“The three elements of electromagnetic interference are the interference source, the interference transmission path, and the interference receiver. EMC conducts research around these issues. The most basic interference suppression techniques are shielding, filtering, and grounding. They are mainly used to cut off the transmission path of interference. The broad electromagnetic compatibility control technology includes suppressing the emission of the interference source and improving the sensitivity of the interference receiver, but it has been extended to other disciplines.
The three elements of electromagnetic interference are the interference source, the interference transmission path, and the interference receiver. EMC conducts research around these issues. The most basic interference suppression techniques are shielding, filtering, and grounding. They are mainly used to cut off the transmission path of interference. The broad electromagnetic compatibility control technology includes suppressing the emission of the interference source and improving the sensitivity of the interference receiver, but it has been extended to other disciplines.
This specification focuses on the EMC design of a single board, with some necessary EMC knowledge and rules. Consideration of electromagnetic compatibility during the printed circuit board design phase will reduce the occurrence of electromagnetic interference in the circuit in the prototype. Types of problems include common impedance coupling, crosstalk, radiation from high-frequency current-carrying conductors, and noise pickup through loops formed by interconnecting wiring and traces.
In high-speed logic circuits, this type of problem is particularly vulnerable for a number of reasons:
1. The impedance of the power supply and the ground wire increases with the frequency, and the common impedance coupling occurs frequently;
2. The signal frequency is higher, it is more effective to couple to the step line through the parasitic capacitance, and the occurrence of crosstalk is easier;
3. The size of the No. loop is compared with the wavelength of the clock frequency and its harmonics, and the radiation is more significant.
4. Impedance mismatch problem caused by signal line reflection.
No.1 General Concept and Considerations
1. The 515 rule, that is, when the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB board must be a multi-layer board.
2. Different power planes cannot overlap.
3. Common impedance coupling problem.
Since ground plane currents may be generated by multiple sources, the induced noise may be higher than the over-mode electrical sensitivity or digital electrical immunity.
①The analog and digital circuits should have their own loops, and finally ground at a single point;
② The wider the power cord and the return line, the better;
③ Shorten the length of the printed line;
④ Decoupling of power distribution system.
4. Reduce the area of the loop and the cross-link area of the two loops.
5. An important idea is: the EMC on the PCB mainly depends on the Z of the DC power line.
Here are the board layout guidelines:
1. The crystal oscillator is as close to the processor as possible
2. Analog circuits and digital circuits occupy different areas
3. The high frequency is placed on the edge of the PCB board and arranged layer by layer
4. Fill empty areas with land
1. The power line and the return line are as close as possible, and the best way is to walk on one side.
2. Provide a zero-volt return line for the analog circuit, and the signal line and the return line are as small as 5:1.
3. For the crosstalk of long parallel traces, increase the spacing or add a zero-volt wire between the traces.
4. Manual clock wiring, away from the I/O circuit, consider adding a dedicated signal return line.
5. Key lines such as reset lines are close to the ground return line.
6. To minimize crosstalk, use double-sided #-shaped wiring.
7. Avoid right angles on high-speed lines.
8. Separate the strong and weak signal lines.
1. Shield > model:
Shielding effectiveness SE (dB) = reflection loss R (dB) + absorption loss A (dB)
The key to high-frequency RF shielding is reflection, and absorption is the key mechanism for low-frequency magnetic field shielding.
2. When the working frequency is lower than 1MHz, the noise is generally caused by the electric field or the magnetic field (interference caused by the magnetic field, generally within a few hundred hertz), above 1MHz, consider the electromagnetic interference. Shielded entities on a single board include transformers, sensors, amplifiers, DC/DC modules, and more. The larger one involves the shielding between boards, subracks, and racks.
3. Electrostatic shielding does not require the shielding body to be closed, only two points of high conductivity material and grounding are required. Electromagnetic shielding does not require grounding, but requires that the induced current has a path on it, so it must be closed. Magnetic shielding requires a material with high magnetic permeability as a closed shield. In order to cancel the magnetic flux generated by eddy current and the magnetic flux generated by interference to achieve the purpose of absorption, the thickness of the material is required. At high frequencies, the three can be unified, that is, enclosed and grounded with high-conductivity materials (such as copper).
