[TowerTalk] [RFI] Snap-on Ferrites

[jimlux]

On 6/19/17 4:57 AM, kc4pe wrote:
> Chris:
>
> Clarify: When your talking about turn in a snap 0n your referring to the
> large snap, 1.75 X 2.25 with a .75 opening, not the 1.255 X .75 with a
> .312 (5/16_ opening..?
>
> So inquiring minds wonder when you buy a printer/TV set, etc and there
> is a molded Ferrite block on the AC line. what are they trying to block..??
>

Almost certainly they are blocking VHF/UHF spurious emissions that cause 
them to not meet FCC Part 15 limits.  The classic problem is the CPU 
clock higher harmonics coupled into the data or power lines, as well as 
"dot clock" for video displays. I notice you don't see the "filter 
lumps" on things like HDMI cables as much as you do on VGA and 
keyboards- HDMI works at a higher clock rate and needs better RF design 
all around, just to work at all (balanced pairs, etc.).