Common mode inductance has been heard many times, but what principle do you really understand?

author ： Mr. Yibo technology Expressway member   David

Common mode inductance , We will see more in communication products , It generally exists in the application of interfaces , for example USB,PCIE,HDMI Such as the interface . You must know its name , Its function is related to the common mode impedance . Its shape is probably as long as the bottom , Connect a pair of differential lines in series .

Yes , It's that simple , Because of its simple use and small size , about PCB It won't add any difficulty to the design , Therefore, it is indispensable in the schematic diagram of many interfaces . Many people can use it , But it doesn't mean that everyone can understand its principle , The case Mr. Gao wants to talk about next is about its application .

A communication board designed by our company for customers , One of them USB The interface module of , from USB Chip to interface , And then through cable Connect to device. After the customer's board is processed and tested , Just asked us such a question ！

The customer first performed a passive test on the completed board , It turned out to be “ mixed ” ah ！ Let's first look at the test scenario of this board , The customer connects the network analyzer to USB fixture , Again from USB The interface is connected to our board , To test the impedance and return loss of the board .

  The customer first tested the differential impedance and return loss of the link , I found that everything was good , Can meet the requirements of the agreement .

  When everything seems to be going in a good direction ,USB There is also an index of common mode return loss in the protocol to meet , So the customer also tested this indicator , It turned out to be embarrassing , This indicator is surprisingly low-frequency, but ！！！

This is why the customer came to Mr. Gao as soon as possible after the test . Because of this board USB yes 3.2 The signal , yes 10Gbps Rate , Not very high , Therefore, Mr. Gao speed is not required to intervene in the simulation in the early stage , The customer chose to trust our colleagues in the design department to deal with it directly ！ Now the indicators are nothing more than , The customer immediately asked Mr. Gao for help ！ Of course, Mr. Gao also believes our colleagues in the design department , There should be no big problem in the design of this rate .

So Mr. expressway is on one side check While designing the board , Let the customer do the active test again , Let customers conduct eye diagram test and function test .

 

  Okay , There's no problem with the function , The eye diagram of the test is also very good , Then why can't the common mode return loss index be better ？ Mr. Gao passed PCB Design has found the answer ！ Mr. Gao found the network tested above , It is found that after entering the interface, it will pass through a series of resistance and capacitance devices , One of the devices we have today “ Pig feet ”, Common mode inductance

Mr. Gao immediately pointed the common mode inductor at the common mode return loss index ！ So check the common mode inductor datasheet, Sure enough, I found a preliminary answer . Seeing the electrical parameters of this common mode inductor, you will find , It plays a very different role in the transmission of common mode and differential mode signals , It can be seen from the impedance curve , The differential mode impedance is relatively small , The common mode impedance is relatively large , In fact, it is a device that blocks common mode signals and allows differential mode signals to pass ！

Then we made a bold attempt , Let the customer short circuit the common mode inductor , Then test whether the function ok, And the customer soon tried and gave the answer ！ And the surprise is that not only the current project that came to us is no problem , Even the previous similar project is the same , After the common mode inductance is short circuited, it is the test index pass Of ！ And more “ Weird ” What's more, the eye diagram tested after removing the common mode inductor is a little better than using the common mode inductor , You didn't hear me wrong ！ Yes, it's better ！

Okay , From the parameters of this common mode inductor and some operations of the test, we can see , Common mode inductance itself has a large common mode impedance , Therefore, testing common mode back loss is certainly easy to understand , In addition, it can be seen that the common mode inductor can work well after it is removed ！ But Mr. Gao is never satisfied with seeing the phenomenon , Mr. Gao also wants to get the model of this common mode inductor and make a comparison between simulation and test , And make an in-depth analysis of this principle , Due to limited space , Next part The more wonderful part will be left for the next article to share with you , Let's answer all the questions in this article ！ That's all for this article , Please continue to pay attention and look forward to ！