Implement low-frequency radio in active RFID systems
RFID (Radio Frequency Identification) is pervasive in asset tracking and logistics applications. Most applications use passive tags, which are low cost and have no power source, so they generate their energy from the electromagnetic radiation emitted by the reader. Thus the range of passive tags is limited.
Some RFID systems, however, are required to be read at great distances or need to operate in environments that are challenging to RF transmissions, such as inside metal shipping containers and buildings with walls made of masonry.
For such applications, an active RFID system is needed. Active RFID tags include their own battery power source. This enables the use of an active transmitter (e.g. UHF) to reply back to the reader, which consistently can achieve longer range than a passive tag.
Active tags must have long battery life and a long range – two requirements that seem to be in conflict with each other. This article will examine the basic architecture and the tradeoffs of active RFID tag design. It will also discuss how the overall power consumption of the tag can be reduced by careful design of the low frequency wakeup receiver.
These constraints drive the system architecture shown in Figure 1. The interrogator (base station) is made up of an LF transmitter and a UHF receiver, while the tag consists of an LF wake-up receiver and a UHF transmitter. The interrogator periodically transmits (typically once a second) an LF pattern. After the transmission, the UHF receiver is switched on to check for replies from tags. At the tag, only the wake-up receiver is active in normal operation. The receiver wakes up the UHF transmitter (uplink) on detection of a valid pattern that it recognizes. Only then does the UHF transmitter transmit the information required to unambiguously identify the tag to the interrogator. This architecture allows the UHF radio to stay in power-down mode almost continuously.
The LF receiver is key to the overall power consumption since it is the only element that must always be active. Sensitivity is another key parameter of the LF receiver, because the signals received may be attenuated by distance and physical obstructions.
The industry requirement for battery life is three years (minimum) from a simple coin battery such as a CR2032. This constraint implies that the tag’s current consumption should be limited to little more than the battery leakage current while in Receive mode. This makes the selection of operating frequency extremely important. To meet the low power requirement, the receiver must operate at < 300 kHz; RFID systems typically use the 125 kHz or 134 kHz frequencies.
At such low frequencies, the wavelength is large and requires a correspondingly large antenna. Successful designs use loop antennas, which mainly sense magnetic fields (H). A loop antenna is essentially an inductor made up of coils of ferrite rods tuned with a parallel capacitor at the wanted frequency. As Figure 2 shows, the LF transmitter in the base station and the LF receiver in the tag work like a transformer, where the inductor at the transmitter is the primary coil and the one at the receiver is the secondary coil.
Effective tuning is one measure that increases the sensitivity of the LF receiver, but the use of loop antennas poses another problem. Since the antennas sense a magnetic field, the orientation between the base station (transmitter) and the receiver has an important effect. Electromagnetic theory dictates that if the two coils show a 90-degree angle shift in space, the induced voltage on the secondary coil is in theory zero. If the mutual orientation is not fixed and predictable, the receiver requires a three-dimensional antenna array, comprised of three antennas orthogonal to each other.
The combination of carrier frequency tuning and three-dimensional antennas not only extends receive range, it also allows the production of reliable Received Signal Strength Indicator (RSSI) measurements. RSSI information can be useful in some applications because it provides an estimate of the distance between the tag and the base station. Another advantage of using LF wake-up receivers is that the RSSI signal is always stable and consistent since it is not affected by wave reflection and EM absorption.
In order to save power and keep the UHF transmitter off as much as possible, the wake-up receiver must be able to reject false wake-up calls generated by noise or disturbance. Implementing a code or pattern-generation capability in the interrogator, and a pattern-recognition capability in the receiver can prevent this.
Active tag performance in practice
Typically active tags use single coin batteries of around 200 mAh capacity, and the minimum expected lifetime is three years. This implies an average total current draw of approximately 7.6 µA. Assuming that on average half of the current is used by the UHF transmitter, the wake-up receiver can draw a maximum current of 3.8 µA.
The main constraint on the range of the system is the sensitivity of the wake-up receiver. An LF wake-up receiver should have a sensitivity of at least 100 µV, which allows the active tag to achieve a 10 to 15 meter range with just a simple transmitter. Active tags can have very robust performance even in hostile environments: the low frequencies used can penetrate even extremely thick walls. The tag’s UHF transmissions typically cover the required range even at output power as low as 0 dBm.
The latest approach to implementing LF wake-up receivers is exemplified by austriamicrosystems’ AS3933 LF wake-up receiver. The AS3933 has a typical current draw in three-channel listening mode of just 1.7 µA and typical wake-up sensitivity of 80 µVrms. The wake-up interrupt can be triggered by frequency detection only. However, to guarantee false wake-up rejection the device includes an integrated correlator that detects programmable 16- or 32-bit Manchester wake-up patterns. Avoiding false wakeups saves precious power.
As discussed, frequency tuning is an important technique for improving sensitivity and effective range. austiamicrosystems’ AS3933 LF wake-up receiver implements an automatic tuning function using on-chip tuning capacitors. This reduces bill of materials cost, since external high-precision tuning capacitors are not required. It also enables antenna failure detection on the production line. Furthermore, the on-chip antenna tuning capability provides a means for end users to check the connection status between the tag and reader.
For more information, go to http://www.ams.com/eng/Products/RF-Products/Low-Frequency/AS3933