Bluetooth is widely predicted to become the ubiquitous wireless connection that links every type of electronic device-from mobile phones to PCs to personal digital assistants (PDAs) to vending machines. Where will it end? Incumbent players in these markets that are not savvy in RF or Bluetooth technology will have to make some changes if they want the advantages Bluetooth wireless connectivity can bring to their product offerings. Today's solutions provider must offer a complete Bluetooth module that portable equipment makers can implement without knowing the black magic of RF.
Successful Bluetooth products offer superior radio performance, low current consumption, a small form factor, ease of use and, of course, low cost. These requirements are being individually addressed in many ways in the Bluetooth market. But higher levels of silicon integration, clever module design and the use of advanced substrate materials such as LTCC are all being employed as well, adding to the difficulty in meeting the diverse market demand for Bluetooth wireless technology.
However, an integrated radio modem IC represents innovations in design, process and packaging for Bluetooth communications. The radio modem IC, combined with separate digital chips-integrated RF logic and a Bluetooth protocol stack-can provide a low-cost, low-power solution for a wide variety of Bluetooth applications. The chip is a fully integrated 2.4-GHz radio transceiver with a Gaussian frequency-shift keying (GFSK) modem contained on a single chip. The separation of radio and baseband functions enables these devices to be optimized for high performance and provides fast time-to-market. Digital tuning eliminates many passive components and allows the Bluetooth module to be packaged on standard FR-4 printed-circuit-board materials.
For example, Silicon Wave's integrated radio modem, IC Odyssey SiW1502, can act as a standalone IC or can work in tandem with the company's Odyssey SiW1601 Link Controller IC and a standard microprocessor to complete a Bluetooth wireless communications system. The lower-layer Bluetooth protocol stack supports up to the HCI layer and is optimized for the Odyssey chip. The software stack is available by licensing, can be ported to standard or custom microprocessors and can also be integrated with upper-layer protocol stacks for a single software offer.
In a Bluetooth environment, good radio performance is characterized by sensitivity and interference rejection. Sensitivity is basically the minimum signal that the receiver can detect, or pick up, and still operate. Low sensitivity means that the radio can detect lower-strength signals and gain better range. It is also required to overcome any limitations in the antenna design.
Bluetooth will be integrated in devices that will not be antenna-friendly. Generally miniature antennas are not as efficient as larger ones, but they are attractive and a must for smaller-form-factor products. This means that sufficient margin on sensitivity is required to ensure good range for the Bluetooth product. In addition to range, interference rejection is also an important element in the overall performance. Imagine, for example, a scenario where a user has a PDA within range of his or her desktop computer, but the radio will not make a connection and allow the devices to synchronize. Interference is a common cause of this type of problem.
In addition to the invasion of Bluetooth devices onto the airwaves, several systems already inhabit the 2.4-GHz ISM band, along with our old friend the microwave oven. The Bluetooth product must operate in the midst of all these potential sources of interference.
Several metrics determine the interference rejection performance of the Bluetooth radio. The most important of these is the rejection of close-in signals in the 2.4-GHz band. The receiver must be able to reject signals in nearby channels. The key metrics include the rejection of signals in the next or adjacent channel, and in the rejection of signals two to three channels away. These have the most effect on the Bluetooth connection.
Thanks to image rejection, a transceiver can still operate with an interfering signal at twice the level of the wanted signal. Due to the growth in the number of wireless communication devices, Bluetooth operation will, in many cases, be limited more by interference than by range. Therefore, interference rejection will become a key differentiation in the products. All channel filtering in the SiW1502 is performed on-chip with no need for off-chip high-Q filters or production tuning. The device inherently guarantees interference rejection in a small form factor.
Sensitivity and interference rejection depend on both the radio performance and the performance of the modem. For example, the noise figure of the receiver and the signal-to-noise (S/N) requirements of the modem determine sensitivity. This means that overall performance is determined not only by analog performance but by digital performance as well. Previously these functions had been integrated in different devices. The radio was a separate chip, and the modem was integrated into a purely digital processor-based device.
An all-RF, single-chip Bluetooth radio is essential to make devices easy to use and provide guaranteed system performance. A radio transceiver and syn thesizer are integrated on the same die as a full-burst digital modem. The input is 2.4 GHz RF, and the output is a bitstream with timing recovery. With a digital interface, a radio modem should be easy to integrate with purely digital devices.
Other key elements of the Bluetooth system, such as the digital link controller and the protocol software, are available from the same source. The split at the modem interface allows Bluetooth product manufacturers to use the SiW1502 as a radio platform from which to build without compromising performance or requiring in-house RF circuit expertise to design 2.4-GHz circuits. This ultimately leads to a high-performance Bluetooth product and rapid volume production.
The radio modem split is important to guarantee system performance. But to remove the need for RF design knowledge, high levels of integration and innovative system and circuit design are required. The SiW1502 uses a direct-conversion architecture in both receive and transmit. All active RF elements are integrated into one chip. High levels of radio performance are achieved without the need for off-chip, low-noise amplifiers, transmitter amplifiers or off-chip surface acoustic-wave filtering. A few passive components are all that is required, of which decoupling capacitors make up the majority. Not only are the receiver and transmitter functions integrated, the VCO and resonator are also fully integrated on-chip. No tuning is required during production since the VCO calibrates itself automatically.
The output of the SiW1502 is balanced, and the RF matching circuitry can be lumped or distributed. Standard FR4 can be used due to the high levels of silicon integration although the passive circuitry may be integrated into more-advanced substrates such as LTCC or glass. Standard tolerance components are used and no production tuning of any kind is required. Power control has been implemented in the chip, making the SiW1502 ready to use with a +20-dBm design.
By reducing the component count and eliminating off-chip tuning, the ease of manufacture is increased and OEMs achieve greater cost savings in both time and materials needed to implement other solutions.
Metal on board
The Odyssey Radio Modem is currently packaged in an MLF 48-pin, 7mm-x-7mm form that can be mounted directly to a standard FR4 PC board. The silicon is packaged in a way that allows the metal to sit directly on the board. Due to its flush leads, the MLF package used for Silicon Wave's Odyssey Radio Modem has a smaller footprint than a conventional thin-shrink small-outline package and has roughly half the parasitic lead inductance. Lower parasitics lead to improved RF performance, which is critically important at the Bluetooth specification's 2.4-GHz operating frequency. The MLF package also has a lower manufacturing cost than a conventional thin-shrink small-outline package.