Member of Mr. High Speed - Huang Gang
Of course, this article is still for AC coupling capacitors for high-speed signals, not decoupling capacitors for power supplies. At the same time, the inspiration for this article comes from the previous article about the difference in the effect of capacitors with different capacitance values on high-speed signals. Why do we want the capacitor package to be smaller when we design at high speeds? The smaller the pad of the capacitor, the better the impedance is optimized. At the same time, we can easily notice that the smaller the package, the smaller the corresponding capacitor body. The PCB package of 0402 and 0201 is indeed quite different!
Why is the inspiration for this article derived from the previous article? Because in the previous article, didn’t Chris analyze the effect of capacitors of different values on the signal from the principle? Then he found a test board he had made before, on which there was a 0402 capacitor package test fixture, which allows us to solder different capacitors to test various passive indicators, such as TDR impedance, return loss, etc. This is the test fixture in the figure below!
Then Chris took out the sealed bag containing different capacitors in the lab, and planned to take out a few of the capacitor strips with different capacitance values in 0402 packages to see if the soldering test can also restore the effect of the previous article. Because it is very small, it is not clear to the naked eye unless you get close, so Chris called Xiaojie, a welding expert in the group, to help with soldering. Similarly, Chris asked Xiaojie to solder a 1nf 0402 package capacitor mentioned in the previous article first. Before Chris came back to his senses, Xiaojie soldered it in two strokes. Then Chris took it to the instrument for testing. Well, it really corresponds to the principle. There is obvious loss drop in the very low frequency band! Then as a comparison, Xiaojie took out a 0.22uf capacitor for comparison. Well, the 0.22uf capacitor will not have the phenomenon of low-frequency drop. Finally, the test can match the previous theoretical verification.
Originally, Chris just wanted to verify the DC drop problem of this small capacitor. After finishing, he was about to pack up his things in the laboratory and leave, when he suddenly saw other capacitors of different capacitance values on the table. At the same time, he saw Xiaojie was eager to solder, so Chris asked Xiaojie to solder a few more capacitors of different capacitance values.
Chris accidentally saw Xiaojie take out a 10uf capacitor, which is considered to be a very large capacitor in 0402. Although it still looks very small, it always feels like it is a little bit bigger than the previous capacitor. The intuitive comparison between it and 1nf and 0.22uf seen by Chris's naked eyes is as follows.
It's so small, I really don't know if I can see it clearly. There is indeed a little difference in size. I took out the laboratory magnifying glass and saw it clearly. I was shocked to find that the same 0402 package capacitors, with different capacitance values, the size of the capacitor itself is actually different!!!
What will be the impact? I believe that everyone should know without Chris saying it this time. We tested the TDR impedance of these three capacitors separately, and the results really verified Chris's conjecture.
It can be seen that after different capacitors are soldered to the fixture, the impedance difference at the capacitor position is still very large! Since the TDR difference is large, the corresponding return loss must be the same.
Although in principle, the function of capacitors in high-speed serial links is to block DC, the previous article saw the impact of different capacitor values, and this article found the difference in SI performance of capacitors with different capacitance values in the same package. It seems that a device with such a simple principle actually has many details (pitfalls) in application. Chris has been studying small capacitors for a day and is tired, so today's article will stop here!
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