360°Comprehensive Analysis of Electromagnetic Interference and EMC Design of Radar Circuits

Modern radars have higher and higher requirements for signal spectrum quality, and require the radar to work reliably in harsh electromagnetic interference environments. This puts forward higher requirements for the anti-electromagnetic interference capability and electromagnetic compatibility design of radar circuit systems. Since the parasitic output of the radar signal is not only generated in the process of signal conversion, but also related to the external interference of the system, the interference between circuits, the structural design, process design and signal transmission matching of the circuit system, it is necessary to develop electromagnetic Compatible requirements of the circuit system, in addition to a reasonable solution, design is correct, but also must pay attention to the following points.

​​a. Take effective measures such as reasonable grounding of circuits and decoupling between circuits to suppress all irrelevant signals.

​​b. Assembly design, circuit layout and arrangement must be correct and reasonable.

​​c. Advanced process design should be adopted, and yongucase design should be beautiful and reasonable for electronic equipment enclosures

​​d. Strengthen the EMC shields electronic supply isolation between unit circuits and cables and between circuit systems.

Electromagnetic Compatibility Analysis and Design of Circuit System

When a circuit system is composed of high-quality unit electronic circuits to complete a certain function, in addition to the correct system scheme, the electromagnetic compatibility design is also very important. Especially for the highly stable signal system and some complex circuit systems in modern radars and circuit systems working in harsh electromagnetic environments, the design of electromagnetic compatibility is even more important. The following will analyze the electromagnetic compatibility problem of the circuit system in detail, and propose the relevant implementation measures and methods.

​​1. Electromagnetic compatibility of power system

​​There are three forms of interference in the power supply system: the first is the interference from outside the system, such as the interference signal caused by the interference from the AC power grid and the interference magnetic field; the second is the interference signal generated by the system itself, such as rectification filter After the ripple interference, the sharp pulse generated by the thyristor voltage regulation, the high-frequency pulse caused by the switching power supply, the leakage of the high-frequency power supply and the noise generated by the Zener tube, etc.; the third is the field interference signal on the system connection. . To suppress or weaken these interference signals, the power system must be carefully designed for electromagnetic compatibility.

​​a. Add EMC electronic shielding and isolation measures to the power transformer

​​Each power transformer should be electrostatically shield electronics between the primary and secondary to isolate the interference from the primary grid. Important power supplies should also cover the entire transformer with high magnetic permeability materials for magnetic shielding to suppress the interference caused by the transformer magnetic field and external magnetic fields. These electrostatic isolation and magnetic shielding of electrons should be reliably grounded.

​​b. The power transformer should be as far away as possible from the electronic circuit, so as to minimize the interference of the power frequency and the magnetic field of the AC power supply.

​​c. In the circuit system, if there are analog circuits and digital circuits at the same time, the analog circuits and digital circuits must be powered separately. Prevents fast rise time transients in digital circuits from affecting analog circuits through the power supply.

​​d. Assemble the circuit reasonably, correctly ground, wire and lay cables correctly. The selection of appropriate magnetic and electromagnetic EMC shielding mesh materials must take into account size, weight and cost.

​​2. Electromagnetic compatibility of signal transmission system

​​There are two main aspects of interference during signal transmission. One is that when the signal passes through the transmission line, an electromagnetic field is generated around it, and these electromagnetic fields will induce current in the surrounding conductors to form an interference signal; the other is that the signal is generally reflected during the transmission process, and the reflected signal is serialized into other circuits to form interference Signal. The following measures can generally be taken to overcome the interference caused by transmission.

​​a. Strengthen the electromagnetic shielding. For different frequencies and different types of unit circuits, the combined shielding can reduce the mutual influence.

​​b. Strengthen the matching and isolation of the transmission signal, so that the input and output of the signal have matching networks, and appropriately increase the isolation level to reduce reflection interference.

​​c. Suppress all useless signals, even if they fall outside the frequency band of the circuit, suppression measures should be taken. Because ordinary amplifiers, frequency multipliers, frequency mixers and frequency dividers all have nonlinearities, it is possible to convert out-of-band signals into the effective frequency band range of the circuit. Therefore, if necessary, various filters should be added to suppress unwanted signals.

