## 4 Simulation and Experimental Results
### Experimental Results
Conversely, when the data signal is low, the voltage amplitude drops to 8.45 V, resulting in a transmitted load power of 66 mW. In this mode, the input power is 85.8 mW, combined with the dissipated power of 10 mW from the circuit chips, thus achieving a power transmission efficiency of 68.9%. The MI for achieving ASK modulation, as indicated by the experimental waveforms, is observed to be approximately 10%, aligning with the theoretical design value. Compared with the simulation results, the voltage amplitude of the receiver coil is a difference of 1.6 V when the data signal was at a high level, whereas it was a difference of 1.16 V when the data were at a low level. This is because the simulation results are ideal and do not consider the circuit's ESR. The circuit's ESR was measured at 16 MHz by an impedance analyzer; the ESR was about 4.8 \(\Omega\) at a high-level data signal, and the other was about 4.2 \(\Omega\). According to the simulation results shown in Figure 11, the transmitter coil current was 95 mA and 76 mA for high-level and low-level data signals, respectively. As a result, the ESR results in a loss of the receiver coil's voltage amplitude are 1.57 V and 1.1 V. If these error voltages are subtracted from the \(V_{\text{out}}\) of the simulation results, the \(V_{\text{out}\_H}\) is 10.33 V, and the \(V_{\text{out}\_L}\) is 8.5 V. The \(V_{\text{out}}\) results between the simulation and experiment are very close. Therefore, the experimental results verify the feasibility of the proposed circuit design methodology. To evaluate the BER, the FPGA transmits several million random bits, which are subsequently captured by the FPGA through the data recovery circuit. After performing computations within the FPGA, it was determined that the BER is less than \(10^{-7}\) for a data rate of 2 Mbps. Additionally, the measured BER at 3 Mbps is lower than \(10^{-6}\). Figure 13 illustrates the experimental results for data rates of both 2 Mbps and 3 Mbps, including the receiver coil voltage, the transmitted data, and the demodulated data. It was observed that the demodulated data closely matched the transmitted data. The total power transmission efficiency and output power were investigated at a data rate of 3 Mbps for different distances \(d\) between the receiver and transmitter coils. The experimental results are presented in Figure 14. When \(d\) = 6 mm, the power transmission efficiency reaches its maximum value due to the Class-E PA being designed for the tuned state. The maximum power transmission efficiency and output power reach 70.8% and 84.1 mW, respectively. As \(d\) increases from 6 mm, the coupling coefficient decreases, resulting in a reduction of the induction voltage of the receiver coil and a detuned state for the Class-E PA. Consequently, both power transmission efficiency and output power exhibit a decreasing trend as \(d\) increases. Conversely, as \(d\) decreases from 6 mm, the coupling coefficient \(k\) increases, and the reflected resistance \(R_{\text{ref}}\) increases. Meanwhile, the Class-E  
Figure 13: Measured experimental waveforms with different data rates. (a) 2 Mbps (b) 3 Mbps. PA also operates in a detuned state. As a result, both power transmission efficiency and output power exhibit a decreasing trend with decreased \(d\). The proposed WPDT circuit has been compared with previous works, and the characteristics of this circuit are outlined in Table 2. The comparison demonstrates that the WPDT developed in this study surpasses the reported circuits in terms of both PTE and data rate. In this cochlea implant application, permanent magnets ensure the best coil alignment. However, the coil's position is prone to change in other IMDs. The system's power transmission efficiency is sensitive to the coil's misalignment in other IMDs. Therefore, enhancing the power transmission efficiency in link misalignments can be explored to extend this work for other IMD applications, which has potential for future investigation.