On Thursday, July 21, 2022 at 11:52:24 AM EDT, Jim Lux <jimlux@...> wrote:
On 7/21/22 6:12 AM, Magicbean wrote:
Hello - Totally new to VNA here...
I want to measure the characteristic impedance of 120 ohm shielded twisted pair wire. I know that you can measure it for 50 ohm coax using the NanoVNA but I wondered if there was any reason why it wouldn't work for non-50 ohm twisted-pair?? The purpose of measurement is to make sure I fit the best termination resistors for long runs (100m ish) of cable for CAN bus cable.
It should work fine.? That STP has a "reasonably well controlled"
impedance, but i wouldn't be surprised by 5% changes.? It might be most
useful to do the TDR type display, rather than a S11.? Then you can see
the dips and peaks along the cable. (Fun experiment, tie a knot or kink
it in the middle of the cable)
I do have a Siglent SSA3032X plus spectrum analyser for which there is a return loss bridge (RB3X25) but at around ?400. I'm not sure whether that would be useable for that kind of measurement, especially as the cable isn't 50 ohms.
No, that's not the right tool.? The NanoVNA is what you want.
I assume the NanoVNA probably is suitable for measuring the 120 ohm pair and can probably offer more functionality even if some of the specs of the Siglent are rather better!? If so, what's the shortest length of twisted pair I could sensibly make measurements on? I have some 1m samples of cable which I would like to evaluate. The final cable runs will be in the order of 100m.
An interesting question.? The "resolution" in TDR type displays is
determined by the frequency span. So 1 GHz gives you 1 ns (20-30cm)
resolution. The unambiguous range is set by the lowest frequency.
If you want to distinguish the "cable" from the "ends" (both at the VNA
end and the termination end) you probably want the cable to be long
enough that it's several resolution points.? The NanoVNA can go up to
900 MHz, so a 3-4 m piece should be long enough.
Before you get into fixturing and all that, try it with a 2-3 m piece of
50 ohm coax, and see if you can distinguish the termination (i.e. do the
cal with no cable, put the cable on, and then look at TDR with the
short, open, and load at the far end)
BTW I know I can also fire a pulse down the cable and match with a variable resistor when the cable is easily accessible (and I might do that too) but I am interested to know if the NanoVNA would be useful for this and also, in some circumstances, I won't be able to get to the far end of some cables. Would it be necessary to normalise to something other than 50 ohms? Would I need any other components or equipment to do the test (apart form connectors to physically connect)?
NanoVNA does that nicely.
Whether you normalize or not depends on whether you need to have actual
values, or if you're just doing a qualitative test.? If you get a
reflection from the end vs no reflection, you don't really care what the
value is.
I have some other applications for the NanoVNA too, so I will probably get one (I was thinking NanoVNA H4 although there seems to be a bewlidering number of variants). I'd just like to know if it is suitable for this measurement as well. Or... if I could do everything with a Siglent RF bridge, maybe that would be a better focus as I already have the SA.
Thanks.
