Electrostatic coupling is also called capacitive coupling,NoiseThere is a capacitive path between the source and the interfered circuit.Electrostatic coupling can be realized bywirewayDistribution formed betweencapacitanceBreak into the system.The distributed capacitance is formed between two parallel conductors, such asPrinted circuit boardBetween upper printed linestransformerDistributed capacitance will be formed between winding wires.
Coupling refers to the process of signal transmission from the first stage to the second stage, and generally refers to AC coupling when not specified.
DecouplingIt refers to taking further filtering measures on the power supply to remove the interference of signals between two stages through the power supply.The coupling constant refers to the time constant corresponding to the product of the coupling capacitance value and the second level input impedance value.
Decoupling has three purposes:
1. Remove the high-frequency ripple in the power supply, and crosstalk the high-frequency signals of multistage amplifiers through the power supplyaccessCut-off;
2. When the big signal works,CircuitThe increasing demand for power supply causes power supply fluctuation, and the influence of power supply fluctuation on input stage/high voltage gain stage in case of large signal is reduced through decoupling;
3. Form a levitation ground or a levitation power supply, and complete all parts of the ground wire orPower SupplyThe high frequency switching noise generated by the coordinated matching active device will spread along the power line when switching.
Decoupling capacitorIts main function is to provide a localDC power supplyActive devices are provided to reduce the transmission of switching noise on the board and guide the noise to the ground.[1]
Jamming mode
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The interference signal generated by the interference source iscouplingThe channel has electromagnetic interference effect on the electronic control system.The coupling mode of interference is nothing more than acting on the electric control system through wires, space, public wires, etc.After analysis, there are mainly the following.
1. Direct coupling: This is the most direct way of interference intrusion and the most common way in the system.If the interference signal passeswirewayDirectly invade the system and cause interference to the system.For this coupling modewave filteringThe decoupling method can effectively suppress the incoming of electromagnetic interference signals.
2. Common impedance coupling: this is also a common coupling mode.It often happens that the current of two circuits has a common path.Common impedance coupling includes common ground impedance and power impedance.To prevent such coupling, the coupling impedance should approach zero, and there should be no common impedance between the interference source and the interfered object.
3. Capacitive coupling: also called electric field coupling or electrostatic coupling, it is a coupling mode due to the existence of distributed capacitance.
4. Electromagnetic induction coupling: also calledMagnetic coupling。It is a coupling mode due to the induction of internal or external space electromagnetic field. The common method to prevent this coupling is to shield the devices or circuits that are vulnerable to interference.
5. Radiation coupling: the radiation of electromagnetic field will also cause interference coupling, which is an irregular interference.This interference is easily transmitted to the system through the power line.In addition, when the signal transmission lines are long, they can radiate interference waves and receiveInterference wave, called the big line effect.
6. Leakage coupling: so-calledelectric leakageCoupling is resistive coupling.This interference often occursinsulationOccurs when lowering.[1]
Explanation of related terms
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1. Electrostatic coupling
Electrostatic coupling is also called capacitive coupling,noise sourceThere is a capacitive path with the interfered circuit.The electrostatic coupling can enter the system through the distributed capacitance formed between two wires.The distributed capacitance is formed between two parallel conductors, such asPrinted circuit boardThe distributed capacitance will be formed between the upper printed lines and the winding lines of the transformer.
2. Magnetic coupling
The magnetic field coupling in space comes in through mutual inductance coupling between conductors.Magnetic field will be generated around any current carrying conductor, which will cause alternating magnetic field when current changesmagnetic fieldIt must generate induced potential in the closed circuit around it to cause interference.Inside the equipment,coilOr the magnetic leakage of transformer will also cause interference;Outside the equipment, the two parallel wires will also produce interference, which is caused by the induced electromagnetic fieldcoupling, the induced voltage can be calculated.
3. Common impedance coupling
The common impedance coupling interference is caused by the current flowing through the common impedance between circuits, so that the voltage drop generated by the current of one circuit affects the other circuit.staycomputer control system Common coupling impedance is common, such as power leadpcbGround and public power lines, bus bars, etc.These busbars all have a certain impedance, which is a common coupling impedance for multiple circuits.
Interference suppression measures
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1. Eliminate interference sources
The active and active measure to suppress interference is to eliminate interference sources.To eliminate interference sources, you must first determine where the interference source is.When the interference source cannot be eliminated, suppression measures can be taken. The closer to the interference source, the better the interference suppression effect will be.
In general,electric currentOr the place where the voltage changes dramatically is the interference source, specifically,relayOn/off, capacitor charging, motor operation, integrated circuit switch operation, etc. may become interference sources.
2. Cut off interference coupling path
For interference intruded in the form of "circuit", such as improving insulation performance and isolationtransformer, optical coupler, etc;Use decoupling, filtering and other means to guideinterference signalTransfer of;Change the grounding form and cut off the interference path.For interference intruded in the form of "radiation", various shielding measures are generally taken, such as electrostatic shieldingelectromagnetic shielding , magnetic shielding, etc.
3. Improve anti-interference ability
In order to weaken the sensitivity of the receiving circuit to interference, the anti-interference ability of the detection device must be improved.Generally speaking, circuits with high input impedance are more susceptible to interference than circuits with low input impedance;Analog circuitThe anti-interference ability is worse than that of digital circuit.A well-designed detection device should be sensitive to useful signals and insensitive to interference signals.[2]
application
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Electrostatic shielding: When a charged object approaches the input end of the circuit under test, electrostatic coupling and interference will occur.At low impedance, due tochargeIt dissipates rapidly, so the influence of interference is not obvious.However, high resistivity materials do not allow rapid charge decay, which may produce unstable measurement results.Because incorrect readings may be caused by DC or AC electrostatic fieldsElectrostatic shieldingIt helps to minimize the impact of this electric field.
DC electric field may produce noisy readings or undetectable errors.Movement near the experimental circuit (for example, the movement of the operator of the instrument or other movements in the adjacent area)electrometerThe fluctuation of the display reading reflects the existence of this field.In order to quickly check the existence of interference, a charged plastic object, such as a comb, is placed near the circuit.A large change in the reading of the instrument indicates that the shielding is not perfect.
AC electric fields can also cause problems.AC electric fields are often supplied byPower SupplyAnd RF field.If the AC voltage at the input terminal is very large, part of the signal will be rectified, so an error will occur in the measured DC signal.useOscilloscopeThis can be checked by observing the analog output of the electrometer or picoammeter.The limiting waveform indicates that the electrostatic shielding needs to be improved.[1]
