Experimenting with a homebrew time domain reflectometer

Have a big coil of wire you don’t know the length of? don’t bother measuring it, just measure how long it takes for a signal to go through it and multiply by the speed of light and there you go.


Here is a simple one chip time-domain reflectometer. It consists of a pulse generator that can drive pulses with a rise time on the order of a nanosecond. I hook it to just one end of the unknown wire and set it off, the first step you see on the screen is my generated pulse. the next step is my signal being reflected off the other end of the cable and coming back to hit me. I have the meter set to 25 nanoseconds a division, so it took about 75 nanoseconds for the signal to come back, rule of thumb for the speed of light is a foot a nanosecond in vacuum and about 2/3 of that for an electric signal in copper. so 75 feet * 2/3 = 50 feet, and since the signal had to go down the wire and come back, you end up with 25 feet of wire in my unknown length coil.

The circuit is based on the one on this page  http://www.epanorama.net/circuits/tdr.html which also has more discussion on how a TDR works.

Comments 1

  1. Bob wrote:


    I am working with TDR and I have built a circuit with a coax (with short termination) to measure the reflection from the mismatch. I used a 7GHz Transistor to generate a fast pulse. The rise time is 10ns.

    But in this case, I have pure water to test and I cannot see the reflection from the water-air Interface but only the end of coax generates a reflection. I had an idea to Change the Transistor to higher frequency.

    And also to verify my results, I would like to know the S-parameters for thhe reflection and Absorption.

    I request your advice on this.


    Posted 25 Feb 2015 at 4:22 am

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