Shielding design is one of the more important designs in electromagnetic compatibility

Shielding design is a very important aspect of electromagnetic compatibility design. Shielding is an important means of suppressing all unrelated signals, and can generally be divided into three types: magnetic shielding, electromagnetic shielding, and electrostatic shielding.

1. Magnetic shield

When current flows through inductive coils and transformers, a magnetic field is generated around them. The magnetic field propagates through mutual inductance in the circuit, and the magnetic lines generated by the current are induced to voltages in other circuits through mutual inductance.

Especially under low-frequency conditions below 3KH, the main interference is caused by magnetic fields, but solving magnetic shielding is often expensive and difficult. In radar, magnetic shielding is mainly used for power transformers and high-voltage regulators. Generally speaking, transformers for low ripple power supplies are usually shielded with permalloy, otherwise the effect is not good.

2. Electromagnetic shielding

Any 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 sensor. In radar electronic circuit systems, the operating frequency is generally higher, and above IKHz, aluminum can generally be selected as the electromagnetic shielding material. The shielding isolation of 300 MHz signals exceeds 100 dB. When the frequency is lower than KHz, the magnetic field is mainly shielded, and materials with high magnetic conductivity should be selected.

In electronic circuit systems, in order to facilitate internal wiring and removal and placement of circuits, the shield assembly box has a cover plate, sometimes for ventilation. For cooling and other needs, the designer should carefully consider drilling holes in the shield plate and opening seams, which can lead to shield breakpoints, signal leakage, and interference. Correctly position the components so that the joints and holes do not cut off the induced current. If necessary, the waveguide can be switched off to further weaken the hole radiation.

3. Electrostatic shielding

Any two electrically charged objects in space can generate electrostatic fields, and changes in voltage in one of them will inevitably lead to changes in the other and electrostatic disasters. The mechanism of the electrostatic lotus is caused by the clumsy cooperation of capacitors between circuits.

The best way to overcome electrostatic interference is to use a metal plate as an electrostatic shield. Using a metal plate to isolate the two sources of interference, or installing all components on a circuit on one side of the metal plate, just like surface mounted circuits, can achieve good electrostatic shielding. At the initial stage of design, specific shielding methods should be fully considered, especially for stray capacitance hazards.

Shielding method: Place a metal plate between the two shielded circuits, and connect each metal plate to the ground, as shown in the figure. In this way, the power cord emitted from a single point is blocked by the shield plate, which is electrostatic shielding.

Increasing the A.B distance to reduce scattered capacitance can also reduce cooperation in capacitance disasters, but this method is generally not available due to volume limitations. Pay special attention to the good contact between the electrostatic shield and the ground. If the contact is poor, there will be a potential difference between the shield and the ground, affecting the shielding effect. Therefore, the shielding box should be subject to conductive corrosion treatment, and screws, rivets, etc. should not be fixed too thin. Try to make good electrical contact, reduce ground resistance, and reduce ground inductance.

4. Shielding 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.

Generally, the following points should be noted:

Take measures to prevent unnecessary feedback and disasters between circuit levels and levels.

B. The shielding design of internal circuits should not only prevent the leakage of RF energy from the electronic circuit itself, but also prevent the impact of external electromagnetic energy on it.

C. The smaller the RF ground resistance, the better.

D. Transmit radio frequency signals within and between electronic assemblies.

Of course, requirements such as volume, weight, and cost should also be considered.

5. Conclusion

When the circuit requires high attenuation of parasitic fields, it is best to use an assembly box structure. It can be in the form of one isolation chamber or multiple isolation chambers, that is, a shielding box, divided into several isolation chambers. Such an assembly box can better isolate electrostatic and electromagnetic fields. Aluminum is the best material for manufacturing assembly boxes. It's cheap and light. For radar circuit systems, the isolation effect is generally very good.

When assembling these shields, leakage from long seams is often encountered, and necessary measures should be taken to ensure that there are many contact points on the long seams. Measures such as adding screws, adding elastic plates, and lining conductive pads can be taken. However, attention must be paid to corrosion prevention, especially electrochemical corrosion prevention, otherwise satisfactory results cannot be maintained for a long time.

It is also necessary to consider the interference of radio frequency conductors, reasonably select the range of transmission signals, and correctly arrange the system cable direction and pulse signal lines. AC signal lines should not be bound to each other, especially large pulse signals that are significantly different from high purity signals during transmission.

In short, shielding design is a more complex design in electromagnetic compatibility design, which requires not only mechanical design knowledge, but also familiarity with the involved zombies and various knowledge.