​​d. Reasonable wiring and reasonable arrangement of circuits. The transmission lines on the printed board may become both transmitting antennas and receiving antennas. Therefore, they must be arranged reasonably and their lengths should be shortened as much as possible to reduce their mutual interference.

3. Electromagnetic compatibility of ground wire system

​​As the ground wire of the circuit system, the primary task is to make good contact and minimize the grounding inductance and grounding resistance.

​​Make the ground current less pot and reduce mutual induction.

​​In the circuit system, the analog signal ground, the digital signal ground and the noise ground should generally be separated, and some systems also need a separate shielding ground. The analog ground is used for analog circuits and their power supplies; the digital signal ground is used for digital circuits and their power supplies; the noise ground is used for electrostatic shielding of AC power transformers and transformer shielding, AC power supply line shielding and transmitters, etc. These ground wires should not be confused and connected in the circuit system, so that they are connected to the ground at a single point outside the system.

4. Electromagnetic compatibility design of circuit technology and structure

Electromagnetic compatibility can not only be regarded as a matter of circuit designers, but also must be coordinated by process and electron shielding enclosure  structure personnel to complete a reasonable design together. Because whether it is the setting of the ground wire, the arrangement of the cable, or the appearance design of the shielding body of the electronic combination, the arrangement of the electronic combination and the placement of the transformer, and the selection of various materials are all electronic structure issues.

​​The processing of the shielding body, the manufacturing and dispatching of the printed board, and the assembly of the circuit will directly affect the shielding, radiation and conduction effects, which is a process problem. The following issues should be paid attention to when designing.

​​a. Reasonably divide and combine unit electronic circuits so that they can be combined into different functional block circuits according to their functions. Especially for the periodic pulse signal circuit, it is best to let it start and end in the same functional block, that is, assembled in the same EMI shielding enclosure.

​​b. The digital circuit and the analog circuit must be assembled separately, and the connection between them should be isolated. If necessary, photoelectric coupling devices can be used to completely isolate them.

​​c. For cables that transmit highly stable signals, if necessary, add emi shield sleeves to the cables, or use semi-rigid cables and rigid cables.

​​d. The arrangement of circuits and components should be reasonable, do not make the signal circuitous, and minimize the output, input and mutual coupling in various situations.

​​e. Try to choose a plane installation circuit, do not ground a large area, its resistance to electromagnetic field radiation is better than that of a three-dimensional circuit, which can greatly reduce the field radiation.

​​f. Pay special attention to noise circuits, the assembly position of noise components, and deal with their ground wires, such as: relays, power transformers, high-power and high-current devices, and high-voltage pulse circuits.

​​g. Do not install high-power and high-current components on the shield to prevent their return current from causing unnecessary coupling interference through the electromagnetic interference shielding.

5. electro magnetic shielding design

 In the design of electromagnetic compatibility, the appearance and structural design of the shield is a very important aspect. Shielding is an important means to suppress all irrelevant signals. Generally, it can be divided into three types: electrostatic shielding, magnetic shielding, and electromagnetic shielding.

1. Electrostatic shielding

​​Any two charged objects in space can generate an electrostatic field, and a change in the voltage of one of them will definitely cause a change in the other, resulting in an electrostatic calamity. The mechanism of electrostatic coupling is caused by the capacitive blunt action that exists between the circuits.

​​The best way to overcome static electricity is to use a metal plate as a static shield. Isolate the two sources of interference with a metal plate, or install all components on the circuit on one side of the metal plate, as in surface mount circuits, to obtain good electrostatic shielding. The specific shielding method, especially the shielding of the stray capacitance catastrophic effect, should be fully considered at the initial stage of design.

​​Shielding method: place a metal plate between the two shielded circuits, and make electrical connections between the metal plates and the ground, as shown in Figure 1. In this way, the power lines from one point are blocked by the shielding plate, which plays the role of electrostatic shielding.

​​It can also be made into a shielding cover, as shown in Figure 2, and a good shielding effect can also be obtained. It can be seen from Figure 2 that the power line at point A cannot reach point B, so it can also play a good role in electrostatic shielding.

 Increase the distance between A and B to reduce the stray capacitance and also reduce the catastrophic effect of capacitance, but this method is limited by volume and generally cannot be used. Special attention should be paid to the good contact between the electrostatic shield and the ground. If the contact is not good, there will be a potential difference between the shield and the ground, which will affect the shielding effect. Therefore, it is required that the shielding box should be treated with conductive and anti-corrosion. The screws and rivets used for fixing should not be too thin. Try to make the electrical contact good, reduce the grounding resistance and reduce the grounding inductance.

