Bluetooth PCB Design Methodology and Tips

Key Takeaways

• A discussion of Classic Bluetooth, including security and architecture
• Analysis of low energy Bluetooth and Bluetooth applications
• Tips and notes for designing Bluetooth PCBs

Bluetooth is a communication protocol we all know and love. It enables our speakers, keyboards, and other devices that we use on a daily basis.

Bluetooth occupies the 2.4 GHz spectrum and contains less bandwidth and range than typical WiFi A-Band configurations that also operate on the 2.4 GHz. It is often used for personal area networks (PAN) as opposed to WiFi’s wireless area networks. In other words, Bluetooth is used for one-to-one (device-to-device) communication, whereas WiFi is better utilized in a device-to-hub configuration.

As WiFi and Bluetooth share a part of the frequency spectrum, there is a chance for interference if both transmission paths are within 10 meters of each other. However, this is minimized in newer versions of Bluetooth, which can learn which radio channels work well and which ones are experiencing more interference. Then Bluetooth communication can dynamically switch to use channels with less interference—known as adaptive frequency hopping.

Present-day Classic Bluetooth achieves up to 3 Mbps while Bluetooth Low Energy (LE), discussed further below, can achieve up to 2 Mbps. Bluetooth is managed by the Bluetooth Special Interests Group (SIG), a collection of companies that work together to advance Bluetooth technology. For a manufacturer to market a Bluetooth device, it must meet Bluetooth SIG standards.

In this article, we will explore Bluetooth PCBs.

Bluetooth Connectivity Process

Bluetooth devices use a primary and secondary model to be able to communicate. A single primary device, such as a smartphone, computer, console, etc., can connect to multiple secondary node devices.

Every Bluetooth device has a unique 48-bit address, usually represented as a 12-digit hexadecimal value. BLE devices that want to be discovered send out messages in a process known as advertising. Advertising messages contain information about the device, including this unique ID. At the same time, another capable device will scan for packets and select an appropriate one.

After connecting, a device can be in one of a couple modes:

• Active mode, where the device is receiving or transmitting data
• Sniff mode, where the device sleeps and listens for a signal at a set period 
• Hold mode, where the device sleeps for a specific period then returns back to active mode
• Park mode, where the device sleeps until the master device wakes it back up

Bluetooth Security 

Bluetooth devices use encryption to ensure a secure connection between devices

In general, Bluetooth is a fairly low-power, reliable, secure, and widely supported communication standard that is great for small peripherals. Generally, Bluetooth devices have a range of 1 cm to 100 meters, with each connected device requiring device approval using unique codes. Bluetooth data exchanged between devices can be encrypted, keeping information from being picked up by eavesdropping devices. Furthermore, it can also change the address that acts as the device identity and which is included in wireless data exchanges, reducing the risk of any device being tracked.

Bluetooth Hardware Architecture

In any Bluetooth PCB device, there are two parts that work together to create Bluetooth connectivity. The first is a radio device that modulates and transmits a signal. The second is a digital controller; these may or may not be physically separate.

The digital controller is usually a CPU that runs a Link Controller and interfaces with the host device. This Link Controller does the processing of the baseband and management of physical layer FEC protocols. Additionally, it deals with transfer functions (asynchronous and synchronous), audio coding, and data encryption.

Low-Energy (LE) Bluetooth

Low-energy Bluetooth (BLE) is an additional Bluetooth standard that has allowed for many electronic developments and innovation. Specifically, many IoT and medical devices utilize Bluetooth LE for communication.

Bluetooth LE is built to be low energy and utilizes less than 15 mA of current while in active mode. BLE devices spend most of their time in a low power sleep mode and only wake up to send data. Compare that with classic Bluetooth, which usually spends more time “on” and actively communicating.

BLE can reach up to 150 meters in open area, and is a cost-effective alternative to Bluetooth, as it also provides a significant increase in battery life. It can also be used for broadcast or mesh networks.

