[RFI] 22 inch lcd monitor power supply noise

Tim Groat tcgroat at mesanetworks.net
Tue Feb 24 22:13:53 EST 2009

The symptoms you describe suggest capacitive coupling from the internal 
high-frequency parts to nearby metal and wiring (that's not the only 
possibility, but it seems likely). There are many construction tricks 
the power supply manufacturers use to pass FCC specs without a shielded 
case. Even a few pF of extra capacitance will couple considerable RF 
current when a switching power supply transistor swings about 500V in 

If it is a small stray capacitance that's coupling the noise, the source 
impedance will be high and it takes a high impedance choke to make a 
difference. A multi-turn choke wound on a mix 31 toroid would be a good 
place to start. Since you have ferrite on order, go and ahead and try 
what you already paid for. If it works, don't worry about getting the 
"optimum" core.

--Tim (KR0U)

> "W0MU Mike Fatchett" <w0mu at w0mu.com>
> It appears that the power supply brick that powers my X2Gen 22 inch monitor
> when close to metal or certain cables emits a nasty s3 hash into the K3.
> I have moved it to a place that is quiet.  I wonder if that will last.
> Would ferrites help?  I have a batch coming in from Allied.
> W0MU

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