Cable interference and Countermeasures

Cables are the main cause of electromagnetic compatibility problems in the system. Therefore, in practice, it is often found that when the external drag cable on the device is removed, the device can pass the test smoothly. When electromagnetic interference occurs on the site, if you unplug the cable, the fault will disappear. This is because the cable is an efficient receiving and radiating antenna. In addition, the distance between the wires in the cable is the longest, so there is a large branch capacitor and mutual inductance between the wires, which leads to Signal Crosstalk between the wires.

One of the main ways to solve the cable problem is to shield the cable, but it is effective to shield the cable, and a series of problems are common concern and fuzzy problems. This section discusses cable radiation, electromagnetic interference to cables, Signal Crosstalk between wires, and countermeasures.

Cable radiation problems

Radiation problems of cables are one of the most common problems in engineering. More than 90% of devices are mainly devices with pulse circuits. The failure to pass the radiation emission test is caused by cable radiation. There are two kinds of cable Radiation Mechanisms: one is the signal current difference mode current in the cable) The difference mode radiation produced by the circuit, the other is the wire in the cable includes the shielding layer) the common mode current. The cable radiation mainly comes from common mode radiation. Common mode radiation is produced by common mode current, and the loop area of the common mode current is formed by the cable and the earth or adjacent to other large conductors. Therefore, it has a large loop area, it produces strong radiation.

How common-mode current is generated is often a problem that many people are confused about. To understand this problem, we first need to clarify that the common mode voltage is the root cause of the common mode current, and the common mode voltage is the voltage between the cable and other large conductors in the earth or adjacent areas. Starting from the common mode voltage, it is easy to find the cause of the common mode current, and once the cause of a problem is clear, it is not very difficult to solve this problem. The common mode current on the cable is caused by the following reasons: common mode current caused by differential mode current leakage. even if the cable contains a signal return line, it cannot guarantee that the signal current is 100% returned from the return line. In particular, when the frequency is high, various stray parameters in the space provide the third line for the signal current, more return paths. Although this common mode current accounts for a small proportion, due to the large area of the radiation loop, radiation cannot be ignored.

Do not try to reduce the common mode current by disconnecting the circuit board from the ground or the ground between the ground and the ground to reduce the common mode radiation. Disconnecting the circuit from the Earth can only reduce the common mode current at the low frequency. At the high frequency, the path formed by the parasitic capacitor has little impedance. Common-mode current is mainly generated by stray capacitors. Of course, if the common mode radiation problem occurs mainly in the low frequency, disconnecting the circuit board or chassis from the earth will have a certain effect. According to the mechanism of common mode current generation, the effective method to reduce this common mode current is to reduce the impedance of the differential mode circuit, so that most of the signal current is returned from the signal ground.

Generally, the closer the signal line is to the return line, the smaller the impedance of the differential current loop. A typical example is the coaxial cable. Because the return current of the coaxial cable is evenly distributed on the outer skin, its equivalent current is coincident with the axis, so the loop area is zero, and the differential mode impedance is close to zero, almost 100% of the signal current is returned from the outside of the coaxial cable. The common mode current is almost zero, so the common mode radiation is very small. On the other hand, because the area of the differential current loop is almost zero, the radiation of the differential mode is very small, so the radiation of the coaxial cable is very small. For high-frequency signals, use coaxial cables to avoid radiation. In fact, this is essentially the same as traditional coaxial cable for transmitting high-frequency signals to reduce signal loss. Because the loss of the signal is small, it naturally shows that the leakage of components is less, and this part of the leakage is the radiation of the cable.

Common Mode current caused by ground noise of the circuit board. The signal ground line is the reflux line of the signal. Therefore, there must be voltage between two points on the ground line. For high-frequency circuits, these are high-frequency noise voltage, it acts as the common mode current on the common mode voltage drive cable, resulting in common mode radiation. The circuit board design chapter provides various design methods to reduce the ground impedance, which can be used to reduce the noise on the ground and thus reduce the common mode voltage. One recommended method is to set "clean" on the cable port ". The so-called clean ground is that there is no noise circuit on the ground line, so the local potential on the ground line is almost equal. If the chassis is a metal chassis, connect it to the metal chassis cleanly. Common Mode current caused by electromagnetic space sensing in the chassis.

