Achieving energy efficient wireless performance in smartwatches
Smartwatches are some of the most intimate and personal technology products available on the market. They are literally tied to their users with a strap, and kept on the person almost all day, every day.
These intimate devices pose a number of design challenges: they need to fit in seamlessly with users’ lifestyles, habits and be exceptionally ergonomic. Failing on any of these three measures can be costly, and the difference between a thunderous success or flop.
Power efficiency is fundamental to these devices. They are meant to be kept on all day and provide all-day connectivity for consumers. However, in such a small device, there’s a fundamental design trade-off: how can you ensure an antenna doesn’t draw unnecessary power throughout the day? In this article, we’ll review the steps you should take to maximise battery life through your wireless performance.
Close-packed components detract from efficiency
Smartwatches are some of the most compact wireless devices on the market with plenty of functionality. Space is at a premium, as components are tightly packed on the circuit board, and an entire side of the housing is usually made up of an LCD screen. Here are a smattering of components that may need to be considered in your design:
Heart rate sensors
User input of some kind, either buttons or a touchscreen
Gyrometer(s) or other components to measure movement
Highly efficient antennas are key to the functionality of a wearable. When used as health and activity monitoring devices, their connectivity is fundamental. For example, an activity tracker may be reliant upon high levels of GNSS connectivity to provide accurate data, and then WLAN/BT connectivity to communicate this information to other devices.
The presence of multiple antennas on an already compact circuit board causes a number of issues:
Limit antenna coupling
In compact devices, it is absolutely essential that antenna coupling is avoided. Failing to do so can cause antenna inefficiency and vastly increase power consumption – making wireless performance worse and causing inefficient power usage.
On small circuit boards, antenna coupling is not uncommon, and there are some steps you can take to increase isolation. For example, increasing the physical distance between antennas, use antennas with different polarisations and select antennas based on radiation patterns (so that they point in different directions).
Our Intelligent Antenna Selector considers the polarisations of antennas and suggests the optimum antenna placement to reduce isolation. The app also considers the size of your PCB to help you select the most appropriate antennas for your device.
Design sufficient ground plane space
The necessary length of a ground plane will depend on the wireless technology being used by your smartwatch. Wi-Fi, Bluetooth, Zigbee and the like are seen as easier to integrate within wearables, as they often require less ground plane to effectively work.
Sub-GHz frequencies are more challenging to work within these compact form-factors, however. They often require 90-120mm of ground plane (in length) to achieve acceptable levels of performance. This creates a number of issues for wearables: should you create extra functionality, at the sacrifice of performance, or leave out the functionality altogether?
Just a 20mm reduction in ground plane length could drop efficiency by up to 20%, whereas 1.7-2.2GHz antennas may only suffer a 5% drop.
Layout your board to maximise wireless performance
Even if you select the most suitable antenna and design sufficient ground plane space, there are some other guidelines to follow during the board layout stage of your design. For example:
Do not attempt to save space by using long, narrow transmission lines
Place antennas first, and always place them according to their guidelines in their technical specifications
Ensure the top laer groound is totally filled – if you are using multiple grounds, ensure these are linked with adequately spaced vias
Do not run digital tracks or wires underneath the antenna (either above or below)
Place high speed switching components away from any antenna
If using a ground-plane free antenna, ensure no metallic objects are placed underneath (including batteries)
Optimise your enclosure
Whilst smartwatches need to be tactile, rugged and sweat resistant, dense enclosures and certain materials can inhibit wireless performance. We recommend you opt for a plastic enclosure, using polycarbonate or ABS, however, even these materials will detune the antenna if there is an insufficient gap. We recommend introducing a gap of 3-5mm (or greater) between the antenna and you product housing.
In the case of wearables, the gap between housing and antenna is especially important. Users effectively act as transducers, and will detune an antenna to some degree – the smaller the gap, the more significant the detuning effects. Utilising plastic materials in your product housing and leaving a sufficient gap will make detuning effects much more manageable to counteract.
Review battery performance
Users expect smartwatches to fit in with their lifestyle, and don’t anticipate having to shoehorn in extra charging sessions throughout the day. Wireless solutions and designs play a fundamental role in determining the longevity of your device on a single charge.
Even an inefficient wireless design can work – function wirelessly – but the suboptimal operating environment will drain the battery more quickly. Poorly considered component layouts, inadequate antenna placement, poor isolation and a lack of ground plane length can all drain battery life, as the wireless module works harder to compensate.
In smartwatches, efficient wireless performance is essential. Users expect these devices to last all day and remain connected all day. With the added challenges of packing multiple antennas within an ultra-compact form, it is absolutely vital you consider your wireless solution first.
If you are kicking off your design project, we recommend you use our Intelligent Antenna Selector. By entering your PCB dimensions and selecting antennas, you will see a visual representation of the optimum antenna placement – based on a number of design-in guidelines.
If you have already begun design, then consider how the form factor will impact wireless performance. If you’re planning on using an Antenova antenna, get in touch with our expert RF engineers, send over your design files and then get individual feedback on your RF design.