The working principle and use method of electromagnetic jammer
There are two kinds of electromagnetic interference (EMI): conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiated interference means that the portable jammer couples (interferes) its signal to another electrical network through space. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may all become radiated interference sources with antenna characteristics, which can emit electromagnetic waves and affect other systems or other subsystems in this system. normal work.
The existence of electromagnetic interference
"Common mode" interference refers to the interference that exists between the line (including the power line and signal line) to the ground. Among them, the power line refers to the interference between the live line and the neutral line to the ground. For three-phase circuits, common mode interference exists between any one phase and the ground. Common mode interference is sometimes referred to as longitudinal mode interference, interference interference and ground interference. This is the interference between the current-carrying conductor and the ground.
"Differential mode" interference is interference between lines (including between power lines, signal lines and its ground return line). For power lines, differential mode interference refers specifically to the interference between the phase and neutral lines; for three-phase circuits, differential mode interference also refers to the interference that exists between the phase and phase lines. Differential mode interference is sometimes called normal mode interference, transverse mode interference, or symmetric interference. This is the potential difference between current-carrying conductors.
The mode in which the interference exists suggests the relationship between the interference source and the coupling path. For example, common mode interference indicates that the interference is coupled into the circuit by radiation or crosstalk. Such as lightning, electric arcs near the equipment, nearby radio stations, and other high-power radiating devices. The interference on the power line also includes the interference of the internal lines of the chassis or other cables on the power line. Due to the induction from space (electromagnetic radiation, inductive coupling and capacitive coupling), the effect on each wire is the same. The differential mode interference indicates that the jammer originated in the same power line (direct injection). Such as motors, switching power supplies, thyristors, etc. working in the same line, the interference they generate on the power line is differential mode interference.
Usually, these two components of the interference voltage on the line exist at the same time, and due to the imbalance of the line impedance, the two components will change each other during transmission. After the interference is transmitted on the line for a long distance, the attenuation of the differential mode component is greater than that of the common mode component. This is because the impedance between the lines and the line-ground impedance are different. On the other hand, the frequency of common mode interference is generally distributed above 1 ~ 2MHz, so while common mode interference is transmitted on the line, it will also radiate to the surrounding adjacent space (this is because the line-to-earth impedance is large, plus common mode interference The frequency is relatively high, so it is easy to escape the transmission line and form a space induction). The radiation of the power cord, especially after entering the equipment, can be further coupled to the signal circuit to form interference, so it is difficult to prevent. The frequency of differential mode interference is relatively low, and it is not easy to form radiation. In addition, in the general circuit, there have been many measures when dealing with differential mode interference (for example, a large capacitor has been used in the voltage regulator circuit; on the printed circuit board, between the power line and the ground line Decoupling capacitors are also commonly used), so the chance of equipment malfunction due to differential mode interference is relatively small. Therefore, most of the equipment sensitivity problems are caused by common mode interference. At present, common mode interference is the focus of our consideration, which can be confirmed from the content of commonly used immunity tests. Among them, static electricity test, high-frequency radiation electromagnetic field test, electric fast transient pulse group test, line-to-ground lightning surge test and conduction test caused by RF field induction, etc. All they feel is common mode interference.
Types of electromagnetic interference
The second reason for the complexity of electromagnetic interference (especially the complexity of power line interference) is that there are many forms of wifi jammer, from peak interference with a short duration until the grid is completely de-energized. It also includes voltage changes (such as voltage sags, surges, and interruptions), frequency changes, waveform distortion (including voltage and current), continuous noise or clutter, and transients.
Not all electromagnetic interference will cause trouble for electronic equipment. In fact, there are only two very important reasons: short-term spike interference and long-term voltage drop. Peak interference can be coupled to the system through crosstalk or directly into the power supply, causing false triggering of internal logic circuits. The voltage drop can cause the loss of storage circuits or other volatile data. However, other disturbances, such as slight overvoltage, harmonic distortion, or frequency offset, usually do not cause the computerized system to malfunction.