The chassis is always filled with electromagnetic waves. These electromagnetic waves will sense the common mode voltage on the cable. In addition, there will be some circuits that generate high-frequency electromagnetic fields near the cable port, these circuits have capacitive coupling and electrical inductive coupling between the cables to form a common-mode voltage on the cables. Common Mode Voltage produced by electromagnetic induction. It should be noted that the electromagnetic waves in the chassis are mostly caused by the differential mode radiation of the circuit. In the circuit board design chapter, we discuss the spectrum of the differential mode radiation of the pulse signal, the frequency range is wide. This leads to frequent common-mode voltages much higher than we expected.

Ii) Cable Length: use short cables whenever possible to meet the requirements. However, the cable length is often limited by the connection distance between devices and cannot be shortened at will. In addition, when the length of the cable cannot be reduced to less than half the wavelength, the cable length is not significantly reduced; Increase the impedance of the common mode current loop: The purpose is to reduce the common mode current, because when the common mode voltage is fixed, increasing the impedance of the common mode current path can reduce the common mode current. Reducing the common mode voltage aims to reduce the common mode current. When the common mode circuit impedance is certain, reduce the common mode voltage to reduce the common mode current; low-pass filter: The purpose is to reduce the high-frequency common mode current composition, these high-frequency common mode current radiation efficiency; cable shielding: the purpose is to provide a small loop area path for common mode current. The following describes how to apply the above concepts in actual projects.

1. Increase the impedance of the common mode current loop.

After the device is assembled, the common mode voltage generated on the device cable is fixed. At this time, the method to reduce the common mode current on the cable is to increase the impedance of the common mode current loop. However, many engineers are confused about how to increase the impedance of common mode circuits. They often try to increase the impedance of the common mode loop by disconnecting the circuit board from the chassis, or connecting the chassis to the safe location. The results are often disappointing. These methods are only effective for low frequencies, and the low-frequency common-mode current is not the main cause of radiation.

The practical and effective method is to connect the common mode throttling on the cable. The common mode throttling can form a large Impedance for the common mode current without affecting the differential mode signal. Therefore, it is very easy to use, and the common mode throttling can be directly added to the cable without grounding. The whole bundle cable passes through a ferrite magnetic ring to form a common mode throttling. You can also round the cable several turns around the magnetic ring as needed. For the convenience of engineering, many manufacturers provide split magnetic rings, which can be easily stuck on cables. After the ferrite magnetic ring is mounted on the cable, the increase of the radiation intensity depends on the impedance of the original common mode current loop. It is easy to derive the following conclusions from the formula of common mode radiation, when the common mode voltage remains unchanged ):

Common mode radiation improvement = 20lgE1/E2) = 20lgICM1/ICM2) = 20lgZCM2/ZCM1) = 20lg 1 + Z/ZCM1) Formula: E1 = radiation intensity before ferrite is added, E2 = radiation intensity after ferrite is added, ICM1 = common mode current before ferrite is added, ICM2 = common mode current after ferrite is added, ZCM2 = Common Mode loop impedance after ferrite is added, ZCM1 = Common Mode loop impedance before ferrite is added, Z = the impedance of the common mode throttling.

For example, if the common mode current loop impedance is 100 W without a common mode throttling, and the common mode throttling impedance is 1000 W, the common mode radiation is improved to 20 dB, if the current loop impedance of the common mode is 1000 W, the improvement is only 6 dB. To achieve the expected interference suppression effect, pay attention to the following issues when using ferrite magnetic rings:

A. Selection of Ferrite Materials: Select Ferrite Materials with different material composition and magnetic permeability Based on the frequency of interference suppression. High-frequency properties of Nickel-zinc ferrite due to manganese-Zinc Ferrite Materials, the higher the permeability of Ferrite Materials, the higher the Low-Frequency Impedance, and the lower the high-frequency impedance. This is because the conductivity of Ferrite Materials with high magnetic conductivity is high. When the conductor is worn out, the parasitic capacitance between the cable and the magnetic ring is large.

B. Dimensions of ferrite magnetic ring: the larger the difference between the inner and outer diameter of the magnetic ring, the longer the axial direction and the larger the impedance. But the internal diameter must be packed with tight wires. Therefore, to obtain a large attenuation, try to use a large magnetic ring when the inner diameter of the magnetic ring is packed into the cable.