4. For low frequency and high conductivity materials, the absorption attenuation is reduced, and the shielding effect of the magnetic field is not good, so materials with high magnetic permeability (such as galvanized iron) should be used.
5. Magnetic field shielding also depends on thickness, geometry, and maximum linear size of the hole.
6. The noise voltage of magnetic coupling induction UN=jwB.A.coso=jwM.I1, (A is the area when the circuit 2 is closed; B is the magnetic flux density; M is the mutual inductance; I1 is the current that interferes with the circuit. Reduce noise Voltage, there are two ways. For the receiving circuit, B, A and COS0 must be reduced; for the interference source, M and I1 must be reduced. Twisted pair is a good example. It greatly reduces the loop of the circuit road area, and at the same time generate an opposite electromotive force on the other core wire of the twist.
7. Empirical formula for preventing electromagnetic leakage: gap size
1. Below 300KHz, generally single-point grounding, above multi-point grounding, the mixed grounding frequency range is 50KHz ~ 10MHz. Another way of dividing is:
2. Good grounding method: tree grounding
3. Grounding of the shielding cover of the signal circuit.
The ground point is selected on the ground wire of the output terminal of the amplifier, etc.
4. For cable shielding layer, L
5. For the grounding of the radio frequency circuit, the grounding wire is required to be as short as possible or to achieve grounding without wiring at all. The best ground wire is flat copper braid. When the ground wire length is an odd multiple of the wavelength of λ/4, the impedance will be very high, and at the same time, it is equivalent to a λ/4 antenna, radiating out interference signals.
6. There are multiple digital grounds and analog grounds on a single board, and only one common location is allowed.
7. Grounding also includes the use of wires as the power return line, bonding and so on.
1. Select the EMI signal filter to filter out the high-frequency interference components that are not needed for work on the wire, and solve the high-frequency electromagnetic radiation and receiving interference. It must be well grounded. Breakout board mount filters, feed-through filters, connector filters. From the circuit form points, there are single capacitor type, single Inductor type, L type, π type. The π-type filter has the best transition performance from the pass band to the stop band, and can best ensure the quality of the working signal.
The spectrum of a typical signal:
2. Select AC and DC power filters to suppress conduction and radiation interference on internal and external power lines, which not only prevents EMI from entering the power grid and harms other circuits, but also protects the equipment itself. It does not attenuate power frequency power. DM (Differential Modulation) interference dominates at 1MHz frequency.
3. Use ferrite beads to be mounted on the leads of the components for decoupling, filtering and parasitic oscillation suppression of high-frequency circuits.
4. Decouple the power supply of the chip as much as possible (1-100nF), and filter (uF) the DC power supply entering the plate and the output of the voltage regulator and DC/DC converter.
Pay attention to reduce the lead inductance of the capacitor, increase the resonant frequency, and even use a four-core capacitor for high-frequency applications. The selection of capacitors is a very important issue, and it is also a means of single-board EMC control.
The interference suppression of a single board involves a wide range of aspects, from the impedance matching of the transmission line to the EMC control of the components, from the production process to the wiring method, from the coding technology to the software anti-interference and so on. The conception and birth of a machine is actually EMC engineering. The most important thing is that engineers need to inject EMC awareness into the design. In the process of experimental testing, we often encounter such a situation: although the design engineer has connected a power filter to the power line of the device, the device still cannot pass the “conducted disturbance voltage emission”. During the test, the engineer suspects that the filtering effect of the filter is not good, and the filter is constantly replaced, but the desired effect cannot be obtained.
Two aspects to analyze the reasons for equipment exceeding the standard
1) The harassment generated by the equipment is too strong;
2) The filtering of the equipment is insufficient.
