Basics of Noise Countermeasures
Lesson 13: Using Common Mode Choke Coils for Signal Lines

7/22/2014
Application

Category: Noise suppression filter Room

This article -provides information on the use of common mode choke coils for signal lines.

1. Skew-improvement function of common mode choke coils
The main reason common mode choke coils are used in signal lines is to eliminate common mode noise, but since common mode choke coils utilize the operating principle of a transformer they can be expected to provide a skew correction function in a differential transmission circuit. Ideally, both lines of a differential transmission circuit are designed to be balanced, but factors such as manufacturing inconsistency can cause them to be unbalanced. This can cause a gap between the signal arrival times of the two lines, resulting in skewing of the transmission signal (figure 1).


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Figure 1 Signal skew

In such cases, inserting a common mode choke coil can reduce the amount of skew.
Figure 2 shows how a common mode choke coil improves the skew characteristics.


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Figure 2 Skew correction using a common mode choke coil


A common mode choke coil has the same configuration as a transformer, so when the timing of the rising and falling edges of the two lines is unbalanced, the common mode choke coil can ensure the currents are balanced by generating induced electromotive force on the opposite side. This behavior has the result of aligning the timing of the differential signals, thereby improving the skew characteristics.

Figure 3 shows actual experimental test results.

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Figure 3 Results of skew improvement using common mode choke coil


After measuring the wave forms of differential transmission lines with signal paths that were deliberately made unequal in length, we can see that the timing of the DOUT+ and DOUT- rising and falling edges is skewed when no filter (common mode choke coil) is used. Adding DOUT+ and DOUT- should result in a fixed value when the two lines are balanced, but in this case there is some degree of variation because the balance is disturbed.
Inserting a common mode choke coil aligns the timing of the rising and falling edges of the two lines, so adding DOUT+ and DOUT- results in a roughly constant level, showing that the skew characteristics have been improved.


2. Common mode choke coil equivalent circuit diagram
"old hands" is somewhat usual slang expression, for this article. And might not be globally understood. I suggest alternative term, such as "Industry veterans" will already know about this, but since we sometimes receive questions we will take this opportunity to explain the meaning of the black dots in the common mode choke coil equivalent circuit diagram.
Figure 4 below is a diagram of a common mode choke coil equivalent circuit.

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Figure 4 Common mode choke coil equivalent circuit diagram

The configuration is basically the same as that of a transformer. In the diagram, there are two black dots at one side of the coil.
We are sometimes asked if these dots indicate where the coil winding start, but in fact they don't tell us that something is present at the location of the dots; rather, they indicate the directionality of the magnetic coupling of the two coils. The structure of a common mode choke coil was described in an earlier article . In order to function as a common mode choke coil the magnetic flux generated by the two coils must reinforce the common mode current and cancel out the differential mode current.
Therefore, if the winding direction of the two coils is not the same they will function in the opposite manner.

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Figure 5 Different operation due to different equivalent circuit diagram

When the black dots are aligned on the same side of the coils in the equivalent diagram, as in the top diagram in figure 5 above, the magnetic coupling functions as a common mode choke coil, but when the dots are on different sides, as in the bottom diagram, the coils do not function as a common mode choke coil.
In this way, the position of the black dots indicates the directionality of the magnetic coupling of the coils, and they don't tell us that anything in particular is present on the side where the two dots appear.
Note that the original purpose of the black dots was to indicate the voltage polarity in the case of a transformer.


Written by: Yasuhiro Mitsuya, Component Business Unit, Murata Manufacturing Co., Ltd.