October 2018 - San Francisco Circuits, a leading
provider of printed circuit board fabrication and assembly, has published an application
note titled, "How to Improve PCB Design for Bluetooth Circuit Boards." Their experience
in assisting customers in successful layouts for substrates operating in the RF frequency
realm.
See the original article here.
How to Improve PCB Design for Bluetooth Circuit Boards
Bluetooth Circuit Board Design Guidelines
 Printed circuit boards with Bluetooth technology
can be problematic with interference, lost data and poor signal integrity if certain
precautions aren't taken. We'll outline many rules and guidelines to consider when
choosing Bluetooth technology for a given application and, more specifically, designing
it into a circuit board.
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 controller 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 utilizes it
differently and depending on the connection type, it's up to the design engineer to incorporate
basic principles to optimize signal integrity and overall device effectiveness.
Bluetooth is not a very fast wireless choice when compared to Wi-Fi, however it is
getting faster. It also doesn't do well through walls and other nearby obstructions and
has a poor range.
It is still a good choice, despite being a work-in-progress (5.0 is the most recent
update and is a decent improvement over 4.2). For the most part, it is a fairly low-power,
reliable, secure, widely supported option that can easily be implemented on a wide range
of small peripherals.
Bluetooth has been around for 20+ years and is still evolving and although it's improved
in speed, power, range, security and other attributes over the years, it still seems
to have some of the same issues since its conception in the mid-90's, 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:
Use Certified Modules
 1. If you're incorporating Bluetooth
into a product and are limited on resources, consider using a pre-certified, fully contained
module to help accelerate development and time to market. It may drive cost a little
in the end but will usually prevent several headaches that can arise from antenna placement/design
and EMI susceptibility.
There are several reasonably priced certified modules in the market today and most
incorporate a small ARM processor, such as the RN4020 or RN4870 from Microchip, or the
BT121 or BGM113 from Silicon Labs. Having the processor on-board gives it more flexibility
and power, such as controlling simple peripherals via GPIO, SPI, I2C, PWM and so on,
in addition to its Bluetooth stack.
Check Your Bluetooth Device Selection
2. Make sure you're choosing an appropriate Bluetooth device for
the application and that the antenna has also been appropriately sized and tuned.
If you're going for a simple beacon application where you only need to advertise location
or data in short spurts/intervals, then a low-power (Bluetooth Low Energy or BLE), cost-effective
solution with minimal features and peripherals could be used to save on-board real estate
and end cost.
If you're looking for more of a higher throughput, audio-streaming or data-exchanging
Bluetooth application, then you may need something that has a bit more Tx power, higher
Rx sensitivity and a faster data-rate (although slowing down the data rate can typically
help with minimizing dropped packets).
If you're looking for a sort of all-in-one chip, consider using the chipsets containing
powerful or secondary processors that include available UART, SPI, I2C, PWM, ADC, DAC
and GPIO pins.
If you're working on something heavily dependent on an RSSI reading, make sure it
has enough dB resolution on its RSSI monitor.
Separate or Remove Copper Signals & High Energy Components
 3. When designing in a Bluetooth chipset
or module, keep the antenna region completely free from nearby copper signals or components
carrying significant amounts of energy (especially power paths that are switched such
as boost or buck converters).
This also includes keeping the area (and board layers) free of planes and polygon
pours. Most Bluetooth chipset manufacturers will provide layout guidelines that should
be followed closely during PCB design. If you're 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 enough room for tuning elements (a ground plane is required for printed and
ceramic antennas).
Use ground stitching vias to prevent unwanted radiation from the PCB edge as it could
penetrate nearby Bluetooth signals. If you can, try to make the board shape optimized
to the Bluetooth device and where it's antenna is, keeping it on the edge and far away
from nearby components and signals. If using analog-based signals such as Audio, make
sure that the analog and digital ground planes are separated.
And it's always a good idea to shield the electronics (not the antenna of course)
to prevent cross-coupling and minimize noise picked up.
Power Supply Notes
4. Make sure that the rail supplying power to the Bluetooth module
or chip is clean and use bypass (1.0 uF) and decoupling capacitors (0.1uF and 10nF) where
needed. Also feel free to use ferrite beads on the power rail entering the Bluetooth
area of the board to reject high frequency noise.
Tools & Analysis
5. If you're designing the antenna area, make sure you have the proper
equipment (such as a network analyzer) to analyze and tune the matching network, or consider
sending the design to a 3rd party RF test lab.
Consider Real-World Obstructions
6. There are a wide variety of things that can cause obstruction
or de-tuning during a Bluetooth connection, including nearby water (humans too… we're
made mostly of water), metallized objects, smart phones/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.
It's hugely susceptible to signal loss even when paired at close distance (1-2 meters).
If there is higher risk of these kinds of things impacting signal quality, then choose
a higher power device and operate at a slower speed to minimize packet drop. Or if the
electronics are inside of an enclosure, make sure that the metallized materials are minimized
and far away from the BLE module. The relationship between Bluetooth signal strength
and distance is not a linear relationship. In fact, it's very non-linear and somewhat
unpredictable based on the surrounding environment but does follow a general pattern
(shown below).
Whether you're designing a small, simple Beacon module or a data-streaming, power
hungry Bluetooth hub, following these considerations could save you 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.
At San Francisco Circuits, we're excited to help bring these new concepts into reality.
Partnering with an experienced PCB manufacturing and
PCB assembly provider with engineers that have long-standing relationships
with component suppliers is crucial to the success of your next Bluetooth PCB fabrication
and assembly needs.
Visit their website for more information on their
PCB assembly capabilities.
About San Francisco Circuits
San Francisco Circuits is a leading provider of printed circuit board fabrication
and assembly. Their specialty is in the complex, advanced technology of PCB fabrication,
producing high quality multi-layered PCBs from elaborate layouts. With San Francisco
Circuits and their partner network, customers gain the benefit of unparalleled technical
expertise at competitive prices and the most progressive solutions available. San Francisco
Circuits has locations in San Diego and headquarters in San Mateo, California.
Contacts
Alex Danovich President San Francisco Circuits, Inc. Phone: 1-8001732-5143
Web: www.sfcircuits.com
www.motava.com
Posted October 29, 2018
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