Bluetooth RF test builds, slowly

When the Bluetooth wireless communications link was introduced, it was promoted as a seamless way to connect electronic devices. Ambiguities in the spec, however, gave rise to interoperability problems. The Bluetooth Special Interest Group (SIG; www.bluetooth.com) addressed these problems by tightening the specs and establishing a qualification procedure, but RF test remains a challenge because dedicated Bluetooth RF test equipment has been slow to emerge.

Tests of Bluetooth devices can be separated into two layers: radio and other. Radio testing covers the physical performance of receivers and transmitters, while the other testing covers system behaviors such as communication protocols and link management. The Bluetooth SIG has qualified nearly 50 test instruments that can correctly handle the test requirements, yet only a handful of these perform RF testing.

Part of the challenge is that the Bluetooth RF spec requires the use of a "dirty" transmitter. Angus Robinson, microwave product manager at Anritsu (Morgan Hill, CA; www.anritsu.com), points out that the RF portion of Bluetooth was developed with an eye toward CMOS process implementation, which means that a transmitter can have relatively large variations in its RF performance and still be within spec. It also means that receiver tests must use a signal source that can be impaired in a controlled manner, such as by inserting frequency drift. Implementing impairments in generic RF test instruments, which are typically designed to produce high-quality signals, can be difficult.

A few systems have appeared that can generate the necessary signals. Agilent Technologies (Palo Alto, CA; www.agilent.com) introduced its GPIB-based E1852B test set in late 2001 and added test management software a year later. CETECOM (Munich, Germany; www.cetecom.com) adapted its BITE RF conformance tester to handle Bluetooth, and Rohde & Schwarz (Munich, Germany; www.rohde-schwarz.com) released its validated TS8960 tester in 2002. In November 2003, the Anritsu MT8852A received test system validation from the SIG.

Unfortunately, most of these systems are large, expensive, rack-mounted configurations used in Bluetooth Qualification Test Facilities. Only the Agilent and Anritsu devices are small and inexpensive enough for easy use on a manufacturing floor.

The differences between the two sizes of test systems stem from the number of tests each can handle. The full Bluetooth RF layer test spec includes 15 tests of transmitter and receiver parameters. Transmitter tests include spectrum, modulation, and drift. Receiver tests include sensitivity, carrier-to-interferer performance, and intermodulation performance. The large systems perform all 15 tests, which are needed for design certification.

The smaller testers only provide about half of the tests, focusing on the ones needed most frequently in manufacturing. These testers can help designers perform pre-qualification testing, and they can rapidly measure performance parameters that are production-sensitive. Once a design is validated, these testers help keep production within spec.

With the Bluetooth spec currently at Version 0.91, it is clear that RF-layer testing will continue to evolve. Test equipment manufacturers have anticipated that evolution and are keeping their instruments current with software upgrades. But new instruments are rare. Part of the reason may be that Bluetooth production is still relatively small. With interoperability problems being resolved, however, Bluetooth manufacturing will finally begin to ramp, and more tools may follow.