 2. Magnetic shielding

​​When the current flows in the wire, when it flows through the inductor coil and the transformer, a magnetic field is generated around it. The magnetic field is propagated through the mutual inductance in the circuit. The magnetic field lines generated by the current induce voltages in other circuits through the mutual inductance.

​​Especially at low frequencies below 3KH: the main interference effect is caused by the magnetic field, but solving the magnetic shielding is often expensive and difficult. In radar, magnetic shielding is mainly aimed at power transformers and high voltage modulators. Generally, the transformer of low ripple power supply is often shielded by permalloy, otherwise it will not achieve good results.

3. Electromagnetic shielding

​​Any kind of AC circuit produces alternating electric and magnetic fields. Electromagnetic shielding is related to the nature of the electromagnetic field, the frequency of change, and the distance between the radiation source and the receptor. In the radar electronic circuit system, the operating frequency is generally higher, and above IKHz, aluminum is generally used as the electromagnetic shielding material. The aluminum shielding box made of cutting technology can shield and isolate 300MHz signals above 100dB. When the frequency is below ]KHz, the magnetic field is mainly shielded, and materials with high magnetic permeability should be selected.

​​In the electronic circuit system, for the convenience of internal wiring and taking out and placing the circuit, a cover plate is added to the shielding assembly box. Sometimes, for ventilation and heat dissipation, holes are punched and slits in the shielding plate, resulting in discontinuous points in the shielding body. Cause signal leakage and cause interference. When you encounter this problem, please feel free to leave it to Yongucase, which is a manufacturer of electronic equipment enclosures for 18 years, and has very rich experience in the design of shielding enclosures. You only need to provide the design ideas of the enclosure and the size of the pcb, You can tailor the ideal shielding box for you. We will arrange the positions of the components correctly so that the slits and holes do not cut off the induced current. If necessary, the holes can be changed to cut-off waveguides to further weaken the hole radiation.

6. Shield assembly design

​​In addition to shielding design, assembly technology is also very important, especially for the assembly of RF systems, which should be carefully designed.

​​In general, the following points should be noted.

​​a. The shielding design of the internal circuit should be able to prevent the leakage of radio frequency energy of the electronic circuit itself, and also prevent the influence of external electromagnetic energy on it.

​​b. Take measures to prevent unnecessary feedback and mishaps between circuit stages.

​​c. Add filtering measures to the power supply to attenuate and suppress the conduction of radio frequency signals within the electronic combination and between the electronic combination and the combination.

​​d. The smaller the RF grounding resistance, the better.

​​Of course, the requirements of volume, weight, cost, etc. must also be taken into account.

​​When the attenuation of the parasitic field between the circuits is relatively high, it is better to use the assembly box structure. It can be made into a single isolation room, or it can be designed into multiple isolation rooms, that is, a shielding box, which is divided into several isolation rooms. This kind of assembly box has better isolation for electrostatic field and electromagnetic field. The best material for making the assembly box is aluminum, which is cheap and light, and generally has a good isolation effect on the radar circuit system. Aluminum has good corrosion resistance, strong plasticity, recyclability, and good heat dissipation effect, making it the best choice for industrial equipment enclosures. Yongucase has ready-made shielding boxes with multiple specifications and sizes, and the size can be customized, anodizing, hole-opening services, etc.

​​When assembling these shields, long seam leaks are often encountered, and necessary measures should be taken to allow for many contact points on the long seam. Measures such as adding screws, adding elastic sheets and lining conductive pads can be used. However, we must pay attention to anti-corrosion, especially electrochemical anti-corrosion, otherwise the satisfactory effect cannot be maintained for a long time.

​​The interference of the radio frequency wire should also be considered, the amplitude of the transmission signal should be reasonably selected, and the direction of the system cable and the pulse signal line should be correctly arranged. AC signal lines should not be tied to each other, especially large-scale pulse signals should be strictly distinguished from high-purity signals during transmission.

​​In short, shielding design is a relatively complex design, which requires not only knowledge of mechanical design, but also knowledge of the various aspects of the electro-corpse involved.