Phase-sensitive detection circuit working principle and working process
There are two main types of phase-sensitive detectors. The first type consists of a transformer and a diode bridge, but this design is bulky and lacks stability. The second type uses analog multipliers, which significantly improves performance, although it comes at a higher cost and requires more complex debugging. In the development of an atmospheric electric field instrument, a simpler and more stable phase-sensitive detector was designed using a photoelectric switch, a four-channel analog switch, and an operational amplifier. This approach takes into account the structural features of the probe and the specific requirements for signal detection in atmospheric electric field measurements. Additionally, to accurately determine the polarity of the electric field signal, the position of the photoelectric switch was adjusted so that the induced voltage signal and the synchronous pulse signal remain in phase, ensuring maximum rectified output and precise polarity identification.
1. What is a phase-sensitive detection circuit?
A phase-sensitive detection circuit is a type of detection system that has the ability to select both the phase and frequency of a modulated signal, allowing for accurate discrimination of the signal from noise or interference.
2. Why use phase-sensitive detection?
Envelope detection has limitations—it cannot distinguish the phase of the modulated signal, and it also fails to differentiate between signals of different carrier frequencies. This can lead to errors in signal interpretation. Phase-sensitive detection overcomes these issues by incorporating a reference signal, enabling the circuit to identify both the phase and frequency of the input signal, thus improving overall accuracy and anti-interference capability.
3. What is the main difference between phase-sensitive and envelope detection circuits in terms of function and structure?
The key difference lies in their functionality: phase-sensitive detection can detect the phase of the signal, which allows for determining the direction of change, while envelope detection only captures the amplitude. Structurally, phase-sensitive circuits require a reference signal in addition to the modulated signal, which enables phase and frequency discrimination, whereas envelope detection circuits do not have this feature.
4. What are the similarities and differences between phase-sensitive detection and amplitude modulation circuits?
Both circuits involve multiplication of signals, with the phase-sensitive circuit resembling the amplitude modulation process in structure. However, while amplitude modulation combines a low-frequency signal with a high-frequency carrier to produce a modulated signal, the phase-sensitive circuit multiplies a high-frequency modulated signal with a reference carrier, then filters it to recover the original low-frequency signal. This difference in input and output coupling leads to variations in filter design and circuit parameters.
Phase-Sensitive Detection Circuit Working Principle
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 fed into the detector. Depending on the phase relationship between these signals, the analog switches inside the MC14066BCP control the output. For example, during the negative half-cycle of V1(t), if VC(t) is low, switch 1 is off and switch 4 is on, resulting in a negative half-cycle output. When V1(t) is positive and VC(t) is high, switches 1 and 3 are activated, and the output becomes a negative half-cycle again. After filtering, the final output V3(t) becomes a DC signal, whose polarity indicates the direction of the electric field. This working principle is illustrated in the provided diagrams.
In practical applications, the initial angle φ’ must be carefully chosen to ensure that the induced signal remains in phase with the reference signal. Experimental results show that setting φ’ to around 37° significantly improves the accuracy of polarity detection compared to a 0° setting. When φ’ is too small, the detected signal remains AC, and the DC output is unstable. With the correct angle, the output becomes a pulsating DC signal that reflects the actual polarity of the measured electric field.
Multifunctional Phase-Sensitive Detection Circuit
A phase-sensitive detection circuit, when combined with a filter, can demodulate amplitude-modulated waves back to their original form and also detect the phase of the signal. A typical example is a diode-based phase-sensitive detector, which uses four identical diodes connected in a bridge configuration. The circuit includes resistors for balance and connects to secondary coils of two transformers. One transformer receives the modulated signal, while the other provides the carrier signal. The output depends on the phase relationship between these inputs, making it useful for precise signal processing in various applications.
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