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Instant wireless?

Do you design your own rf module or is it better to use a predeveloped module? By Graham Pitcher.

The current downturn in demand for electronics components has been brought about, at least in part, by a drop in demand for mobile 'phones. In the UK, for example, market saturation means that sales are slow and the traditional 'churn' – as users trade up for better models – is not helping much. Many manufacturers are now looking to 3G communications as one way to boost the industry's fortunes.

And there has been quite a lot of attention paid to Bluetooth and to IEEE802.11 wireless communications. Whilst addressing different areas of the communications business, applications of these two emerging technologies are set to increase dramatically over the next few years.

Yet these technologies represent only part of the wireless communications business. Alongside gsm, Bluetooth and wireless lan, a number of different approaches are available to those companies who want to offer the ability to communicate without wires.

A significant proportion of these products access the licence free industrial, scientific and medical (ISM) band, which centres on 2.4GHz. But it is only in the last decade that this area of the spectrum has been opened up; initially in the US, with the UK following in the mid 1990s. And the attraction of the band is that Bluetooth and IEEE802.11 devices also operate at 2.4GHz.

One of the advantages of using the ISM band is that your products are almost certain to work anywhere in the world, so long as there is some equipment with which to communicate. The other benefit is that, so long as your product has been approved by a body such as the US Federal Communications Commission or ETSI in Europe, it will generally be accepted worldwide.

Having decided to design a wireless communications enabled device to run at 2.4GHz, the next decision to make is whether to start with a clean sheet of paper or to adopt existing technology.

According to Jim O'Callaghan, vice president for oem sales and marketing with US based Aerocomm (www.aerocomm.com), it can be beneficial to take the latter route. "We can assist oems by becoming, in effect, their rf engineering team. This can be a significant business driver considering the time to market pressures which companies experience today. We fill the void by providing the technology capability and a price competitive product with approvals."

Aerocomm, which has been in existence since1990, introduced its first wireless product in 1995. It claims to be the first company to gain FCC approval for a 2.4GHz direct sequence spread spectrum (dsss) radio in 1994. In 1996, it changed its focus to frequency hopping spread spectrum (fhss) technology. According to Aerocomm, this was because fhss technology allowed it to produce radios which consumed less power, had greater resistance to interference, were smaller and – importantly – were lower cost.

O'Callaghan continued: "We're about 'instant' wireless, working with oems who are looking to create robust industrial applications which benefit from fhss technology." In fact, O'Callaghan claims Aerocomm is the largest supplier of 2.4GHz fhss rf modules.

Aerocomm's oem products are fhss transceivers, which transmit signals by sending a packet of data on one frequency and then 'hopping' to another frequency to transmit the next packet. This hopping continues throughout the transmission. For the transmission to be successful, both transceivers must be synchronised, so they hop to the same frequency and remain there for the same amount of time. Aerocomm's products hop over 77 channels, with each channel in use for 100ms. O'Callaghan said that, because of the frequency hopping technique, higher power can be used.

With an operating temperature range of -40 to 80oC, the modules can withstand the harsh conditions of both outdoor and factory applications. Each radio is calibrated to assure consistent performance, even at extremes of temperature.

Aerocomm says fhss technology helps reject interference in rf hostile areas, whilst its proprietary protocol further ensures data security and integrity. All frequency hopping, synchronisation and rf system data transmission/reception is performed by the module.

Although the 2.4GHz ISM band is attracting a lot of attention – including those developing Bluetooth and wireless lan applications – there is a limit on how much power a device using this band can radiate. In the UK, the maximum emitted power is restricted to 100mW. O'Callaghan said, despite this, fhss communications in the ISM band could still span a reasonable distance. "One of our AC3124 transceiver modules can transmit data at 115kbit/s over a distance of 1km." Higher powered versions for use in the US extend that distance to some 3km. Indoors, the transmission distance is reduced – probably to 10% of the outdoors figure.

AC3124 transceivers (available in the UK from Low Power Radio Solutions, www. wireless-radio.org) are suitable for integration into portable and fixed devices and can be supplied in a 10mW version for battery powered applications and in a 200mW version. The devices are complete rf modules with a choice of approved antennas.

The transceiver operates in a point to point or point to multipoint client/server architecture. One transceiver is configured as a server; there can be one or many client transceivers. An assortment of radios is available to suit the varying range and network configurations of stationary as well as portable applications

Aerocomm's ConnexSync technology enables high data throughput, lower power consumption and increased cost efficiency. Advanced algorithms allow multiple AC3124 transceivers to synchronise three times as quickly as competitive devices. Precise timing then keeps the radios in sync, resulting in higher bandwidth.

Problem solving
US based Hunter Engineering develops and markets wheel alignment systems. Its DSP250 and DSP300 sensors measure the primary wheel alignment angles – camber, caster and toe – to an accuracy of 0.01o. This data is transmitted via an Aerocomm transceiver to a main console.

Initially developed for use in the US, Hunter's line of high frequency spread spectrum sensors were compliant with FCC standards. But the need to address other markets required the use of spread spectrum technologies in the 2.4GHz band. Hunter planned to introduce its wheel alignment system at a major international show but, in order to do so, had to bring its rf technology into compliance with international standards in just four months.

This responsibility fell to Hunter R&D engineer Jim Smith. He could either modify Hunter's rf technology to bring it into compliance or he could look for an existing technology and attempt to integrate it. "Neither option was attractive," said Smith.

"No matter which direction we turned, we seemed to be faced with an engineering mountain to move. Starting from scratch would require us to overcome a steep learning curve, whilst prepackaged solutions tend to favour simple applications. We needed tight integration with our dsp sensors and that comes from custom engineering."

The environment is challenging for rf communication. Wheel alignment is measured in small bays. Whilst the distances rarely exceed 15m, the nature of the testing facility contributes to multipath fading: walls and other objects – including the vehicle – reflect the signals. And reflected signals result in a small time lapse, creating an echo effect for the receiver.

"Aerocomm provided an ideal transceiver for our needs and offered the experience and commitment to make sure our project was a success," Smith concluded.

And that is the major benefit of the oem module approach, said O'Callaghan. "It gives a faster time to market than the use of chipsets, it saves r&d cost and time and it saves agency and lab fees."

Author
Graham Pitcher

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