Telecoms, Datacoms, Wireless, IoT


UHF RFID explained

14 May 2008 Telecoms, Datacoms, Wireless, IoT

Anyone can buy an RFID system nowadays. In the past you would have had to be a specialist that either understood radio or computer issues, but nowadays you click on the order button, submit your payment details and the system arrives in the post.

That does not, however, mean that you are going to understand its operation or make it part of a successful application.

Anyone buying computer equipment in a computer store, will realise that the end user is being treated as a fool and that the specifications he is given about the equipment he is buying seldom go beyond the mains voltage needed to operate. All those specifications that are deemed necessary to inform him of the product, need to fit on the side of a small box and sometimes in nine languages. This is a far cry from buying computer equipment in the past when the pricelist along with the options, ran into 76 pages.

However, the equipment is not getting simpler, the users are just being told less.

UHF RFID is a technology that also has many options that affect its performance, and it is important that end users understand these choices so that they can get the right equipment for their application.

One of the most used choices, with all the hype about very low-cost transponders, is price. Not understanding the issues, users buy the cheapest transponders with the worst performance and end up with applications that will not work.

RFID performance is dominated by the choice of the operating frequency, and RFID operating in the UHF frequency band offers the best of most worlds with long range performance and potentially the lowest manufacturing costs.

There are two major classes of UHF RFID, namely those where the tag-talks-first (TTF) and the other where the reader-talks-first (RTF).

If you have an application for measuring slow-moving items passing a control, where you want very short operating range and you are labelling items that need to be sold in a retail store, then you want RTF such as EPC Gen2 type tags. These tags are relatively cheap, but the protocol causes the reader to generate a lot of radio interference for other users in the vicinity and so very few readers can operate in close proximity.

Usually these readers will be switched off most of the time and will only be activated when a pallet is passing so that other readers can use the spectrum. You cannot use these types of transponders for high speed situations, or for sports timing situations, or for theft control, as the time when the reader will successfully communicate with a transponder is uncertain as a result of the RTF protocol.

The other protocol (TTF) allows critical situations to be continually monitored, allowing fast moving tags to be measured, and can provide repeatable, accurate measurements even in situations where multiple readers are in use at the same time.

The reader emits a continuous energy field which provides power to the tags and a frequency reference for them to use for communication. As the energy field is constant, little interference is caused and many readers can operate simultaneously and continuously in close proximity. The tag responds when entering this energy field and receiving enough energy to operate.

This response can come within thousandths of a second of it receiving enough power, which means that it is suitable for sports timing in many situations, can handle transponders attached to speeding items (up to 300 kph), and can be used to monitor access points for anti-theft, asset and monitoring purposes as the energising field is continuously active and goods will not be able to pass through undetected. TTF tags also usually have longer operating ranges, providing a larger coverage field in front of readers.



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