[time-nuts] Noise and non-linear behaviour of ferrite transformers

Ackermann, John R jra at febo.com
Thu Jul 24 12:00:28 EDT 2014

On 7/20/2014 6:41 AM, John Miles wrote:

> I often find that when I use coaxial baluns to cut down on ground loop
> noise, I end up with more noise and interference than I started with.  Not
> always, but often enough that I'm leery of them.
> Due to skin effect, most signal propagation in a coaxial cable takes place
> between the outer surface of the center conductor and the inner surface of
> the braid.  Ideally, the outer surface of the braid just underneath the
> jacket will act like an equipotential shield to keep external EMI away from
> the signal path inside the cable.
> But that's really only true when the cable connects two devices in
> well-shielded enclosures that are themselves at a similar ground potential.
> When you "lift the ground" with a coaxial balun such as an FTB-1-1+, you can
> no longer pretend that the coax braid is at ground potential along its
> length.  From an RF perspective the braid is floating at one end, which
> makes it an antenna.

Just FWIW, the TADD-1 uses transformers to provide DC isolation, but the 
shield side of the coax goes to ground through a 0.1uF cap.  The hope is 
that this reduces the issue that John's referring to (and which I've 
seen plenty of times using baluns).


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