Short bursts make for tough tests

With the rising interest in wireless connectivity, the question of bandwidth use becomes increasingly important. Proponents of pulse radio have an answer to the problem. Their technique, most commonly referred to as ultra-wideband technology, uses a lot of bandwidth, but only for a little while.

Ultra-wideband (UWB) technology seeks to generate a signal that supports high data rates without shouldering anyone out of its target frequency band. The trick is to use extremely short pulses, a few cycles long at most, along with a position modulation scheme to carry the data. Fourier analysis reveals that these short pulses yield a broad-spectrum signal with very little energy in any given frequency. That keeps them from interfering with more conventional RF signaling schemes that use the same frequency channels.

In February 2002, the Federal Communications Commission (FCC) (http://www.fcc.gov) gave the go-ahead for commercial deployment of UWB. Several companies, including XtremeSpectrum (http://www.xtremespectrum.com), Multispectral Solutions (http://www.multispectral.com), and Time Domain Corp. (http://www.timedomaincorp.com), have developed products for using the technology, including chipsets, signal generators, and complete systems. This is paving the way for more widespread adoption of the technology.

Test engineers working with UWB systems will find they differ from conventional RF systems, according to Bob Cutler, a senior member of the technical staff at Agilent Technologies (http://www.agilent.com). He notes that phase noise, for instance, is a common measurement in RF systems that UWB systems don't need. In contrast, UWB systems need jitter measurements that RF systems don't. Cutler says these differences arise because UWB systems use time separation, not frequency differences, to establish channels.

The FCC's approval of UWB for communications systems allows the systems to be used in the 3.1 GHz to 10.7 GHz band and also specifies that their bandwidth be no less than 20% of the center frequency. This combination of high frequency and wide bandwidth pushes test tools to their limits. For example, the interference tests that the FCC mandates call for a spectral analysis of peak power using a 50-MHz resolution bandwidth.

That's out of reach for all but one commercially available analyzer, according to Eric Boll, senior applications engineer at XtremeSpectrum. The FCC does provide an algorithm for calculating the power spectrum using a narrower resolution bandwidth, one more commonly available. Boll cautions, however, that the method makes assumptions about the type of signal being measured, assumptions that may not be valid for every system.

Spectrum analysis alone is not enough for testing a UWB system, notes Boll, especially during system development. Test engineers will also need a digital sampling oscilloscope with at least a 6-GHz bandwidth. Instruments such as the Tektronix TDS6604 and the Agilent 86100 are suitable for the task, Boll says.

Agilent's Cutler cautions, however, that at the upper range of the approved band, even a 20-Gsamples/s instrument such as his company's 86100 is hard pressed to capture a single pulse. Cutler recommends that a test mode providing repetitive signals be included in the design.

Pulse shapes and system bandwidth aside, test engineers must also show that a UWB communications system is transmitting data correctly. Cutler notes that the kinds of tools used for 40-GHz optical systems are also applicable to UWB communications. Boll adds that companies such as XtremeSpectrum offer test suites that help validate end-to-end communications.