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EETE JANUARY 2013

RFID TECHNOLOGIES Graph 4: Tag 9 minimum power Graph 5: Tag 9 minimum power Graph 6: Tag 9 modulation depth response versus frequency to get a response versus frequency to get a versus frequency. correct RN16 response. correct RN16 response. frequency range standard 860 to 960MHz. Graphs 6 and 7 show the modulation depth versus frequency for tags 7 and 9, Graph 7: Tag respectively. 7 modulation One can see that tag 9 presents the best modulation depth depth versus at 880MHz. In other words, at 880MHz the transmitter load frequency. presents a load real: short / open. Looking at the shape of the modulation depth, we can say that the associated transmitter load is like an LC resonator. at 880MHz the load is real, around it the load is real and imaginary. Tag 7 presents the same behaviour, centred at 890MHz. Its peak modulation depth is smaller. In that case, the switch in tag 7 used to connect or disconnect the inductor L is too resistive. Active RFID systems that combine robust RF performance and low power consumption By shawn Rezaei RADIO FREqUENCyIDENTIFICATION (RFID), as an automatic masonry or liquid. sometimes, wrongly oriented tags will limit wireless data collection technology, is commonly used in ap- readability. plications such as asset tracking, access control and inventory Under these conditions, an ‘active RFID’ system, in which management. the common way to implement the technology each active tag carries its own battery power source, offers far is in the form of passive RFID systems, in which an RFID reader more robust communications performance. the battery power transmits a modulated RF signal to RFID tags each consist- source enables the tag to support high-power transmission, ing of an antenna and an IC. the chip receives power from the providing for greater range, and the ability to connect even antenna and responds by varying its input impedance, and thus through solid barriers of metal or masonry. modulating the backscattered signal. the ability to backscatter But this very property of an active tag is also its main limita- is strongly dependent on the surface on which the tag is mount- tion: high-power UHF transmitters are power-hungry devices. ed. A conductive material close to the tag can adversely affect so how can an active RFID system be designed to provide ex- performance by detuning the tag and limiting the read range. cellent communications performance while operating for several as a result, passive RFID systems have limitations due to any years on a cheap, small battery? one or more of the following factors: this article describes the operation of an active tag RFID tag power is limited (the power at the tag determines its read system, and then introduces a reference design which demon- range). Tag reflection affects the signal that is scattered back strates a way to square the circle of high RF output power and Surface on which the tag is mounted affects performance. low system power consumption. Environmental factors can include multi-path fading and inter- fering readers. above a certain speed, moving tagged objects The operation of active RFID systems cannot be interrogated by a reader. there is mutual coupling Active RFID usually operates at 455MHz, 850MHz, 900MHz, between tags. Performance degradation can be due to low 2.4GHz or 5.8GHz. It is suited to applications such as asset and reader transmit antenna gain and low reader Receive sensitivity. people tracking, access control, passive keyless entry in cars, some systems have the inability to transmit through metal, rock, parking management systems and temperature monitoring. as stated above, active RFID tags have greater transmission power shawn Rezaei is Field application Engineer (Us) at ams - than passive tags. Another difference in active RFID systems www.ams.com is that an active tag using its own battery power source can 42 Electronic Engineering Times Europe January 2013 www.electronics-eetimes.com


EETE JANUARY 2013
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