Basic Wireless Communication for Microcontrollers
Chapter 4 - Design Project 3: 900MHz Automatic Error-Correcting Data Link
Prebuilt RF Modules
You saw in the second design project how much design effort goes into producing an effective transmitter and receiver for an RF data link. If the focus of your project is on RF design, then all this effort is worthwhile. If you simply want to add wireless communications to a project as quickly as possible, it probably isn't worthwhile. Several manufacturers make prebuilt RF modules of varying complexity and function.
Most RF modules are just a transmitter and a receiver, along with a simple modulator and demodulator designed for digital communications. Some require a few external components to be added (a few resistors, capacitors, and possibly inductors) and others require no external components other than an antenna and a power source. Once supplied with what it needs, the module accepts digital data in and transmits it, as well as outputing data that it receives.
Some modules are even more sophistocated and include a microcontroller to assemble data into packets and handle some kind of basic protocol for communication, which may or may not include error detection/correction. These are called "packetized" modules as opposed to "non-packetized" ones which are just radios.
The goal of RF module manufacturers is to help those with little RF knowledge to put together an effective RF link simply and easily. Since most electronics enthusiasts and engineers are familiar with how to communicate over short distances on wired connections, the companies try to make their modules emulate a wired connection as much as possible. Non-packetized ones emulate a wire farily well at close range where the signal is strong and overwhelms both noise and interference. As the distance increases, though, errors start to come in and need to be taken into account. Packetization helps fix the errors, but causes extra delays in transfer which make the "wired" approximation break down again.
Advantages and Disadvantages of Using Modules
Clearly the greatest advantage of using prebuilt modules is the simplicity. The disadvantages are that they are often hard to find or out of stock and quite expensive. You are also constrained to the particular frequencies they use (which may be only a few within a whole band which the law allows) and to what specifications they have, such as sensitivity, selectivity, and bandwidth.
Links
Here are a few links to companies which make or sell prebuilt RF modules. Also included are some links to other sites with information about such modules.
http://www.oricomtech.com/teklink2.htm#RF2
http://www.linxtechnologies.com
http://www.rfm.com
http://www.radiometrix.co.uk
http://www.aerocomm.com/oem_products/oem_products.htm
http://www.rfdigital.com/
http://www.parallaxinc.com/
http://www.rfsolutions.co.uk/
http://www.radiotronix.com
http://www.sages.com.au/rf.htm
http://www.laipac.com/msg5.htm
http://www.rentron.com/rf_remote_control.htm
http://www.rotrax.co.za/products.asp
http://www.oem-barcode.com/index2.htm
http://www.neosid.de/rf-mod.htm
http://www.orbitcoms.com/rf_modules.htm
http://www.radiodata.co.uk/Products/rdt-rfmodules.htm
http://www.imst.de/ism-link/ism-link.htm
http://www.commlinx.com.au/rf_modules.htm
http://www.ishtronics.com/msg5.htm
http://www.apgate.com/elec/pselect/ps_1625.htm
http://www.dipinternational.co.uk/products/communications/rfmodules.html
http://abacom-tech.com/
How to Select a Prebuilt Module
You should probably first consider what is actually available and affordable. This will narrow down the choice to only a handful in most cases. Then, you need to decide what frequency range you want to operate at (you should consider how crowded the band is and what type of modulation/data can be sent there: see Spectrum and Regulatory Information). You also need to consider what type of antenna you can use (both due to legal and space constraints) and the amount of power you can transmit versus the distance you need to cover (see Radio Link Analysis). Doing the same kind of simple analysis which you would perform on a custom design (and as we performed in the second design project, using transmitter output power, antenna type, and receiver sensitivity) will show you whether a particular module will be able to cover the distance you need to cover. Some modules combine both transmitter and receiver in one package and others have them separate, which may require separate antennas as well.
Obviously, you need to check the module datasheet to see what data rates it can handle, as well as for any other unexpected "gotchas". The final consideration is whether the module is packetized. If your application is not very demanding about latency and changes in latency (see section Communication Protocols and Networks) and you need a very low output error rate, a packetized module may be best. If you need low latency or have other requirements, implementing your own protocol is the way to go.
Our Design Choice
For our design, we will use the TR-916-SC, a 916MHz transceiver (transmitter/receiver combination). You can take a look at the datasheet here. I chose this module because it is available though DigiKey and is very easy to use, requiring no external components. Unfortunately, it is expensive (about $45 in single quantities). It is not packetized, which will give us the opportunity to use this final project to focus on protocol design.
The 900MHz band has many advantages over the 430MHz band, one of the other common license-free low-power bands. Among the advantages are that resonant antennas are even smaller (about 3 inches long), there is no restriction on the type of data which can be sent (430MHz has restrictions), higher power is allowed than some of the other bands (such as 49MHz), there is less interference from household appliances as the frequency increases, the smaller wavelength means less interaction between the antenna and surrounding objects. The trade offs are that 900MHz modules tend to be more expensive due to the more critical circuitry inside, and that there is theoretically shorter range for the same power level and omnidirectional antennas, when compared to lower frequencies. This only applies in open space, though, not inside buildings or in most practical circumstances.
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