# Miniature Coil for Wireless Power and Data Transfer through Aluminum +
## 4 Through Metal Communication System
### Inside Subsystem  
The inside subsystem is made of a harvesting subsystem, Tx subsystem, Rx subsystem and microcontroller ATtiny104 Xplained Nano from Microchip Technology. Communication is based on amplitude shift keying (ASK) modulation due to its simplicity for wireless data transfer through metal [34]. Figure 13(b) shows the Tx Subsystem which consists of a low power mixer using a NPN transistor, part number BC547 from Fairchild, and the power amplifier, part number TPA301 from Texas Instruments. The Rx subsystem uses a low noise amplifier, part number MAX4475 from Maxim Integrated, a passive envelope detector, and the built-in comparator module of the microcontroller for demodulation.  
Figure 13(b) shows the harvesting subsystem used to rectify and regulate the harvested AC power from outside to supply DC power for the operation of the inside subsystem. The harvesting subsystem consists of a Schottky diode full wave rectifier, UPS115 UE3 from Microchip Technology, to convert the AC harvested signal to DC. Using this full wave rectifier, the total forward voltage drop is \(V_{F}<0.2\) V. The voltage at the output of the rectifier is only 1 Vrms and needs to be upscaled to at least 4 Vrms to charge the storage capacitor and supply power to the inside subsystem. To boost the voltage, a low-voltage boost converter, BQ25505 from Texas Instruments, was used. The boost converter has efficiency of >80%. Finally, a voltage regulator, S-1318D28 from ABLIC, was used to supply a constant 2.8 V supply for the inside communication subsystem for operation. The voltage regulator has a current consumption \(I_{ss}<100\) nA.  
Figure 15 shows the input impedance \(Z_{in}\) seen by the inside coil. Impedance \(Z_{in}\) has an important effect in received voltage, rectifier efficiency and harvested power [35]. To  
Figure 14: Fully integrated communication system: (**a**) outside, and (**b**) inside subsystem.  
measure received voltage, rectifier efficiency and harvested power, the outside transmitted power, \(P_{Tx}\), was fixed to 35 W at 2 kHz. The coil setup was the same for the rest of this paper, and the barrier is a 1 mm-thick aluminum plate. Additionally, the load \(R_{DC}\) on the inside subsystem was modified in the range of 10-1200 \(\Omega\) to change equivalent input impedance \(Z_{in}\). The impedance \(Z_{in}\) was calculated by measuring the voltage and current at the input of the full wave rectifier.  
Figure 16 shows the harvested AC and DC power and total rectifier efficiency versus equivalent input impedance \(Z_{in}\). When the equivalent input impedance \(Z_{in}\) is between 1-10 \(\Omega\), the harvested AC power ranges from 200-356 mW with a peak of 440 mW at 4 \(\Omega\). In the same impedance range, the total rectifier efficiency is between 6% to 21% due to the low received voltages, typically \(V_{rx}=0.5\)\(V_{rms}\). As a result, the harvested DC power goes from 6 mW to a maximum of 75 mW at 10 \(\Omega\). Input impedance \(Z_{in}=10\)\(\Omega\) is the optimal value that provides a balance in rectifier efficiency and AC harvested power. When input impedance increases further, \(Z_{in}\geq 10\)\(\Omega\), the total rectifier efficiency increases to almost 25% while the AC harvested power degrades rapidly. Hence, when \(Z_{in}\geq 10\)\(\Omega\), the DC harvested power is reduced to 1 mW.