are making progress, albeit slowly.

Time-and frequency-domain measurements have become more intricately linked with the onslaught of RF signals. Digital and RF designers now use oscilloscopes for time-domain measurements (used by many digital designers), as well as spectrum analyzers for frequency-domain measurements (employed by many RF designers) to look at signal integrity and EMI signals. Digital designers are finding out that, due to harmonics, a low-frequency clock with a very fast rise time can cause more EMI problems than a signal with a higher clock frequency.

Wideband high-performance oscilloscopes are proliferating these days. Last year, LeCroy released the WaveExpert 9000 and the SDA 100G. Both devices offer 100-GHz bandwidths.

Spectrum analyzers aren't skipping a beat either. Tektronix's 80-GHz RSA 3408A real-time spectrum analyzer provides 36-MHz real-time triggering, improving RF measurement resolutions by a factor of 2000. Several powerful tools let users view signal characteristics in time, frequency, and modulation domains.

By seamlessly capturing signals into memory, and by using DSP techniques to process the sampled data, the RSA 3408A shows how RF signals change over time via a spectrogram display (Fig. 1). This graphical representation lets engineers measure bursted, transient, and frequency-hopping signals.

Low-cost RF T&M instruments also debuted last year. Signal Forge's SF800 high-performance digitally synthesized signal generator costs less than $1000. In 1-Hz increments, it combines a 1-GHz range with ac-coupled, differential, and digital outputs, suiting it for digital-and RF-system testing.

One of last year's biggest low-cost surprises was Agilent Technologies' high-performance CSA N1996A spectrum analyzer. The 3-GHz version costs $8950, and the 6-GHz version runs a few thousand more. Rohde & Schwarz showed off 3- and 6-GHz versions of its FSL303/306 RF spectrum analyzers at slightly higher prices. These developments represent the lowest prices yet for high-performance spectrum analyzers.

PC-based plug-in cards also are coming to the rescue, advancing the cause of virtual instrumentation. Engineers can use National Instruments' 26.5-GHz PXI plug-in switch modules, the 5660 2.7-GHz RF vector signal analyzers, and the Spectral Measurements Toolkit for LabView and LabWindows/CVI to develop PXI-based RF and microwave test applications.

Last year, Acqiris introduced 10-bit, 4-Gsample/s, 3U PXI digitizers with input bandwidths of up to 3 GHz. The dual-channel DC152 and single-channel DC122 digitizers target synthetic instrumentation systems, replacing standard digital multimeters, oscilloscopes, power meters, and frequency counters in RF and microwave test systems.

RFsystem designers often build their own makeshift testing systems, relying on synthetic instrumentation systems based largely on software. A number of leading T&M companies advocate a synthetic instrument approach, including Aeroflex, Agilent Technologies, DRS Technologies, Honeywell, Phase Matrix, and Teradyne.

National Instruments, a proponent of the virtual instruments approach, offers software and hardware products that conform to this concept (Fig. 2). The concept is particularly valid for production-level automatic-test-equipment (ATE) systems in military/aerospace, automotive, and communications applications.

Adding to the complexity, new wireless devices enter the market each day, calling for equipment that can effectively monitor a crowded communications field. This would help ensure that minimal or no interference occurs between the millions of mobile handsets out there.

The proposed mobile equipment identifier (MEID) standard underscores the industry's concern for the crowded RF spectrum. The Telecommunications Industry Association (TIA) calls this standard the IS2000 Release D specification.

For about two decades, the electronic serial number (ESN) standard mandated by the U.S. Federal Communications Commission was sufficient enough for T&M equipment to track and identify all mobile handsets. Now, the ESN standard is running out of room. While adoption of the proposed MEID standard is years away, a pseudo-ESN (pESN) is being developed as a stopgap measure. But even this effort has yet to be finalized.

The advent of system-on-a-chip (SoC) devices highlights the need to effectively test a wide range of circuit types, including analog circuits, high-speed interfaces, embedded memories, mixed-signal circuits, and RF circuits. Older pass/fail criteria no longer suffice using production ATE systems.

Now emerging are failure modes that can cause variations in signal delay, crosstalk among signals, and spurious transients, among many other faults. Testing such failure modes becomes difficult to define and gets more exacerbating as frequencies rise into the RF range. As a result, there's a lot of pressure on ATE system manufacturers to constrain the costs of their platforms while offering more testing capabilities.

RF ATE system vendors follow an open-architecture approach, where a basic ATE platform can be customized with add-on features. This enables more cost-effective testing.

For more, see "Test For RF Trends" at www.elecdesign.com, Drill Deeper 11766.

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