The direct object of EMC testing is the port to which the cable and its cable are directly connected

EMC testing begins with connector cables. EMC testing is the measurement of electronic products. EMC standards not only specify the test levels of various electronic products, but also specify test methods and means. EMC design and EMC problem analysis must be based on relevant standards EMC testing based. Table 1 and Table 2 are the requirements for EMC testing items for wireless base station equipment in the wireless base station product standards, including Table 1 for disturbance testing items and Table 2 for immunity testing items. The author is free to choose from existing product standards.

Table 1. Harassment Test Items

Table 2. Immunity Test Items

As can be seen from the table above, the direct objects of EMC testing are cables and the ports to which they are directly connected. For disturbance testing, the disturbance of electronic products is transmitted from cables or cables. Even for radiation testing, most problems often occur on the cable or internet of the product. For anti-interference testing, interference is mainly injected from the cable. Even for ESD testing and radiation immunity testing, the main problem is that the cable becomes a receiving antenna, receives radiation interference, and sets the interior through the cable and interface. Imagine that products without cables (including power cords) also have significantly lower EMC requirements.

In practice, we often find that when the external cable on the device is removed, we can pass the test. When encountering electromagnetic interference on site, as long as the cable is unplugged, the fault phenomenon will disappear. This is because cables are not only a receiving and radiating antenna, but also a channel for interference and disturbance in and out. In addition, the parallel transmission distance of the conductors in a cable is the longest, so there are large parasitic capacitance and mutual inductance between the conductors, resulting in signal crosstalk between the conductors.

From the description of EMC testing technology in Chapter 1, it can be seen that in EFT/B testing and conducted immunity testing, interference is always injected into various cable ports of the product in a common mode manner; For testing and conducting immunity testing, ESD testing, and radiation immunity testing, when testing is conducted, the cable always receives electromagnetic interference in the form of a common mode (of course, interference from poor molds cannot be ignored, such as interference caused by loops); The main issues and difficulties in conducting and radiated disturbance testing also lie in common mode issues. Common mode problems are often complex, with unclear interference transmission paths, and relatively simple differential mode interference problems.

With the development of electronic system design technology and multilayer board circuit design technology, the clock frequency is usually tens of MHz or hundreds of MHz, or even higher, and the front and rear edges of the signal pulses used are in the Asian ns range. The rate at which signal interfaces transmit data is typically tens of Mbit/s or hundreds of Mbit/s. For example, high-quality video circuits are also sub sub ns pixel rate transmission signals. The oscillation rate (rise/fall time) dV/dt becomes faster, and the voltage/current. In this case, the negligible parasitic capacitance C will flow through a larger parasitic current r=CdY/dt, which is mostly the common mode current in the parasitic EMC problem, making the common mode problem even more serious.

For example, although in circuit design, in order not to generate or introduce unexpected interference, the signal loop is always designed to be minimal and some necessary differential mode filtering is made, capacitive coupling noise interference always occurs in the input/output (I/O) connector to the housing or ground plane of the connecting cable, and RF voltage can appear on the cable, resulting in dozens of μ The RF current of A is sufficient to exceed the allowable emission level; Or some interference (such as interference) EFT/B, ESD interference) can introduce into the circuit, and a few V transient voltage is sufficient to cause abnormal operation of the circuit.

It can be seen that cables and interfaces are the earliest relationship between EMC interference and test equipment and the most direct factor leading to electromagnetic compatibility; Common mode problems are common mode problems. The most important issues in EMC testing.