Shenzhen City HuaJieTai Technology Co., Ltd.

Nowadays, the clock frequency of the electronic system for several hundred MHz, the pulse before and after along in the subnanosecond range, high quality video circuit is also used to sub nanosecond pixel rate. These high processing speeds represent a constant challenge to the project. So how to prevent and solve the problem of electromagnetic interference connector is worthy of our attention. The oscillation rate on the

circuit becomes much faster (rise / fall time), the voltage / current amplitude becomes larger, the problem becomes more. Therefore, it is more difficult to solve electromagnetic compatibility (EMC) today than before.

in the circuit of the two wave nodes before the fast change of the pulse current, said the so-called differential mode noise source, the electromagnetic field around the circuit can be coupled to other components and the intrusive connection part. The inductive or capacitive coupling noise is the common mode   interference. The radio frequency interference current is the same with each other, and the system can be modeled as a noise source, a "victim circuit" or "receiver" and a circuit (usually a bottom board). Several factors are used to describe the size of the disturbance: the &nbsp of the noise source, the magnitude of the current surround area, and the change rate of the interference current.

so, although there is likely to produce an interference in the circuit, the noise is almost always a common model. Once the input / output (I/O) connector and the casing or ground plane are connected to the cable, there is a certain RF voltage which causes a few Ma of RF current to be sufficient to exceed the allowable emission level. The coupling of

noise and the propagation of

common mode noise are generated due to the unreasonable design. Some of the typical reasons are that the length of the line is different, or the distance between the power plane and the casing is different. Another reason is the defects of components, such as magnetic induction coils and transformers, capacitors and active devices (for example, application of special integrated circuits (ASIC)). < br > < br > magnetic elements, especially the so-called "core choke coil" type energy storage inductor is used in the power converter, always produces an electromagnetic field. The air gap in the magnetic circuit is equivalent to a large resistance in series circuit, where more &nbsp is consumed. So, iron core choke coil, winding in the ferrite rod, in around a bar produced a strong electromagnetic field, in the vicinity of the electrode has the strongest intensity. In the switch power supply with the back trace structure, the transformer must have a gap, &nbsp, which has a strong magnetic field. One of the most suitable elements to maintain the magnetic field is the spiral tube, which causes the electromagnetic field to be distributed along the length of the tube core. This is one of the reasons for the optimization of the helical structure of the magnetic components operating at high frequency.

inappropriate decoupling circuit is usually also a source of interference. If the circuit of pulse current, and local decoupling cannot guarantee small capacitance or require very high resistance, generated by the power supply circuit voltage decline. This is equivalent to a ripple, or equivalent to a fast change in the voltage between the terminals. Due to the stray capacitance of the package, the interference can be coupled to other circuits, causing the common mode problem.

when the common mode current is polluted by the I/O interface circuit, the problem must be resolved before the connector is passed through the. Different applications, it is recommended to solve this problem with different methods. In the video circuit, where the I/O signal is a single ended &nbsp, and the common common circuit, to solve it, with a small LC filter to filter out noise. In the low frequency series interface network, some stray capacitance is enough to shunt the noise on the bottom board. A differential drive interface, such as an Ethernet, &nbsp, is usually coupled to the I/O region through a transformer, which is provided by a center tap on one side or both sides of the transformer. The center tap is connected with the bottom plate through a high voltage capacitor, and the common mode noise is split on the bottom board so as to make the signal not distorted.

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in the I/O region of the common mode noise

does not have a general solution to solve all types of I/O interface problems. The main goal of the designers is to design the circuit, and often ignore some of the simple details. Some of the basic rules can cause the noise to drop to a minimum of

1 before reaching the connector. The decoupling capacitor is set to the next load.

2) fast changing pulse current along the front and back edges, the size of the loop should be minimized.

3 () enables large current devices (i.e., drives and ASIC) away from the I/O port.

4) determination of signal integrity, to ensure that impulse and undershoot minimum, especially for large current key signals (such as clock, bus).

5) the use of local filtering, such as RF ferrite, can absorb RF interference.

6) to provide low impedance lap on the bottom board or in the I/O area on the base plate. & nbsp; RF noise and connector < br > < br > even engineers to take many of the above listed the preventive measures to reduce RF noise in the I / O area, but also can not guarantee that these precautions can successfully enough to meet the emission requirements. Some noise is conducted interference, namely, &nbsp; the circuit board on the common mode current flow. The source of this interference is between the base plate and the circuit. Thus, the RF current must pass through the minimum impedance (between the base plate and the carrier signal line). If the connector is not rendered &nbsp; low enough impedance (with the overlap of the base plate), the RF current flows through the stray capacitance. When the RF current flows through the cable, it is inevitable that the. The other mechanism of the common mode current injected into the I/O region by

is the coupling of strong interference sources in the vicinity. Even some of the "shield" connectors are useless, because the source of interference in the vicinity of the connector, such as the PC machine environment. If there is a gap between the connector and the base plate, the RF voltage