How to increase the anti-interference ability of film capacitors

In order to better solve the problem of high frequency of thin film capacitors and particularly fast bus cycles, and to increase the system's anti-electromagnetic interference capability, the following measures can be taken:

1. Reduce distortion in signal transmission:

Microcontrollers are mainly manufactured using high-speed CMOS technology. The static input current of the signal input end is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output end of the high-speed CMOS circuit has a considerable load capacity, that is, a considerable output value. If the output end of a gate is led to the input end with a relatively high input impedance through a very long line, the reflection problem will be very serious, which will cause signal distortion and increase system noise. When Tpd＞Tr, it becomes a transmission line problem, and signal reflection, impedance matching and other issues must be considered. The delay time of the signal on the printed circuit board is related to the characteristic impedance of the lead, that is, it is related to the dielectric constant of the printed circuit board material. It can be roughly considered that the transmission speed of the signal on the printed circuit board lead is about 1/3 to 1/2 of the speed of light. The Tr (standard delay time) of the commonly used logic telephone elements in the system composed of microcontrollers is between 3 and 18ns.

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

2. Select a microcontroller with a low frequency:

Selecting a microcontroller with a low external clock frequency can effectively reduce noise and improve the anti-interference ability of the system. For square waves and sine waves of the same frequency, the high-frequency components in the square wave are much more than those in the sine wave. Although the amplitude 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 and become a noise source. The most influential high-frequency noise generated by the microcontroller is about 3 times the clock frequency.