[Elecraft] Computer Noise ruining 20 & 30m

[Bill W5WVO]

One thing to be aware of -- and I too have had quite a bit of undesired 
experience in this area -- hash from switching power supplies in computers is 
typically common mode noise, not differential noise. Consequently, a 
differential line filter often won't knock it down very much, if at all. What 
you need is ferrite toroids -- big ones -- around and through which you wind 
the power cord. In my case, the computer power supply is a 470W job in an ATX 
form factor, and because of the high power components, there wasn't any room 
in the PS case for filtering! So the RF noise from this PS was horrendous. It 
took TWO large ferrite donuts in series, with the ENTIRE power cord wound 
around them, to suppress the noise on 6M. I had bought a rather expensive 
differential noise filter for the line, and it did absolutely nothing. 
Brute-force common-mode filtering using ferrite toroids will usually cure the 
problem, but you may need a lot more saturability than you think.

Bill W5WVO


G4ILO wrote:
> Alan D. Wilcox wrote:
>>
>> Hello,
>> I gave away the feeble old PC I had by my radio equipment and
>> replaced it with a newer one ...
>>
>> Now I have S9+ noise wiping out 20 and 30 meters.
>>
>> Using same keyboard, mouse, monitor; just the PC in a new case.
>> Evidently it has a
>> noisy switcher supply in it. Tried 110V in-line filter, also
>> different 110V house circuit,
>> but still have noise. Grounds don't help. Don't happen to have a box
>> of toroids for chokes.
>>
> It's very likely to be the power supply, especially if it is a home
> build or a local PC store build rather than a branded model. Most of
> the PC cases and power supplies that home builders and small
> assemblers use are made in China, and the manufacturers often leave
> out all the components they can get away with, like anything to do
> with interference suppression. I remember someone a few years ago
> took apart a noisy power supply and could see all the holes in the PC
> board where the suppression components should have been. It was quite
> an eye opener.
>
> Replace it with a power supply from a decent manufacturer and all
> should be OK.
>
> -----
> Julian, G4ILO K2 s/n: 392  K3 s/n: ???
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