Electronics > Projects, Designs, and Technical Stuff
Cable tester/TDR strange result
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RoGeorge:
The simulation is unrelated with what I was talking.
StillTrying:
Does the time delay accurately give the cables length, I make it 4.4 m.
Could the cable's dielectric be that bad with a ~2V change in voltages, somehow.
It must be the 74LVC14's 3.3V changing between the open circuit and short circuit tests. :)
jmw:
I think I found the culprit: it's bad power! I probed the LDO regulator output and it's trash; not flat at all. I bodged a connection from the 3V3 net to a lab power supply and the voltage levels are the same for both open and short. Now I need a find a better LDO that doesn't tap out at 50 mA...


tggzzz:
Do you have any comments on figures 14 and 15 in the NCP551 data sheet?
jmw:
I can see now why there's a problem from those charts. The average current draw from the shorted configuration was 50 mA, and the duty cycle is close to 50% so it's switching about 100 mA.

I haven't done a wide survey of LDO parts. That was a part family I've used before, so I will look for parts with faster load regulation and also ones with a higher max current so I'm derating it sufficiently. Any recommendations off hand?
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