Analysis and Design of Phase-Sensitive Detector in Atmospheric Electric Field Instrument

introduction

The atmospheric electric field is a vector. On sunny days, there is a negative electric field in the atmosphere that is vertically downward. On rainy days, due to the influence of thunderstorm clouds, there is a positive electric field in the atmosphere that is vertically upward. When the atmospheric electric field instrument is used to monitor the atmospheric electric field on the ground, it is necessary not only to measure the intensity of the measured electric field, but also to identify the polarity of the measured electric field. The polarity of the electric field is usually distinguished by the phase-sensitive detection method, so it is necessary to add a phase-sensitive detector to the preamplifier circuit of the electric field instrument. There are two commonly used phase-sensitive detectors: one is composed of a transformer and a diode bridge, which has a large circuit volume and poor stability; the other is composed of an analog multiplier, which has greatly improved performance, but is expensive and troublesome to debug. For this reason, in the process of developing the atmospheric electric field instrument, according to the structural characteristics of the atmospheric electric field instrument probe and the requirements for the detector in the atmospheric electric field test, a phase-sensitive detector with simple structure and stable performance is designed by combining a photoelectric switch, a four-channel analog switch and an operational amplifier. At the same time, in order to effectively and reliably identify the polarity of the electric field signal, according to the phase-sensitive detection theory, the setting position of the photoelectric switch will be adjusted to ensure that the induced voltage signal and the synchronous pulse signal are in phase to obtain the maximum rectification output, thereby accurately identifying the polarity of the measured electric field.

1 Phase-sensitive detection circuit design

The sensor probe of the atmospheric electric field instrument is shown in Figure 1. The moving piece is similar in shape to the small blade, and the upper and lower positions correspond to each other. They are both fixed on the motor shaft and driven by the brushless motor to rotate simultaneously at a certain frequency. The induction piece is four separate pieces, and the two opposite pieces form a group, which are divided into two groups A and B. The shape of each group is exactly the same as the moving piece. The moving piece and the induction piece are made of brass.

1.1 Induced weak voltage signal and synchronous pulse signal

When the motor in the probe drives the moving piece and the small blade to rotate, an AC induced current signal is generated on the induction piece. After the AC current signal passes through the IV conversion circuit, an AC induced voltage signal V1(t) is obtained. Its expression in one cycle T is:

Where: I is the amplitude of the induced current signal output by the electric field instrument probe; R and C are the feedback resistor and feedback capacitor of the IV conversion circuit respectively; T is the time for the moving piece to expose and cover the sensing piece A or B once; VRC is the equivalent amplitude of the induced voltage signal when t=T/2; K is a constant,

While the moving piece rotates, the small blade passes through the groove of the photoelectric switch at the same frequency ω cycle, and the light path of the light-emitting diode is cut off or passed periodically, so that the phototransistor is in the on and off states. Therefore, in the first cycle T, the synchronous pulse signal is Vc(t), and its expression is:

Circuit experiments have shown that this detection circuit can filter out harmonic components very well. At the same time, by observing the positive and negative DC voltage after filtering, the polarity of the measured electric field can be identified. In the actual circuit, both op amps A1 and A2 are dual-power op amps OP07DP.

1.3 Working principle of phase-sensitive detection circuit

If the electric field meter probe is in a positive electric field, the probe's induced voltage signal V1(t) and synchronization signal VC(t) are input via the detector in FIG. 3 , respectively.

When V1(t) is in the negative half cycle and VC(t) is at a low level, the voltage at point A is in the negative half cycle and the voltage at point B is in the positive half cycle. The analog switch 1 and switch 3 in the MC14066BCP are disconnected, and the analog switch 4 is turned on. Then the output of the O4 pin is the negative half cycle of V2(t), and the output of the O3 pin is in a high-impedance state. When V1(t) is in the positive half cycle and Vc(t) is at a high level, the voltage at point A is in the positive half cycle and the voltage at point B is in the negative half cycle. The analog switch 1 and switch 3 in the MC14066BCP are turned on, and the analog switch 4 is disconnected. Then the output of the O4 pin is in a high-impedance state, and the output of the O3 pin is in the negative half cycle of V2(t). In the entire signal cycle T, the detection output signal V2(t) is always in the negative half cycle. After being filtered by the inverting filter, the output signal obtains a positive DC voltage signal V3(t). Therefore, according to the polarity of V3(t), it can be concluded that the measured electric field is a positive electric field. Its working waveform is shown in Figure 3(a).

Assume that in a period T, the angle of rotation of the moving piece is φ, that is, the angle of rotation of the small blade is φ, then:

Since the influence of inductance on the IV conversion circuit is not considered, in actual operation, the optimal value of the initial angle φ′ should be found between -φ／(2π)ωRC{ln[1+exp(-π／(ωRC))]／2)～φ／4 through experiments. At the same time, since the error of the analog switch is not considered, calibration and compensation through software are also required in the atmospheric electric field instrument.

In the design of the electric field meter, the initial angle φ′=33.23° can be obtained according to formula (5), but φ′=37° is actually selected. Figure 4 shows two sets of comparative experimental waveforms of the phase-sensitive detection circuit when the measured electric field is -600 V/m and the initial angle φ′=0° and φ′=37° are selected respectively. Waveform 1 is the induced voltage signal after IV conversion, with a frequency of 40 Hz, and its amplitude is proportional to the intensity of the measured electric field; waveform 2 is the full-wave detection signal output from the analog switch, that is, the input signal of the low-pass filter. The polarity of this voltage signal is opposite to the polarity of the measured electric field. By observing the two sets of experimental waveforms, it can be found that when the initial angle φ′=0°, due to the different phases of the weak induced voltage signal V1(t) and the synchronous pulse signal Vc(t), the waveform after full-wave detection is still an AC signal, which does not have a single direction. It will be filtered out after the low-pass filter, and no stable DC voltage signal can be obtained. When the initial angle φ′ is set to 37°, the full-wave detection is a single positive direction pulsating DC voltage signal, which ensures that the weak induced voltage signal V1(t) and the synchronous pulse signal Vc(t) are in phase. Therefore, after the low-pass filter outputs a negative polarity DC voltage signal, it can be judged that the measured electric field is a negative electric field, thereby realizing the accurate identification of the polarity of the measured electric field.

3 Conclusion

A phase-sensitive detector consisting of a photoelectric switch, a four-channel analog switch and an operational amplifier is designed. Its working principle is analyzed by combining the characteristics of weak induced voltage signals and synchronous pulse signals. At the same time, it is verified through experiments that the correct placement of the photoelectric switch, that is, setting the initial angle φ′ of the small blade, can achieve the same frequency and phase of the two input signals, thereby obtaining the maximum rectifier output. The designed phase-sensitive detector has been applied in the designed atmospheric electric field instrument. It has stable performance in actual monitoring and work, and can well identify the polarity of the measured electric field.