[RFI] Computer birdies

Jim Brown jim at audiosystemsgroup.com
Tue Jun 26 12:45:20 PDT 2012

On 6/26/2012 11:49 AM, charlie at thegallos.com wrote:
> I personally found, with my router and computers that the computer was the really bad end, but putting cores on both ends helped too, as there was some low level stuff coming from the router

Yes. Each end of a cable connected to ANY digital electronics should be 
viewed as a potential noise source, but as several have noted, some 
products are much noisier than others.

Steve -- many thanks for posting your careful experimental work. I did 
the same many years ago when I first diagnosed the problem and learned 
enough about ferrite chokes to realize that they might be a solution.

Another point. These Ethernet birdies (most typically around 14030, 
21052, and on the bottom end of 10M CW) are produced by by free-running 
clocks that are synchronized to another clock in the system, which is 
also free-running.  The birdies will be CLOSE to 14030, 21052, etc., but 
the precise frequencies will vary from one Ethernet system to another.  
Once you've gotten your own system quiet you'll start hearing your 
neighbors' systems, each a few hundred Hz from you and each other. You 
can identify which is yours by killing power to your router or Ethernet 

And yet another point.  SOME of this noise may be radiated by wiring 
(and circuit traces) internal to the box due to poor design.  Nothing 
you do to choke the cables will reduce that part of the noise.  You CAN, 
however, further reduce the cable part of the noise by adding a second 
choke in series. A second choke having the same choking Z should help by 
about 6 dB -- but ONLY on the part of the noise carried on that cable.

73, Jim K9YC

More information about the RFI mailing list