Ultra-wideband (Uwb) communications is seen by many as an enabling technology for many short-range applications. Low signal-to-noise transmissions enable it to avoid interference with co-located wireless signals. Specifically, UWB signals must have low spectral densities just above the thermal noise floor. To achieve this goal, the UWB transmitter power is restricted to low levels and spread over wide bandwidths. UWB signals also have redundancy built into them, for interference immunity to strong narrowband signals as well as impressive multipath capabilities. Tektronix delves into these details and explains their impact on performance and test in the 17-page application note, "Ultra- Wideband Technology and Test Solutions."
Several different approaches exist for generating UWB signals. Three of the more popular methods for modulating this signal include: Time Hop UWB (TH-UWB), Direct Sequence UWB (DS-UWB), and Multi-Band Orthogonal Frequency Division Multiplexing UWB (MBOFDM). The WiMedia Alliance (www.wimedia.org) has chosen an MB-OFDM signal as its high-speed multimedia UWB data-link standard. That signal comprises an OFDM modulation with 128 carriers using either quadrature phase shift keying (QPSK) or dual-carrier modulation (DCM) on each carrier. With this format, at least eight data rates can range up to 480 Mb/s.
To generate and analyze ultra-broadband test signals for UWB, a direct digital synthesizer (DDS) or high-performance arbitrary waveform generator is required. In addition, verybroadband digital phosphor oscilloscopes are needed to support the UWB signal's bandwidth requirements. Most typical laboratory signal generators can only generate a few tens or hundreds of megahertz of bandwidth, which falls far short of the 1.5 GHz of bandwidth needed for most UWB signals. UWB pulses also can be distorted by the spectral amplitude and phase variations of both the test signal generator and signal-analysis instruments. Such pulse distortion effects will alter the spectral properties of UWB signals. The limited number of measurement bandwidth options that are available also pose a problemas do the time frequency codes (TFCs) that spread the UWB signal.
The note cautions that it can be problematic to use the UWB's own system software to generate test signals. It may not be functioning properly or be able to add impairments. Rather, engineers should use a known software tool that can reliably synthesize both general-purpose and standards-based signals with or without impairments. The note closes by explaining other major test concerns and ways of resolving them. The note helps define UWB in its various forms and how to test it.
Tektronix, inc., 14200 Sw Karl braun dr., p.o. box 500, beaverton, or 97077; (800) 835-9433, internet: www.tektronix.com.