[time-nuts] Isolated jacks and ground loops

[Bruce Griffiths]

Corby Dawson wrote:
> Hi,
>
> Can any one steer me to an explanation of the following setup.
>
> I think I understand the capacitor but not the resistor.
>
> In the 106D DMTD unit I have, the BNC jacks are of the isolated variety.
>
> The center pin and the shield both come out to pins on the rear of the
> connector.
>
> On the mixer input a coax runs from these two pins all the way to the two
> floating mixer input pins on the RF side.
>
> So far so good but under the BNC mounting nut is a serrated lug that is
> connecting to the chassis ground.
>
> Connected between this chassis ground and the BNCs shield pin are a 50
> ohm resistor in parallel with a .1uf capacitor.
>
> My understanding is that the capacitor will shunt any RF on the shield to
> chassis ground, but what does the resistor do?
>
> I'm getting the hardware together to change over to SMA on the critical
> connections and ran into this question.
>
> Thanks!
>
> Corby Dawson
>    
Corby

The 50 ohm resistor reduces the low frequency circulating ground current 
due to ground loops as well as ensuring that the the BNC shield stays 
close to chassis potential.
The capacitor and resistor also provide an ESD discharge path from the 
BNC shield to chassis.

DMTD systems are very sensitive to low frequency (eg 60Hz ) ground loop 
currents.

Bruce