# Design of a Novel RFID Reader for Oilwell Downhole Applications
## 2 Design of Downhole HDX Reader Hardware
### Hardware Design of HDX Reader  
Considering the high temperature environment in the downhole, there is currently no corresponding temperature-resistant HDX integrated RF (Radio Frequency) chip available. Then, to implement the half-duplex communication protocol, it was decided in this RFID reader design to generate intermittent radio frequency signals by the microcontroller to simulate the half-duplex communication protocol. These signals are then power-amplified by a power amplifier and transmitted from both ends of the antenna. During the intervals when the radio frequency signals stop transmitting, signals from the electronic tags are received, demodulated using a demodulation circuit, and then sent to the microcontroller for processing. Based on the above discussion, the hardware block diagram of the RFID reader can be designed as shown in Figure 2.  
In this design, Frequency Shift Keying (FSK) modulation is employed. FSK modulation refers to frequency modulation of the already modulated pulse waveform. FSK modulation is often used in cases where the frequency is below 135 kHz and exhibits strong anti-interference capabilities. Here, a modulation signal of 134.2 kHz is generated by the microprocessor and then amplified by the MIC4452 driver chip for power amplification before being transmitted to the antenna. \begin{table}
\begin{tabular}{c c c c c} \hline
**Reference** & [11,12] & [13] & [15] & **This Work** \\ \hline Protocol & FDX & HDX & FDX & HDX \\ Frequency & 125 kHz & 134.2 kHz & 125 kHz & 134.2 kHz \\ Antenna & Tightly wound & Coil & Tightly wound & Sparse \\  & solenoid & Coil & solenoid & solenoid \\ Recognition Ability & Low & Low & Low & High \\ of Moving Tag & Low & Low & Low & High \\ Works in Downhole & Yes & No & Yes & Yes \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of the Proposed Reader with Previous Studies. The MIC4452V is a high-performance, easy-to-use CMOS MOSFET driver known for its resistance to interference. Its features include a wide operating voltage range of 4.5 V to 18 V, a low delay time of 30 ns, and low output impedance. The operating temperature of the MIC4452 VM ranges from \(-40\)\({}^{\circ}\)C to \(+125\)\({}^{\circ}\)C, with a power consumption of 1040 mW and a decay coefficient of 8.3 mW/\({}^{\circ}\)C. The circuit connection of the MIC4452 in the circuit is illustrated in Figure 3.  
After receiving data through the antenna, this design adopts the XR2211 chip for demodulation. The XR2211 is a FSK demodulator, utilizing a single-phase-locked loop system with a frequency range of 0.01 Hz to 30 kHz. The dynamic range of the input analog voltage ranges from 10 mV to 3 V. To enable the demodulation function of the XR2211, several resistors and capacitors in the peripheral circuit of the XR2211 need to be configured. Among the various models of XR2211, XR2211M operates within a temperature range of \(-55\)\({}^{\circ}\)C to \(+125\)\({}^{\circ}\)C, consumes 900 mW of power, and has a derating factor (Derate above 25 \({}^{\circ}\)C) of 5 mW/\({}^{\circ}\)C for Small Outline Integrated Circuit (SOIC) packaging. According to the working principle of the XR2211, the demodulation circuit can be designed as shown in Figure 4. Based on the XR2211D chip manual and relevant information, the resistors and capacitors in the circuit can be calculated. Among them, the role of C15 and R9 is to set the PLL center frequency. Since the high level corresponds to a frequency of 134.2 kHz, the a frequency of 124.2 kHz and the low level corresponds to a frequency of 134.2 kHz, the center frequency is 129.2 kHz. Therefore, R9 is set to 20 k\(\Omega\), and C1 is set to 360 pF. R13 is used to set the system bandwidth, set to 270 k\(\Omega\) here, and C1 is used to set the time constant and damping factor of the loop filter, set to 91 pF here. C19 and R14 are used to filter the output data, with R14 set to 510 k\(\Omega\) and C19 set to 310 pF. R10 acts as a pull-up resistor to raise the output voltage. Figure 3: Circuit connection diagram of MIC4452.