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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
