Hi, I've got a couple of queries here, and both relate to the calibration of TDR (Time Domain Reflectometer) or more specifically, verifying the accuracy of them. A performance verification if you like rather than an actual calibration.
Firstly, what would be the best method of verifying the accuracy of a TDR ?
Up until now I have been using fixed coax cable lengths (40m, 85m, 350m) I've used these cable lengths as a reference to test the TDR against. How do I know exactly what length the cables are? well...
I verify the cable lengths using the following method.
1. I physically measure a short 1m length of the same type of cable.
2. I then measure it's electrical length using a Fluke DTX cable analyser, in this case velocity propagation is 0.67
3. I then note the error between what I physically measured, and what I measured electrically.
4. I then measure my reference lengths (40m, 85m, 350m) using the Fluke DTX and apply the error correction.
5. That then gives me the length of my reference cables
The problem I can see with this is that Physical length and Electrical length are two different things. it might physically measure exactly 1m, but electrically could read differently. It's no good knowing exactly how long my reference cables are (Physically) if I am using them for a (electrical) length test. I need to know the precise electrical length of the reference cables. I guess that's independent of any physical measurement or reference.
That leads me to my next question,
Assuming the reference cable length is best method to use. What is the best method of accurately measuring the length of my reference cables ?
Would I be better off using simple maths along with a pulse generator and counter to simulate a physical cable ? The outgoing pulse triggers the pulse generator, and it sends a delayed pulse back. Pulse delay is calculated according to the length and propagation factor you want to simulate. Counter verifies accuracy of the pulse delay.
I've heard a Vector Network Analyser can measure cable length far more accurately than a TDR ?