The Bluetooth specification is a de facto standard containing the information needed to ensure that diverse devices supporting the technology can communicate with each other worldwide. It includes specifications for the radio, baseband, link manager, transport layer and service discovery protocol. It also specifies interoperability protocols and procedures required for communication with the various types of Bluetooth applications.

When Bluetooth devices are brought within range of one another, they seek each other out and form what is known as a piconet. Piconets consist of a master device, such as a laptop computer, and such slave devices as a printer, personal digital assistant, overhead projector, digital camera and second laptop computer. A master device can actively communicate with up to seven slave devices; at the same time, more than 200 slaves can be registered in a noncommunicating, power-saving mode.

A unique pseudorandom frequency-hopping sequence is established for each piconet, which prevents interference from other nearby piconets and Bluetooth devices. The master device's clock is used to synchronize all other devices in the piconet, and the piconet is defined by its unique hopping sequence.

Like any radio, a Bluetooth radio consists of the host device, transmitter, receiver and baseband. The host runs the application that is sharing information. The transmitter upconverts baseband information to the frequency-modulated carrier, and the receiver downconverts and demodulates the radio frequency signal.

The type of modulation used in a Bluetooth system is two-level Gaussian frequency-shift keying (GFSK), a type of binary frequency-shift keying (2FSK) in which the modulated carrier shifts between two frequencies representing a logical one and a logical zero, respectively. The system uses two different frequencies for the zero and one values of each bit. A base frequency and carrier deviation are used, with the sum of the two frequencies forming a logical one and the difference forming a logical zero. The receiver measures the deviation of the signal with respect to the reference frequency to determine which bit value was transmitted.

Because each value is present for only a short period, system-timing synchronization is critical if the receiver is to accurately determine when each bit is transmitted, and the deviation of the signal must fit within the band allocated to it, which for Bluetooth is a 1-MHz window. Radio-frequency test equipment such as spectrum analyzers and vector signal analyzers are useful for making spectrogram measurements and for testing such phenomena as carrier frequency drift, burst profile and frequency modulation characteristics.

Much information has been published on performing high-frequency measurements of Bluetooth radio components. Equally important for the integration of a Bluetooth chip set is the verification and measurement of the kilohertz- and megahertz-ranged signals present in the digital baseband (the functional component of a Bluetooth device in which incoming data from the radio component is slowed and signals are processed).

Oscilloscopes and logic analyzers are the tools of choice for time-domain analysis of electrical signals. In its most simple operation, an oscilloscope measures the changes in electrical signals and displays an on-screen plot of voltage vs. time, where the vertical (Y) axis represents voltage and the horizontal (X) axis represents time. The intensity of the brightness of the display is often considered a third (Z) axis. Oscilloscopes are typically used for verifying waveform shapes; measuring voltages and frequencies; calculating signal transition times; detecting overshoot, ringing and other noise; and general-purpose debugging of electrical system behavior.

Logic analyzers, like oscilloscopes, provide time-domain analysis of electrical signals. One major difference between logic analyzers and oscilloscopes is that logic analyzers use only a 1-bit encoding of the signal value, storing only whether the signal value is high or low (above or below a threshold), while oscilloscopes record a byte or more of data to represent the signal value. Another contrast is the number of channels. While oscilloscopes typically contain two or four input channels, logic analyzers typically support dozens or even hundreds of input channels.

Best of both

A mixed-signal oscilloscope combines all of the capabilities of a digitizing oscilloscope with some of the functionality of a logic analyzer. The user thus can analyze the analog characteristics of electrical signals while simultaneously viewing the logic timing interactions of many digital signals. A mixed-signal oscilloscope can therefore be an optimal tool and invaluable resource for debugging Bluetooth baseband circuitry at the system level.

All Bluetooth signals are transmitted and received through the RF portion of a Bluetooth device. Information is then transferred to the digital baseband for processing via the transmit and receive serial lines.

An oscilloscope can be used for real-time verification of the matching and delayed values of the transmit and receive lines. That starting point in circuit debugging allows an initial verification that communication between the devices is occurring.

