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data at a pre-determined rate back to the S ra eM u tnocfu continually transmit its identity and other cally configured with an interval of around a d P u - nIgln- reader or a base station – tags are typi from a reader before it can initiate a data Fig. 1: ams active RFID reference design Ta s o m rserf/zH008n00r4--his gives the system designert one or two seconds between transmis sions. considerable extra flexibility compared to a passive RFID system, in which the pas sive tag must first receive a transmission transmission. In an active RFID system, – base station block diagram. tity to a reader over a relatively long range (up to 100m) even if the tag’s receive path Now...- by contrast, a tag can broadcast its iden is compromised, for instance because of interference or low antenna sensitivity. the high-power UHF transmitter is required to operate every one or two seconds. an up tohis mode of operation, however, entails relatively high power consumption, sincet alternative approach allows the active tag to remain in a deep power-down state al- most permanently. to achieve this, the system requires a low-frequency (LF) wake-up 150 receiver, which waits to receive an incoming signal from a nearby reader before initiat- ing a UHF transmission. Low-frequency transmissions are little impaired by reflections from materials such as rock and liquids, and also support operation at a lower current Watts than UHF transmissions. a well-designed LF receiver can operate continuously on a tiny current similar to a battery’s leakage current, and thus has a negligible effect on the tag battery’s life. • 0.4 Watts to 150 Watts Now, however, the range of the system is the smaller of the receive range of the Power Transformers tag’s LF receiver and the transmit range of the tag’s UHF transmitter. Clearly this means the sensitivity of the LF receiver is of high importance. this low-power archi- • 115V/26V-400/800 Hz Primary tecture can be evaluated in a reference design kit developed by ams. so how can this implementation of the low-power architecture described above ensure years of opera- • S.5V nd3ry VoltagesV00aotoce2 tion on a small battery? • Manufactured to MIL-PRF 27 Architecture of the ams reference design: base station Grade 5, Class S, The base station consists of an LF wake-up transmitter, a 2.4GHz RF transceiver (the (Class V, 1550C available) AS3940) and a microcontroller - see figure 1. The base station is powered via its USB • Surface Mount or Plug-In interface. As stated above, the effective range of the system is limited by the receive range of the tag’s LF receiver. In order to maximize the base station’s LF transmit • Smallest possible size range, a power management IC (PMIC) is used, supporting a high voltage input to the network (MN) and the PCB antenna. LEDs indicate the status of the base station. www.picoelectronics.comleyltaidemmigolataClufs’ociPeeShe LF transmitter is based on discrete transistor circuitry, a matchingt low-power microcontroller (MCU) controls the operation of the LF anda LF antenna. UHF protocols. the base station’s primary task is to continually transmit its LF wake-up pattern and its own identification data. It also collects return signal strength indicator (RSSI) information and identification data from tags that are within receive range, and com- municates this to a host device or controller. Architecture of the ams reference design: active tag PICO Electronics,Inc. The tag consists of an LF (15-150kHz) wake-up receiver, the AS3933, the AS3940 143 Sparks Ave. Pelham, N.Y. 10803 2.4GHz UHF frequency shift keying (FSK) transceiver, and an ultra-low power MCU - E Mail: info@picoelectronics.com see figure 2. As stated above, the sensitivity of the wake-up receiver is a crucial factor in determining the effective range of the complete system. This isaddressed in a clever design which takes advantage of the AS3933’s three-channel input. In the chal- Pico Representatives lenging applications to which active RFID Germany technology is suited, the orientation be- ELBV/Electronische Bauelemente Vertrieb tween the base station and the active tag E-mail: info@elbv.de is normally not fixed. The ams reference Phone: 0049 89 4602852 design thus uses a three-dimensional an- Fax: 0049 89 46205442 tenna system, with an antenna in each of England the x, y and z axes each feeding one of Ginsbury Electronics Ltd. the device’s inputs. these three LF coils E-mail: rbennett@ginsbury.co.uk are combined in a single package. the Phone: 0044 1634 298900 AS3933 offers typical receive sensitivity Fax: 0044 1634 290904 of 80µVrms. the UHF transmit path is implemented Fig. 2: ams active RFID reference design via the 2.4GHz transceiver with its – active tag block diagram. www.electronics-eetimes.com Electronic Engineering Times Europe January 2013 43


EETE JANUARY 2013
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