C. The number of turns in the common mode throttling ring: increasing the number of turns passing through the magnetic ring can increase the low-frequency impedance, but the High-Frequency Impedance will decrease as the parasitic capacitance increases between the turns. It is a common mistake to blindly increase the number of turns to increase the attenuation. When the interference frequency band to be restrained is wider, different turns can be wound on two magnetic rings.

For example, a device has two frequency points of over-standard radiation. One is 40 MHz, and the other is 900 MHz. After inspection, it is determined that the common mode radiation of the cable is caused. When a 1/2-turn magnetic ring is mounted on the cable, the interference of MHz is obviously reduced, and the frequency of 40 MHz is still exceeded. 3 turns the cable around the magnetic ring, with 40 MHz of interference reduced and no longer exceeding the standard, but MHz of interference exceeded. To solve this problem, two ferrite magnetic rings, one 1/2 turns and the other three turns, were used.

D. Number of ferrite magnetic rings on the cable: increasing the number of ferrite magnetic rings on the cable increases the Low-Frequency Impedance, but the High-Frequency Impedance decreases. This is because the parasitic capacitance between the cable and the magnetic ring increases.

E. Position of ferrite magnetic ring installation: usually close to the interference source or sensitive source. For cables shielded from the chassis, the magnetic ring should be as close as possible to the cable import and export of the chassis. Because the effect of ferrite magnetic ring depends on the impedance of the original Common Mode loop, the lower the impedance of the original loop, the more obvious the effect of the magnetic ring. Therefore, when the common mode filter capacitor is installed on both ends of the original cable, the effect of the magnetic ring is more obvious due to its low common mode impedance.

3) Influence of electromagnetic field on cables

When the cable is in an electromagnetic field, noise voltage is induced on the cable. In contrast to the cable radiation, the voltage induced by the electromagnetic field on the cable is also divided into common mode and differential mode. The common mode voltage is generated by the electromagnetic field in the circuit between the cable and the earth. The differential mode voltage is generated by the electromagnetic field in the loop formed by the signal line and the signal ground. When the circuit is a non-balanced circuit, the common mode current is converted to a differential mode voltage to interfere with the circuit. Because the circuit area formed by the signal line and the signal ground is very small, the noise voltage is still dominated by common mode.

1. Voltage induced by the electromagnetic field on the cable

When the cable is very close to the ground: the electric field component is perpendicular to the ground, and the magnetic field component is perpendicular to the wire-ground loop, the strongest induction.

When the cable is far away from the ground: the electric field is parallel to the ground, and the magnetic field is perpendicular to the wire-ground loop, the strongest induction.

The voltage induced by the electromagnetic field in the wire is in the common mode. the voltage on the load is based on the public conductor or Earth in the system as the reference point. Generally, the reference ground surface in the system is used as the reference point. For multi-core cables, this means that all conductors in the cables are exposed in the same field, and the voltage they sense above depends on the impedance between each conductor and the reference point.

2. Suppression of low frequency magnetic fields by cables

Low frequency magnetic field interference is very common in practice, such as near the power cord, near the motor or transformer. When the cable passes through this magnetic field, interference occurs in the circuit connected by the cable. This interference is caused by the variation of the magnetic flux surrounding the circuit area of the conductor. According to the Law of electromagnetic induction, the voltage amplitude induced on the conductor is proportional to the magnetic flux change rate it is surrounded. If the magnetic flux contained in the loop area is j, then:

VN = d j/dt)
If the magnetic field enclosed in loop area A is uniform, that is, the magnetic density B of each point in the loop is equal, then j = a B, then:
VN = AdB/dt)
If the magnetic field changes according to the sine and is expressed:
B = B0e-jwt
Then: VN = j wA B

From the formula, we can see that the induction voltage is proportional to the frequency, magnetic density, and loop area of the magnetic field. Because the frequency of the external interference field is uncontrolled, in order to reduce the induction voltage, we should minimize the magnetic flux density and loop area in the loop. Reducing the magnetic density can only be achieved by increasing the distance between the cable and the magnetic radiation source. Reducing the loop area can be achieved by using appropriate cables and grounding methods. An effective way to overcome the interference of the magnetic field is to reduce the area of the loop, that is, to keep the signal line close to its return line as much as possible.

