In conventional MIPI D-PHY it is necessary to remove common mode noise to avoid having an adverse effect on the differential signals, so 2-line common mode noise filters are used. However, MIPI C-PHY transmits differential signals using three signal lines, so normal common mode noise filters cannot be used as is.
One possible method is to combine three common mode noise filters as shown in the figure left below, but this has a large effect on the signals, so sufficient common mode noise suppression effects cannot be expected. Therefore, MIPI C-PHY noise suppression requires a common mode noise filter that supports 3-line differential signals.
When using a 3-line common mode noise filter that magnetically couples the 3 lines internally, circuit simulation was used to check whether the signals could be transferred effectively.
Using a 2-line filter, the transfer waveform was disturbed; however, when a 3-line common mode noise filter was used, we found that the signals were transferred without disturbance to the waveform.
■ Waveform verification by simulation
— Comparison of 2-line and 3-line common mode noise filters —
■ Noise filters required in MIPI C-PHY
- Must be a 3-line noise filter that supports the 3-line configuration of MIPI C-PHY
- Must not have an adverse effect on signal quality
- Noise filter must be able to eliminate common mode noise over a wide frequency range