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09/02/2009
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A radio is pretty much a radio, so why are there so many types in development?
 According to John Schwartz, technology strategist at Digi International, the short answer is that different radios and protocols fit better in some situations and applications than others.
“Point to multipoint has certain advantages over mesh and vice versa,” Schwartz explained. “More output power equates to longer range, but more current draw and some frequencies are not accepted in certain countries.”
In most vertical markets, there seems to be a trend towards wireless connectivity, so embedding a module can offer a lot of flexibility – especially if the end product will be deployed in several environments with different products. With so many different frequencies, every device that emits radio signals has to comply with government regulations in the country where the device is installed. “If radio modules have the proper certifications, they can save tens of thousands of dollars in required testing and shave months off of time to market.”
Schwartz pointed out that testing modules with precertifications are available to make this process easier. “The US has USM bands at 902 to 928MHz, 2400 to 2483MHz and 5.8GHz still available,” he said. “But because these frequencies allow for use without the need for specific site licences, they tend to be the most popular and easiest to implement.”
“In Europe,” he added, “the ISM bands vary somewhat with 2.4 and 5.8GHz still available, but 868MHz is used instead of 900MHz because of mobile phone frequencies. Lower frequencies have better range and obstacle penetration than higher frequencies. If a 900MHz radio and a 2.4GHz radio had the same output power and receive sensitivity, the wave from the 900MHz would go approximately twice as far or penetrate twice as many walls as the 2.4GHz signal.”
In addition to agency certifications, some module families are designed so they can be interchanged without modification to the host pcb. It is typical for modules to be pin compatible and interface with the host mcu over a 3.3V uart. Digi’s Xbee modules can also operate with either a transparent mode or with a standard, compatible API that allows the modules to be exchanged without having to change the host mcu’s firmware. Because the radio module is essentially a daughter card to the main system – and as long as a similar form factor is used – modules can be interchanged within the same system whenever necessary.
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Author
Chris Shaw
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