Bluetooth radio designs employ a number of system architectures, from conventional intermediate-frequency (IF)-based systems with analog modulation to digital in-phase/quadrature (I/Q) modulator/demodulator configurations. Currently, various forms of modules are being used. Ultimately, circuit-level integration may be required for the lowest BOM. Regardless of how the Bluetooth design is configured, numerous issues must be addressed, including global regulatory requirements, Bluetooth certification, development of simple, high-yield manufacturing and test procedures, and flawless interoperability with designs from other vendors, some of which may perform at the limits of the Bluetooth specification. This article will examine different features of Bluetooth designs, implications for research-and-development (R&D) tests, and the tools that can make development easier. It will also describe how to performance RF measurements and what types of results should be expected.

Bluetooth devices operate in the industrial-scientific-medical (ISM) band from 2.402 to 2.480 GHz, usually on 79 pseudorandomly chosen channels, spaced 1 MHz apart. There are occasions, such as during the inquiry phase, when a reduced set of channels are used. Figure 1 shows how the spectrum usage changes during this period. Bluetooth devices communicate using a digital frequency-modulation (FM) technique known as 0.5BT Gaussian frequency-shift keying (GFSK). This means the carrier is shifted up nominally 157 kHz to represent a digital 1 value or down to represent a digital 0 value, at a rate of one million symbols (or bits) per second. The "0.5" confines the 3-dB bandwidth of the data filter to 500 kHz, thereby setting a limit to the RF spectrum occupied.

Bluetooth employs time-division-duplex (TDD) communication: the transmitter (Tx) and receiver (Rx) alternate transmissions in separate timeslots, one after the other. In addition, a frequency-hopping scheme, with up to as many 3200 hops/s during the inquiry phase, increases the reliability of a Bluetooth link in relatively crowded RF band. Optimum link performance is essential given that recent US Federal Communications Commission (FCC) rulings anticipate that band usage will almost certainly increase.

Figure 2 shows possible timings for sending and receiving a 366-µs DH1 packet, relative to the 625-µs timeslots. The lower traces indicate a settling-time interval. During this interval, the device must hop to the next channel frequency and the voltage-controlled oscillator (VCO) must settle in time for the packet data to be transmitted or received. The start of the packet is not directly related to the rising edge of the RF burst, as shown by the dotted lines representing possible alternative rising edges. Nor is the rising edge of the burst related to the beginning of the timeslot. After transmission of all the packet data, the design may ramp the power down immediately, or wait until near the end of the timeslot. The exact way the burst is sent may impact other Rx designs and the battery current used.

Bluetooth Receiver Layout
An example Bluetooth Rx layout (Fig. 3) employs one downconversion stage (the orange boxes are areas where parts are omitted or swapped in different designs) and a single local oscillator (LO). The output of the LO is frequency doubled and switched between the receive and transmit functions. The use of FSK allows simple direct modulation of the VCO. Baseband data is passed through a Gaussian filter, which is characterized by a constant time delay and no overshoot. Pulse shaping is applied only to the Tx. Either a sample-and-hold (S/H) circuit or a phase modulator can be employed to override attempts by the phase-locked loop (PLL) to strip off phase modulation within its bandwidth. Often the IF will be quite high, to limit the physical size of the filter components and to make sure that the IF is spaced far enough from the LO frequency for proper image rejection. Antenna switching is used when the transmit level would otherwise be high enough to overloading the Rx input. Occasionally separate transmit and receive antennas are used. The reference oscillator is usually included in most combined RF/Baseband modules.

An output amplifier is optional in Bluetooth systems, but if included it will be employed to boost the power required for Class 1 (+20-dBm) versions. The amplifier's level accuracy specification is not demanding, but some attention to power output is needed to avoid excessive power output and to minimize battery drain. Regardless of whether the design delivers +20 dBm or less, the Rx must be ready to provide received-signal-strength-indication (RSSI) information, so that devices from different power classes can operate. To equalize the link budget with a non-class 1 device, it may be necessary to employ an Rx that is more sensitive than required by the basic specification. Power ramping in a design such as this can be readily achieved by controlling the amplifier bias currents, but care needs to be taken with the modulation applied during ramping, otherwise unwanted spectral components can be generated.