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

Bob Camp kb8tq at n1k.org
Sun Jul 20 13:40:47 EDT 2014


Hi

With the Cap to ground, you “short out” the transformer at RF. The resistor takes care of static, and if it’s low enough shorts out the transformer at lower frequency. 

More or less - you break the 60Hz ground loop (sort of). You do very little for RF isolation. 

Bob

On Jul 20, 2014, at 11:38 AM, Ed Palmer <ed_palmer at sasktel.net> wrote:

> I've seen a few pieces of equipment that use a transformer-coupled output and an isolated BNC jack to break any ground loops.  Then they connect the shield to the chassis with a parallel RC network. The C might be in the 1-10 nf range while the R is a few hundred ohms.  I know of one piece of equipment that doesn't use an R at all.  There has been a bit of discussion about this in the past.
> 
> Does this sound like a valid way to avoid the antenna effect?
> 
> Ed
> 
> On 7/20/2014 4:41 AM, John Miles wrote:
>>>> All that said, the real hazard with transformers is that people tend to use them to drive unbalanced coax cables with balanced signals.  This turns the coax shield into an antenna, at which point you may end up with with more noise and spurs than you had before.
>>> Could you explain this a little bit more? Because this would be exactly
>>> what i would like to do.
>> 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.
>> 
>> Put another way, a balun will reject common-mode signals in favor of
>> differential signals.  That's fine if you're using it with a twisted pair or
>> other balanced line (you're probably aware that this is how UTP Ethernet
>> cables work).  RF interference in such a line is picked up equally by both
>> conductors and rejected by the balun.  But a length of coax cable is as far
>> from a balanced line as you can get.  One conductor is well-shielded, the
>> other has its outer surface flapping in the breeze.  The balun can't tell
>> the difference between desired signals on the inside surface of the braid
>> and undesired signals on its outside surface.  They both look like
>> differential-mode signals, relative to the inner conductor.
>> 
>> The same thing happens with instruments that allow you to lift the ground at
>> their input jacks.  Apart from the unwanted-antenna effect, this is almost
>> always a bad idea because it's very hard to properly ground the jack's outer
>> shell to the chassis.  Few things in EMC are more important than ground
>> integrity at the point of entry to an enclosure.
>> 
>> When fighting ground loops, a good first step is to minimize the loop area
>> if you can.  Try plugging your DUT, reference, instrumentation, and computer
>> into a single power strip.  That will take care of most of your power-line
>> interference problems.  Baluns can help too, but don't be surprised if they
>> don't.
>> 
>> -- john, KE5FX
>> Miles Design LLC
>> 
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