Recent laptop PCs have become slim and streamlined. During the 1990s, PCs were like large lunch boxes, and it seems hard to believe how large and heavy they were. The interface section was also large and equipped with various types of specialized connectors for the mouse, printer, and other devices. It later changed to a general-purpose interface which lead to considerable miniaturization.
The connector miniaturization was achieved by reducing the number of signal wires through a speed-up of the signal. However, when the signal frequency is simply accelerated, the EMI noise increases, which is a problem. The adoption of differential transmission has significantly contributed to resolving this problem. This article explains the features of differential transmission and methods of noise suppression.
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3-2. Noise Suppression in Differential Transmission
When a cable is connected, noise is easily emitted from the cable regardless of whether you are using differential transmission.
3-2-1. What is Differential Transmission?
Differential transmission is a method which basically uses two signal wires as a pair of transmission lines. As shown in Figure 2-1, the current flows along both wires in the opposite direction. Therefore, the magnetic flux is canceled out as shown in Figure 2-2(a), and the EMI noise is reduced.
In addition, differential transmission determines the logic by the electric potential difference between the signal wires. Therefore, as shown in Figure 2-2(b), the externally applied noise is canceled, so it is notably unlikely to malfunction even if the signal amplitude is reduced. Reducing the amplitude not only further decreases the noise but also has the advantage of accelerating the signal.
3-2-2. Suppression of Common Mode Noise
While differential transmission is noted for the low level of noise due to the signal, the noise radiation from the cable is still a problem. A major cause is the common mode noise generated within the electronic circuit. As shown in Figure 2-3, this noise flows through all conductors in the same direction. For example, when conducted through a cable, the magnetic flux of the current is not canceled out due to this common mode noise, so strong radiation noise is generated from the signal wires and shield.
To take measures against the common mode noise, the generation of the common mode noise is reduced at the noise source through methods such as noise current suppression by mounting ferrite beads on the signal lines and ripple noise suppression by mounting a bypass capacitor on the power line. In addition, the common mode noise which conducts the GND can be reduced by strengthening the ground (GND) which connects to the metal of the printed circuit board and chassis, etc.
However, in the event that the common mode noise generated within the IC is conducted to the differential transmission line, a filter must also be mounted to the cable junction as a countermeasure. Using a common mode choke coil reduces the common mode noise without affecting the signal as the filter. Figure 2-4 shows an example of mounting a common mode choke coil to a USB 3.1 gen2 to reduce the radiation noise at the secondary higher harmonic of 10GHz for the fundamental signal frequency at 5GHz.
The reason why the common mode choke coil can counteract the common mode noise without affecting the signal can be understood from the direction of the magnetic flux generated within the common mode choke coil. As shown in Figure 2-5, the magnetic flux due to the signal current is canceled out and impedance does not occur, so the coil does not affect the signal waveform. Meanwhile, the magnetic flux due to the common mode noise is added and impedance occurs, which reduces the common mode noise.
For the reason explained above, a common mode choke coil is a suitable filter for differential transmission.
3-2-3. Noise Suppression via Skew
Up to this point, we have treated the differential transmission waveform as an ideal waveform for the purpose of discussion. However, in reality, a so-called “skew” sometimes occurs that divides the waveforms into rising and falling signals, as shown in Figure 2-6.
The occurrence of skew means that the signals D+ and D- are no longer symmetrical. This means that the current flowing through the two signal wires is not symmetrical, the magnetic flux is not properly canceled out, and noise problems will occur. The total sum of the D+ and D- signal waveforms is no longer 0, and the signal distortion due to the waveform ringing also increases.
Common mode choke coils are also an effective way to reduce the skew which causes such problems. Figure 2-7 shows an example of improving skew by mounting a common mode choke coil.
The structure of a common mode choke coil is the same as a transformer, so it balances the current between the signal wires using the electromotive force to improve the skew as shown in Figure 2-8. However, please note that the waveform rising and falling times are not improved by using a common mode choke coil.
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3-3. Characteristics Required by Common Mode Choke Coils
Thus far, we have introduced noise suppression for differential transmission. Ideal common mode choke coils remove only the common mode choke noise without affecting the signal waveform, but unfortunately real components do not work that way. Therefore, the impact of the common mode choke coil on the signal waveform and the common mode noise suppression effect must be checked. For this reason, the following section will introduce how to represent the common mode choke coil characteristics and the effect that those characteristics have on the signal waveform.
3-3-1. Representing the Electrical Characteristics of Components
S-parameters are typically used to represent the characteristics of electronic components. S-parameters indicate the signal relationship between the terminal pairs (ports) which input and output signals to the circuit. Figure 3-1 shows how to measure the S-parameters for a component with two signal terminals. For example, when a signal is input to Port 1, the ratio of the magnitude of the signal output from Port 2 and the phase difference is represented as S21. When loss occurs, the polarity of S21 becomes negative. Meanwhile, when the insertion loss is positive, it means that loss is occurring. S21 corresponds to the insertion loss, but be sure to note that the polarity is the reverse. S11 represents the signal output from Port 1 when a signal is input to Port 1, so it is equal to the reflection coefficient.
