RF interference evaluation of flexible flat cables for high-speed data transmission in mobile devices

This paper proposes a radio-frequency interference (RFI) evaluation method for flexible flat cables (FFCs), which are commonly employed for high-speed signal transmission in modern mobile devices and become a major path of noise coupling to antennas. The noise coupled from differential-mode and common-mode signals flowing through FFCs is obtained in terms of transfer functions based on S-parameter measurement. Considering the source power from a driver IC on the main board sent to the FFCs, the noise power at the antenna was calculated to evaluate the noise coupling through the FFCs based on a criterion of RFI noise power received at an antenna port. To measure the coupled noise in terms of transfer function, a dedicated evaluation board is designed and fabricated. Several low-voltage differential signaling (LVDS) FFCs used in a real tablet were evaluated based on the proposed method and validated by experimental measurement on RFI noise power at Wi-Fi antenna port using both the evaluation board and the real tablet. A good correlation was achieved between them.