# A Magnetotelluric Signal Acquisition and Monitoring System Based on a Cloud Platform
## 3 Development of the Magnetotelluric Monitoring Receiver  
### Hardware Circuit Design  
As shown in Figure 3, the magnetotelluric monitoring receiver consists of amplification and filtering circuits, an FPGA (Field Programmable Gate Array) control system, a DSP (Digital Signal Processing) system, and an embedded control system. Five channels of potential difference signals are amplified differentially and then fed into an ADC (Analog to Digital Converter). The entire analog to digital conversion and data transmission processes are controlled by the FPGA, which then sends the data to the DSP for processing, including sampling, notch filtering, low-pass filtering, and high-pass filtering. Finally, the DSP sends the processed data in three frequency bands to the embedded system for storage. The embedded system has both wired and wireless networking capabilities, allowing it to receive control signals from an upper computer to control the entire acquisition process while saving the acquired waveform data on flash memory. Figure 1: Design of the IoT-based magnetotelluric signal acquisition and monitoring system. Figure 2: Information flow of the IoT magnetotelluric monitoring system. To ensure the reliability of the acquisition system, several additional functions were incorporated. First, lightning protection and electrostatic discharge protection circuits were added, as shown in Figure 4, ensuring that the internal circuits of the measurement system can continue to operate during a lightning strike. Second, various test circuits were included, such as ADC test circuits, amplification and filter test circuits, and ground resistance test circuits. The entire test circuit is controlled by the FPGA, which generates the required test signals via a DAC (Digital to Analog Converter). In the magnetotelluric monitoring receiver, the embedded system is at the core, as shown in Figure 5. It controls the DSP for magnetotelluric signal data processing and the FPGA for data acquisition. It also receives control signals from the upper computer, ensuring that the measurement system operates with the set parameters. The core of the embedded system is an ARM SoC (Advanced RISC Machines System on a Chip), SDRAM (Synchronous Dynamic Random-Access Memory), NAND flash, and NOR flash. The SoC has more functions than a CPU, with NAND flash used for storing measurement waveforms and NOR flash for storing programs. The SoC controls the FPGA via an SPI interface and communicates with the DSP via serial and synchronous serial interfaces. Using the SoC's rich interface resources, the ARM can connect to a GPS OEM board for timing and navigation information, a 4G/5G module for wireless networking, and an Ethernet interface chip for communication with a PC. The serial port can be used for debugging the embedded system, and the USB interface can be used for expanding storage or WiFi communication. The PC can communicate with the embedded system  
Figure 4: Block diagram of the signal conditioning circuit in the magnetotelluric monitoring receiver. Figure 3: Block diagram of the magnetotelluric monitoring receiver. via WiFi, Ethernet, or serial ports, meeting the needs for debugging, development, and remote control. Based on system functionality and stability requirements, the magnetotelluric monitoring receiver was ultimately divided into two circuit boards: one responsible for magnetotelluric signal conditioning and ADC and DSP processing (see Figure 6), and the other for the embedded subsystem (see Figure 7), including the ARM SoC, FPGA, ZigBee Pro module, GPS module, memory, and flash. Both circuit boards were custom-designed for this system, achieving optimal coupling between the embedded system and the acquisition system. Figure 5: Block diagram of the embedded subsystem in the magnetotelluric monitoring receiver. Figure 6: Signal acquisition circuit board of the magnetotelluric monitoring receiver. Figure 7: Embedded system circuit board of the magnetotelluric monitoring receiver.