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

Charles Steinmetz csteinmetz at yandex.com
Fri Jul 25 14:07:51 EDT 2014

Stefan wrote:

>>shield side of the coax goes to ground through a 0.1uF cap.
>For optimum results with respect to high RF frequencies, I'd expect 
>that you would need this cap to be annular, so that the cable can 
>pass through the middle, and the outside connects to the chassis 
>hole all around.

The coax itself remains coaxial through the connector -- it is just 
the point on the shield that is bypassed to chassis that is not 
annular.  The low impedance of the shield makes this non-critical up 
to frequencies where you should be using a waveguide anyway.  It is a 
well-known and proven technique for bypassing shields to chassis with 
no galvanic connection.  (In some cases, the capacitor is paralleled 
with a resistor of 10 ohms to 1k ohms -- this provides some DC/LF 
continuity while limiting the possible ground loop current to levels 
that [hopefully] don't cause noise problems.)

Rarely is the cap as large as 0.1uF -- 0.01uF is most common, and 1nF 
is also quite common.  You want the cap to have low inductance (high 
first self-resonant frequency), and you need to keep the leads very short.

Best regards,


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