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How to prevent and solve the problem of electromagnetic interference of connectors

Author: Date:4/26/2020 1:52:43 AM

Today, electronic systems have clocks of a few hundred megahertz, use pulses in the sub-nanosecond range, and high quality video circuits use sub-nanosecond pixel rates.These high processing speeds represent ongoing engineering challenges.So how to prevent and solve the problem of connector electromagnetic interference is worth our attention.

 

The rate of oscillation on the circuit becomes faster (rise/fall time), the voltage/current amplitude becomes larger, and the problems become more numerous.As a result, solving electromagnetic compatibility (EMC) is harder today than ever.

In front of the two wave nodes of the circuit, the rapidly changing pulse current represents the so-called differential mode noise source, the electromagnetic field around the circuit can be coupled to other components and intrude into the connection part.Inductive or capacitively coupled noise is common mode interference.The rf interference currents are identical to each other and the system can be modeled as consisting of a noise source, a "victim circuit" or a "receiver" and a loop (usually a baseplate).The magnitude of the disturbance is described by several factors: the intensity of the noise source, the area of the disturbance current, and the rate of change.

Thus, noise is almost always co-modal, despite the likelihood of unwanted interference in the circuit.Once the cable is connected between the input/output (I/O) connector and the housing or ground plane, the presence of certain RF voltages can cause a few milliamps of RF current to be sufficient to exceed the allowable emission level.

Coupling and propagation of noise

Common mode noise is caused by unreasonable design.Some are typically due to the different lengths of individual wires in different pairs, or the different distances to the supply plane or housing.Another reason is defects in components such as magnetic induction coils and transformers, capacitors and active devices (such as asics for special applications).

Magnetic elements, especially so-called "core choke" type storage inductors, are used in power converters and always generate electromagnetic fields.The air gap in the magnetic circuit is equivalent to a large resistance in a series circuit, where more energy is consumed.

So an iron-core choke coil, which is wound around a ferrite rod, generates a strong electromagnetic field around the rod, and the strongest field near the electrode.There must be a gap in the transformer with a strong magnetic field in the switching power supply using a backtrace structure.The most suitable element for maintaining the magnetic field is the spiral tube, which distributes the electromagnetic field along the length of the tube core.This is one of the reasons why magnetic elements operating at high frequencies prefer helical structures.

Improper decoupling circuits also often become interference sources.If the circuit requires a large pulse current, and local decoupling does not guarantee a small capacitance or a very high internal resistance, then the voltage generated by the supply loop drops.This is equivalent to a ripple, or a rapid change in voltage between terminals.Due to the packaged stray capacitance, the interference can be coupled to other circuits, causing common mode problems.

When common mode current contaminates the I/O interface circuit, this problem must be resolved before it can pass through the connector.Different applications suggest different ways to solve this problem.In a video circuit, where the I/O signal is single-ended and shares a common loop, the noise is filtered out with a small LC filter.

In a low frequency series interface network, some stray capacitance is sufficient to divert the noise to the baseplate.Differentially driven interfaces, such as the ether, are typically coupled to the I/O region via a transformer, providing coupling at a central tap on one or both sides of the transformer.These center taps are connected to the baseplate by a high-voltage capacitor, which diverts the common-mode noise to the baseplate so that the signal is not distorted.

Common mode noise in the I/O region

There is no general solution to all types of I/O interfaces.The designers' main goal is to design the circuit well, and they often overlook the details that are considered simple.There are some basic rules to minimize noise before it reaches the connector:

1) set the decoupling capacitor close to the load.

2) the size of the loop shall be minimum for the pulse current with fast changing front and back edges.

3) keep large current devices (i.e. drives and asics) away from I/O ports.

4) determine the integrity of the signal to ensure the minimum overshoot and undershoot, especially for the critical signal of large current (such as clock and bus).

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

6) provide low impedance lap to the baseplate or reference in the I/O area on the baseplate.

Even if engineers take many of the precautions listed above to reduce RF noise in the I/O region, there is no guarantee that these precautions will be successful enough to meet the emission requirements.Some of the noise is conductive interference, that is, the flow of a common mode current on the internal circuit board.The interference source is between the baseplate and the circuit, etc.

Thus, this RF current must flow through the path of the lowest impedance (between the baseplate and the carrier signal line).If the connector does not present a sufficiently low impedance (the overlap with the baseplate), this RF electricity flows through the stray capacitance.When this RF current flows through the cable, emission is inevitable.