Today, the clock frequency of electronic systems reaches hundreds of MHz, and the front edge and back porch of the pulse used are in the subnanosecond range. High quality video circuits adopts subnanosecond pixel rates. These high processing speeds indicate the continuous challenges in engineering. So how to prevent and solve the problem of connector electromagnetic interference is worth our attention.
The oscillations on the circuit become faster (rise/fall time), the voltage and current amplitude become larger, and the problems become more numerous. Therefore, it is more difficult to solve electromagnetic compatibility (EMC).
Before two nodes of the circuit, the pulse current that changes rapidly represents the so-called differential mode noise source. The electromagnetic field around the circuit can be coupled to other components and intrude into the connecting part. The noise of inductive or capacitive coupling is a kind of common mode interference. The RF interference current is identical to each other, and the system can be modeled as it consists of a noise source, a "victim circuit" or receiver and a loop (usually a baseplate). Several factors are used to describe the magnitude of the interference: the intensity of the noise source, the size of the surrounding area of the interference current, and the change rate.
Thus, although unwanted interference is likely to occur in the circuit, the noise is almost always a common mode. Once the cable is connected between the input/output (I/O) connector and the housing or the ground plane, a few milliamps of RF current is sufficient to exceed the permissible emission level when some RF voltage appears.
The coupling and spreading of noise
The common mode noise is caused by the unreasonable design. Some typical reasons are that the length of individual wire in different line pairs is different, or the distance to the power plane or housing is different. Another reason is the defect of components, such as the defect in magnetic induction coils, transformers, capacitors and active devices ( specific integrated circuit (ASIC) is required).
Magnetic components, especially the so-called iron-core choking coil type energy storage inductors, always produce electromagnetic fields when they are used in power converters. The air gap in the magnetic circuit is equivalent to a large resistance in a series circuit, where more electric energy is consumed.
Therefore, the iron-core choking coil is wound on the ferrite rod, and a strong electromagnetic field generates around the rod. The strongest electromagnetic field is near the electrode. There must be a gap on the transformer when using the switching power supply with the retracing structure. There is a strong magnetic field in the gap. The most suitable element to maintain the magnetic field is the shelix tube, which makes the electromagnetic field distribute along the length of the die. This is one of the reasons why the helical structure is preferred for magnetic elements working at high frequency.
Improper decoupling circuits often become interference sources. If the circuit requires a large pulse current, and the need for small capacitance or very high internal resistance cannot be guaranteed when the circuit is partially decoupled, the voltage generated by the power supply circuit will drop. This is equivalent to the ripple wave, or to a rapid change in voltage between terminals. Due to the packaged stray capacitance, the interference can be coupled to other circuits, causing the common mode problem.
When the common mode current contaminates the I/O interface circuit, the problem must be solved before the circuit flows through the connector. Different methods are suggested to solve this problem in different applications. The I/O signals are single ended and share the same common circuit in video circuits, so small LC filters can be used to filter out the noise to solve this problem.
In low frequency series interface networks, it is enough for some stray capacitances to shunt the noise to the baseplate. In the interface of differential drive such as Ethernet, the noise is usually coupled to the I/O area through a transformer, and the coupling is provided at the central tap on one or both sides of the transformer. These central taps are connected to the baseplate via a high voltage capacitor, and the common mode noise is shunted to the baseplate so that the signal will not be distorted.
The common mode noise in I/O area
There is no universal solution to all types of I/O interfaces. The main goal of designers is to design the circuit well, but they often ignore some simple details. Some basic rules can minimize the noise before it reaches the connector:
1) Set the decoupling capacitor close to the load.
2) The loop size of the fast changing front edge and back porch pulse currents should be the smallest.
3) Keep high current devices (i.e. drivers and ASIC) away from I/O ports.
4) Measure the signal integrity to ensure the minimum overshoot and undershoot, especially for critical signals with high current (such as the clock and bus).
5) The RF interference can be absorbed by local filter, such as RF ferrite.
6) Provide low impedance lapping or the standard of the I/O area to the baseplate.
Even if engineers take many of the precautions listed above to reduce RF noise in the I/O area, there is no guarantee that these precautions will be successful enough to meet the emission requirements. Some of the noise is conducted interference, which flows on the internal circuit board according to the common mode current. This interference source is between the baseplate and the circuit.
Thus, the RF current must flow through the access with the lowest impedance (between the baseplate and the carrier signal line). If the connector does not present a sufficiently low impedance ( at the lap joint with the baseplate), the RF current will flow through stray capacitors. When this RF current flows through the cable, the emission is inevitable.
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