An RF module is nothing but a small electronic device which is used to communicate between two devices, wirelessly, using radio waves. This is one of the most preferable methods of wireless communication as it does not require line of sight. Radio circuit design is a complex field and the performance highly depends on accuracy of components, a perfectly designed layout and carefully monitored manufacturing process. Also, the design needs to be then certified after conformance testing, as radio circuits are subject to limits on radiated emission. Due to all these, designers prefer to use off-the-shelf RF modules which save a lot of money and reduce time-to-market to a great extent.
These RF modules come with different carrier frequencies allowed by national and international regulations, such as 315MHz, 433.92MHz, 868MHz, 915MHz and 2400MHz. Other than frequency, these modules are also differentiated based on the protocol standards they use for communication, such as Wi-Fi, Zigbee, Bluetooth or other proprietary protocols.
Wi-Fi and WLAN have been interchangeably used in general English. Wi-Fi is a local area wireless technology that allows electronic devices to exchange data or connect to the Internet using 2.4GHz UHF and 5GHz SHF radio waves. Wi-Fi Alliance defines Wi-Fi as any wireless local area network (WLAN) products that are based on the IEEE 802.11 standards. Most modern WLANs are based on these standards, which make both these terms a synonym for each other.
Wireless revolution enters its next phase of deployment with recent advent of the Internet of Things (IoT). Now WLAN capabilities have been added even to the consumer products, which give users the comfort of controlling these devices with a smart phone or over the Internet. To achieve WLAN capabilities, of-the-shelf available Wi-Fi modules are the best solution for the reasons stated above.
There are different kinds of Wi-Fi modules available. The vendors normally categorise the modules by parameters such as data rate, range, RF band, certification and packaging type. These parametric filters will allow you to refine your search results to find the most suitable part for your application. Below are some parameters and their brief description. Table I shows some manufacturers of Wi-Fi modules.
Protocol standards. The original version of the standard IEEE 802.11 was released in 1997 and clarified in 1999, but it stands obsolete today. The wireless LAN standards in use these days include 802.11 a/b/g/j/n/p/ac/ad. Each protocol standard has different data rate, frequency, modulation scheme, range and power consumption. The newer modulation methods and coding rates are generally more efficient and sustain higher data rates, but older methods and rates are still supported for backwards compatibility.
Operating frequency band. Table II shows operating frequency bands for different Wi-Fi protocol standards. Some Wi-Fi modules support dual frequency bands. For example, while 802.11n solution supports both the 2.4GHz and 5GHz bands, 802.11g supports only 2.4GHz band. Each range is divided into a multiple channels. Countries apply regulations on number of channels as well as users and maximum power levels within a frequency band.
Data rate. Each Wi-Fi standard is rated according to its maximum theoretical data rate. Different standards have different data rates due the technology used for each standard. Practically, these data rates are never reached. These are only the theoretical maximum value for a particular technology. The theoretical maximum data rate of a Wi-Fi module could be from 1Mbps (802.11b) to 6.75Gbps (802.11ac). You need to decide the required data rate for your application. Higher data rate would not always be the best choice as sometimes high data rates would not even be sustainable with large number of clients.
Transmit range. The range of a Wi-Fi network is always limited and majorly depends on the specific protocol used and the transmission power. Interference and obstruction from environment also plays some role. For example, approximate maximum indoor range for 802.11g is around 38 metres.
OS support. It is highly desirable that the module quickly gets connected and configured with Android, iPhone, Linux and other embedded OS. If you need to build in this feature into your product, you need to check the driver level support, and configuration and management support from the Wi-Fi module carefully.
Antenna. Both omni-directional and directional antennae are available in Wi-Fi modules. Omni-directional antennae are typically used for wireless coverage providing connectivity to Wi-Fi devices inside office, home, warehouse, etc. Directional antennae on the other hand are used for focusing the wireless signal in a specific direction, resulting in a limited coverage area. The typical applications are wireless connection from one building to another. Through use of directional antennae, the connection can be extended with many kilometres between stations.
