[time-nuts] SMD TADD-1 distribution amplifier
Charles Steinmetz
csteinmetz at yandex.com
Fri Dec 18 12:27:33 EST 2015
Poul-Henning wrote:
>A significant reason for the TADD-1 existing in the first place was to
>break groundloops. This is incompatible with tying all the BNC's together.
It is perfectly possible to ground coax connector bodies at RF
without grounding them at power-line frequencies (and expected
harmonics) or DC. Indeed, that is what the TADD-1 attempts to
do. The BNC bodies are connected to the enclosure with 10nF capacitors.
However, the implementation in the TADD-1 is not as ideal as I'd
like. The bypass isn't directly to the enclosure, but rather to the
ground plane of the PC card, which appears to be connected to the
enclosure only by fasteners at the corners of the PCB -- some inches
away from the BNCs. That is why I will almost never approve a design
that uses isolated, PC-mount coax connectors. The loop from each BNC
body, through the capacitor, to the enclosure should be no more than
1/2" (~ 1cm) long, preferably less.
I like to add a resistor in parallel with each capacitor, to shunt
any unexpected DC or LF leakage current. This resistor needs to be
large enough (in value) to limit any circulating ground currents to
low values that will not cause significant voltage drop in the
shields (i.e., ground loops), but at the same time will effectively
terminate any expected leakage currents. Generally, this will be
from 10 to 100 ohms, depending on the sensitivity of the most
sensitive inputs in the system and whether guarding is employed.
Best regards,
Charles
More information about the time-nuts
mailing list