Electromagnetic Interference Analysis of Cables

In electronic equipment and systems, various cables are essential links for signal transmission, and cables are also the main factor causing various electromagnetic compatibility (EMC) problems. The main reason why cables cause electromagnetic interference (EMI) is that there is interference current on the cables. Two types of interference currents are introduced. Combined with the problems that often occur in electromagnetic compatibility testing, the causes of each current are analyzed, and specific methods for reducing these interference currents are proposed. The operability of the method is verified through experiments, providing a reference for product design.

1 Interference current

The power line, signal line and other communication lines of the equipment, and the communication lines that exchange with other equipment or peripherals have at least two wires. These two wires are used as round-trip lines to transmit current or signals. However, there is usually a third wire in addition to these two wires, namely the ground wire. There are two types of interference currents in cables: one is that the two wires are transmitted as round-trip lines respectively, which is called "differential mode"; the other is that the two wires are used as the outgoing path and the ground wire is used as the return path, which is called common mode.

The differential-mode interference current and common-mode interference current on the cable can be directly conducted into the circuit module or other equipment of the electronic equipment through the cable, and can also generate electromagnetic fields in space to form radiation interference.

Usually, the differential-mode component and common-mode component on the line exist at the same time, and due to the unbalanced impedance of the line, the two components will transform into each other during transmission. After the interference is transmitted over a long distance on the line, the attenuation of the differential-mode component is greater than that of the common-mode component, because the impedance between the lines is different from the line-to-ground impedance.

The frequency of common-mode interference is generally distributed above 1 MHz. During transmission, it will radiate to the adjacent space and couple into the signal circuit to form interference, which is difficult to prevent. The frequency of differential-mode interference is relatively low, and it is not easy to form spatial radiation. Treatment measures can be taken to reduce its interference.

2 Methods to reduce interference

2.1 Reduce differential-mode
interference The main reason for the generation of differential-mode interference current on the cable is that the noise current generated during the operation of the circuit or device is directly conducted to the cable, and then conducted to other circuits or equipment to affect them.

These are unwanted clutter currents. There are two ways to eliminate these currents: change the circuit structure or select high electromagnetic compatibility devices to suppress the generation of noise from the root; use filtering methods to prevent interference currents from entering other devices or circuits to cause interference.

The design of circuit structure requires the accumulation of basic experience and knowledge. The price of electronic devices with good electromagnetic compatibility is often much higher than that of similar products, and the actual effect is related to the specific usage. Therefore, filtering methods are usually used to reduce differential mode interference during design. Good filtering can directly suppress the flow of interference energy on the wire, so it can also play a significant role in suppressing the radiation interference through the current-carrying wire.

There are many kinds of filter components. The filters used in the power supply part include power supply filters, magnetic rings and magnetic beads; the filters used on the signal line include signal filters, magnetic rings and magnetic beads, through-hole capacitors, filter connectors, etc.; the filters used on the printed circuit board include decoupling capacitors, sheet (surface mounted) filters and magnetic beads, etc.

2.2 Reduce common mode interference
The common mode interference current on the cable can be coupled to other cables through distributed capacitance and mutual inductance, and then affect the devices and equipment connected to it through the cable; it can also directly interfere with other equipment through space electric field radiation.

These methods are often used to reduce the common mode interference of cables to the outside world: reduce the capacitive coupling and inductive coupling between wires; shorten the cable length, use shielded cables and ground when practical conditions permit.

2. 2. 1 Capacitive coupling and mutual inductive coupling
When there is voltage or current flowing on a wire, electromagnetic energy will radiate and reach nearby wires. This is crosstalk between wires. Crosstalk belongs to near-field inductive coupling. The reason is that there are stray capacitance and mutual inductance between wires, that is, capacitive coupling and mutual inductive coupling, as shown in Figure 1.

Figure 1 Capacitive coupling and mutual inductance coupling

Figure 1 (a) is a schematic diagram of capacitive coupling. If there is a distributed capacitance C12 between two conductors, there will be a voltage between the two conductors, that is, the voltage is connected in series with the disturbed conductor. The conductor has a distributed capacitance C1G and C2G per unit length to the ground, and the distributed capacitance per unit length between the conductors is C12. The distributed capacitance between the conductors is also called coupling capacitance. Without considering C1G, the interference voltage U2 generated by the interference power supply U1 in the conductor 2 circuit is:

In the formula:

Z2 is the capacitive reactance of C2G in parallel with R, that is:

From formula (1), it can be seen that reducing the interference voltage U2 can be achieved by reducing Z2 and increasing the value of XC. The specific methods are as follows:
① Reduce the resistance R of the disturbed wire to the ground;
② Reduce the distributed capacitance C12. The capacitance is maximum when the two wires are parallel, and the capacitance is minimum when the direction is changed to vertical. Increasing the distance between the wires and reducing the length of the wires within a certain range can reduce the capacitance between the wires;
③ Make the disturbed wire close to the ground wire, or set the ground wire near the receiving wire, so as to increase the capacitance of C2G.

In addition, shielding the disturbed wire can also reduce the capacitive coupling between the wires.

