How to improve the technology of electromagnetic compatibility?
How to reduce the various EMI of the communication system and improve the EMC capability of the system is the premise to ensure the normal operation of the system. There are several methods:
Reduce radiation interference
The transmission mode of electromagnetic radiation is as large as the distance between the systems far apart, and the small distance within the system. Therefore, at the beginning of equipment design, the large signal and small signal should be strictly separated, sub-unit design or isolation, to the greatest extent. Electromagnetic interference between each other in a small space. The main methods are:
1. Maintain good shielding between signals
Limit the influence of power lines or magnetic lines to a certain range, prevent power lines or magnetic lines from entering a certain range, and separate external interference from the measuring device. The measurement signal is not affected by the external electromagnetic field. When using shielded wires and shielded cables, care must be taken that the shield and the outer layer cannot flow current, and the ground cannot form a loop.
2. Perform physical isolation
That is to increase the distance between the signal conductor and the interference source, the weak signal conductor should avoid parallel design with the strong signal conductor. Avoid two wires of the same signal loop, and the weak signal loop and the strong signal loop share the ground wire. For various components and wires of different performance, they should be classified according to their different levels, power and anti-interference ability.
3. Balance circuit voltage
Using the balanced relationship on the circuit, the two wires that transmit the same signal have the same interference voltage. The interference voltage can be self-cancelled on the load of the two wires. In this way, electromagnetic interference of the external circuit can be effectively suppressed.
4. Correct grounding
The ground potential is very complicated. In order to minimize the interference, “one-point grounding” and “multi-point grounding” should be applied correctly.
Reduce grounding interference
The general requirements for the ground plane are: the ground plane should be zero potential; the ideal ground plane should be a zero-resistance entity, there is no voltage drop when the current passes; there is a large distributed capacitance between the ground plane and the wiring, and the lead inductance of the plane itself Will be small; grounding requirements to minimize the interference voltage generated by the common ground impedance of multiple circuits. At the same time avoid unnecessary loops.
When designing the grounding of small-signal circuits, especially for circuit design with large signal variation range and large frequency variation range, the following aspects should be noted:
1. Multi-stage circuit selects one point to ground
A low level circuit is the most susceptible circuit. Especially for multi-stage connected circuits, the input signal is very small compared with the output signal. Therefore, a grounding should be selected to minimize the interference of the low-level circuit.
2. Cable shield grounding point selection
For low frequency and high frequency circuits, different grounding methods should be used, especially high frequency shielding wires, so that the loop current only passes through the low resistance shielding layer and not through the inner conductor of the cable. It can effectively suppress the interference of the ground loop.
3. Grounding selection of resonant circuit
The grounding of the resonant tank and the filter circuit is the most easily overlooked problem. Due to the misdirection of the grounding method, many designers have grounded the capacitive inductance of the resonant circuit. In the parallel resonant circuit, the inductor and capacitor should be grounded a little, so that the resonant circuit itself forms a closed loop, at which time the high-frequency large current will not pass through the ground plane. And suppress the ground loop interference. The correct resonant loop and filter circuit can effectively suppress interference.
4. Weak signal amplifier and signal source select one point to ground
For the signal source and the circuit to which the amplifier is connected. If the source and amplifier are grounded at different points, there is a potential difference between the two places. For the output of the amplifier, this potential difference is small and negligible; but for the input of the amplifier, this potential difference should be much smaller than the output signal of the signal source, otherwise it will cause a lot of interference on the amplifier.
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