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

Bob Camp kb8tq at n1k.org
Mon Jul 21 07:10:28 EDT 2014


If you have an electrostatic field to worry about, a simple shield does a fine job. If it’s a magnetic field - maybe not so much. A propagating electromagnetic field is going to be a bit tough to stop.

Again it comes back to Bert’s question - what is the objective? 


On Jul 21, 2014, at 12:24 AM, Chuck Harris <cfharris at erols.com> wrote:

> Hi Bob,
> I agree, but most of the time, you can use good design
> practices to keep the currents flowing through the outside
> of the shield to a minimum... avoiding ground loops, stuff
> like that.
> Simple coax is used for shielding very high gain circuits
> from 60Hz noise all the time in PA systems.
> -Chuck Harris
> Bob Camp wrote:
>> Hi
>> Yup
>> It is impractical to make coax that has a shield thickness of 1/3”. Even if you
>> do, it’s not going to be very flexible. For a real world system that needs good
>> isolation, coax is not the way to go below 100 KHz. There are a few other issues
>> that come up, but skin depth is a big part of the problem.
>> Another part of the equation is (as Bert points out in another thread) “how good
>> do you need?”. Skin depth simply the point that you have knocked out 2/3 of the
>> current. That probably isn’t what you are after when you ask for “good isolation”.
>> The “inside” of the coax should be below 170 dbm/ Hz to be “quiet” when
>> terminated. If you have -70 dbm / Hz noise signals running around here and there,
>> you need quite a bit of isolation. You might have a spur spec rather than a noise
>> floor spec to meet and that would give you different numbers to go after.  In most
>> cases you will need multiple skin depths (like 10 or more) to get the job done in
>> a noisy environment.
>> Bob
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