Why reserve a capacitor station on the clock output?

Hello everyone , Here is the big talk hardware .
Today, I want to share an article about the big talk Hardware Group , The problem of reserving capacitor station for clock .

01 The problem background

Some time ago, someone asked a question in the dahua Hardware Group ,SPI A resistor is connected in series on the clock signal line of , And a capacitor is reserved C Work station , How to explain ？

Several people in the group answered this question , Agreed yes EMC The needs of . stay RE When testing , The reserved capacitor can play a very big role in welding .

How to explain the detailed principles and reasons behind it ？

02 Single board clock radiation

During electromagnetic compatibility radiation test , There are many cases where the clock radiation on the board exceeds the standard . For example, the following picture , High frequency radiation obviously exceeds the standard . Without subsequent analysis , It can be judged preliminarily that ,80% The probability is caused by clock radiation .

According to Fourier transform , The clock signal we see , It is actually composed of many fundamental waves . Only the fixed periodic signal is tested in the time domain , In the frequency domain, the test is discrete spectrum .
For example, in the following figure （ This is from the Internet ）, On the left is the time domain square wave , On the right, we see the frequency domain square wave .

In the actual circuit , The clock is a square wave , Data is also square wave , But we generally recognize