For product managers and design engineers, the question has changed from "when will we implement Bluetooth?" to "how will we implement Bluetooth in our designs?" What are the pitfalls? What is the most efficient, cost-effective way forward? For designers with limited wireless experience, the road ahead could look very intimidating.
What is Key to Know?
Bluetooth is a short-range wireless networking technology that operates in one of three power classes: Class 1 (+20-dBm maximum power, with a range limit of approximately 100 m), Class 2 (+4-dBm maximum power, with a range limit of approximately 20 m), and Class 3 (0-dBm maximum power, with a range limit of approximately 10 m).1
Last November, the Bluetooth SIG adopted Bluetooth Version 1.2. What does that mean to the designer? It means that components meeting this specification offer better interference protection and improved coexistence with other 2.4-GHz products (such as WLANs, some cordless telephones, and microwave ovens) as well as improved voice connections as compared to previous versions. These advances are due in large part to the addition of adaptive frequency hopping (AFH) technology, enhanced voice processing, and faster connection set ups. Of course, Bluetooth 1.2 products are backward compatible with Version 1.1 products.
Once designers have chosen to integrate Bluetooth technology into their products, there are many issues that should be considered beyond simple performance figures in a vendor's datasheet. For instance, what are the usage scenarios for the product? Is the product embedded or an external add-on? What operating systems need to be supported? What profiles/services are required to satisfy the requirements of the product applications? Does the product packaging design allow for an embedded or external antenna? What is the product lifecycle and how many variants are planned?
How Can I Be Most Efficient?
The original Bluetooth design was a number of discrete ICs with supporting circuitry. It eventually evolved into a module (Figure 1).
Since then, the level of integration has increased and the number of external parts has decreased as the technology progressed. As a result, the latest module solutions offer designers the means to implement Bluetooth technology with significantly reduced design risk, making the reality of a drop in "complete Bluetooth solution" even closer (Figure 2).
A major consideration when adding Bluetooth functionality to a design is to find out if the module or chipset meets the current Bluetooth standard, version 1.2, and/or if it is upgradeable. A good way to avoid delayed design cycles is to select a product that does not require additional Bluetooth certification or FCC/CE regulatory certification.
Designers should also consider choosing a Bluetooth solution that has leading edge performance and on-board memory. This will make the product more likely to last through a few generations of designs. Selecting a module with moderate performance characteristics might cut costs initially, but will likely require a complete redesign in the next generation.
In addition to the expected RF and baseband functionality, some of the latest Bluetooth modules offer additional functions, including a dedicated microcontroller, antenna, and connector integration, as well as on-board flash memory, voltage regulators, filters, and crystals. These features offer simplified designs, lower development costs, and improved time to market.
If they are not using one of these highly integrated modules, designers must pay very careful attention to the layout of the Bluetooth printed circuit board (PCB). Component placement, tracking, decoupling, grounding, shielding, and board material are all factors that impact performance, and they are crucial in ensuring good RF performance. When using a pre-designed, pre-certified module, designers avoid these issues as well as the need to worry about these factors in their final design.
Product Development Issues
Once a designer understands the basic application needs of the product, it is important to keep a watchful eye on power dissipation (battery life), physical limitations of the Bluetooth circuitry in the existing product design (how much real estate is available), and the transmission rate limitations/needs. For instance, most Bluetooth Internet connections are limited to 732.2 kbit/s, which can have an impact on audio quality.
Here are five considerations that are critical to the development process:
1. Module vs. Chipset
The main advantage of taking chips and designing them directly on the PCB is saving on footprint. But the advantage pretty much ends there. Single chip/chipset approaches require RF design resources to provide filters, amplifiers, matching networks, oscillators, clocks, and antenna for both the transmit and receive paths. It also requires extensive test equipment suites to verify and synthesize the design. And, in addition to significant design time, RF expertise, and verification, the design will also have to be certified for use in Bluetooth products.
2. Antennas
Most Bluetooth handheld devices require antennas that radiate in a spherical pattern so they can connect in any direction. Good antenna design for Bluetooth products is important, and designers selecting an external antenna need to consider space, cost, and antenna patterns. The surrounding components, casing, and proximity to the ground planes also all play a role in the ultimate performance of an antenna on a product.