The principle of inductive coupling is shown in Figure 1 (b). When an alternating current flows through the interference loop 1, alternating magnetic energy will be generated. The alternating magnetic flux passes through loop 2 and generates an induced electromotive force in loop 2, which can be expressed as:

Where, M is the mutual inductance between the two circuits, M=Φ/I1(H), Φ is the magnetic flux generated in loop 2 when current I1 flows through circuit 2, I1 is the current of the interference loop; ω is the angular frequency of the alternating current.

It can be seen from formula (2) that the induced electromotive force is mainly related to the mutual inductive coupling. Reducing the value of Φ can weaken the mutual inductive coupling. The specific methods are as follows:
① Reduce the loop area of ​​the disturbed circuit. The signal line and the return line of the disturbed circuit are as close as possible, such as using twisted pair or coaxial cable;
② The magnetic field decays very quickly with the increase of distance, and the distance between the disturbing wire and the disturbed wire can be increased. The relative angle between the two loops can also be adjusted.

Avoiding the signal return lines sharing a common path can also reduce inductive crosstalk.

2. 2. 2 Spatial Radiation
The reason why cables radiate electromagnetic waves is that there is a common-mode voltage at the cable port. Driven by the common-mode voltage, the cable generates a common-mode current. The cable with common-mode current is like a monopole antenna, generating electric field radiation.

Common-mode electric field radiation can be simulated by a short monopole antenna with a length less than 1/4 wavelength excited by the ground voltage. For a short monopole antenna with a length of L on the ground plane, the electric field strength at the far field r is:

where L is the antenna length (m).

It can be seen from formula (3) that common-mode electric field radiation is proportional to frequency f, common-mode current I and antenna length L. Limiting f, I and L respectively can control common-mode electric field radiation, and limiting common-mode current I is the basic method to reduce common-mode radiation. The specific effective measures that can be taken are as follows:
①Try to reduce the source voltage that excites this antenna, that is, the ground potential, and the cable is routed close to the ground plane;
②Inside the equipment, the cable length is as short as possible to avoid routing around the circuit;
③Provide high common mode impedance in series with the cable, use a common mode choke, such as ferrite magnetic rings often used on the external connection lines of computers;
④Use capacitors and other devices to bypass the common mode current to the ground;
⑤Use shielded cables, and the cable shielding layer and the shielding shell are terminated 360 degrees.

3 Test result analysis

The differential mode interference test results are shown in Figure 2. Figure 2 (a) shows the excessive power line conducted emission caused by the operating frequency of the switching power supply and its high-frequency harmonic components. The 200 kHz peak is the operating frequency of the switching power supply. Selecting a suitable power input filter can attenuate these interference currents. Figure 2 (b) is the effect curve after using the filter.

Figure 2 Test curve for reducing differential mode interference

The test curves for reducing capacitive coupling and mutual inductance coupling are shown in Figure 3. Because of the existence of capacitive coupling and mutual inductance coupling, test results such as Figure 3 (a) often appear. This is because the input and output lines of the power line filter are bundled together, and the clutter current on the filter output line is directly coupled to the input line through capacitance and mutual inductance, making the filter ineffective. To solve this problem, you only need to shorten the input and output lines of the filter and keep them as far away as possible, keep a certain angle, and fix the lines on the chassis shell, so that the interference current on the input and output lines cannot be directly coupled, and the interference current on the power line is suppressed after passing through the filter.

Figure 3 Test curves for reducing capacitive coupling and mutual inductance coupling

The test curve for reducing spatial radiation is shown in Figure 4. Figure 4 (a) is a test curve for a device with an external shielded cable. The shield layer is twisted into a pigtail (also known as a Pigtail) at the place where the cable is connected to the interface connector, and then fixed to the screw of the connector. Shielded cables can control common-mode radiation because the common-mode current provides a low-impedance path, allowing the common-mode current to flow back to the common-mode voltage source through the shield layer. The impedance of the common-mode current path provided by the cable shield layer consists of two parts: the impedance of the cable itself and the overlap impedance between the cable and the metal chassis. Because the overlap impedance between the cable and the metal chassis (the impedance of the Pigtail) is large, a large common-mode voltage is formed on the shield layer. Driven by this voltage, a common-mode current will be generated on the cable shield layer, so the shield layer becomes an antenna to radiate outward. Change the Pigtail connection to 360 crimped on the metal shell of the connector to ensure low-impedance overlap. The test results are shown in Figure 4 (b).

Figure 4: Test curve for reducing space radiation

4 Conclusion

Cables are the main factor causing the failure of electromagnetic compatibility tests of equipment or systems. EMI problems caused by cables involve all aspects of product equipment from design to assembly, including the design, assembly and assembly of the routing on the circuit board, the cables between the circuit boards and the cables between the equipment. During the test process, electromagnetic interference problems caused by improper cable assembly are often encountered, which can be readjusted. As for the design of the routing in the circuit board and other issues, they must be fully considered in the electromagnetic compatibility design stage, otherwise once the design is completed, the impact caused will be difficult to eliminate.