The reader in an RFID system is primarily composed of a control unit, a high-frequency transceiver module, an antenna, and various interfaces that connect to the backend device. A transponder, commonly referred to as a tag, serves as the source of RFID data. It mainly consists of an antenna and a microelectronic chip. The reader plays a critical role in the RFID system, and the core technology behind it lies in the receiving circuit. This paper focuses on analyzing and constructing the receiving circuit for a UHF passive RFID reader.
1. Basic Architecture
The circuit structure of the RFID reader is illustrated in Figure 1, where the upper half represents the transmit path and the lower half represents the receive path. The reader operating in the UHF band first applies PIE coding to the baseband signal and then performs 80% to 100% ASK modulation. On the receiving side, the circuit must support demodulation in the form of PSK or ASK, while also enabling Miller or FM0 subcarrier decoding of the baseband signal.
The transmitted signal can be described as follows:
$$ f(t) = m(t) \cdot \cos(\omega_c t) $$
Where:
- $ f(t) $: Signal received by the tag
- $ m(t) $: Baseband signal
- $ \omega_c $: Carrier frequency
During the backscattering process, the receiving end receives the following signal:
$$ g(t) = B(1 + m'(t)) \cdot \cos(\omega_c t + \phi) $$
Where:
- $ g(t) $: Signal received by the receiver
- $ m'(t) $: Baseband signal at the tag end
After down-conversion, the demodulated result is shown below:
$$ \text{Output} = \text{LPF}(f(t) \cdot s(t)) $$
Where $ s(t) = \cos(\omega_c t) $ represents the clock signal. The low-pass filter output is denoted as $ \tilde{f}(t) $.
In Figure 1, the circulator isolation typically provides 20–30 dB of isolation. Alternatively, using two independent antennas for transmission and reception can also achieve good isolation. It's important to note that during the receiving process, the same frequency carrier is transmitted simultaneously. Therefore, the noise in the system includes environmental interference, leakage from the transmitter to the receiver, and internal device noise, along with interface losses.
Using the radar equation:
$$ P_r = P_t \cdot G_t \cdot G_r \cdot \frac{\sigma}{(4\pi R)^2} $$
Where:
- $ P_r $: Received power
- $ P_t $: Transmitted power
- $ G_t $: Transmit gain
- $ G_r $: Receive gain
- $ \sigma $: Radar cross-section
- $ R $: Distance between transmitter and receiver
From this, a logarithmic communication equation can be derived:
$$ \text{Pr (dB)} = \text{Pt (dB)} + G_t + G_r - L_c $$
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 tag’s signal is $ g(t) = B(1 + m(t)) \cos(\omega_c t + \phi) $, then the leakage from the transmitter to the receiver is $ l(t) = C \cos(\omega_c t) $. When mixed with the clock signal $ s(t) = \cos(\omega_c t) $, the results are:
$$ l(t) \cdot s(t) = C \cos^2(\omega_c t) $$
$$ g(t) \cdot s(t) = B(1 + m(t)) \cos(\phi) $$
It becomes clear that the DC component from the TX-RX leakage is much larger than the tag’s return signal, making DC offset a significant challenge. Thus, the main interference comes from frequency leakage, which needs further investigation and suppression techniques.
2. System Simulation and Theoretical Verification
A Simulink simulation was conducted to model the transceiver link, as shown in Figure 2. The top part represents the tag’s received signal and transmission path, while the bottom part shows the combined signal after down-conversion. Based on the ISO18000-6C standard, the tag uses Miller or FM0 subcarrier modulation followed by ASK. In the simulation, the repeated "1101001101" sequence was used for FM0 coding.
Since Miller or FM0 codes have a small DC component, it gets filtered out during processing, reducing the DC drift caused by down-conversion. With a distance of 1 meter between the reader and tag, 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 signal is shown in Figure 3. The upper and lower waveforms represent the baseband information from the tag and the demodulated signal, respectively. Figure 4 displays the spectrum before and after DC filtering. The upper spectrum shows the signal without DC, while the lower one contains a large DC component.
As seen in Figure 4, a DC-blocking filter must be placed at the end of the receiver mixer. Since this does not affect the integrity of the FM0 signal, the system requirements are met. However, due to reverse leakage from the circulator, the receiver may experience interference, potentially leading to blocking.
3. Conclusion
In summary, based on the ISO18000-6C protocol, this paper studies and designs the UHF RFID reader’s receiving circuit, implementing a theoretical analysis of the zero-IF receiving structure and evaluating its advantages and disadvantages. Through simulation, the signal 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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