The phase-sensitive detection circuit is an essential component in various measurement systems, especially when it comes to detecting and analyzing modulated signals. There are two main types of phase-sensitive detectors: one uses a transformer and a diode bridge, which is bulky and less stable, while the other employs analog multipliers, offering better performance but at a higher cost and with more complex debugging. In the development of atmospheric electric field instruments, a simpler and more stable design was implemented using a photoelectric switch, a four-channel analog switch, and an operational amplifier. This setup ensures accurate detection of the electric field's polarity by aligning the induced voltage signal with the synchronous pulse signal. **1. What is a phase-sensitive detection circuit?** A phase-sensitive detection circuit is a specialized detection system that has the capability to select both the phase and frequency of a modulated signal, making it highly effective for precise signal discrimination. **2. Why use phase-sensitive detection?** Envelope detection, while commonly used, has limitations. It cannot distinguish the phase of the modulated signal and lacks the ability to differentiate between signals of different carrier frequencies. To overcome these issues and improve the system’s anti-interference capability, a phase-sensitive detection circuit is necessary. **3. What is the main difference between phase-sensitive and envelope detection circuits?** The key difference lies in their functionality. A phase-sensitive circuit can detect the phase of the signal, allowing it to determine the direction of change, while an envelope detector only detects amplitude. Additionally, a phase-sensitive circuit includes a reference signal to identify both phase and frequency, enhancing its accuracy. **4. How do the structures of phase-sensitive and amplitude modulation circuits compare?** Both circuits involve multiplication operations. However, in amplitude modulation, a low-frequency signal is multiplied with a high-frequency carrier to produce a modulated signal. In contrast, a phase-sensitive circuit multiplies a high-frequency modulated signal with a reference signal, then filters the result to extract the original low-frequency information. **Working Principle of Phase-Sensitive Detection Circuit** When the electric field probe is exposed to a positive electric field, the induced voltage signal V1(t) and the synchronous signal VC(t) are input into the circuit. Depending on the phase relationship between these signals, the analog switches control the output, ensuring that the detected signal remains in phase with the reference. After filtering, the output becomes a DC voltage that reflects the polarity of the measured electric field. In practical applications, the initial angle φ’ plays a crucial role. Experimental results show that setting φ’ to 37° rather than 0° significantly improves signal clarity and accuracy. When the initial angle is not properly set, the detected signal may remain AC, leading to errors in polarity determination. By adjusting this angle, the system can effectively align the induced voltage with the reference signal, ensuring reliable detection. **Multifunctional Phase-Sensitive Detection Circuit** A phase-sensitive detection circuit, when combined with appropriate filtering, can demodulate amplitude-modulated signals and also detect their phase. A typical example consists of four identical diodes arranged in a bridge configuration, along with resistors to balance the circuit. The output is derived from the interaction between the modulated signal and the carrier, making it ideal for applications requiring high precision and stability.

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