Secure Wi-Fi authentication schemes. In 2001, a group from the University of California, Berkeley presented a paper describing weaknesses in the 802.11 Wired Equivalent Privacy (WEP) security mechanism defined in the original standard. The IEEE then set up a dedicated task group to create a replacement security solution. The Wi-Fi Alliance announced an interim specification called Wi-Fi Protected Access (WPA) based on a subset of the then current IEEE 802.11i draft. WEP, WPA, WPA2, WPA2-Enterprise, WPS, WMM, WMM-PS are typical Wi-Fi security types these days. Each type has different advantages and disadvantages. These security schemes help protect information from Wi-Fi devices on a network.
Packaging type. Both surface-mount (SMT) and through-hole packages are available for these modules. SMT packages are mostly used for their known advantages, but through-holes are more suitable for testing and prototyping purpose.
These are tiny blue chips that can communicate wirelessly with each other. They can do simple things like replacing a couple of wires in serial communication. There are lots of different modules available but all of them come with similar pin-outs. Power, ground and TX/RX lines are in the same place, making the chips pretty interchangeable for most of the simpler applications.
You will encounter two terms XBee and Zigbee while working with these modules. XBee is nothing but Digi’s (a company) own Zigbee-based protocol. These modules use the IEEE 802.15.4 networking protocol for fast point-to-multipoint or peer-to-peer networking. They are designed for high-throughput applications requiring low latency and predictable communication timing. To select the right module for your application you need to understand things like difference between Series1 and Series2, different antenna types available and what Pro models are.
Series1 (XBee 802.15.4). A Series1 module does not indicate series on it. Other series modules do indicate it on the chip. So if nothing is mentioned, your module is Series1. Note here that Series1 and Series 2/2.5/ZB hardware are not compatible with each other.
Series2 (Znet 2.5). Series2 modules require configuration unlike Series1 modules. These modules are not compatible with Series1 modules and are not sold any more in the market.
Series2 replacement (ZB). With Znet2.5 discontinued, ZB is often called Series2 module. Though this is not technically correct, it does distinguish these from Series1. ZB is nothing but Znet2.5 with a new firmware which allows it to run in a transparent mode or work with API commands. You also do this upgrade yourself with a conversion kit available on the internet.
Latest Series2 (2B). These modules come with improved hardware as compared to ZB modules with better power usage. These modules run the ZB firmware itself.
Antenna. Antennae available in these modules are chip, wire, u.FL and RPSMA antenna. Chip antenna is nothing but a small chip that acts as an antenna. It is being replaced by trace antenna that is directly printed on the PCB. Wire antenna is just a wire sticking out of the module. u.FL and RPSMA on the other hand are both connector-based antenna systems where your own antenna can be connected through a connector. These are very useful when your object is enclosed in a chassis and you want your antenna outside the enclosure.
Regular vs Pro. The Pro normally consumes more power which directly improves its range. Use the Pro version only when you require better range. But this is a trade-off when it comes to a battery-operated device. So select wisely for the application at hand. Table III show some manufacturers producing these modules.
Bluetooth is another technology standard for connecting devices wirelessly. The radio-frequency range for Bluetooth is between 2.4GHz and 2.48GHz. It is mostly used for transmitting data in a secured way over short distances, wirelessly. Bluetooth uses a radio technology called Frequency-Hopping Spread Spectrum (FHSS). The transmitted data is divided into packets and each packet is transmitted on one of the 79 designated Bluetooth channels. Each of these 79 channels has a bandwidth of 1MHz, starting at 2402MHz and continuing up to 2480MHz in 1MHz steps. Bluetooth 4.0 uses 2MHz spacing, which allows only 40 channels.
Bluetooth LE, marketed as Bluetooth Smart, is intended to provide considerably reduced power consumption and cost while maintaining a similar communication range as compared to Bluetooth classic. Bluetooth 4.0 specs allow devices to implement Bluetooth LE, Bluetooth classic or both. The devices that implement both the systems are known as Bluetooth 4.0 dual-mode devices. Bluetooth Smart is v4.0 LE only and Bluetooth Smart Read supports dual mode (refer Fig. 1).
The parameters for selection are more or less the same, as already discussed for Wi-Fi and Zigbee. These are data rate, range, power consumption, package and certifications. Table III show some manufacturers for Bluetooth modules. With all the above information, you can easily decide on the wireless protocol and select the best module for your application.
The author is a technical editor at EFY