A The first thing to do is simplify the system. The interfering signals could be coming from a variety of sources, including any peripheral devices (printers, monitors and so on).
Start by switching off as many external devices as possible and disconnecting their cables from your computer. If possible, use just the computer, keyboard and monitor. This is probably the most straightforward way to uncover the sources of your interference.
Figure 1—You can make a common-mode choke by wrapping several turns of the offending cable through a ferrite torroid.
You can often suppress the interference substantially by installing common-mode ferrite chokes (see Figure 1) at the proper points—preferably as close to the computer or peripherals as possible. Just make sure you use toroids made of Type 43 ferrite material. Figure 2 shows the points in a typical system where it may be most effective to install common-mode chokes. You’ll find much more information about computer-generated interference— and how to get rid of it—in The ARRL RFI Book.
Figure 2—You don’t have to place chokes at all of the points shown, but these are the most effective locations in a typical station computer system to suppress interference.
From QST December 1999
