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The radiation problem of cable

Information Source:Henan Huadong Cable Co., Ltd  Release date:2014-03-26

Radiation cable is one of the most common problems in engineering, more than 90% of the equipment (mainly including pulse circuit equipment) can't launch test by radiation are due to cable by radiation. Cable producing the radiation mechanism has two kinds, one kind is the signal of the current in the cable (differential mode current loop) differential mode radiation generated, another is the cable in conduit (including the shield) on the common mode current. Cable radiation mainly comes from the common mode radiation. Common mode radiation is produced by the common mode current, common mode current loop area is composed of cables and earth (or close to other large conductor) form, so it has a larger area of the loop, will have the strong radiation.

The common mode current is how to produce is often many people confused the issue. To understand this issue, first clear the common mode voltage is the fundamental cause of the common mode current, common mode voltage is cable and earth (or near other large voltage between conductor). Starting from the common mode voltage, looking for the cause of common mode current is easy, and the cause of a problem once clear, to solve this problem is not difficult. Causes of common mode current on the cable has the following several points: differential mode current leakage caused by common mode current. Even if the cable contains a signal loop, also cannot guarantee the signal current of 100% return signal source from the loop, especially in the high frequency of the occasion, space of various stray parameters for signal current provided third, or even more return path. The common mode current, although the proportion of small, but due to radiation loop area, radiation can not be ignored.

Don't try to the circuit and the earth "disconnect" (the wire between the circuit board and the chassis is disconnected, or the earth between the chassis and the earth off) to reduce the common mode current, thereby reducing the common mode radiation. The circuit and the earth opened only to reduce the common mode current in low frequency, channel parasitic capacitance formed at high frequency is very small impedance. The common mode current is mainly produced by stray capacitance. Of course, if the common mode radiation problems occurred mainly in the low frequency, the circuit board or the chassis and the earth off the meeting has certain effect. The mechanism resulting from the common mode current, the effective methods to reduce the common mode current is reduced impedance differential mode loop, so that most of the signal current return signals from the ground.

The general signal line and the loop impedance is closer, differential mode current loop is small. A typical example is the coaxial cable, coaxial cable because the return current distribution in the skin, the equivalent current and axis coincide, so the loop area is zero, differential mode impedance is close to zero, the signal current is almost 100% of the return signal source from the casing of the coaxial cable, the common mode current is almost zero, so the common mode radiation very small. On the other hand, because of the differential mode current loop area is almost zero, the differential mode radiation is very small, so the radiation of the coaxial cable is very small. For high frequency signals, using coaxial cable transmission can avoid radiation. In fact, this and the high frequency signal transmission with our traditional coaxial cable, has the same essence to reduce signal loss to. Because of the loss of signal is small, the leakage of natural ingredients less, and this part is the radiation leakage cable.

The common mode current ground noise circuit board cause. Return wire, signal wire is the signal therefore, certain voltage between two ground, for high frequency circuit, these are high frequency noise voltage, it as a common mode voltage driving the common mode current cables, leading to common mode radiation. Design method provides a variety of reducing ground impedance in a chapter circuit board design, it can be used to reduce noise on ground, thus reducing the common mode voltage. Methods a recommended is clean settings "in the cable port". The so-called clean is the ground no circuit can generate noise, so the local potential ground almost equal. If the case is a metal box, the clean and the metal chassis connected. The common mode current electromagnetic induction box in space.

The chassis is always full of electromagnetic wave, the electromagnetic wave will be the common mode voltage in the cable on the other, near the cable port, there will be some high frequency electromagnetic field circuit, there is a capacitive coupling and inductance coupling between the circuit and cable, the common mode voltage in the cable. Common mode voltage generated by electromagnetic induction. Need to pay attention to is, electromagnetic wave within the enclosure consists mostly of differential circuit to design radiation, a chapter on the circuit board, we discuss the differential mode radiation spectrum of pulse signal, the frequency range is very wide. This leads to the common mode voltage frequency is often far more than we expected value.

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