八大胜

Many people should have encountered the problem of interference in the process of using the burst generator. When we encounter the problem of interference, how can we solve this problem? Next, let’s talk about the general processing method of pulse group generator interference

The first is absorption and filtering

The original intention of the burst generator test is to carry out the common mode interference test, but the waveform front of the interference pulse is very steep and the duration is very short, so it contains extremely rich high-frequency components, which leads to a part of interference escaping from the transmission cable to form radiation in the transmission process of the interference waveform, so the final interference received by the equipment is the combined interference of conduction and radiation.

For the interference of pulse group generator, one of the most common suppression methods is to use filtering (filter with power line and signal line) and absorption (absorb with ferrite core). The ferrite core absorption scheme is very cheap and effective.

The second is transient interference suppression device

Because the signal of the burst generator is high-frequency and high-voltage pulse, the transient voltage absorption device can also be used to absorb the burst signal for the suppression of the burst signal.

There are many kinds of transient voltage absorbing devices, but they mainly include gas discharge tubes, metal oxide varistors, silicon transient voltage absorbing diodes and solid discharge tubes. Considering the high-frequency and high-speed characteristics of pulse group signals, there are only two suitable transient voltage absorbing devices: metal oxide varistors and silicon transient voltage receiving diodes.

In principle, these two devices can be used for interference suppression of power lines, but in contrast, varistors may be more commonly used because of their low price and fast response speed. In addition, the problem of large inherent parasitic capacitance of varistors does not have any bad effect on 50Hz power lines.

For signal lines, these two devices can also be used, but silicon transient voltage absorption diodes may be more applicable, because in the primary protection of printed circuit boards, silicon transient voltage absorption diodes have many voltage grades, and there is a lot of room for choice; At the same time, the limiting voltage is relatively accurate (the residual voltage when the diode acts is low, which is close to the breakdown voltage), especially suitable for the protection of electronic components such as semiconductor integrated circuits. In addition, the parasitic capacitance of silicon transient voltage absorption diode is smaller than that of varistor, and it will cause less damage to the transmission signal when used on the line. For high-speed transmission signals, there are specially processed silicon transient voltage absorption diodes with small parasitic capacitance that can be selected (high-speed diodes are used in series with silicon transient voltage absorption diodes. Because the high-speed diodes have pf level junction capacitance, the equivalent capacitance of silicon transient voltage absorption diodes after series processing is also pf level), which is particularly suitable for the protection of these lines.

Then shielding

Considering that the front edge of the pulse group generator signal is very steep and the pulse width is also very narrow, it contains extremely rich harmonic components, and the frequency with large amplitude should reach at least 60MHz. For the power line and signal line for the test, even if the length is only 1m, because the length can be compared with the wavelength of the transmission frequency, it cannot be treated as an ordinary power line and signal line. When the signal is transmitted on it, part of it enters the test object through the line (conduction); Part of it should escape from the line and become a radiation signal into the test object (radiation). Therefore, the interference received by the test object is actually the combination of conduction and radiation, that is, when doing the pulse group test, there is actually a radiation field around the test wire.

For radiated interference, the more common suppression method is shielding, including shielding of equipment shell, local shielding inside equipment, and shielding of transmission line (shielded wire, coaxial cable, twisted pair, etc.).

Finally, structure and grounding

Structure and grounding are also very important aspects of suppressing pulse group interference.

As an electronic equipment with non-metallic shell, firstly, the power line is far away from the grounding point, and the grounding path is long, which greatly reduces the grounding effect.

Secondly, when the signal line 1 is far away from the grounding point, causing the interference pulse to be applied to the power line and signal line 1, the common mode interference current of the pulse group should pass through the interconnection line and the circuit board below, and finally enter the ground through the grounding terminal. Because the impedance of the interconnection line is large and the grounding path is long (the grounding impedance is also large), the risk of electromagnetic compatibility is greatly increased.