Connector EMI problems how to prevent and solve?
Today's electronic systems have clock speeds in the hundreds of megahertz, with pulses in the sub-nanosecond range, and high quality video circuits at sub-nanosecond pixel rates. These higher processing speeds indicate constant engineering challenges. So how to prevent and solve the connector electromagnetic interference deserves our attention.
The oscillation rate on the circuit gets faster (rise / fall time), the voltage / current amplitude gets bigger and the problem gets more. Therefore, it is even harder to solve Electromagnetic Compatibility (EMC) today than before.
Fast changing pulse currents, before the two nodes of the circuit, represent so-called differential mode noise sources and the electromagnetic fields surrounding the circuit can be coupled to other elements and intrude into the connection. Perceptual or capacitively coupled noise is common mode interference. The RFI currents are identical to each other and the system can be modeled as a noise source, "victim circuit" or "receiver" and a loop (usually the backplane). Several factors are used to describe the magnitude of the disturbance: the intensity of the noise source, the size of the surrounding area of the disturbing current, and the rate of change.
Thus, the noise is almost always common-mode, despite the unwanted interference that is most likely to occur in the circuit. Once some of the RF voltage appears on the cable between the I / O connector and the chassis or ground plane, a few milliamperes of RF current is sufficient to exceed the allowable emission level.
Noise coupling and propagation
Common-mode noise is due to unreasonable design. Some typical reasons are different lengths of individual wires in different pairs, or different distances from the power plane or cabinet. Another reason is component defects, such as magnetic induction coils and transformers, capacitors, and active devices (such as application specific integrated circuits (ASICs)).
Magnetic components, especially the so-called "iron-core choke" types of energy storage inductors, are used in power converters and always generate electromagnetic fields. The air gap in the magnetic circuit corresponds to a large resistance in the series circuit, where more power is consumed.
As a result, the iron core choke, wound around the ferrite rod, produces a strong electromagnetic field around the rod and has the strongest field strength near the electrode. In the use of switch-mode switching power supply, the transformer must have a gap, during which there is a strong magnetic field. The most suitable element in which the magnetic field is held is a spiral tube, which distributes the electromagnetic field along the length of the die. This is one of the reasons why a preferred helical structure of a magnetic element operating at high frequencies is achieved.
Inappropriate decoupling circuits also often become sources of interference. If the circuit requires a large pulse current, and the local decoupling can not guarantee a small capacitance or very high internal resistance required, the voltage generated by the power supply circuit will drop. This is equivalent to ripple, or equivalent to rapid voltage changes between terminals. Due to the stray capacitance of the package, the interference can be coupled into other circuits, causing common-mode problems.
When the common mode current contaminates the I / O interface circuit, the problem must be solved before passing through the connector. Different applications, it is recommended to use different methods to solve this problem. In video circuits, where I / O signals are single-ended, and share the same common loop, to solve it, filter out the noise with a small LC filter.
In low frequency serial interface networks, some stray capacitance is enough to shunt noise to the backplane. Differential-driven interfaces, such as Ethernet, are typically coupled to the I / O area by transformers, providing coupling at the center tap on one or both sides of the transformer. These center taps are connected to the backplane via high voltage capacitors, shunting common mode noise to the backplane so that the signal is not distorted.
The oscillation rate on the circuit gets faster (rise / fall time), the voltage / current amplitude gets bigger and the problem gets more. Therefore, it is even harder to solve Electromagnetic Compatibility (EMC) today than before.
Fast changing pulse currents, before the two nodes of the circuit, represent so-called differential mode noise sources and the electromagnetic fields surrounding the circuit can be coupled to other elements and intrude into the connection. Perceptual or capacitively coupled noise is common mode interference. The RFI currents are identical to each other and the system can be modeled as a noise source, "victim circuit" or "receiver" and a loop (usually the backplane). Several factors are used to describe the magnitude of the disturbance: the intensity of the noise source, the size of the surrounding area of the disturbing current, and the rate of change.
Thus, the noise is almost always common-mode, despite the unwanted interference that is most likely to occur in the circuit. Once some of the RF voltage appears on the cable between the I / O connector and the chassis or ground plane, a few milliamperes of RF current is sufficient to exceed the allowable emission level.
Noise coupling and propagation
Common-mode noise is due to unreasonable design. Some typical reasons are different lengths of individual wires in different pairs, or different distances from the power plane or cabinet. Another reason is component defects, such as magnetic induction coils and transformers, capacitors, and active devices (such as application specific integrated circuits (ASICs)).
Magnetic components, especially the so-called "iron-core choke" types of energy storage inductors, are used in power converters and always generate electromagnetic fields. The air gap in the magnetic circuit corresponds to a large resistance in the series circuit, where more power is consumed.
As a result, the iron core choke, wound around the ferrite rod, produces a strong electromagnetic field around the rod and has the strongest field strength near the electrode. In the use of switch-mode switching power supply, the transformer must have a gap, during which there is a strong magnetic field. The most suitable element in which the magnetic field is held is a spiral tube, which distributes the electromagnetic field along the length of the die. This is one of the reasons why a preferred helical structure of a magnetic element operating at high frequencies is achieved.
Inappropriate decoupling circuits also often become sources of interference. If the circuit requires a large pulse current, and the local decoupling can not guarantee a small capacitance or very high internal resistance required, the voltage generated by the power supply circuit will drop. This is equivalent to ripple, or equivalent to rapid voltage changes between terminals. Due to the stray capacitance of the package, the interference can be coupled into other circuits, causing common-mode problems.
When the common mode current contaminates the I / O interface circuit, the problem must be solved before passing through the connector. Different applications, it is recommended to use different methods to solve this problem. In video circuits, where I / O signals are single-ended, and share the same common loop, to solve it, filter out the noise with a small LC filter.
In low frequency serial interface networks, some stray capacitance is enough to shunt noise to the backplane. Differential-driven interfaces, such as Ethernet, are typically coupled to the I / O area by transformers, providing coupling at the center tap on one or both sides of the transformer. These center taps are connected to the backplane via high voltage capacitors, shunting common mode noise to the backplane so that the signal is not distorted.
