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Short-range devices pose tradeoffs

Posted: 17 Aug 2007 ?? ?Print Version ?Bookmark and Share

Keywords:home networking? short-range devices? SRD radio transceiver?

As low-power wireless technologies compete for dominance in replacing wires and cable in home networking, automation and sensor systems, the importance of short-range radio transceivers has grown. And, as this market for short-range devices (SRDs) expands, the idea of a one-size-fits-all transceiver is simply out of the question. Designers of these new systems need to consider the device trade-offs and their impact on link robustness and battery life.

A recent survey of the market found that many leading companies, such as Analog Devices Inc., AMI Semiconductor, Atmel Corp. and Freescale Semiconductor, are involved in developing and supporting SRD transceiver lines that operate in the industrial, scientific and medical (ISM) band. (This is not intended to be an exhaustive list of suppliers for SRD radio devices.) Designers will find a variety of transceivers available for use in home and building automation, wireless sensors, alarm systems, remote automotive applications and home networking.

Going wireless
"Signal range and link robustness are critical components to consider when selecting RF transceivers for the design of ISM and SRD applications. These features are important because low current consumption and link reliability are key when switching from a wireline to wireless design," said Dave Boylan, product line director for RF and wireless systems at ADI.

As with most other RF devices, the trend with SRD transceivers is to increase integration with each new generation. At AMI, industrial RF system architect Craig Christensen and industrial wireless product manager Paul Pulley said: "Newer transceivers are becoming easier to use and have added features that improve their versatility and performance. In many applications, they can be an attractive alternative to running a cable or wire."

Prime considerations
So what are the key specifications for an SRD transceiver? Top contenders include frequency range, modulation scheme, receiver sensitivity, interference blocking (image rejection) and current consumption. Designers should also ensure that the devices conform to regulatory requirements, such as those from the FCC and the European Telecommunications Standards Institute. Boylan contended that "ADI's ADF70xx transceivers offer the best compromise between regulatory compliance and system performance."

Belkin's USB Hub and dongle

As with most other RF devices, the trend with SRD transceivers is to increase integration with each new generation.
(Click to view image.)

There are other design considerations beyond the numbers. The level of functionality integrated into the device, for example, directly addresses ease of use, which is particularly important if designers are not well versed in RF engineering. It also addresses cost, in terms of keeping the design simple and the BOMs smaller.

For instance, J. Mark Foster, senior RF applications engineer at Atmel, said ATA542x ASK/FSK multichannel transceiver ICs are highly integrated, requiring only 13 passive external components and one crystal.

Additionally, the AMI Semiconductor AMIS-53050 is capable of autonomous transmit operation without an MCU and external memory, which means multiple channel sensor data can be wirelessly transmitted using the company's intellectual property, an integrated ADC, programmable transmit times and duration and on-chip memory.

At Freescale Semiconductor, Ryan Kelly, global marketing and applications manager, observed that his company's designs only require an off-chip antenna and crystal. "This provides for reduced size, simplicity of board design, and reduced manufacturing cost," Kelly said.

Sleeping, design issues
In a typical wireless low-power network, the transceiver spends most of its time in sleep mode and its power consumption while sleeping is crucial in terms of battery life. At predetermined intervals, the transceivers awaken, listen for a signal from the master, respond if a valid signal is detected and then go back to sleep. For example, the Atmel ATA542x consumes less than 10nA in off (or deep sleep) mode and 220?A in idle mode (the stage just before the radio becomes active).

Unlikely to have the option of developing a fully customized device, designers must carefully weigh the performance tradeoffs when using a general-purpose transceiver for their applications.

Christensen and Pulley stated: "It is also important that the RF transceiver vendorespecially if it is a chip or device providerdemonstrate compliance to applicable government regulations with respect to the operation of the device."

Vendors are also stepping up to help designers complete their SRD systems. Many offer reference de- signs/boards and tech support.

In addition, ADI recently announced its SRD Design Studio, a free software download that assists with the design of SRD systems through desktop simulation.

- Janine Love
RF DesignLine

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