The simulation application of common mode inductance is here, full of dry goods for everyone

Author: Mr. Huang Gang, member of Yibo Technology Co., Ltd.

First of all, let's clarify the questions that need to be solved in this article. There are two main ones.First, what is the role of the common mode inductor itself?The second is why the test eye diagram after adding the common mode inductance is not as good as without the common mode inductance?These two problems will be told to you through the simulation verification of this article!
 
首先我们先解决第一个问题，共模电感到底起的是什么作用。Let's take a look at the datasheet about the common mode inductor in last week's article. As you can see from the figure, the common mode impedance presented by the common mode inductor is very large, that is to say, if the common mode signal wants to pass through this inductor, it willIt is severely suppressed, but it is very friendly to differential mode signals, and the differential mode impedance is not large, so it can still pass the differential mode signal relatively smoothly.

So after this, Mr. Gao found various channels and successfully obtained the model (S parameter) of this common mode inductor, so he can go to do detailed simulation verification for this case.
 
首先我们还是先验证下客户测试共模回损fail的情况，我们把共模电感的参数加入到这条仿真链路里去，结果仿真得到的共模回损果然和测试The old image!Low frequencies just can't get through.

Of course, in order to prove that the test requirements can be met after removing the common mode inductance, we also directly short-circuit the common mode inductance in the simulation and then simulate again, eh!Indeed, there is no problem with the common mode return loss, and the customer also reported that the test can pass.

Okay, after solving the customer's passive test problem, let's take a look at the use of this inductor from the time domain.From the previous analysis, we already know that the common mode inductor is mainly to suppress the common mode signal generated in the differential line, then we will simulate a pair of differential signals. If a common mode signal is generated for various reasons, look at this inductor pairWhat is the use of common mode signals?As shown in the figure below, we input the differential excitation (the clock signal is given, the peak-to-peak value is 1V, and the rate is the same as the USB transmission rate of this case) into the link model we extracted, and the output after passing it ishow.

We additionally inject a common-mode noise into this pair of differential signals, with a high-frequency noise of 100mV amplitude. Let's first see how this common-mode noise reaches when it passes through a link with and without a common-mode inductor.situation at the receiving end.

Well, it can be seen from this simulation result that when a high-frequency common-mode noise is generated from the differential pair and transmitted along the differential line, it encounters a link with a common-mode inductance, and the common-mode noise is greatly increased.The suppression of , the amplitude decreases rapidly, if there is no common-mode inductance link, the common-mode noise can pass unscrupulously.Therefore, a conclusion can be drawn here on the effect of common mode inductance. It is a device that exists to suppress common mode noise, because in interface devices, common mode noise is easily passed in with the interface, so once it is in the differential lineAfter the existence of the differential line, it will interfere with the surrounding signal or radiate into the air with the path of the differential line, resulting in crosstalk and EMI interference, which will have a serious impact on the signal and system.After adding the common mode inductor, the noise can be suppressed in a small range, and the amplitude of the noise can also be attenuated, so as to improve the signal-to-noise ratio of the system.
 
恩，第一个遗留的疑问已经解决了，现在再看看第二个疑问，为什么加了共模电感后的信号质量反而没有不加共模电感的好呢？We still use simulation to verify this problem.Similarly, we compare the simulation results of the full-link differential loss with and without inductors, and we can see that although the common-mode inductor can suppress common-mode noise well, it will also bring certain effects to useful differential-mode signals.It is impossible to be so perfect, completely suppressing the common mode noise while retaining 100% of the differential signal.

Therefore, we can see in the time domain that when the same differential signal passes through the link, whether there is a common mode inductance has a different effect on the amplitude of the differential signal. Indeed, when there is no common mode inductance, the amplitude will beHigher, that is, the signal quality is better.

Well, this simulation verification is also consistent with the customer's feedback. The common mode inductance does affect the quality of the differential signal slightly.Therefore, it is also recommended that you not only pay attention to the suppression effect of the common-mode inductor on the common-mode signal when selecting the device, but also check the impact of the inductor on the differential-mode signal, because after all, the signal required by the final system is the differential-mode signal.It can transmit well to work properly.