BNC questions

[Tom Dressel]

All the NIM components dealing with signal manipulation are connected in a chain with BNC connectors. Is there anything special about the cables or connectors? (Other than the special hige voltage BNC connectors). I assume that the cables should be as short as possable within reason.

Can I use Radio Shack stuff? Is soldered better than crimped.

Everyone talks about proper termination, don't essentially all the modules have a 50 Ohm input impedence?

Tom Dressel

[guest]

Any junk BNC cables will do. They abound at hamfests. Some of them are real primo gov't items. Ideally you need 50 ohm coax (RG-58U) however, 75 ohm video cables (RG-59U) will suffice for all but the most critical apps.

Crimped BNC cables are prefered, but soldered is OK.

One of the nice things about 50 ohm lines is that as long as you play by the rules (terminate, supply proper cables, etc), the length of the cable is pretty unimportant. Longer cables introduce delays between modules, but at our level of usage, a 6 inch cable is as good as a 6 footer. A crude estimate is 1 nanosecond delay per foot of cable. I personally prefer 12" cables and if one won't reach, I move the module to a slot where it will. I just got a bunch of top drawer 36" milspec 50 ohm silver braid cables at a recent hamfest and will soon use those over all others.

It is true that most NIM modules outputs are terminated in the characteristic impedance of 50 ohms, but for hooking up scopes and other external stuff beyond the bin, you should terminate at that location with a BNC "Tee" with 50 ohm terminator or you'll not necessarily see what the NIM amp sees. Likewise, the last module in a NIM daisy chain should be terminated also. In short, if you have a NIM module with a 50 ohm output feeding any other device which ends there and doesn't "loop" over to another unit, then you must terminate at the input of that last module. All of this is much more critical in timing situations such as coincidence detectors, delay line amps and the like.

Richard Hull

[Tom Dressel]

Thanks Richard

I got everyyhing I need to make the cables at Radio Shack, including the terminators.

You said to terminate at the INPUT of the last NIM component, not the OUTPUT of the last component, even if the last component already has an intrinsic INPUT impedance of 50 Ohms? Or maybe I am confused, do the NIM components have an input impedance of 50 Ohms, or output impedance of 50 Ohms, or are both 50 Ohms?


Tom Dressel

[Richard Hull]

Most all Nim modules have input impedances of about 1k ohm to 10k ohms. (reduced loading). Many have outputs with a 50 ohm resistor in series. (50 ohm output impedance). This matches the driving line or coax impedance. All lines must also end in their characteristic impedance to avoid reflections, and un balanced reactive loadings which can alter waveform edges or create ringing. Again, all this is hyper critical in ultra fast lines where absolute fidelity of the original pulse must be maintained. For simple, smaller NIM system lashups, this is not too critical.

50 ohm terminators can be obtained from digi-key, Newark electronics or any of a number of electronic jobbers in big cities. Hamfests have them at the booths manned by the big wire and cable sellers.

You can roll your own by wiring a 47 ohm resistor across a male BNC connector.

Richard Hull