Six steps for limiting interference and data loss.

Printed circuit boards (PCBs) utilizing Bluetooth technology can encounter issues like interference, lost data and poor signal integrity if certain precautions aren't taken. This guide outlines essential rules and guidelines for selecting Bluetooth technology for various applications and, more specifically, for integrating it into circuit boards.

A variety of applications utilize Bluetooth, including:

• Beacons used in shopping malls
• Eddystone frames for industrial sensing applications
• Headsets and audio/stereo products
• Remote peripheral devices such as video game controllers or computer mice/keyboards
• Home automation systems
• Wireless consumer electronics applications, including cameras, printers and phones.

Each application incorporates the same common Bluetooth technology but adapts it based on connection types. Design engineers must apply fundamental principles to optimize signal integrity and enhance overall device effectiveness.

While Bluetooth is not as fast a wireless choice compared to Wi-Fi, it is getting faster. Its performance declines when obstructed by walls or nearby objects, and range is limited.

Despite these challenges, Bluetooth remains a strong choice. The latest version, 5.0, shows significant improvements over 4.2. Overall, Bluetooth provides a low-power, reliable and secure solution that integrates easily into a range of small peripherals.

Bluetooth has been around for more than 20 years and is still evolving, and although it’s improved in speed, power, range, security and other attributes over the years, some of the issues seen at its conception in the mid-90s remain, including its susceptibility to signal interference.

So, what can be done from a PCB design standpoint to optimize signal integrity, minimize interference and lost data packets?

Here are a few Bluetooth circuit design considerations and general rules of thumb:

1. Use certified modules. When incorporating Bluetooth into a product with limited resources, consider a pre-certified, fully contained module to accelerate development and reduce time-to-market. While this choice may slightly increase costs, it often prevents headaches related to antenna placement, design issues and EMI susceptibility. Many reasonably priced certified modules are available today, typically featuring a small ARM processor such as the RN4020 or RN4870 from Microchip or the BT121 or BGM113 from Silicon Labs. The on-board processor provides greater flexibility and power, allowing control of simple peripherals through GPIO, SPI, I2C, PWM and more, in addition to managing the Bluetooth stack.

Figure 1. Sample Bluetooth boards.

2. Check the Bluetooth device selection. Ensure an appropriate Bluetooth device is chosen for the application and the antenna is appropriately sized and tuned.

For a simple beacon application requiring short bursts of location or data advertising, opt for a low-power device using Bluetooth Low Energy (BLE). This cost-effective solution minimizes features and peripherals, helping to save on-board space and reduce overall costs (Figure 2).

Figure 2. For simple beacon applications, a low-power BLE is a cost-effective solution and can save board real estate.

If a higher throughput, audio-streaming or data-exchanging Bluetooth application is needed, select a device with increased transmit (Tx) power, higher receiver (Rx) sensitivity and a faster data-rate (although slowing the data rate can typically help with minimizing dropped packets).

Need an all-in-one chip? Consider chipsets containing powerful or secondary processors with available UART, SPI, I2C, PWM, ADC, DAC and GPIO pins. If the project relies heavily on received signal strength indicator (RSSI) readings, ensure the device provides adequate decibel (dB) resolution on its RSSI monitor.

3. Separate or remove copper signals and high-energy components. When designing in a Bluetooth chipset or module, keep the antenna completely free from nearby copper signals or components carrying significant amounts of energy (especially switched power paths, such as boost or buck converters).

This includes keeping the area (and board layers) free of planes and polygon pours. Most Bluetooth chipset manufacturers will provide layout guidelines to follow closely during PCB design. When manually laying out the antenna area, use a ground plane as appropriate to keep a good bandwidth at the input and make sure to build in room for tuning elements (a ground plane is required for printed and ceramic antennas).

Ground stitching vias prevent unwanted radiation from the PCB edge which could otherwise penetrate nearby Bluetooth signals. When possible, optimize the board shape to the Bluetooth device’s antenna location, keeping it on the edge and far away from nearby components and signals. If using analog-based signals such as audio, separate analog and digital ground planes.

And it’s always a good idea to shield the electronics (not the antenna, of course) to prevent cross-coupling and minimize picked up noise.

4. Consult power supply notes. Ensure the rail supplying power to the Bluetooth module or chip is clean. Use bypass capacitors (1.0µF) and decoupling capacitors (0.1µF and 10nF) where needed. Additionally, incorporate ferrite beads on the power rail entering the Bluetooth area of the board to effectively reject high-frequency noise.
5. Tools and analysis. If designing the antenna area, make sure have the proper equipment (such as a network analyzer) is available to analyze and tune the matching network, or consider sending the design to a third-party RF test lab.
6. Consider real-world obstructions. A wide variety of things can cause obstruction or de-tuning during a Bluetooth connection, including nearby water (humans too … we’re made mostly of water), metallized objects, smartphones/tablets, computers, devices operating on the same ISM band (such as microwave ovens or WLAN technologies), power sources, wireless RF video, office lighting and home phones.

The device is highly susceptible to signal loss, even at distances of just 1-2 meters. If there is a higher risk of these kinds of things impacting signal quality, choose a higher power device and operate at a slower speed to minimize packet drop. When electronics are housed in an enclosure, ensure that metallized materials are minimized and positioned far away from the BLE module. The relationship between Bluetooth signal strength and distance is nonlinear and somewhat unpredictable based on the surrounding environment, though it generally follows the pattern shown in Figure 3.

Figure 3. A graph showing the relationship between Bluetooth signal strength and distance.

Whether designing a small, simple Beacon module or a data-streaming, power-hungry Bluetooth hub, following these considerations could save tons of headaches during the test/implementation stage of design.

With the expansion of Bluetooth PCB assembly, it’s an exciting time to incorporate wireless communication and control into products, and the future will only bring smaller, faster, cheaper and stronger Bluetooth components.

Andrew Gonzales is VP of engineering at San Francisco Circuits (sfcircuits.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. Jason Metzner is a senior systems engineer at Arthrex (arthrex.com).