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1503 TDR

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Chris.benoit
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Joined: May 12th, 2011, 2:44 pm

1503 TDR

Post by Chris.benoit » May 12th, 2011, 3:31 pm

I would like to know if there is a formula to determine the VP factor for cable lengths. I have setup a controlled experiment to help me determine this but it just doesn't seem to add up. I am using an Tektronix 1503B TDR which hasn't been calibrated since 2000 but I have to work with what I got.

I have referenced my 100' test lead so the marker is at zero before the cable to be tested.

a 35' piece of 500p3 cable has a propagation factor of .84
a 85' piece of the same type of cable I have a propagation factor of .88

I was under the impression the propagation factor shouldn't have to be changed once the type of cable has been determined, am I wrong?

Chris B
Tektronix Applications
Tektronix Applications
Posts: 657
Joined: December 17th, 2010, 3:17 pm

Re: 1503 TDR

Post by Chris B » May 16th, 2011, 8:02 am

Yes, your Vp should be the same for both cables, if the same type. Vp is derived from the capacitance and inductance in the cable itself, so if one of the cables has some imperfections in it, this may explain the differences in results you are seeing. Although it is possible that your unit has some error in it, I would suspect that it would be relatively the same regardless of your cable length.

There is some discussion of the relations bewteen L, C, Vop, etc. here:
http://www.-.com/technical/paper2.html

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