If there is no reference ground at all, guess if you can control the impedance?

author ： Mr. Yibo technology Expressway member David

 

As a SI The engineer ,PCB Design engineers, even hardware engineers and test engineers , Calculating transmission line impedance must be one of the necessary skills . Mr. Gao also talked about the theory and practice of impedance calculation in many previous articles , Let's review together here , The impedance of the transmission line is mainly related to the following factors , Impedance is inversely proportional to linewidth 、 It is inversely proportional to the dielectric constant of the medium 、 And line spacing （ If it's a difference line ） In direct proportion to 、 Directly proportional to the distance from the reference plane . Today's factor is the distance from the reference plane ！

We use the impedance calculation software to study the relationship between the transmission line impedance and the distance of the reference plane , Set the line width of a pair of difference lines to 5mil, The line spacing is 9mil, The dielectric constant of the medium is 4, That is, ordinary FR4 The level of . At this time, we are on the stripline model , Impedance can be easily achieved by controlling the distance from the reference plane 100 Ohm's goal . At this time, the thickness of the upper and lower parts is 5mil and 6.1mil, Impedance can be calculated 100 Ohm .

Okay , It's simple , Then put it another way , What if it's like this ？ Only the ground plane of the lower floor is used as the reference plane , Then the ground above disappeared , Only the media is on it , This is called embedded microstrip line .

Of course , We change H1 and H2 Distance of , It is certain that the impedance can also be calculated as 100 ohm , After all, there are reference planes . This is the time H1 and H2 Adjust the value of , It is also very simple to calculate 100 了 .

Then we will slowly enter the core content of this article , If there is no ground level below , Guess you can control 100 Ohm's differential impedance ？

Is there much room for thinking ？ According to the theory of impedance and reference distance , There is no reference plane , That is to say H infinity , It must be impossible to control the impedance , Impedance doesn't fly into the sky ？

In the process of thinking , Mr. Gao's team did meet a customer , This customer is not normal , Think it's OK ！ I can control ！

you 're right , What the customer said , That's what Mr. Gao Xin just asked you . The application scenario of this customer is FPC Soft plate , It is the wiring on the single-sided flexible board , It has coverlay Cover , In fact, it is equivalent to a layer of glue , The following is also media , There is no ground level up and down , And customers can calculate through this model 100 Ohmic differential impedance ！

Mr. Gao passed the relevant impedance calculation software , It's really possible to calculate 100 Ohmic impedance , For example, in this case .

Line width 12mil, Line spacing 4.7mil.

Mr. Gao is suddenly interested , I want to use it 2D The simulation software is definitely impossible to verify , Only up to 3D The simulation software of , So according to the customer's parameters, the transmission line model is built .

After setting the simulation parameters , Run this model .

Come out , See the simulation results of impedance immediately , Why ！！！ It can really control the impedance ...

Look at the insertion loss by the way , I didn't expect that without the ground plane , It not only controls the differential line impedance , It's amazing that the loss at high frequency can be so linear ！

Without the help of the ground plane , This pair of differential lines can also stably control the impedance ？？？ This looks really incredible , How did it do it , Why can it be done ？ Then there is a problem of thinking value , It must be a question of this issue to let everyone speak freely , Mr. Gao will decide whether to speak in the next article according to the answers of his fans ！

The problem is coming. ：
Why is there no reference ground plane , The difference line can also be controlled 100 Ohmic impedance ？