The same via can have different impedances???

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The same via can have different impedances??? [Copy link]

Author: Huang Gang, a member of Yibo Technology Expressway Media

Two members of Mr. High Speed conducted impedance tests on the processed PCB board. The impedances they tested were very different. Can you believe it?

This is a test board with an SMA coaxial connector, which is used by Mr. High Speed to test various PCB structures. The performance of the 2xthru de-embedding routing we did is particularly important, as it determines the accuracy of our de-embedding. Mr. High Speed has already described the specific principles of testing de-embedding in an article, so I won't repeat it here.

After the PCB board came back, Master Chris asked his apprentices Lei Bao and Tang Niu to test the test board. Not long after, Chris went to the laboratory to see the test results of Lei Bao and Tang Niu. As soon as he entered the laboratory, he heard Lei Bao and Tang Niu having a very "fierce argument".

Lei Bao: The impedance of this 50-ohm SMA via is not bad, reaching 45 ohms, meeting the 10% requirement;

Tang Niu: How can it be so good! I clearly measured less than 42 ohms, which obviously did not meet the requirements.

Then they did show Chris their test results, and one test reached 45 ohms, and the other less than 42 ohms!

What is going on? It is obviously the same via, and the same network analyzer is used for testing. Why are the test results of the two people so different?

Just when some fans of Lei Bao, Tang Niu and Mr. Gaosuo were puzzled, the knowledgeable Chris found a possible point. He immediately sat in front of the network analyzer and operated on the 2xthru trace, and then Chris found the reason for the difference between Lei Bao and Tang Niu.

What did Chris operate on the network analyzer? He quickly switched the test stop frequency, and then tested and compared the same 2xthru trace at different test frequencies, setting the stop frequency to 10GHz, 20GHz, 30GHz, and 40GHz respectively, and then performed impedance tests on the same signal!

As a result, something incredible happened! We looked at the impedance of this via at different test frequencies and found that there was indeed a significant difference!

The impedance of the vias at 10GHz is 47 ohms, 45 ohms at 20GHz, 43 ohms at 30GHz, and only 41 ohms at 40GHz. I

later asked Leibao and Tangniu and found that Leibao tested at 20GHz, while Tangniu tested at a higher 40GHz, which is why they had a "dispute".

However, the change in impedance does not mean that there is a problem with the S parameters they tested. Whether you use 20GHz or 40GHz for frequency domain testing, the measured S parameters are the same, with only a difference between 20GHz and 40GHz!

Chris looked back at Lei Bao and Tang Niu and found that they seemed to understand the phenomenon of different impedance values. So, fans of Mr. Gaosu, do you understand?

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Are the test conditions good?

Is the frequency domain impact so great?

btty038

This is considered a relatively good optimization parameter. The resonance points are all in the same position. Are the parameters getting better and better each time they are optimized?