[RFI] Keyboard RFI

[Jim Brown]

On 8/22/2013 10:03 AM, Charlie Gallo wrote:
> One odd thing I found - every keyboard around the house (and I have different brands from cheap to Unicomp Model M keyboards) has horrid RFI_IF_  you are close to it (like the hand held within say 6-9 inches)

Most trash like this is radiated as a common mode signal by the cable -- 
in other words, the cable is a transmitting antenna. Before spending big 
bucks on a new keyboard, I would try a suitable common mode choke on the 
cable. "Suitable" means wound on a suppression core with the number of 
turns that makes it resonant in the frequency range of the interference. 
Bear in mind that the typical circuit Q of the resonance is on the order 
of 0.4 to 1.  Fair-Rite #31 is the weapon of choice below 5 MHz, and is 
quite effective to about 200 MHz. Fair-Rite #43 is a bit better above 20 
MHz, and quite effective down to about 5 MHz.

Some of the trash may be radiated by wiring within the keyboard itself, 
and a common mode choke won't make a dent in that part of it.

73, Jim K9YC