The follow-up is coming. Whether it's OK without reference, let's make it clear to everyone at once!

author ： Mr. Yibo technology Expressway member David

From the simulation results of the previous article , A pair of differential lines can indeed control the impedance well by refluxing each other , So why are all design rules , Even from our cognition, we all feel that we need a reference place to be a complete differential line transmission structure ？ For many engineers , They may not be able to give specific reasons , But the concept of land has gone deep into their hearts , So today, , Mr. Gao wants to make it clear to all of you ！


First of all, let's take the model established in the last article and continue to expand it , Why can impedance be controlled without ground , Theories and formulas tell us , The impedance is proportional to the distance from the reference plane , At the same time, I know , If the distance from the reference plane （ Below H Parameters ） Too close , In the same case, the impedance is low , So let's verify it first , If the reference plane changes from relatively close to slowly far away , How will the impedance of the differential line change .


Sure enough , We scan H This parameter , You'll find that with H The increase of , Impedance does slowly increase , But at the same time, we also need to pay attention to the most important point , Is that when H Increase to a certain value , The increase in impedance becomes very small ...


therefore , If H Add more , It's very close to the last article without land 100 Ohm , Of course, this model also adds a pair of side wiring ,gap by 3W, Therefore, it will also be affected by the running line next to Yichui , The impedance will also be slightly lower . as for , Why should we add a pair of lines next to this article , You will know later ！

We know , If it is a single ended signal , It must have a return path , That is, the impedance can only be controlled by the reference plane we commonly call , Ensure signal transmission , Differential signals can be reciprocal return paths , To control the impedance and forward transmission . For single ended signals , It will only have right “ The earth ” The impedance of , But as a difference line , There are two forms of differential impedance and common mode impedance , The so-called differential impedance is what we usually call the impedance of the differential line , It's the difference line P and N As the impedance in the case of differential mode transmission , But the difference line also has common mode impedance , That is, the difference line P and N Each of them is right “ The earth ” The impedance of , Used to transmit the same mode signal of the differential pair . Normally , The ideal difference line does not have the same mode signal , But only in an ideal situation , Under non ideal conditions, the same mode signal will be generated more or less , Therefore, the common mode signal will be transmitted and crosstalk with the differential pair to other signals .

Said so many theories , Is there any specific risk without land ？ Let's continue with the above model . In addition to the differential mode impedance of the differential pair , Let's casually look at its common mode impedance , According to the normal 100 Ohmic differential impedance control , The ideal common mode impedance is 25 ohm （ That is, single ended 50 Ohm parallel impedance to ground ）, You can see from the figure below that , As the distance between reference planes gets farther and farther , The common mode impedance increases significantly , And more importantly ,40mil In this way, it seems that there is a long distance and no land , Common mode impedance is not a very close magnitude , There is still a big difference .

That's the question , Does the increase of common mode impedance have any effect ？ This is why our model was built 2 The reason for the difference line . You can see , Or with the increase of ground level distance , The common mode crosstalk between the two pairs of wires becomes very serious . This is the biggest problem when there is no ground plane , After the common mode impedance increases , Reflection becomes serious , In addition, there is no ground level package , Common mode energy can easily spread around , Therefore, crosstalk will increase dramatically .

To sum up , First, when there is no ground plane , The differential mode impedance of the differential line can indeed be controlled by the backflow between the differential lines , But the difference line will have a form of common mode impedance , The common mode impedance needs to be controlled by the ground plane , Ideally, there really is no common mode impedance in the difference line , But in the non ideal case, once the common mode component is generated , It can easily spread to other signals on the board and even to space , Form serious crosstalk and... To other signals EMI radiation , It is a parameter that many fans may not pay attention to . So as the rate increases , The ground plane is basically an integral part of the difference line , Don't try this special design easily .


The problem is coming.
You can think about , Under what circumstances will the differential line produce the same mode signal to interfere with others ？