Receiver Circuit Design of UHF RFID Reader Based on ISO18000-6C Protocol

The reader in an RFID system is typically composed of several key components: a control unit, a high-frequency transceiver module, an antenna, and various interfaces that connect to the backend system. A transponder, commonly referred to as a tag, serves as the data source for RFID reading. It mainly consists of an antenna and a microelectronic chip. The reader plays a crucial role in the RFID system, and the core technology behind it lies in the receiving circuit. This paper focuses on analyzing and designing the receiving circuit of a UHF passive RFID reader. 1. Basic Architecture The circuit structure of the RFID reader is illustrated in Figure 1, with the upper half representing the transmit path and the lower half representing the receive path. The UHF reader first applies PIE coding to the baseband signal and then performs 80% to 100% ASK modulation. On the receiving side, the system must support demodulation using PSK or ASK, and it can also implement Miller or FM0 subcarrier decoding of the baseband signal. The transmitted signal in the system can be represented as: $$ f(t) = m(t)\cos(\omega_c t) $$ Where $ f(t) $ is the signal received by the tag, $ m(t) $ is the baseband signal, and $ \omega_c $ is the carrier frequency. During the backscattering process, the receiving end receives the following signal: $$ g(t) = B(1 + m'(t))\cos(\omega_c t + \phi) $$ Here, $ g(t) $ represents the signal received at the receiver, and $ m'(t) $ is the baseband signal from the tag. After down-conversion in the receiving circuit, the demodulated signal is obtained. The result of this process is shown in the figure below: $$ s(t) = \text{LPF}(g(t) \cdot \cos(\omega_c t)) $$ Where $ \text{LPF} $ denotes the low-pass filter. In the circuit, the circulator isolation device usually provides 20–30 dB of isolation. Using two separate antennas for transmission and reception can further improve isolation. It should be noted that during the receiving process, the same frequency carrier is transmitted simultaneously, which leads to different types of noise in the system, such as environmental interference, leakage from the transmitter to the receiver, and internal device noise, along with interface losses. Using the radar equation: $$ P_r = \frac{P_t G_t G_r \sigma}{(4\pi R)^2} $$ Where $ P_r $ and $ P_t $ are the received and transmitted power, respectively; $ G_t $ and $ G_r $ are the transmit and receive antenna gains; $ \sigma $ is the radar cross-section; and $ R $ is the distance between the transmitter and receiver. From this, a logarithmic form of the communication equation can be derived: $$ P_r = P_t + G_t + G_r - L_c $$ Where $ L_c $ represents the spatial loss, calculated as: $$ L_c = 20\log_{10}\left(\frac{4\pi R}{\lambda}\right) $$ If the unmodulated carrier is $ f(t) = A\cos(\omega_c t) $, and the signal transmitted by the tag is $ g(t) = B(1 + m(t))\cos(\omega_c t + \phi) $, then the leakage from the transmitter to the receiver (TX-RX Leak) is related to $ f(t) $, given as $ l(t) = C\cos(\omega_c t) $. When mixed with the clock signal $ s(t) = \cos(\omega_c t) $, the resulting signals after mixing are: $$ l(t) \cdot s(t) = \frac{C}{2} [1 + \cos(2\omega_c t)] $$ $$ g(t) \cdot s(t) = \frac{B}{2} [1 + m(t)] [1 + \cos(2\omega_c t)] $$ It can be observed that the amplitude of the TX-RX leakage is much larger than the tag's return signal, leading to a significant DC offset during demodulation. This DC offset is a critical issue in the system, and thus, suppressing the leakage from the transmitter to the receiver is essential for improving performance. 2. System Simulation and Theoretical Verification To verify the system design, a Simulink simulation was conducted, illustrating the transceiver link structure as shown in Figure 2. The upper part of the figure represents the tag’s received signal and transmission path, while the lower part shows the combined signal from the circulator’s coupling and the tag’s return, followed by the down-conversion process at the receiver. According to the ISO18000-6C standard, the tag returns information using Miller or FM0 subcarrier modulation, followed by ASK modulation. In the simulation, a repeated "1101001101" sequence was used for FM0 coding. Due to the small DC component in the Miller or FM0 spectrum, it gets filtered out during processing, reducing the DC drift that occurs during downconversion. If the distance between the reader and the tag is 1 meter, the calculated spatial loss is 18 dB, and the circulator isolation can be set to 20 dB. After demodulation, the time-domain waveform of the tag’s return information is shown in Figure 3. The upper and lower waveforms represent the baseband signal from the tag and the signal after demodulation by the receiver, respectively. Figure 4 displays the spectrum before and after DC filtering in the receive chain. The upper graph shows the spectrum without the DC component, while the lower graph includes a large DC component. As seen in Figure 4, a DC-blocking filter is necessary at the output of the receiver mixer. Since this does not affect the integrity of the FM0 signal, the system requirements are satisfied. However, due to reverse leakage from the circulator, the receiver experiences relatively high interference, which may lead to blocking issues. 3. Conclusion In summary, this paper studies and designs the UHF RFID reader receiving circuit based on the ISO18000-6C protocol. It presents a theoretical analysis of the zero-IF receiving architecture and evaluates its advantages and disadvantages. Through simulation, the waveform and spectral characteristics of the zero-IF structure were obtained, demonstrating that it meets the requirements of the RFID system.

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