Corby wrote:
My NBS 106D DMTD originally had all BNC connectors but after finding out BNCs are not the best for DMTD units I retrofitted it with insulated SMA connectors. I used nylon washers on both sides. See PIX.
Note that this can cause problems with RF noise and interference (both ingress and egress) if it is not carefully done.
The problem is that RF can get into (or out of) the enclosure on the shield of a connected cable, on the signal conductor(s) of a connected cable, or (usually) both. One needs to bypass (short/shunt) the RF energy on the cable shield and and on the signal conductor(s) to the instrument's RF shield (i.e., the conductive enclosure) *without* causing appreciable ground loop currents to flow between the instrument and other instruments and devices connected to it (DC and LF -- usually the first 10-30 harmonics of the mains frequency ).
This is a much more involved topic than can readily be covered here, but there are some fundamental principles.
Generally speaking, it involves (i) connecting the isolated cable shield(s) to the instrument enclosure with low-impedance RF paths (e.g., capacitors) *very close* to the connectors; (ii) introducing high-impedance RF paths in series with the signal conductors (e.g., series resistors, chokes, ferrite beads, and/or common-mode RF chokes); and (iii) bypassing the signal conductors with low-impedance RF paths (e.g., capacitors) *very close* to the connectors, on the instrument side of the high-impedance series RF element(s).
As the frequencies of concern get higher, connecting the isolated cable shield(s) to the instrument enclosure effectively with low-impedance RF paths gets harder and harder. I often use two or three capacitors equally spaced radially around the connector as a useful approximation. Leads must be kept *short* to minimize the effective antenna loop -- 2mm or less for high VHF to low-GHz signals (or use SMT parts to build out the connector-to-enclosure link).
There is another approach, which I often prefer. You can bond all connector bodies (cable shields) together on the smallest area of the enclosure (RF shield) possible, then make sure you don't make *any* mistakes in the entire system that create ground loops. When done correctly, this method can often provide superior results. I'll try to post a general overview of that solution in the future, modeled on the above -- but there are far more variables and opportunities to go wrong on that side of things, so the designer must be correspondingly more diligent.
Best regards, Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