The twisted pair and the same axis have good effects in reducing the magnetic field interference. Twisted Pair wires: twisted pair wires can effectively suppress magnetic field interference, not only because the two wires of twisted pair wires have a small loop area, in addition, because each of the two adjacent return wires of the twisted pair has the opposite direction of the current, They offset each other. The closer the twisted pair is, the more obvious the effect is.

However, if the two ends of the circuit are grounded, the above features are no longer available. At this time, each wire and the ground plane constitute a large loop, in which the induced current will be generated. Due to the imbalance between the two wires, the differential mode voltage is generated. Coaxial cable: When the coaxial cable is properly connected, the suppression effect on magnetic field interference is ideal. Because the signal current and Backflow on the coaxial cable can be equivalent to a geometric coincidence, and the area is 0.

In order to maintain the characteristics of the coaxial cable, the non-coaxial part at both ends of the cable should be kept as small as possible. That is, the connection of the screen layer should be as short as possible. The actual coaxial cable, because the core line and the outer layer are not necessarily completely concentric, will have a certain equivalent area, affecting its suppression effect. Similar to the twisted pair, the two ends of the same axis cannot be grounded. Otherwise, the current will be generated in the loop of the core line and the earth and in the loop of the outer and Earth. Due to the imbalance of the circuit, the difference mode noise is generated.

Due to the symmetry principle of the antenna, if the above-mentioned cables have low reception efficiency, their radiation efficiency is also low. Therefore, the radiation of twisted pair cables and coaxial cables is also small. This feature can reduce the magnetic field radiation of the cable. The effect of the shielded cable is closely related to the grounding of the Screen Layer and circuit. Especially when the external interference is a magnetic field, the effects of different connection methods vary greatly. This set of data is obtained from tests on different grounding structures in the magnetic field:

Structure:

A non-magnetic shielding sleeve is mounted on the signal line and grounded at a single point. For the magnetic field, the magnetic field in the signal circuit does not change when the shielding layer of the non-magnetic material is grounded at a single point, so the magnetic field induction is the same, that is, this structure has no shielding effect. In this case, the shielding effect is defined as 0 dB, which serves as a reference point.

Structure B:

Ground the two ends of the shield layer in. In this case, we can provide a certain shielding performance. Because the current is also induced in the loop consisting of the shield layer and the ground plane, this current produces a magnetic field opposite to the original magnetic field, which weakens the magnetic field in the signal loop and reduces the induced noise.

Structure C:

The twisted pair wires should provide better shielding effect because the current direction induced in the adjacent twisted joint is opposite and offset each other). However, due to the grounding at both ends of the circuit, the actual induction circuit is not small, so the effect is poor.

Structure D:

A single-ended grounding shielding layer is added to the twisted pair. Because the single-ended grounding shielding layer has no shielding effect on the magnetic field, the shielding efficiency of the twisted pair is not improved.

Structure E:

After the two ends of the shielding layer are grounded, the antimagnetic field generated by the current in the shielding layer weakens the original magnetic field, and the shielding efficiency is improved. Note: structure C is a common error and should be avoided in practice.

Structure F:

The circuit is only grounded at a single point, and the shielding layer of the cable is used as the reflux path, which greatly reduces the area of the induction circuit, thus greatly improving the shielding efficiency. The ideal coaxial cable loop area is 0 and will not sense any noise voltage. The shielding effect of the actual coaxial cable depends on the deviation between the core line and the Outer Axis.

Structure G:

Due to the fact that the twisted pair has a very small induction circuit and the induced current in the adjacent twisted joint is eliminated, it shows a high magnetic field shielding effect. The actual inhibition effect is higher than that of 55, because some electric fields are sensed here. This can be seen from the structure H. In the structure H, the single-ended grounding shielding layer suppresses the Electric Field Induction and increases the shielding effect to 70.

Structure H:

A single-ended grounding shielding layer is added on the basis of G, which eliminates the influence of the electric field generated by the experimental device. The shielding effect here is not F high because the loop area of the twisted pair does not have a small coaxial cable. Increasing the twist link density can further improve the inhibition effect.

Structure I:

After the two ends of the shield layer in H are grounded, the shielding efficiency decreases. This is because after the two ends of the shielding layer are grounded, the induced current is generated on the shielding layer. This current is induced on the twisted pair wires. Because the circuit is not balanced, the differential mode voltage is generated.

