## 3 Design of the Stress Wave-Based Communication System
### Hardware Design
#### 3.2.2 PPM Signal Generator Design  
As shown in Figure 4, the PPM signal generator is composed of a MCU, a driver circuit, and a power converter circuit. The MCU receives data from the PC, encodes data, and then sends out the encoded data to the driver circuit. The driver circuit combines a low-voltage pulse control circuit with a high-voltage charge-discharge circuit to generate high-voltage pulse signals. The low-voltage circuit controls the high-voltage circuit via an isolation transformer, thereby improving the driving capacity of the metal-oxide-semiconductor field-effect transistor (MOSFET). The high-voltage capacitor and transducer are instantaneously turned on to form up a discharging loop when the MOSFET is turned on, thus actuating the transducer. The power converter circuit converts an input AC voltage into a DC voltage to serve as a low-voltage DC power supply for the MCU control circuit and a high-voltage DC power supply for the driver circuit as well. Microcontroller Unit (MCU) Selection  
The 16-bit single-chip microcomputer C8051F060 is selected for the microcontroller unit, since it features a high-speed CIP-51 core that is compatible with the 8051 microcontroller, an enhanced universal asynchronous receiver-transmitter (UART) serial interface with hardware address recognition for asynchronous serial communication, a general-purpose timer, and a 16-bit analog to digital converter (ADC). As shown in Figure 5, the unit consists of one C8051F060 microcontroller, one clock circuit, one reset circuit and one debug interface. The clock circuit includes C44, C45 and Y1, which uses a 22.1184 MHz crystal oscillator to clock the microcontroller and peripherals. The reset circuit uses a 10 \(\mu\)F capacitor C34 to achieve power-on reset. The port connecting to UART receives data from the PC, the I/O port P0.7 is working as the signal transmission pin to output the encoded data, and the timer T1 is applied while encoding data based on the data transmission protocol. \begin{table}
\begin{tabular}{c c} \hline \hline
**Name** & **Parameter** \\ \hline Resonance frequency & 17–21 kHz \\ Static capacitance & \(1000\pm 20\%\) pf \\ Electromechanical coupling coefficient & 0.52 \\ Operating temperature & \(-40\sim 200\) \\ Size (outer diameter \(\times\) inside diameter \(\times\) height) & \(53\times 46\times 41\) mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the piezoceramic transducer. Figure 4: Pulse position modulation (PPM) signal generator design. Driver Circuit Design  
As shown in Figure 6, the driver circuit consists of a low-voltage pulse control circuit and a charge-discharge circuit. The low-voltage pulse control circuit is composed of an NPN transistor Q2, a transformer T2, and a Zener diode D46. As presented in the left part of Figure 6, if the input pulse voltage at J11 is high, then the transistor Q2 will be turned on, and the current will pass through R55. The current will be generated at the output of T2 due to induction, and then passes through the resistor R13 to drive the MOSFET. The diode D46 acts as a regulator to limit the voltage across R13. On the contrary, if the input pulse voltage at J11 is low, the transistor Q2 is turned off and thereby the driving circuit will not drive the MOSFET. As shown in the right part of Figure 6, the voltage of DC input at high voltage (HV) is 150 V for the charge-discharge circuit. The output voltage of this circuit needs to be higher than 1000 V to actuate the transducer. Thus, the charge-discharge principle of the capacitor is used to enhance the output voltage. When the MOSFET is grounded, the capacitor C36 will discharge and then a current loop will form up to increase the output voltage of the transformer T1. When the input pulse voltage  
Figure 5: The schematics of microcontroller unit (MCU) control circuit. Figure 6: The schematics of the driver circuit. at J11 is low, neither the MOSFET nor the transformer T1 works. R70 is a power resistor prone to burn out if the power through it is small.