Testing Ultrawideband Part 1: learn more about what's involved in testing UWB devices to the applicable regulations

EE-Evaluation Engineering, May, 2005 by Roland Gubisch

Ultrawideband (UWB) technology provides low-cost communications and measurement capabilities for a number of applications including short-range data exchange in excess of 100 Mb/s, imaging through walls and into the ground, and automotive speed and distance reporting and control.

The technology for generating UWB signals generally is simple and inexpensive. Short electrical pulses on the order of picoseconds or nanoseconds in duration are directed through band filtering to an antenna. The narrow pulses, which may be modulated in time position, repetition rate, pattern, or polarity, generate a wide spectrum of emissions whose bandwidth is inversely proportional to the duration of the pulse.

Because UWB transmissions have the potential to interfere with many licensed and unlicensed radio services, restrictions must be placed on the UWB signal power depending on the specific application. In the United States, the FCC has established Sub-part F of Part 15 to govern unlicensed UWB devices. The rules were established in 2002 after careful investigation of interference issues such as multiple UWB transmitters and coexistence with global positioning satellite (GPS) receivers.

The resulting Part 15 emissions limits for UWB devices are equivalent to the Part 15 general emissions limits or decades lower depending on the application and the frequency band (Table 1). Such low emissions limits present a unique challenge for regulatory compliance testing, especially when they are combined with measurement frequencies above 1, 10, or 50 GHz. Nevertheless, the FCC has certified about 40 UWB devices to date, with many more in process.

Jurisdictions outside the United States are reviewing the appropriate regulatory response to the growing popularity of UWB technology. The European Telecommunications Standards Institute (ETSI) has published several UWB reference documents and standards.

The standards include procedures and limits similar to FCC Part 15, but they have not been harmonized to the European Union (EU) Radio and Telecommunication Terminal Equipment (R&TTE) Directive. As a result, the standards are not widely acceptable for demonstrating equipment compliance in the EU. Outside of the United States and the EU, no other UWB standards have been established yet.

UWB Testing

There is no single set of FCC emissions limits for all UWB devices. There are general requirements in section 15.521, which includes the 15.209 limits below 960 MHz and a 0-dBm equivalent isotropically radiated power (EIRP) peak emissions limit for all UWB devices operating above 960 MHz. Otherwise, each category of UWB device has its own emissions limits. The restricted band limits of 15.205 do not apply to UWB devices.

The different categories of UWB transmitters also restrict the use or user of the device. For example, surveillance systems operating under section 15.511 may only be for fixed applications and operated only by law enforcement, fire, or emergency rescue organizations or by manufacturer, petroleum, or power licensees.

Before testing begins, the engineer must be sure that the appropriate set of limits is being applied to the UWB transmitter. An incorrect choice of limits may not be an easy error to correct after testing has been completed. Moving from one UWB device type to another can involve different upper frequency ranges, antennas, and low noise amplifier (LNA) choices.

In addition to 50 MHz for peak EIRP, the FCC has designated three different measurement bandwidths for UWB devices:

* 1 kHz for emissions in the GPS receiver bands.

* 120 kHz for emissions in the 30-MHz to 960-MHz range.

* 1 MHz for emissions above 960 MHz.

The use of dBm EIRP as a limit may be unfamiliar. The FCC provides a formula for converting from field strength at 3 m to dBm EIRP:

dBm EIRP = dB[micro]V/m - 95.2

For other test distances, Figure 1 relates field strength to values in dBm EIRP. Spectrum analyzer readings may be converted between dB[micro]V and dBm using the formula:

dBm = dB[micro]V - 107

Measurement Parameters

Three types of radiated emissions detectors are required for UWB devices:

* Quasipeak Detector: 120-kHz resolution bandwidth (RBW), 30 to 960 MHz. Video bandwidth must be at least as large as the resolution bandwidth, and trace averaging is not used.

* rms Average Detector: 1-MHz RBW, above 960 MHz except in GPS bands where it is [greater than or equal to]1 kHz, 1 ms or less averaging time. Trace averaging is not used. The [less than or equal to]1-ms averaging time is achieved by selecting a sweep time on the spectrum analyzer at least as fast as 1 ms times the number of screen data points or bins.

An rms average detector was selected by the FCC for UWB emissions because it most accurately reflects the interference potential of the signals. Spectrum analyzers are readily available with rms detectors. An alternative to the rms detector is provided in procedure (3) in Appendix F of the UWB First Report and Order (ET Docket No.98-153).

[FIGURE 1 OMITTED]

* Peak Emissions in 50 MHz: This measurement is centered on the frequency at which the highest emissions occur in the UWB band used. Most spectrum analyzers are not capable of a 50-MHz RBW so the FCC has provided a formula for redefining the peak emissions limit of 0-dBm EIRP in terms of the RBW actually used: