Wideband Passive Components Pose Measurement Challenges

Wideband wireless technologies are going through a tremendous growth period. At the same time, frequency spectrum has opened for unlicensed low-power wireless technologies and there have been significant advances in imaging radars. As a result, the need for spectrum bandwidth for radio and radar systems has increased substantially. When engineers try to test and select the components for these new technologies, however, several challenges present themselves.

An example is commercial Ultra-Wideband (UWB) radio technology, which is just starting to proliferate in the personal-area-network (PAN) environment. With Wireless Universal Serial Bus (WUSB) technology commercially available, other wireless data and video standards are soon to follow. Examples include TransferJet and wireless-video applications. The promise of being able to conveniently upload and download high-bandwidth multimedia content will make these technologies abundantly available if they can provide good value and an enjoyable user experience. Currently, these radio technologies operate at bandwidths greater than 500 MHz with several gigahertz planned for future applications.

As for imaging radar, this technology is driven by synthetic- aperture-radar (SAR) and inverse-SAR (ISAR) innovations. The most common implementations of wideband radar employ chirp pulses, which simplify the processing required to determine the delay of an echo. By definition, the delay of an echo is the time it takes for a signal to make a round trip from the transmitter to the target and back again to the receiver. Improvements in imaging resolution are directly proportional to the width of the chirp pulse. It is common for chirp radars to have bandwidths greater than 1 GHz.

Both wireless radio and radar applications rely on the right component choice. Often, the selection of passive components for wideband radio systems can entail the assessment of system-level performance and quality measurements at the component level. For radios, it might be important to understand the error-vector-magnitude (EVM) contribution of filters, up/downconverters, in-phase/quadrature (IQ) modulators, and other passive components that might be used in the transceiver chain. The EVM is a qualitative assessment of a signal's magnitude, phase, and frequency errors relative to an ideal signal.

For the radar components used in chirp pulse applications, it is extremely important to minimize group-delay fluctuations. Variations in delay can lead to errors in target ranging as well as complex radar functions like imaging and active jamming. To test the qualitative performance of passive components, modern trends for wideband assessment are moving from the frequency-domain approach with network analysis to a time-domain assessment that incorporates a stimulus-response representative of the end product. Such representatives comprise complex modulation signals for radios and chirp pulses for radars. The time-domain assessment provides the ability to see all of the spectral components and nonlinearities that might be missed by the continuous-wave (CW) stimulus response of network analyzers.

To address the different stimulus requirements of these wideband applications, signal generators are needed that can instantaneously create the wideband complex modulation waveforms and chirp pulse patterns (Fig. 1). These test patterns may be imported from common digital-signal-processing (DSP) tools or synthesized directly. Alternatively, they may be the playback of a previously captured waveform. Either way, flexible signal-generation tools might need to generate the baseband (IQ), intermediate-frequency (IF), or RF signal directly.

When testing passive components, waveforms might not need the necessary coding and bit patterns of the end device. It is often sufficient to have a representative spectrally correct signal with the proper time-variant statistics. The added flexibility to replay captured waveforms can enable the pre-emphasis or correction of non-ideal responses in test setups or device response.

The analysis of wideband radio and radar performance requires the real-time acquisition bandwidth to exceed the occupied bandwidth of the signal of interest. This capability allows spectral-regrowth products and other nonlinearities to be measured and correlated to the stimulus signal. Complex modulation analysis, which is often found in vector signal analyzers, will allow radios to be measured in a qualitative manner (Fig. 2).

Pulse measurements also are important for evaluating chirp-pulse quality. Most modern oscilloscopes have built-in pulse measurements. However, the type of pulse measurements for envelope-modulated radar signals are quite different than the built-in functions for measuring the rise time and pulse parameters of RF pulse carriers. As a result, the built-in oscilloscope measurements will not work for RF-pulse-carrier measurements.

Tektronix, Inc., 14200 SW Karl Braun Dr., Beaverton, OR 97077; (800) 835-9433, Internet: www.tektronix.com, e-mail: [email protected].

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