Method for increasing the anti-interference ability of film capacitors

In order to better solve the problem that the film capacitor frequency is high and the bus cycle is extremely fast, the following measures are taken to increase the system's anti-electromagnetic interference capability:

1. Reduce distortion in signal transmission:

Microcontrollers are primarily manufactured using high speed CMOS technology. The static input current of the signal input terminal is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output of the high-speed CMOS circuit has considerable load capacity, that is, a considerable output value, and the output of one gate passes a very long period. The long line leads to the input with a relatively high input impedance, and the reflection problem is very serious, which causes signal distortion and increases system noise. When Tpd>Tr, it becomes a transmission line problem, and signal reflection, impedance matching, etc. must be considered.

The delay time of the signal on the printed board is related to the characteristic impedance of the lead, which is related to the dielectric constant of the printed wiring board material. It can be roughly assumed that the transmission speed of the signal on the printed circuit board leads is between about 1/3 and 1/2 of the speed of light. The Tr (standard delay time) of a conventional logical telephone component in a system composed of a microcontroller is between 3 and 18 ns.

When the rise time of the signal is faster than the signal delay time, it must be processed according to fast electronics. The film capacitor manufacturer believes that the impedance matching of the transmission line should be considered at this time. For the signal transmission between the integrated blocks on a printed circuit board, it is necessary to avoid the occurrence of Td>Trd. The larger the printed circuit board, the less the speed of the system. fast.

2. Select a microcontroller with low frequency:

The use of a microcontroller with a low external clock frequency can effectively reduce noise and improve the system's anti-interference ability. For square waves and sine waves of the same frequency, the high frequency components in the square wave are much more than the sine waves. Although the amplitude of the wave of the high-frequency component of the square wave is smaller than the fundamental wave, the higher the frequency, the easier it is to emit the noise source. The most influential high-frequency noise generated by the microcontroller is about three times the clock frequency.