If selecting a module without an antenna, the designer must understand the environment where the product will be used. Severe signal strength losses can occur if the feed between the radio and the antenna is not well matched, and the signal is reflected back into the circuit.
Recently, modules became available with integrated antennas. If a designer selects a module with an on-board antenna, then the significant antenna matching work has already been done. If a module without an antenna is selected, this matching needs to be done by the designer and then the solution needs to be re-certified by the Bluetooth Qualification Review Board (BQRB).
3. Antenna Matching
Most Bluetooth radio chipsets (and modules) have an on-chip transmit/receive switch in the front-end architecture to allow both the transmission and receive paths to use the same differential port. Typically, the output transceiver architecture is a single combined transmit/receive chain.
When designing the Bluetooth module, a matching network formed by L1 and C1 is used to transform between the impedances of the Tx differential port and that of the balun (Figure 3). The balun converts the differential signal into one single-ended signal that is fed to a band pass filter, then to the antenna port. Filtering formed with the L2/C2 network prevents RF energy from contaminating the circuit. The band pass filter in the signal path provides additional filtering for the Tx signal and suppresses interference in the Rx path.
A shield helps reduce internal and external interference from radiating into the module and other RF sensitive components. RF coupling also needs to be considered, and components should be carefully oriented on the PCB.
As an example, the RF performance of a Bluetooth device with an antenna was measured. Two cases are considered for comparison. In Case 1, a 0-ohm matching network is used to measure the initial antenna state (Figure 4) and in Case 2, an optimum matching network was implemented to enhance antenna performance (Figure 5).
In the cases above, measurements performed included 3D radiation pattern, return loss, and antenna efficiency. For this analysis, the return loss and antenna efficiency will be considered. In these examples, initial antenna measurements showed that the antenna is detuned from the central frequency of 2.45 GHz, so a matching component was required. The results showed that 44-percent total antenna efficiency (at the peak of 2.45 GHz) was achieved when using the optimum matching network (Table 1). Return loss in Case 1 ranged from -2.9 to -5.0 dB and improved to -6.5 to -12.4 dB with matching circuitry (Figure 6).
As this analysis shows, although the antenna is only one component, designing it in a circuit requires other circuits to optimize performance, with special considerations given to filtering requirements, matching, and layout.
4. Connectors
Many Bluetooth modules do not have connectors, but have leaded BGA or SMD packages. Rework on any products that feature these leaded modules requires the use of a BGA or SMD rework station. These stations are costly, and the process can be time consuming when these components need to be extracted from the PCB for test, debug, or repair. In such cases, the main PCB could easily be damaged. And, reworking the leaded module back onto the PCB could be challenging. (For instance, a BGA X-ray machine may be needed to verify whether the pins have connected properly to the PCB.) Working with a module that has the appropriate signals brought out on a connector can be a major time and aggravation savings for a designer.
When a module is equipped with standard connectors offering USB or UART interface, the designer essentially has a plug-and-play module to insert into the existing design, without the need to redesign the PCB to integrate BGA or SMT type components. An on-module connector can also help debugging and testing without the need to have special equipment.
5. Crystal
Some modules do not have an on-board crystal while others have an on-board crystal but the module is not fully tested and certified. In either of these scenarios, some special design considerations apply.
Crystals vary, and each crystal has an optimal bias level that minimizes the phase noise on the oscillator, so, when using a crystal, its bias level must be individually configured. In the case of certified modules with an on-board crystal, the individual bias levels are pre-stored on each module in order to achieve minimum phase noise.
What about the Stack?
Standard Bluetooth devices are minimally equipped with a host controller interface (HCI) stack, which supports basic Bluetooth host protocols for serial communications, such as synchronizing the hopping frequencies used by communicating Bluetooth products. A module manufacturer can load a profile on top of the stack, such as the serial port profile (SPP) which allows for cable replacement. In addition, end users can develop custom stacks or purchase third-party stacks.
What Do I Need to Test For?
