Quote:
Originally Posted by gingpeakin
You can send a square edged pulse down the line with a pot on the end and adjust for the cleanest pulse with scope and a t-piece.
It helps but it's not vital if the sending end has roughly the same impedance as the pot setting. Use a nice low value cermet trimmer.
Ging
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Yes, I've done exactly this too. I used a nice 1k log pot that I found in a drawer at work, and put it in a small diecast box with a BNC socket - it lives at work and students use it during practicals.
Why 1k log?
It means that 50 ohms and 75 ohms are easy to adjust, but by turning it fully clockwise, you can easily and quickly get to what is close enough to an open-circuit for the purposes of demonstrating strong reflections to students. Of course, if you use a lin pot, then 50 and 75 ohms will be cramped down near the CCW end stop.
Having adjusted the pot (I suppose I should say "rheostat") for zero reflections, a DC resistance measurement of the pot with a DMM gives a value that is within an ohm or two of the cable spec, which is more than good enough for determining what type of cable it is.
I've tried the inductance/capacitance method plenty of times before, and find the variable resistance box gives more accurate answers more quickly. My difficulty is that I have 3 ways to measure inductance and capacitance at the sorts of values we're talking about. However, all 3 methods give slightly different answers! This, of course, is the classic problem with having more than one meter
By contrast, the variable resistance method feels more "direct", simply because by killing all of the reflected pulse, you've - by definition - correctly terminated the cable. Job done
The source impedance doesn't really make much difference. If the pulse is fully absorbed in the load, then there is nothing coming back needing to be terminated by the source impedance. There are plenty of instances where the source impedance is not the same as the transmission line and termination - the classic examples being transmitters, where losing power in Zout would not be welcome.
Only caveat: don't use a wirewound pot. I'm sure that's stating the obvious for 99.99% of us
I have thought about building a simple resistance meter into the box - perhaps operated via a changeover switch, or indeed by using a dual-gang pot (assuming the tracking is good enough) to give a live value update. Suitable panel-mount DVM modules are cheap enough from the usual sources. But KISS wins every time - especially when teaching.