If there is no reference ground at all, do you think it is still possible to control the impedance?

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If there is no reference ground at all, do you think it is still possible to control the impedance? [Copy link]

Author: Huang Gang, member of YiBo Technology Expressway Mr.

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As an SI engineer, PCB design engineer, or even a hardware engineer and test engineer, calculating transmission line impedance is definitely one of the essential skills. Mr. Gaosuo has also talked about the theory and practice of impedance calculation in many previous articles. Let’s review it here. The impedance of the transmission line is mainly related to the following factors: impedance is inversely proportional to the line width, inversely proportional to the dielectric constant of the medium, proportional to the line spacing (if it is a differential line), and proportional to the distance from the reference plane. The factor we are going to talk about today is the distance from the reference plane!

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We use impedance calculation software to study the relationship between the transmission line impedance and the distance from the reference plane. Set the line width of a pair of differential lines to 5mil, the line spacing to 9mil, and the dielectric constant of the medium to 4, which is the level of ordinary FR4. At this time, we can easily achieve the impedance target of 100 ohms by controlling the distance from the reference plane on the stripline model. At this time, the upper and lower thicknesses are 5mil and 6.1mil respectively, and the impedance can be calculated to 100 ohms.

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Well, it's very simple. What if it's like the following? There is only a ground plane below as the reference plane, and there is no ground above, only the medium on top. This is called an embedded microstrip line.

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Of course, we can definitely calculate the impedance to be 100 ohms by changing the distance between H1 and H2, after all, there is still a reference plane. At this time, the values of H1 and H2 can be easily calculated to 100.

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Then we slowly enter the core content of this article. If there is no ground plane below, do you think we can still control the differential impedance to 100 ohms?

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There is a lot of room for thought, right? According to the theory of impedance and reference distance, if there is no reference plane, that is, H is infinite, it is definitely impossible to control the impedance, and the impedance will go to the sky, right?

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During the process of everyone's thinking, Mr. Gaosu's team did meet a customer who, contrary to his usual behavior, thought: "Yes! I can control it!"

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Yes, the customer is talking about the type that Mr. Gaosuo just asked you about. The application scenario of this customer is FPC soft board, which is a single-sided soft board with a coverlay covering it. In fact, it is equivalent to a layer of glue on the top and a dielectric underneath. There is no ground plane above or below. Moreover, the customer can calculate a differential impedance of 100 ohms through this model!

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Mr. Gaosuo can indeed calculate 100 ohm impedance through relevant impedance calculation software, for example in this case.

The line width is 12mil and the line spacing is 4.7mil.

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Mr. Gaosu suddenly became interested. He thought that it would be impossible to verify the result using 2D simulation software, so he had to use 3D simulation software. Then he quickly built the transmission line model based on the customer's parameters.

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After setting the simulation parameters, run the model.

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After it runs, I immediately check the impedance simulation results. Wow!!! It really can control the impedance...

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By the way, let’s take a look at the insertion loss. I didn’t expect that in the absence of a ground plane, not only the differential line impedance can be controlled, but the loss at high frequencies can also be made so linear!

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Without the help of the ground plane, the differential line can actually control the impedance steadily? ? ? This seems a bit incredible. How and why can it be done? So the question worth thinking about must be the question of this issue for everyone to speak freely. Mr. Gaosuo will decide whether to expand on it in the next article based on the answers of the fans!

The question is:
Why can the differential line be controlled to 100 ohm impedance without a reference ground plane?