When an external card with a cable is connected, noise is emitted not only from the card itself but also from the notebook PC, conducted through the cable. When the high-frequency impedance between the card's GND and PC's GND is high, the card’s GND becomes unstable. This makes noise suppression difficult. Therefore, the slot fingers in contact with the card’s GND are screwed to the PCB’s GND and metal plate, minimizing the high-frequency impedance between them.

Noise Emission Status Noise Suppression Model     Improving the Case Shielding and GND Connection Installing EMI filters on Interface Cable Ports Installing EMI Filters on the DC Power Supply Input Installing EMI filters in the LCD Panel Improving the External Card GND     Examples of Noise Suppression for USB

If the PC’s noise affects the card, radiated noise cannot be suppressed, even if the card contains a noise suppressor. In this case, the slot is shielded with a metal plate. When the ground connection relies on physical contact, such as when an external card makes contact with the slot, the level of radiated noise varies widely depending on the status of the contact. When taking measures to suppress noise or measure the noise level, care must be taken.