Noise suppression technologies/case study introduction (Consumer)

Noise suppression measures for differential interface--intrasystem EMC (electromagnetic compatibility)-- (1)

1. Popularization of differential interface and wireless communication modules

In recent years, digital devices such as smartphones have been required to transmit high-resolution and large-volume data inside and outside them. Accordingly, various differential interfaces have been adopted.

Chart of the latest standards for differential interfaces
latest standard for differential interfaces

On the other hand, such digital devices have become equipped with wireless communication modules for transmitting data between the devices and the Internet. Of the modules, Wi-Fi modules have been popularized as popular wireless communication modules.

Wi-Fi wireless communication flow chart

2. Noise issues with differential interfaces

The intrasystem EMC has been more noticeable due to the speeding up of differential interfaces and the popularization of wireless communications.

Image of noise issues with differential interfaces

Intrasystem EMC: Phenomenon in which noise emitted from USB 3.2 interferes with the Wi-Fi antenna in a device and deteriorates the quality of Wi-Fi communication

EMC for public standards: Phenomenon in which noise emitted from the actual device or USB 3.2 cable exceeds the limit values of public standards

3. Measures to suppress radiated noise

Insert a common mode choke coil (CMCC) into the differential interface. Each CMCC becomes more effective when it is installed close to the noise source. 

Noise source of Tx signal = USB IC side

Noise source of Rx signal = USB connector side

Image of measures to suppress radiated noise (before)
Before
Image of measures to suppress radiated noise (after)
After

Recommended filter

Due to this function, common mode noise can be effectively eliminated without affecting the signal waveform. Furthermore, the measure has been recommended by several IC makers.

4. Noise picked up by Wi-Fi antenna (1): Investigation with simple board

The noise picked up by an antenna was evaluated during transmission through USB 3.2 using an evaluation board equipped with a USB Type-C connector. The signal source was a USB 3.2-compatible notebook PC with SSD used for its sink device. In addition, an omni antenna was set up 5 cm away from the evaluation board.

The extent that the noise radiated from the evaluation board wirings was picked up by the antenna was investigated during transmission through USB 3.2. 
Furthermore, the effects of this measure were compared with those derived from CMCC.

Image of chart of coupling noise to omni-antenna during USB3.2 transmission

5. Noise picked up by Wi-Fi antenna (1): Results of the investigation with simple board

Green vs. red  (standard)
When a CMCC was placed away from the noise source, the noise was improved by approximately 2 dB. 
Blue vs. red  (standard)
When a CMCC was placed close to the noise source, the noise was improved by approximately 5 dB.

The result indicated that the noise differed by 3 dB, depending on the position the CMCC was installed.

Chart of coupling noise to omni-antenna during USB3.2 transmission
Noise picked up by the omni antenna during transmission through USB 3.2

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