Structure J:

The ungrounded end of the shield layer in H is connected to the public end of the circuit, which further improves the shielding efficiency, but does not reach the F level, because the cable in F is a coaxial cable and has a small induction circuit. Problem: the shielding efficiency of the structure H is somewhat higher than that of the structure G, because the shielding layer of the single-ended grounding eliminates the additional electric field generated by the experimental device, why is the shielding efficiency of structure D not higher than that of structure C?

Balanced circuit:

The two conductors in the balanced circuit have the same impedance to the ground or other conductors as all the circuits connected to them.

The equilibrium circuit's response to the electromagnetic field: the two conductors in the equilibrium circuit have the same geometric size and are very close, so they can be considered to be in the same field strength. Because their impedance is equal to the impedance of any reference object, the current they sense is the same, and the voltage at both ends of the conductor is the same as that at the reference point. Therefore, the voltage between two conductors is 0 V.

If the two conductors are connected at the input end of the circuit, the input signal voltage is provided for the circuit. As there is no noise voltage between them, the external electromagnetic field has no effect on the input of the circuit. The ideal balanced circuit can withstand electromagnetic interference of any intensity.

Balanced circuit performance evaluation: the degree of balance of the balanced circuit is described by the common mode rejection ratio. The common-mode rejection ratio is defined as the ratio of the common-mode voltage to the differential-mode voltage generated by the common-mode voltage.

CMRR = 20 lgVC/VD) dB

For example, if the common-mode rejection ratio of the circuit is 60dB, the common-mode voltage of V can only generate 1 V differential-mode voltage at the input end of the circuit. This circuit has good performance in resisting common-mode interference such as lightning.

The common-mode rejection ratio of a well-designed circuit can reach 60-80 dB. However, at high frequencies, due to the influence of parasitic parameters, the circuit balance is difficult to achieve. Therefore, the balanced circuit does not effectively suppress high-frequency common-mode interference.

Note 1:

When using a balanced circuit, you must not only select a balanced circuit, but also ensure the symmetry of the two wires during wiring so as to ensure the high-frequency balance.

NOTE 2:

Twisted Pair wires are a balanced structure. Therefore, twisted pair wires are often used in balancing systems. The coaxial cable is not a balanced structure. You must pay attention to the connection method when using the balanced system. A coaxial cable can only be used as a conductor, and its outer layer is used as a shielding layer.

The balanced circuit can suppress electromagnetic interference between space and ground. Therefore, it is widely used in communication cables. When the common-mode rejection ratio of the balanced circuit cannot meet the requirements, you can use methods such as shielding and common-mode throttling to improve the performance. However, the shielding method is only applicable to scenarios where space electromagnetic fields cause common mode interference. The common mode throttling method is suitable for any common mode interference, such as common mode interference caused by ground voltage difference.

Shielding: shielding the input cable of the circuit. The shielding layer is connected according to the specifications, which can shield the electromagnetic field, the suppression effect and the circuit balance on the common-mode interference of the spatial electromagnetic field are added together. For example, if the common-mode blocking effect is 30dB and the common-mode blocking ratio of the balanced circuit is 60dB, the total common-mode blocking effect is 90dB. The shielding effect of the cable shielding layer is largely determined by the shielding layer's end connection mode. If the end connection is poor, it is not a 360-degree lap connection mode), and the high-frequency shielding performance will decrease.

Common Mode throttling: the special Bypass Method of common mode throttling determines that it only limits the common mode current, but does not affect the differential mode current required for circuit operation. Therefore, common mode Throttling is an ideal device for common mode interference. The low-frequency common-mode suppression function of the ideal common-mode throttling ring is small, and the suppression effect increases as the frequency increases. This is a high ratio of low-frequency common-mode suppression of the balanced circuit. As the frequency increases, the balance deteriorates and the common-mode suppression ratio decreases, which is complementary. Therefore, after the common mode Throttling is used in the balancing circuit, the circuit can maintain a high common mode rejection ratio within a wide frequency range.

NOTE 1: The actual common mode throttling frequency characteristics are related to the core material, coil winding method and other factors. In actual use, adjust the parameters according to the specific situation.

NOTE 2: the characteristics of common mode throttling are complementary to those of many common mode suppression devices. For example, the isolation transformer, due to the influence of parasitic capacitance between the first and second levels, has a poor effect on high-frequency common mode interference suppression, this defect is improved when used together with the common mode throttling. Another advantage of common mode Throttling is that grounding is not required. This provides great convenience for the design.