EMI Suppression Filters (EMC and Noise Suppression)MIPI C-PHY Immunity Noise Suppression

1About MIPI C-PHY

MIPI is primarily used for data transmission between the display module and an application IC or between the camera module and an application IC. D-PHY was previously adopted as the MIPI standard, but the majority of D-PHY subsystems that are used in commercially available smart phones consist of one clock lane and three data lanes. Meanwhile, it has become necessary to transmit a massive volume of data at high speed due to increasingly high-resolution displays and cameras. Moreover, because smart phones cannot be made larger, it has also become important to reduce the amount of wiring.
Against this background, the introduction of C-PHY, which is high speed and able to reduce the number of lanes, is advancing in the latest smart phones.

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2New noise problem in MIPI C-PHY

In a previous article, we introduced how MIPI C-PHY signals radiate noise from the substrate wiring and the flexible cables (FPC) and the impact on various types of wireless communication.
As we continued our investigation, it became clear that when 5G/LTE/Wi-Fi and other wireless signals couple with the MIPI C-PHY signal line, the noise penetrates the APU, and an immunity problem occurs whereby visible noise is created on the display.

Existing problem

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The C-PHY signal radiates noise and interferes with the 5G/LTE/Wi-Fi

New problem

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Problem whereby the 5G/LTE/Wi-Fi signal couples with MIPI C-PHY and creates visible noise

3An example of the immunity problem

The following introduces an example of the MIPI C-PHY immunity problem.
Using a smart phone equipped with MIPI C-PHY in the camera line, we observed the state of the camera screen during 5G NSA (non-stand-alone) communication. When it communicates over 5G, visible noise occurs on the camera screen. As a result, we can see that a camera line equipped with MIPI C-PHY has an immunity problem with respect to wireless communication signals.

Wireless communication setup

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Camera image during wireless communication

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4Suppression method for the immunity problem

For a problem such as this, we equip a noise filter in between the FPC cable and the APU where it is easy for the noise to couple. As a result, it suppresses the noise conduction from the FPC cable side to the APU and is able to prevent the immunity problem from occurring.

Insert a common mode choke coil

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The filter used in this solution

Common mode choke coil three-line CMCC

Because this is a three-line type of common mode choke coil, it supports a three-line type of signal line.

5Verification of the noise suppression effect

In order to suppress the noise that causes the visible noise, we measured the conductive noise of the MIPI C-PHY line during 5G communication. We compared the level of the 5G signal that penetrates the C-PHY line during 5G NSA communication in the case where a CMCC (common mode choke coil) is inserted into the MIPI C-PHY line to remove common mode noise and the case where a CMCC is not used.
The measurement location is the point in between the camera module connector and the APU on the main board.

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6Noise suppression results

When we took the measurements, we realized that the 5G signal was penetrating the MIPI C-PHY line on the main board as common mode noise, because the noise on the APU side decreased when the CMCC was inserted. The CMCC contributes to noise suppression.

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In addition, we confirmed whether it was able to suppress the visible noise from the reduction in conductive noise. When the camera image was checked, we were able to confirm that the visible noise disappeared when the CMCC was used. Through the reduction in the 5G signal reaching the APU, we were able to confirm that the camera image can be kept clear of noise.

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7Summary

In a MIPI C-PHY line, there is not only a radiation noise problem but also a troubling immunity problem in which the 5G communication signals couple with the MIPI C-PHY line.

  • By inserting a CMCC into the MIPI C-PHY line, we can prevent the wireless signals from penetrating the APU and causing problems.

The filter used in this solution

Common mode choke coil three-line CMCC

Because this is a three-line type of common mode choke coil, it supports a three-line type of signal line.