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

Charles Steinmetz csteinmetz at yandex.com
Fri Jul 25 19:48:52 EDT 2014

Stefan wrote:

>However, when making shield terminations, many would tell you that 
>it is important to terminate the shield 360 degrees to the chassis 
>in order to have the best effect. Surely, that also has to be true 
>for RF frequencies when you choose to raise the near-DC impedance of 
>the shield-to-chassis connection to combat a hum loop?
>I'm just trying to take this to the logical conclusion, which would 
>be an annular capacitor for the shield connection.

Be careful, taking things to their logical conclusion often sets up a 
reductio ad absurdum.

In theory, annular bypassing may be better.  In practice, it doesn't 
seem to make any difference.  Which probably explains why nobody 
makes annular shield-bypass capacitors.

Note that there are two phenomena here.  We have been talking about 
the *conducted RF* case where RF current on the shield (in 
particular, RF current from the signal being carried inside the coax 
that appears on the outside of the shield due to imbalance) needs to 
be bypassed to chassis.  In this case, bypassing (or directly 
connecting) one point on the shield to the chassis works fine.  The 
other case is preventing *radiated* RF unrelated to the signal being 
carried by the coax from entering the chassis.  If there is a gap 
between the shield and the chassis, radiated RF will be admitted 
according to well-known rules.  Annular shield bypassing can help 
this case, but again -- that is not what we've been talking 
about.  In any case, the tiny gaps produced by shield-isolated 
connectors will not admit much RF energy at any sane frequency and 
field strength.

Best regards,


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