For the first case, we can solve it by taking measures at the source of disturbance to reduce the intensity of disturbance, or increase the order of the power filter to improve the filter’s ability to suppress disturbance. For the second case, in addition to the poor performance of the filter itself, the installation method of the filter has a great influence on its performance. This is often overlooked by design engineers. In many tests, we were able to get the device to pass the test by changing the way the filter was mounted.
Below are some examples of the effects of common filter installation errors on filter performance.
1 The input cable is too long
After the power cords of many devices enter the chassis, they go through a long wire to the input of the filter. For example, the power cord enters from the rear panel of the chassis, runs to the power switch on the front panel, and then goes back to the rear panel to connect to the filter. Or the installation position of the filter is far from the inlet of the power cable, resulting in the lead wire being too long. As shown in Figure 1.
Figure 1 Schematic diagram of too long power cord
Because the lead from the power inlet to the filter input is too long, the electromagnetic disturbance generated by the device is re-coupled to the power line through capacitive or inductive coupling, and the higher the frequency of the disturbance signal, the stronger the coupling, causing the experiment to fail.
2 Parallel wiring of filter input and output lines
In order to make the wiring inside the chassis beautiful, some engineers often bundle the cables together, which is not allowed for power cables. If the input and output lines of the power filter are routed in parallel or bundled together, due to the distributed capacitance between the parallel transmission lines, this routing method is equivalent to connecting a capacitor in parallel between the input and output lines of the filter to avoid disturbance. The signal provides a path around the filter, causing the filter to significantly degrade and even fail at very high frequencies (as shown in Figure 2). The size of the equivalent capacitance is inversely proportional to the wire distance and proportional to the length of the parallel traces. The larger the equivalent capacitance, the greater the impact on the filter performance.
Figure 2 The effect of parallel traces on the filter
3 The grounding of the filter is not good, and the housing of the filter is not
It is also common for the chassis to be well lapped. When many engineers install the filter, the connection between the filter housing and the chassis is poor (there is insulating paint); at the same time, the ground wire used is long, which will lead to the deterioration of the high-frequency characteristics of the filter and reduce the filtering performance. Due to the long ground wire, the distributed inductance of the wire cannot be ignored at high frequencies. If the filter is well connected, the interference signal can be directly grounded through the housing. If the connection between the filter housing and the chassis is poor, it is equivalent to a distributed capacitance between the filter housing (ground) and the chassis, which will cause the filter to have a high ground impedance at high frequencies, especially in the case of distributed inductance. Near the frequency at which the distributed capacitance resonates, the ground impedance tends to be infinite. The effect of poor filter grounding on filter performance is shown in Figure 3.
As can be seen from Figure 3, due to the poor grounding of the filter and the large grounding impedance, some disturbance signals can pass through the filter. In order to solve the poor overlap, the insulating paint on the chassis should be scraped off to ensure a good electrical connection between the filter housing and the chassis.
Figure 3 Influence of poor filter grounding on filter performance
4 Examples of ideal installation of mains filters (for reference only)
Figure 4 Example of power filter installation
In this installation method, the housing of the filter is in good contact with the chassis, blocking the opening of the power cord on the chassis, and improving the shielding performance of the chassis; in addition, the input and output lines of the filter are isolated by the chassis shielding. , eliminating the disturbance coupling between the input and output lines and ensuring the filtering performance of the filter.
The installation method of the filter directly affects the filtering effect of the filter. In order to give full play to the performance of the filter, the following principles should be followed when installing the filter:
5 Five-point principle for installing filters
1) Install it near the power inlet, preferably cover the power cord inlet hole on the chassis with a filter housing;
2) The shorter the ground wire, the better;
3) The filter housing and the chassis are well overlapped;
4) The input and output lines of the filter are separated and cannot be parallel or crossed;
5) Avoid strong interference sources near the filter.
This paper mainly introduces the calibration method of electromagnetic pulse sensor under strong field strength. The device under test adopts the analog optical fiber transmission system to transmit the pulse signal, which has the characteristics of low noise, small nonlinear distortion and large dynamic range. The peak response sensitivity to the sensing system was obtained by measurement, and a group of interlaboratory comparisons showed that this calibration method had good consistency.