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

[Chuck Harris]

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