Several EMC rectification methods for circuits
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First of all, it is necessary to diagnose the product according to the actual situation, analyze the source of its interference and the ways and methods of mutual interference. Based on the analysis results, targeted rectification will be carried out.
Generally speaking, the main rectification methods are as follows.
1. Reduce the source of interference On the basis of finding the interference source, the interference source can be attenuated within the allowable range. The methods to attenuate the source generally include the following methods:
a Add a decoupling capacitor between Vcc and GND of the IC. The capacitance of the capacitor is between 0.01μF and 0.1μF. Pay attention to the lead of the capacitor when installing, and make it as short as possible.
b Add an attenuator while ensuring sensitivity and signal-to-noise ratio. For example, the crystal oscillator in VCD and DVD video disc players has a serious impact on electromagnetic compatibility. Reducing its amplitude is one of the feasible methods, but it is not the only solution.
c. Another indirect method is to keep the signal line away from the interference source.
2. Classification of wires and cables In electronic equipment, inter-line coupling is an important way and an important cause of interference. Because of the frequency factor, it can be roughly divided into high-frequency coupling and low-frequency coupling. Due to the different coupling methods, the rectification methods are also different, which are discussed below:
(1) Low frequency coupling Low-frequency coupling is the case where the length of the guide line is equal to or less than 1/16 wavelength. Low-frequency coupling can be divided into electric field and magnetic field coupling. The physical model of electric field coupling is capacitive coupling. Therefore, the main purpose of rectification is to reduce distributed coupling capacitance or reduce For the coupling amount, the following methods can be used:
a. Increasing the distance between circuits is the most effective way to reduce distributed capacitance.
b. Adding a high-conductivity shielding cover, and making the shielding cover single-point grounding can effectively suppress low-frequency electric field interference.
c. Additional filters can reduce the coupling between the two circuits.
d. Reduce the input impedance. For example, the input impedance of the CMOS circuit is very high, and it is extremely sensitive to electric field interference. A capacitor or a resistor with lower resistance can be connected in parallel at the input end within the allowable range. The physical model of magnetic field coupling is inductive coupling. The coupling is mainly coupled by the distributed mutual inductance between the lines. Therefore, the main method of rectification is to destroy or reduce the coupling amount. Generally, the following methods can be used:
a. To add a filter, pay attention to the input and output impedance of the filter and its frequency response when adding a filter.
b. Reduce the loop area of the sensitive loop and the source loop, that is, try to make the signal line or current-carrying line close to or twisted with its return line.
c. Increase the distance between the two circuits in order to reduce the mutual inductance between the lines to reduce the amount of coupling.
d. If possible, try to make the sensitive loop and the source loop plane orthogonal or close to orthogonal to reduce the amount of coupling between the two circuits.
e. Wrap sensitive wires with high-permeability materials, which can effectively solve the problem of magnetic field interference. It is worth noting that it is necessary to form a closed magnetic circuit, and efforts to reduce the magnetic resistance of the magnetic circuit will be more effective.
(2) High frequency coupling High-frequency coupling means that the wiring longer than 1/4 wavelength will increase the coupling due to the standing waves of voltage and current in the circuit. The following methods can be used to solve this problem:
a. Shorten the grounding wire as much as possible, and try to use surface contact with the case for grounding.
b. Rearrange the input and output lines of the filter to prevent coupling between input and output lines to ensure that the filtering effect of the filter does not deteriorate.
c. The shielding layer of the shielded cable is grounded at multiple points.
d. Connect the floating pin of the connector to the ground potential to prevent its antenna effect.
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