The common mode choke coil has four terminals, so four-port S-parameters are used when representing S-parameters as shown in Figure 3-2.
Now, there is a problem with these four-port S-parameters in that it is difficult to understand the common mode characteristics when signals with the same phase are input between the signal terminals or the differential mode transmission characteristics when signals with the reversed phase are input as shown in Figure 3-3. Therefore, mixed-mode S-parameters (Note 1) are used to indicate these characteristics.
(Note 1) Reference documents: David E. Bockelman, William R. Eisenstadt, "Combined Differential and Common-Mode Scattering Parameters: Theory and Simulation", IEEE Tarns. MTT, vol.43, No.7, pp.1530-1539, 1995 July
The method for notating these mixed-mode S-parameters is shown in Figure 3-4. For example, Scc21 represents the ratio of the common mode signal wave output from Port 2 when a common mode signal wave is input to Port 1. When the signal attenuates, the polarity becomes negative. Therefore, the reverse of the polarity is the insertion loss.
Sdd21 represents the ratio of the differential mode signal wave output from Port 2 when a differential mode signal wave is input to Port 1. In other words, Sdd21 corresponds to the differential mode insertion loss.
3-3-2. Selecting a Common Mode Choke Coil
From this point on, we will introduce important points which should be noted when selecting components while indicating actual common mode choke coil characteristics.
Figure 3-5 shows two types of common mode choke coil characteristics.
We can see that the common mode insertion loss characteristics (corresponding to Scc21) are superior at 1GHz for common mode choke coil A while common mode choke coil B is superior at 5GHz. Because the insertion loss differs depending on the common mode choke coil, they are used according to the problematic noise frequency.
When selecting components, attention must be paid to the signal waveform. When the differential mode insertion loss (corresponding to Sdd21) is high, the waveform distortion increases. Therefore, components with a range where the waveform distortion is not a problem must be selected.
The signal waveform is typically evaluated based on the eye pattern. An example of such an evaluation is shown in Figure 3-6. The blue lines are the eye pattern which is formed as the signal waveform is overwritten. It is called an "eye pattern," because the shape resembles a pair of eyes. The red-colored region is the region where the eye pattern must not be present and is called the "mask." Selecting a common mode choke coil with a small differential mode insertion loss can make it so that the eye pattern does not overlap with the mask.
To reduce the differential mode insertion loss, the common mode choke coil adjusts the characteristic impedance between the wires and the transmission line. The impedance between the differential transmission signal lines is generally specified to be 100 ohms as shown in Figure 3-7. Therefore, the characteristic impedance between the signal lines must also be 100 ohms, and the common mode choke coil matches that requirement. Furthermore, impedance between the lines is sometimes set to 90 ohms depending on the standard, so there are also common mode choke coils with a line characteristic impedance of 90 ohms.
Up to this point, we have explained how to use common mode choke coils in differential transmission, but depending on the standard, in some cases a single-ended transmission is included in part of the signal. In such a case, it is important to note that if the common mode impedance of the common mode choke coil is too high, the waveform distortion may increase.
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3-4. Impact of the Printed Circuit Board GND on Differential Transmission Noise
From the perspective of noise, we would like to briefly discuss the design of the printed circuit board GND. The fundamental approach to differential transmission uses two signal wires as a pair of transmission lines and conducts current along both wires in the opposite direction.
Therefore, when a differential transmission line is built, one might think the GND is unrelated as a return current path. However, in reality, it is affected. When designing a circuit board with a realistic line width, the distance between the signal lines and GND (interlayer distance) is shorter than the distance between the signal lines. For this reason, the coupling between the signal lines and GND becomes stronger than the coupling between the signal lines.
To demonstrate the effect of the GND, Figure 4-1 shows the simulation results for the near magnetic field when a slit is placed on the GND side. We can see that the near magnetic field strengthens when the GND slit is introduced. In this way, the GND design can also have an effect on noise, so caution is required. For example, GND separation for the purpose of taking countermeasures against static electricity leads to increased noise.
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In this article, we introduced differential transmission noise suppression measures suitable for high-speed signal transmission. Below is a summary of what we covered on noise suppression.
(1) Attach a filter to the differential transmission line
In differential transmission noise suppression, common mode noise and noise due to signal waveform skew is reduced with a common mode choke coil. Select a common mode choke coil with high common mode insertion loss in the noise band and a low differential mode insertion loss in order to suppress waveform distortion.
(2) Suppression at noise sources in the circuit
The generation of the common mode noise is reduced at the noise source through methods such as noise current suppression by mounting ferrite beads on the signal lines and ripple noise suppression by mounting a bypass capacitor on the power line.
(3) GND reinforcement
Reduce the GND noise level by reinforcing the GND which connects to the circuit board, the GND metal plate of the connector, and the shield case.
This concludes our introduction to noise suppression for differential transmission.