Figure 7 shows a test set up that can be used to test a Bluetooth module for most RF parameters such as receive/transmit signal output, co-channel interference, bit-error rate (BER) performance relative to received signal strength indicator (RSSI), and adjacent channel interference.
The device under test (DUT) shown in Figure 7 is the actual Bluetooth module and it is connected to a PC using an interface board, which in this case requires DC supply. The PC is equipped with test set up software.
The major component of this test set up is the Bluetooth Tester, such as the Agilent E1852A/B or Anritsu MT8850aA52A. (Additionally, a spectrum analyzer can be used to examine the spectral contents of the signal on the RF port of the device for measurements of adjacent channel interference, signal spectrum, harmonics, co-channel interference, and noise.)
The Bluetooth signal generator in Figure 7 is used to generate noise in the particular bandwidth and frequency band of interest. The RF combiner/ splitter is used to combine the noise with the RF signal, and also to split the RF signal where it can be routed to the spectrum analyzer and the Bluetooth tester.
Additional test criteria will vary by application. For instance, Table 2 shows the pass/fail parameters used to evaluate a Bluetooth USB printer adapter.
Some additional tests that can be performed using the test setup in Figure 7 include:
- Frequency versus time and power versus time performance for the module
- Power versus frequency channel number
- Co-channel interference
- Adjacent channel interference
- Spectral contents of the signal
- Harmonics contents
If using a module without an antenna, additional work is needed to determine the matching circuitry. Also, more extensive measurements will be required when the antenna is on board (see Antenna Matching, above), and it may even require the help of the antenna manufacturer if the proper equipment is not available at the design site.
Using a pre-certified fully-integrated module eliminates the need to test for RF conformance testing or data transmission rates, since the product is already certified by the BQB body, CE, and FCC. Bluetooth certification includes RF conformance testing and interoperability testing. So, for the most part, all the designer needs to do is ensure appropriate signal levels are provided to the module and that the correct application or stack is loaded onto the final product.
For instance, using the USB adapter example in Table 2, the designer simply needs to ensure that the proper USB signals are connected to the USB lines of the module. The power, ground, and appropriate signal levels as defined by the module specifications should be applied when integrating the module.
What about Bluetooth Qualification?
Bluetooth qualification allows OEMs to have a product listing on the Bluetooth web site and gives them the ability to use the Bluetooth brand on the product. Qualification helps different manufacturers ensure that their products will operate with other Bluetooth products, and that their products comply with the Bluetooth specification. The qualification process is divided into four parts: radio link requirements, protocol requirements, profile requirements, and information requirements.
The entire qualification program is overseen and administered by the Bluetooth Qualification Review Board (BQRB). Qualification is based on conformance testing (defined by the Bluetooth Qualification Program [BQP]) against a reference test system and functional interoperability tests with another operational Bluetooth product. Testing is performed by a Bluetooth Qualification Test Facility (BQTF) which has been accredited by the SIG's BQRB.
In addition to this testing, manufacturers seeking qualification must submit compliance declarations, which will be reviewed by a Bluetooth Qualification Body (BQB). In addition, Bluetooth products also need to obtain approvals beyond those issued by the BQB, such as FCC and CE.
If designers select a pre-certified module complete with antenna, no re-certification is required. If they select a module that requires additional components, including connectors and antennas, then the design must be re-certified. This process can add several weeks onto the design cycle. The cost of certification for a new design varies greatly and is dependent upon which countries will be receiving the product. For instance, costs can exceed $100,000 for worldwide approvals. Although BQTFs have recently implemented significant price reductions, the use of pre-certified modules still offers considerable cost savings.
Designers implementing Bluetooth into new or existing designs need to be clear on what their application requires, and then carefully select the module or chipset for the design. Fortunately, the technology has evolved so that designers can implement Bluetooth functionality with a limited amount of optimization and RF circuitry design. Some of the latest modules also eliminate the need for certification and extensive design work, so designers can provide a reliable and fully-certified design with a faster time to market.
Author's Note: The author would like to thank Hussein Mehdi for helping create this article.
About the Author
James Kraemer is the director of product development and engineering at Smart Modular Technologies. He can be reached at james.kraemer@smartm.com.
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