Solve high power consumption challenges in active RFID systems
Shawn Rezaei, ams
Active RFID usually operates at 455 MHz, 850 MHz, 900 MHz, 2.4 GHz or 5.8 GHz. It is suited to applications such as asset and people tracking, access control, passive keyless entry in cars, parking management systems and temperature monitoring.
Active RFID tags have greater transmission power than passive tags. The active tag using its own battery power source can continually transmit its identity and other data at a pre-determined rate back to the reader or a base station – tags are typically configured with an interval of around one or two seconds between transmissions. This mode of operation, however, entails relatively high power consumption, since the high-power UHF transmitter is required to operate every one or two seconds.
An alternative approach allows the active tag to remain in a deep power-down state almost permanently. To achieve this, the system requires a low-frequency (LF) wake-up receiver, which waits to receive an incoming signal from a nearby reader before initiating a UHF transmission. Below is a great example to show how to solve the high power consumption by using a different approach.
Architecture of the reference design: base station
The base station consists of LF wake-up transmitter, a 2.4-GHz RF transceiver (the AS3940 from ams) and a microcontroller (Figure 1). The base station is powered via its USB interface. In order to maximize the base station’s LF transmit range, a power management IC (PMIC) is used, supporting a high voltage input to the LF antenna.
A low-power microcontroller (MCU) controls the operation of the LF and UHF protocols. The LF transmitter is based on discrete transistor circuitry, a matching network (MN), and the PCB antenna. LEDs indicate the status of the base station.
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 communicates this to a host device or controller.
Architecture of the reference design: active tag
The tag consists of an LF (15 kHz to 150 kHz) wake-up receiver, the AS3933, the AS3940 2.4-GHz UHF frequency shift keying (FSK) transceiver, and an ultra-low power MCU (see Figure 2). The sensitivity of the wake-up receiver is a crucial factor in determining the effective range of the complete system. This is addressed in a clever design which takes advantage of the receiver’s three-channel input (Figure 3). In the challenging applications to which active RFID technology is suited, the orientation between the base station and the active tag is normally not fixed. The ams reference design thus uses a three-dimensional antenna system, with an antenna in each of the x, y and z axes each feeding one of the device’s inputs. These three LF coils are combined in a single package. The AS3933 offers typical receive sensitivity of 80 µVrms.
The UHF transmit path is implemented via the 2.4-GHz transceiver with its matching network (MN) and the PCB antenna. The LEDs operate as status indicators. In a system based on this reference design, an active tag can be expected to operate for a typical period of three years on a small CR2032 coin cell.
The base station works in the following manner: The MCU initializes the LF wake-up receiver and the UHF transceiver with updated register settings. It then transmits the LF wake-up signal. After the LF transmission, the MCU switches the 2.4-GHz transceiver to receive mode, waiting for active tags to respond.
When a tag responds, it needs first to be temporarily paired to the base station. Once the devices are paired, one streaming data packet is expected. The tag’s ID and RSSI information are contained in this packet. (If pairing is not accomplished, the base station will issue a second temporary pairing command after a period of time.) As soon as the packet has been streamed, the base station switches back to receive mode, waiting for another tag to respond.
The tag’s operation mirrors that of the base station. On power-up, the MCU initializes the LF wake-up receiver and the UHF transceiver. The RC oscillator of the LF wake-up receiver is calibrated and the 3D antenna is automatically tuned by the AS3933. After this, the tag goes into deep sleep mode, waiting for an LF wake-up signal.
Once a wake-up signal is received, the device checks that its pattern corresponds to its reference pattern. It then generates an external interrupt to bring the MCU out of sleep mode. The MCU reads the RSSI and the base station ID contained in the LF signal.
The tag will have a defined UHF transmission time slot assigned to its ID. To conserve power, the tag goes into sleep mode again until this time slot arrives. During the reserved time slot, the tag establishes a 2.4-GHz connection with the base station, pairs with it and streams a data packet. Afterwards, the tag goes into sleep (LF receive) mode again.
Benefits of the ams low-power active RFID implementation
The advantage of the architecture described here is its ultra-low power consumption, the result of keeping the UHF transmitter in almost permanent power-down mode. This is enabled through the use of an LF wake-up receiver, the AS3933, which features a Manchester decoding capability. This allows the implementation of pattern recognition, so that the system avoids false wake-up calls generated by noise or interference. Thus the AS3940 UHF transceiver only operates when in the vicinity of a base station. The rest of the time, the tag draws just a few uA.
The AMS active tag solution (Figure 4) has been successfully implemented in a number of end products, including access control, real-time location systems, and in passive keyless entry systems.
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