RF Hash;
There's 2 basic types of noise emissions that one must be concerned with.
(a) Conducted - This is typically generated inside the computer or power
supply and flows out the power cable or any other cable connected to the
box, i.e. monitor, printer, modem, scanner and etc. This is most often
common mode transmission.
(b) Radiated - Again generated inside but is simply radiated into the air,
often by connected cables, as any RF signal and is thus received by an
antenna. This is most often parallel mode radiation.
Then there's the issue of both common mode and parallel mode signals which
can be associated with both (a) and (b) above. The treatment for each is
different.
Many filter type plug strips employ filtering for both common mode and
parallel mode methods. Common mode is addressed with having all AC lines
(line, neutral and ground) wound through a suitable ferrite donut. I have
found that some filters DO NOT pass the ground through the ferrite donut.
They also employ bypassing with a value of C between all three lines. The
same approach can be used in any computer or power supply. The shortcoming
to the power strip approach is the length of the power cable between the
offending device and the filter. Hence and ideally, the filter should be
located at or inside the offending box.
CORCOM , just one brand, is a good choice in many applications.
For radiated signals, short length leads, thus reduce excessive cable
lengths and use adequate amount of shielding is typically the proper
approach. Adding bypass C at the points where cables attach often works
except take care that the bypassing does not restrict the operation of the
connected device. The biggest challenge is to figure out what type of
emission path(s) exists and then apply the correct solution.
In today's world of high speed computers and data transmission, it will only
get worse. Part 15 only goes part of the way with regard to manufacturer
certification. There is very little enforcement or follow up in that a
manufacturer only has to "certify" that the item complies. To me this says
"we saw it work once, so it must meet the requirement. Put it in production
and ship it".
The ARRL publication RADIO FREQUENCY INTERFERENCE: HOW TO FIND IT AND FIX IT
by Ed Hare & Robert Schetgen is an excellent reference source for this
subject.
73
Bob, K4TAX
----- Original Message -----
From: "Patrick A Thompson Sr" <wa4tuk@softhome.net>
To: "Paul ve1dy" <ve1dy@yahoo.com>; "Bill"
<florida.investigative@verizon.net>; "Tentec Reflector"
<tentec@contesting.com>
Sent: Sunday, December 08, 2002 11:29 AM
Subject: RE: [TenTec] RFI Problem
> For what its worth...
>
> I've been struggling with computer noise for some time now. I have one
> extremely noisy computer and two others in the shack that make very little
> RF hash. I already have an LCD monitor. The noisy computer starts
radiating
> as soon as the power plug goes in the wall because of the "wake-on-lan"
> feature that isn't needed in a home environment. RF hash extends from the
> standard broadcast band as high as 2 meters at S meter strengths of 20
over
> 9 or more depending on which antenna I'm using. I've even seen low level
> trash drifting upward through the picture on TV VHF channel 2 and 3. This
> is of course my fastest and most capable computer so its the one you would
> want to use.
>
> I've tried various clamp-on ferrite devices on the power cord, vga cable,
> Ethernet, and internal to the box etc. with no real improvement. Small
caps
> to ground at the motherboard DC connection made no difference.
>
> I did note that pulling the power cable from the power supply to the
> motherboard ended the hash so I erroneously concluded that the motherboard
> was the source of the noise. Voltage was still present on the cable when
> removed from the PC motherboard so I assumed the supply was still
operating.
> Some supplies will shut down at no load so I've read.
>
> So yesterday because of the write-ups from various folks on this list I
> bought one of Radio Shacks filtered power strips to replace my existing
> strip. $29.95 with colored LED's to look at. Initially I thought there
was
> no improvement but quick comparisons by plugging first into the RS strip
and
> then into an ordinary strip tuned to a noise peak near 7 MHz showed S9 or
so
> compared to S1 with the RS strip.
>
> Down on the bench now I have a Corcom filter salvaged from a defunct piece
> of equipment mated to the 3 pin power plug clipped from an old power cord.
> This filter was part of the AC input assembly and included a 3AG style
line
> fuse. When the RTV I potted the thing in dries I'll try it right at the
> back of the computer. This won't require any modification inside the
> computers box and it might help radiation from the power cord to the
strip.
> The line filter should have been incorporated into the power supply before
> it left the factory.
>
> I've also bought off EBay a couple of Corcom filters for further
> experimentation. You are too late, I bought-it-now! (There are others
still
> for sale or look in your collection of recent boatanchors.)
>
> DISCLAIMER: I don't work for or have a monetary interest in Radio Shack or
> EBay. (I do contribute to their bottom line on occasion!)
>
> Pat
> wa4tuk
>
>
>
> > -----Original Message-----
> > From: tentec-admin@contesting.com [mailto:tentec-admin@contesting.com]On
> > Behalf Of Paul (ve1dy)
> > Sent: Thursday, December 05, 2002 7:31 PM
> > To: Bill; Tentec Reflector
> > Subject: Re: [TenTec] RFI Problem
> >
> >
> > Hi Bill...
> >
> > The Corcom filter really worked for me.
> >
> > I wrote a little page about it on my web site at
> > http://www3.ns.sympatico.ca/ppike/Filter.html
> >
> > This may not be the answer for everyone, but it sure worked for me, and
> > by the sounds of it, for others as well.
> >
> > 73, Paul (VE1DY)
> > Halifax, NS
> >
> > ps... love my 516!
> >
> >
> > --- Bill <fla.pi@verizon.net> wrote:
> > > I have always tried to keep my ham radio separate from my computer
> > > because I play on my radio and work on the computer.
> > >
> > > Since putting a computer near my radio I have noticed that my
> > > computer monitor is providing RFI for my ham rig.
> > >
> > > I know its the monitor. The pitch and intensity of the RFI changes as
> > > I change images on the monitor.
> > >
> > > Monitor is a 19" Compaq, rig is a Ten Tec Delta II.
> > >
> > > Any advice?
> > >
> > > Bill
> > >
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> > > http://lists.contesting.com/mailman/listinfo/tentec
> >
> >
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>
>
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