Acquisition of the transmitted and received signals of a Bluetooth device is achieved at a slow sweep speed that displays the high-level interaction between the transmit and receive signal lines.

Using deep memory and Agilent's MegaZoom technology, the transmitted data packet can be examined in fine detail without retriggering. The initial signal transitions, which were captured at a timebase setting of 1 mill isecond/division, now can be zoomed to 5 microseconds/division. Often, intermittent anomalies may be captured in a big-picture view, but when the signal is recaptured, the anomaly is no longer present. The ability to trigger one time with deep memory allows a large time capture so that signal behavior can be viewed in detail later.

A packet is a single bundle of information transmitted within a piconet. The packet is transmitted on a frequency hop and nominally covers a single time slot, but may be extended to cover up to five time slots.

The first two pulses correspond to the Bluetooth packet preamble, consisting of a fixed zero-one pattern of four binary symbols. The preamble is the first part of the packet access code. Immediately following the preamble is a pattern of ones and zeros comprising the sync word.

Since the preamble is predefined, the bit size can be determined by inspection. Once the bit size is known, the entire bit stream, encompassing the access code, header and payload, can be identified bit-by-bit. MegaZoom lets designers view the high-level transactions to verify that the operation is correct. Users can pan and zoom through the data packet to verify specific bit-stream data values.

A variation would be to use four channels to probe the transmit and receive signals of both Bluetooth devices simultaneously. That lets designers pan and zoom through the acquisition to view correlated waveform activity between the two Bluetooth devices. The technique is useful, for example, to verify that a synchronous-connection-oriented (SCO) link between Bluetooth applications is functioning. An SCO link is a direct, dedicated bandwidth connection for use with time-critical applications such as voice. The verification can identify connectivity errors when integrating two Bluetooth devices into one system.

Transferring a file between two applications is a common operation performed by Bluetooth devices. The universal asynchronous receiver/transmitter (UART) is one of two host controller interface (HCI) transport layers specified for serial communication between Bluetooth devices.

Say the master and slave hosts are both laptops. As the master transmits a text file piecewise in HCI packets, the UART interface is not able to keep pace with the transmit and receive lines. At a slow sweep speed, a segment of UART inactivity is seen. The master continues to resend data packets until the UART catches up.

By definition, the length of each time slot is 625 microseconds. The cursor measurements show a value of 2.40 ms, which corresponds to the master's occupying four time slots. The high-level screen capture capability thus lets the design engineer verify that the constraint is not the Bluetooth device. The graphical image identifies the source of the data bottleneck as the relatively slow UART capabilities of the laptop computer.

Another on-screen measurement would be to measure the amount of time needed for information to transfer from the sending application to the receiving application, in order to estimate and benchmark overall system performance.

Using MegaZoom technology, engineers can zoom in on the packet transfer to the bit-stream level and verify actual file data. With one long-time segment captured in scope memory, each transition sequence of ones and zeroes can be matched with the binary values of an ASCII table to identify the transmission of each alphanumeric file character.

Reversing the file transfer direction can be accomplished by using the zoomed image of the slave transmitting a file and the master receiving the file. Because a device in slave mode only responds when contacted by the master device, the master transmits null packets continuously, allowing the slave to respond. From the timebase range on screen, it is possible to observe the slave transmitting the file data in time-slot packets.

Bluetooth is a powerful and versatile technology that allows instant wireless connectivity among personal information devices. With Bluetooth chip sets' being rapidly integrated into consumer and wireless devices, the functions and measurements illustrated here will result in accurate information for the design engineer.

By Mike Hertz, Applications Engineer, Agilent Technologies, Northville, Mich., Dudi Shmueli, Bluetooth Applications, Engineering Manager, Texas Instruments Inc., San Jose, Calif., Tareq Shahwan, Bluetooth Applications Engineer, Texas Instruments, San Jose, Michael Sharpless, Strategic Programs Manager, Texas Instruments, San Diego

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