On Your Marks for Testing Bluetooth

The imminent market explosion of Bluetooth wireless interfaces on products signals an RF measurement first for many test engineers.

Angus Robinson, Anritsu, Stevenage, UK -- Test & Measurement World, 9/1/2000

The Bluetooth wireless standard is coming into its own, and hundreds of millions of Bluetooth-enabled products will ship by the end of 2002. The Bluetooth technology will be self-contained within many products; for others, it will be an addition in the form of a PC Card that plugs into a mobile device or a dongle that plugs in to a desktop system’s RS-232 or parallel-printer port. As Bluetooth becomes ubiquitous, you’ll find yourself having to test Bluetooth devices at the protocol-stack and RF levels.

A complete Bluetooth module comprises a radio transceiver, a baseband link controller, and a link manager (Fig. 1). The baseband link controller connects the radio hardware to the baseband-processing and physical protocol layer. The link manager performs high-level protocol activities such as link setup, authentication, and configuration. Application layer software sits above the link manager.

A Bluetooth implementation can operate as a single-chip (integrating both radio and protocol stack) or multi-chip (separate radio and protocol ICs) design. Cellular-phone manufacturers may wish to combine the Bluetooth baseband function in the same device as the GSM baseband function, and therefore will want a separate Bluetooth radio. A digital camera manufacturer is more likely to select a single-chip design to simplify assembly.

RF, Protocol, & Profiles
Three aspects of a Bluetooth module need testing: RF, protocol, and profiles. You can perform many of the RF measurements with standard test instruments such as spectrum analyzers with vector demodulation, transmitter analyzers, power meters, digital-signal generators, and bit-error-rate testers (BERTs).

Some of the measurements, however, require the radio to form a standard Bluetooth radio link connection with the test instrument and for the test instrument to have some degree of control over the equipment under test (Fig. 2). For these tests, the test system must be able to support the Bluetooth protocol to make the link. Consequently, you can expect instrument vendors to develop new test instruments similar to the integrated radio test sets for digital cellular radio.

For protocol tests, you can use protocol sniffers that monitor and display data moving between Bluetooth devices. You can also use products such as the Ericsson Bluetooth Development Kit (EBDK).¹ Ericsson will soon release a version of the EBDK, known as a Blue Unit, that will include software for use during early qualification testing. Eventually, some companies should develop complete reference test systems.

Figure 1. A complete Bluetooth module comprises a radio transceiver, a baseband link controller, and a link manager.

Figure 2. Some Bluetooth tests require the test instrument to support the Bluetooth protocol in order to form a standard Bluetooth radio link connection.

For example, the LAN access profile defines a data connection between a data terminal (DT) and a LAN access point (LAP). The profile defines the following services and connection states for the application layer: initialization of LAN access service, shutdown of LAN access service, establishment of LAN connection, loss of LAN connection, and disconnection of LAN connection.

Until a reference test system is available, early Bluetooth profile testing will require product-to-product interoperability testing. To facilitate this, a series of “Unplugfests” have been arranged by the Bluetooth Special Interest Group (www.bluetooth.com). At these meetings, companies with functional products can test product interoperability against products from other suppliers.

RF Measurements
The Bluetooth radio specification states the required performance of the radio and the test that must be made to confirm conformance (Table 1). To measure the performance of a Bluetooth module, the test instrumentation must be able to establish a Bluetooth link with the equipment under test (EUT). It can then put the EUT into test mode. Test mode is a mandatory feature of a Bluetooth module in which the EUT can enter a loopback mode or can disable frequency hopping (for example, for making BER measurements). A Bluetooth test system should also be able to disable hopping, set specific frequencies for tests, and control the transmit power level.

During design and development, you’ll want to test the radio in isolation from the protocol stack that controls it. In these cases, you need to be able to control the radio so you can set frequencies and levels to make raw transceiver measurements. You can then feed the radio output directly into a spectrum analyzer with vector signal analysis or into a transmitter analyzer and power meter to make the measurements (Fig. 3). The measurements you can make will vary according to the control that the radio IC manufacturer provides over the IC connectors. The radio will have inputs controlling data in and out, TX on, clocks, and supply voltage. Without a protocol stack, the radio may not function at all.

Figure 3. In development, you make raw transceiver measurements of a Bluetooth radio IC in isolation from the protocol stack.

Single Chip Measurements
There is no standard for the interface between a protocol stack and a radio, typically known as the radio interface. You must use a test fixture to hold the radio IC and provide it with some baseband control to give Bluetooth functionality. This arrangement lets you then test radio ICs with a Bluetooth link and baseband control.

The connection between the radio and the test instrument will vary depending upon the Bluetooth device implementation. Some radios or integrated modules have printed antennas as part of the design. In this case, it may only be possible to make a connection over the air to an antenna on the test instrument input. If you use this approach, then you must characterize the path loss for each of the 79 Bluetooth frequencies. If the radio IC has an RF output connector, a direct connection to the test instrument simplifies calibrated power and sensitivity measurements. Even if you use a direct connection, though, you should measure and correct for the path loss at each frequency.

Testing OEM Bluetooth Products
OEMs buying commercial Bluetooth chip sets will still need to test. Inevitably, packaging will influence the performance of a finished product because of the positioning of the antenna as well as other internal electronics. In a mobile phone, the other electronics will, by definition, include an interfering radio transceiver. Similarly, PCs have high-speed clocks or noisy buses that could degrade module sensitivity.

In the OEM production environment, the tests will have to validate performance in the shortest possible time. Production engineers will need to select the subset of the conformance test specifications that are appropriate for their products’ requirements.

To confirm that the device will operate over the Bluetooth specification’s 10-m range, engineers will still need to measure sensitivity and power levels. The conformance specification requires receiver sensitivity to be measured as a BER of more than 1,600,000 bits at three frequencies. This test alone would take at least 25 s using standard single-slot (DH1) packets and so, in practice, the test will measure fewer bits even at a reduced number of frequencies.

In addition to RF measurements, OEMs should perform some form of functional test. In the case of a
Bluetooth-enabled digital camera, for example, this could be sending an instruction over the Bluetooth interface to activate a shutter release with flash. This command would need to be created at a high level in the protocol stack (typically the application level) and packaged by the test equipment into the Bluetooth format. Validating the camera’s response to a high-level command would give the manufacturer confidence that the interface was functioning correctly, although it is not necessarily a guarantee of robust performance. T&MW

FOOTNOTE
1. Available from Symbionics. www.symbionics.co.uk/solutions/bluetooth/Bluetoothkit.shtml.