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

Chuck Harris cfharris at erols.com
Mon Jul 21 00:18:58 EDT 2014

Lots of meanings to the word induce.  The one I was using was: to
bring on, or about; effect; cause...  I was not intending to imply
transformer action.  As an engineer I should know better than to
try to use English to describe electrical phenomenon.

My intention was to find out what: " ...the skin depth of the coax
shield gives up well before 60Hz..." meant.

Real life examples show that coax does just fine at shielding all
the way down to DC...  As long as you keep the currents flowing
through the outside of the shield to a minimum.  Which is done
routinely by not connecting the ends of the shield so that current
loops occur.  Hardly a PA system exists that doesn't have a fairly
long bit of unbalanced shielded cable at some high impedance high
gain input.

-Chuck Harris

Alexander Pummer wrote:
> No, the current passing the outside f the shield  will not induce any voltage inside
> of the coax, but the voltage drop caused by the current on the ohmic resistance [!!!]
> of the shield will show upbetween the two ends of the cable -- and that will  show up
> as  it was added to to the voltage which is carried on the center conductor of the coax.
> 73
> Alex
> On 7/20/2014 6:10 PM, Chuck Harris wrote:
>> I'm not sure what you are saying.
>> skin depth = (2.6/sqrt(fhz))inches for copper.
>> So, at 60Hz,   skin depth = 0.336 inches.
>> and at 100KHz, skin depth = 0.008 inches.
>> and at 1MHz,   skin depth = 0.0026 inches.
>> Are you saying that at 60Hz, because the
>> skin depth is deeper than the coax shield is
>> thick, that current passing through the outside
>> of the shield will induce voltage inside of
>> the shield, and that at say, 100KHz where the
>> skin depth is a little less than the shield
>> thickness, or at 1 MHz, where the skin depth
>> is only a small fraction of the thickness of
>> the shield that it won't?
>> Or something else?
>> -Chuck Harris
>> Bob Camp wrote:
>>> 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

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