Bluetooth PCB Applications

Bluetooth has a variety of applications - from health care to audio streaming

Bluetooth capable PCBs are used in a variety appliances and devices. Some Bluetooth PCB applications include:

• Health care devices, such as blood pressure measurement devices, blood glucose monitoring devices, and temperature measuring devices. This includes both wearables and implants that communicate to a smartphone or other external device. 
• Environmental sensing devices, such as illuminance, ambient humidity, pressure, or ambient temperature, capable of communicating this data to a smartphone or other central data-logging device.
• Fitness devices that are equipped with sensors that can measure speed, RPM, scales that monitor body weight, or wearables capable of measuring heart rates.
• Audio streaming devices are also especially benefited by Bluetooth with its lower power and lower range - speakers and headphones that we’re all familiar with.

Bluetooth PCB Design Tips 

There are a variety of tips and pointers necessary to keep in mind when designing Bluetooth devices

When designing Bluetooth PCBs, there are a variety of things to consider to ensure device reliability, function, and safety.

• Power consumption. It’s likely your Bluetooth device will be battery powered. Especially in low-power designs, it’s important to calculate your power usage in advance. Verify that you aren’t suffering from current leakage in your design and that your components are all reliable. Utilizing a microcontroller that is capable of a low-power deep sleep mode can increase your device longevity.
• Power supply reliability. Bluetooth devices typically require a steady voltage between 1.6 V to 3.6 V. Fluctuations in the line can result in issues with transmission and operation. As always, ensuring a stable power rail is critical to device reliability and follows good design practice. Use a bypass capacitor and multiple decoupling capacitors as necessary. Furthermore, using ferrite beads on the power rail can help get rid of high frequency noise. 
• Transmission requirements. Just because a component is Bluetooth-capable, doesn’t necessarily mean it’s fit for your board. Depending on your Bluetooth application, you may require different sized antennas and different transmission power. For example, if you plan on something like a simple beacon application, where communication entails some location or other short datastream, BLE might be a more cost-effective option. Utilizing smaller, more power-efficient ICs with less bells and whistles can save on board space. On the other hand, if your device requires audio streaming or higher data rates, larger, more capable ICs might be the way to go, as they often will draw more power but allow for more sensitive receiving and higher transmission power.
• Electromagnetic interference (EMI). Though Bluetooth operates on the 2.4 GHz spectrum, it can still create EMI issues for surrounding components on your PCB. In order to ensure that the high frequency coupling doesn’t reach these components, use EMI shielding strategies, such as increasing the distance between traces or adding an EMI shield.
• Signal integrity. As we’ve discussed, there’s a lot of room for noise and other interference to enter your board. For this reason, it’s important to keep your antenna region spaced out from nearby copper signals or other high-energy components such as power paths or buck converters—polygon pours and large planes as well. If you’re laying out the antenna area, utilize a ground plane (required for printed and ceramic antennas) to ensure good bandwidth at the input and leave appropriate space for your tuning elements. It’s likely your Bluetooth IC manufacturer will provide layout guidelines for you to follow. In the case of delicate analog signals, consider using separate analog and digital ground planes. 
• Physical size constraints. Your Bluetooth PCB devices will likely be portable, and as such, will likely need to fit in some sort of enclosure, making it necessary to consider these mechanical constraints. Whether it is a smart wearable, consumer device, speaker or anything else, working with PCB design software capable of ECAD-MCAD integration can help you later on in your design process.
• Board real estate management. Your device will most likely be performing additional functions unrelated to Bluetooth. Whether it’s a WiFi card, NFC, analog microchips, or additional sensors, these components also require real estate on your Bluetooth PCB. These components will always be competing for space, so consider IC sizes when choosing components. 
• Utilization of certified modules. If you require your device to be Bluetooth-capable, consider using a pre-certified fully contained module to ease your development process. Although this may increase the cost upfront, it will prevent later challenges involving antenna placement, EMI susceptibility, and figuring out various protocols down the line - ultimately accelerating your time to market. There are all kinds of devices available on the market, so taking time to find one suitable for your required functions can help tremendously. 
• Board layout. Placing large pads, long wires, or other inductors too close to the Bluetooth PCB antenna may change the resonance frequency.