RFID Readers for the HDX Protocol

Description: The upper traces display voltages VOUT, while the lower traces illustrate transients. A frequency transition occurs at 500 microseconds. In Figure 16, the upper trace represents a baseband signal from a simulator operating in FDX mode, retrieved by a reader. The middle trace shows the digital signal on the simulator's LOADMOD line that generated the upper trace, and the lower trace depicts a baseband signal retrieved from an FDX tag. Figure 20 presents input signals VIN12 and VIN2 fed sequentially into the same system, with the traces above the signals indicating the trigger provided by the simulator. The lower section shows superimposed signals for overlap display. Figure 21 examines the impact of transients on bit decoding, with the upper traces showing a signal amplified by the reader, the middle traces indicating transients induced by the transition (noting that only the portion following the transition represents the transient), and the lower traces showing the trigger at transition provided by the simulator. Figure 22 visualizes the frequency transition generated with the tag simulator after applying equalization of bit amplification, with traces having the same meaning as in Figure 21. Previous studies (Gelinotte et al., 2006; Vuza et al., 2007; Vuza et al., 2009) highlighted the contributions of the authors to developing readers for communication with tags according to the FDX protocol. The readers produced by Frosch Electronics have been classified as voltage-driven and current-driven (Vuza et al., 2009), based on which circuit variable is controlled by the reader and which is controlled by the tag (and sensed by the reader). A voltage-driven reader powers the antenna with a constant amplitude AC voltage, while the FDX tag transmits data through load modulation, causing voltage variations at the tap point (the junction between the antenna coil and the tuning capacitor). The reader senses this voltage and extracts the baseband signal containing the data. Conversely, a current-driven reader powers the antenna with a constant amplitude AC current, with the FDX tag again transmitting data via load modulation, modulating the voltage across the entire antenna circuit. The reader extracts the data from this voltage, thus eliminating the need for a tap point connection. Recently, Frosch Electronics decided to enhance existing readers by enabling communication with tags using the HDX protocol, which is particularly useful in applications like animal identification, allowing the same reader to cater to a wider range of applications. In FDX, the tag is continuously powered by the reader, whereas in HDX, the tag is first charged by a limited-duration RF pulse from the reader and subsequently transmits data using energy stored during the initial step, driving its coil with an AC voltage that toggles between two frequencies: fC = 134.2 kHz and fLOW = 123.7 kHz, as prescribed by the HDX standard. Following a brief overview of the two types of FDX readers in section 2, sections 3 and 6 present circuit topologies for achieving the HDX extension for each class of readers. The topologies differ based on the accessibility of the tap point. For voltage-driven readers, the schematic utilizes the TMS3705 circuit as a bit decoder. For current-driven readers, bit decoding may be accomplished either through a dedicated IC or by constructing a custom decoder from discrete hardware components, supplemented by a software component that could provide enhanced control over the decoding process. Section 4 addresses the critical issue of transients, which must be considered to ensure data integrity and reliability.

The electronic schematic described involves a complex interaction between readers and tags operating under different protocols, specifically FDX and HDX. The voltage-driven and current-driven reader classifications highlight the different operational characteristics and requirements of the circuits involved. The voltage-driven reader's design focuses on delivering a constant amplitude AC voltage to the antenna, which necessitates careful consideration of the modulation techniques employed by the FDX tags. Load modulation is a key aspect, as it allows the tag to communicate data by altering the impedance seen by the reader, which in turn affects the voltage at the tap point.

In contrast, the current-driven reader operates by supplying a constant amplitude AC current, which influences the entire antenna circuit's voltage. This approach simplifies the design by eliminating the need for a tap point, thereby potentially enhancing the reliability of signal extraction.

The introduction of HDX protocol compatibility expands the functionality of the readers. The implementation of RF pulse charging followed by energy-efficient data transmission requires precise timing and control within the circuit design. The toggling frequency operation necessitates robust filtering and amplification strategies to ensure that the reader can accurately decode the transmitted signals while maintaining signal integrity in the presence of transients.

The proposed circuit topologies for both voltage-driven and current-driven readers will likely include various components such as operational amplifiers for signal amplification, analog-to-digital converters for digitizing the received signals, and microcontrollers or dedicated ICs for processing and decoding the data. The selection of components will depend on the specific requirements of the application, including power consumption, size constraints, and complexity of the decoding process.

Transients represent a significant challenge in the design of these readers, as they can introduce errors in the decoded data. Therefore, the circuit must incorporate measures such as proper grounding, shielding, and filtering to mitigate the effects of transients. The design should also consider the operational environment, as variations in temperature and electromagnetic interference can affect performance.

Overall, the development of these readers represents a significant advancement in RFID technology, enabling more versatile applications while ensuring reliable communication between the reader and various types of tags.Upper traces show voltages VOUT, lower traces show transients. Frequency transition at 500 us. Figure 16. Upper: baseband signal from simulator in FDX mode retrieved by reader. Middle: digital signal on simulator LOADMOD line that generated the upper trace. Lower: baseband signal retrieved from an FDX tag. Figure 20. Upper: input signals VIN12 and VIN2 fed one after the other to the same system. The traces above the signals show the trigger provided by the simulator. Lower: signals superimposed for displaying overlap condition Figure 21. Effect on transients on bit decoding. Upper traces: signal amplified by reader. Middle traces: transient induced by transition (note that only the part that follows the transition represents the transient). Lower traces: trigger at transition provided by simulator. Figure 22. Scope visualization of frequency transition generated with the tag simulator, after application of equalization of bit amplification.

Traces have same meaning as in figure 21. Previous work ( Gelinotte et al. , 2006 ; Vuza et al. , 2007 ; Vuza et al. , 2009 ) presented the contribution of the present authors to the development of readers for communication with tags according to the FDX protocol. Readers produced so far by Frosch Electronics were classified as voltage-driven and current-driven ( Vuza et al.

, 2009 ), according to which circuit variable is controlled by the reader and which is controlled by the tag (and sensed by the reader). A voltage-driven reader powers the antenna with an AC voltage of constant amplitude. The FDX tag transmits data by load modulation, which causes the variation of the voltage at the tap point (the junction between the antenna coil and the tuning capacitor).

The reader senses the latter voltage and extracts the baseband signal that contains the data. A current-driven reader powers the antenna with an AC current of constant amplitude. Again, the FDX tag transmits data by load modulation, which this time modulates the voltage across the whole antenna circuit. The reader extracts the data from the latter voltage, the tap point connection being not needed in this case.

Recently, Frosch Electronics decided to add a new feature to the existing readers, providing them with the possibility of communicating with tags that use the HDX protocol, of interest in applications such as animal identification, so that the same reader could cover a larger variety of applications. In FDX, the tag is continuously powered by the reader. In HDX, the tag is first charged by an RF pulse of limited duration from the reader, and then it transmits the data using the energy stored during the first step, by driving its coil with an AC voltage whose frequency toggles between two values fC = 134.

2 KHz and fLOW = 123. 7 KHz prescribed by the HDX standard. After a brief reminder on the two types of FDX readers in section 2, we present in sections 3 and 6 circuit topologies achieving the HDX extension for each of the mentioned classes of readers. The topologies are different for the two classes according to whether the tap point is accessible or not.

For voltage-driven readers, the schematic is based on the usage of the TMS3705 circuit as a bit decoder. For current-driven readers, we consider the option of bit decoding by either a dedicated IC or by building a custom decoder from discrete hardware components supplemented by a software component, which could in principle offer more control over the decoding process.

In section 4 we discuss the important issue of transients, which has to be taken into account when striving for data integrity, and hence reliability. Transi