Looking at the heart of wireless regulations
Test specialist John Charters outlines the transmit power and emissions regulations for the three most commonly used low power consumption wireless technologies - Bluetooth, Wi-Fi Direct and ZigBee.
Any device that transmits data is subject to regulatory approval and therefore the technologies covered in this article will all require regulatory approval.
It is important to make this distinction to ensure that when the technologies are being implemented, regulatory approval is still gained. In addition, RF circuit design is also an important consideration and how it meets those regulatory requirements.
This article takes three of the main low power wireless technologies – Bluetooth, Wi-Fi Direct and ZigBee and their device types, and provides guidance on avoiding the most common regulatory pitfalls.
There are three main types of low power wireless technologies which cover a number of battery-powered devices. Firstly, we have Bluetooth smart devices such as heart rate monitors, car key fobs, smart watches and window sensors which use the low energy feature of Bluetooth 4.0.
Most users will recognise the term Bluetooth or its logo and associate it with their mobile headset: a device that is high in power usage and therefore needs regular recharging. Bluetooth’s low energy feature is the latest addition to the technology and promotes longer battery life, plus it isn’t always active – like classic Bluetooth is – meaning it saves power while still operating.
Traditional wireless devices, streaming rich content, like video and audio devices that connect with both classic and smart devices, like tablets and laptops sensor devices, sending small bits of data, using very little energy like sensors and remote controls
Next we have devices which use Wi-Fi Direct, such as mobile phones, cameras and printers. This technology removes the need for an access point in the network, allowing direct communication; instead it creates the access point via the first connected device in the network.
The potential for this technology is broad, however it is currently limited to only those devices certified by the Wi-Fi Alliance.
Finally we have ZigBee, which is by far the most versatile low power wireless technology.
ZigBee is a standards-based wireless technology designed to address the unique needs of low-cost, low-power wireless sensor and control networks. The technology was borne out of a need to improve upon the original Bluetooth and Wi-Fi technologies and has helped to transform home networking.
There are several standards covering different areas of usages such as ZigBee Light Link, for light fixtures and timers, ZigBee Smart Energy for smart energy displays, ZigBee Remote Control for controlling room ambiance and home theatre and ZigBee Healthcare for ECG monitors.
We have established the devices and technologies involved and we can now look at the regulatory pitfalls many organisations find themselves dealing with when working in this area:
Spectral Power Density (SPD)
In Europe the standard EN 300 328 limits spectral power density to 10mW/MHz. The method as laid out in EN 300 328 utilises a duty cycle correction; this duty cycle correction does not work in the favour of low duty cycle modulation schemes such as ZigBee. In order to rectify this you must limit the carrier power in order to meet the power density limit.
For example: EN 300 328 SPD calculation
PD = D + G + 10 log (1/x)
PD= Power density eirp
D= measured power density
G = antenna gain in dBi
x =Tx duty cycle (Tx on/(Tx on + Tx off))
Band edge Compliance
For the USA, the FCC rules require that in restricted bands the spurious emission must comply with the general emissions requirements of CFR47 part 15.209. This requirement causes a potential problem to users of the 2.4GHz band as there is a restricted band at the top band edge.
If the modulation causes the device to fail the limit, reducing the power of the channels near to the band edge is the quickest and easiest method to ensure compliance.
Receiver local oscillator
Most devices that use ZigBee, Wi-Fi Direct or Bluetooth are transceivers and therefore both the harmonics emissions and the emissions from the receiver local oscillator need to be considered.
The latter is often overlooked and can be over the limit. It is also often the case that designers check both these elements but forget to check the harmonics of the oscillator. This can be solved with careful PCB tracking and screening cans.
The use of low power short range RF devices is increasing. As illustrated in this article, a considered approach to design in a few key areas will considerably increase the success rate in passing the regulatory test regimes for global markets.
John Charters is radio product manager at test house, TRaC