How do I control or reduce EMI noise on my display and interface to ensure passing UL Certification requirements?
A. Most noise radiation from the display comes from the dot clock – a synchronization clock that runs at very high speeds. On a VGA display it runs at 25 MHz. For a UXGA display it can approach 140 MHz. The use of small R-C circuits or inductance on the clock lines can soften the edges and lower the radiation. Shielding of the connection cable is another method to reduce noise, but it tends to be more expensive. Another technique for larger displays is to use LVDS or TMDS transmission systems where the TTL signals are transmitted as low voltage differential signals. This is the technique used currently in most laptops. It is important to ground the display frame to a good AC signal ground. This helps contain the noise generated by the display as well as to become a shield for any electronics located behind the display. Sometimes a metal foil is placed behind a display to help shield other noise sources. SHARP displays, by themselves, will generally pass FCC class B tests.
