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

Bob Camp kb8tq at n1k.org
Sun Jul 20 17:09:36 EDT 2014


Hi

The “coax is an antenna” problem comes in well before you get to DC. Even with no transformer involved, the skin depth of the coax shield gives up well above 60 Hz (and likely well above 100 KHz). If you want to do full isolation over a very wide range you need some combination of shielding and balanced lines. 

Bob

On Jul 20, 2014, at 2:02 PM, Tim Shoppa <tshoppa at gmail.com> wrote:

> Common mode choke works fine for RF, hard to reject 60Hz ground loop
> currents using ferrites. (Although there is a website I found once where
> the guy shows an entire bucket of miscellaneous ferrites cobbled together
> for a common mode choke!) For a DC-to-daylight instrument that is suffering
> 60Hz artifacts as a result of ground loops, 60Hz+harmonic reject filters
> seem appropriate and I've seen implemented in both digital and analog
> domains.
> 
> For DC-only digital instruments it is common to choose a sampling frequency
> to reject 60Hz noise. This was being done in the 70's with the very first
> bench DMM's so I'm sure the patents have expired by now.
> 
> Tim.
> 
> 
> On Sun, Jul 20, 2014 at 12:52 PM, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> The simple way to take care of the coax shield issue is to use a common
>> mode choke.
>> 
>> Bob
>> 
>> On Jul 20, 2014, at 12:16 PM, Attila Kinali <attila at kinali.ch> wrote:
>> 
>>> Moin,
>>> 
>>> On Sun, 20 Jul 2014 03:41:44 -0700
>>> "John Miles" <john at miles.io> 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.
>>> 
>>> [...]
>>> 
>>> Thanks for the explanation. Now everything makes a lot more sense!
>>> 
>>> If i understood you correctly, then the right thing to do would be
>>> either to use triax (single core, two isolated shields) with the inner
>>> shield as the ground point for the balun, while the outer shield is
>>> connected to the transformers shield or left open. Or alternativelly
>>> use some differntial, shielded cable.
>>> 
>>> Apropos: does anyone know how "good" Cat5e/Cat6 cable is for timing
>>> applications? Ie, how large is the phase shift on bending or temperature
>>> and humidity change?
>>> 
>>>                              Attila Kinali
>>> 
>>> --
>>> I pity people who can't find laughter or at least some bit of amusement
>> in
>>> the little doings of the day. I believe I could find something ridiculous
>>> even in the saddest moment, if necessary. It has nothing to do with being
>>> superficial. It's a matter of joy in life.
>>>                      -- Sophie Scholl
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