# TDR unexpected results

I am using a signal generator to generate a long square pulse that is fed into a long transmission line consisting of two parallel wires. The transmission line is approximately 1km long. The near end is terminated with a 25$$\\Omega\$$ resistor, and the far end is unterminated. With an ADC I am sampling the incoming signal.

In the below diagram you can see the square pulse (1) I am feeding into the transmission line. (2) shows the signal I measured with the ADC. You can see that the first plateau is at U/2. The 2nd plateau is really small and almost not there.

I would have expected a incoming signal like shown in (3). I don't understand why the plateaus shown in (2) are not at the same voltage level.

Does anyone have an idea?

In the diagram I neglected the attenuation of the cable.

• How is your cable terminated? How long is your cable? What is the design of your cable, i.e. coax, twisted pair, two wires surrounded by copper? Commented Dec 26, 2020 at 13:57
• @MathKeepsMeBusy two parallel wires laying side by side. Around 1km long. The cable has no termination at is ends, it is open circuit Commented Dec 26, 2020 at 14:07
• It is a real cable Commented Dec 26, 2020 at 14:08
• The termination is at the beginning of the transmission line. At the end there is no termination Commented Dec 26, 2020 at 14:09
• @Yoomo did you manage to get microcap working? Commented Dec 26, 2020 at 14:11

When the far-end reflection returns, it causes a source reflection due to mismatch between source and cable impedance.

If you use thinner pulses, you can more easily see what happens: -

The simulation above uses a source impedance of 25 Ω and a line that is 50 Ω with an open far-end. The line is 10 metres long and is loss-less. If I make the pulse 200 ns you see this: -

Now, because you have reflections overlapping the original and reflections overlapping reflections, it becomes that much harder to analyse but, all the information is in there should you be prepared to work at it.

If I made the pulse 300 ns, the picture changes again but, the basic information is all in there: -

To change the cable parameters in microcap you need to access this by double clicking on the transmission line part and editing the line highlighted: -

This line has a length (LEN) of 5 metres with C/m and L/m also stated.

• Thanks for your insights! Btw. I couldn't install microcap. I went with a demo version but I can't change the parameters of a transmission line Commented Dec 26, 2020 at 14:37
• @Yoomo That's a shame. I have run the demo version in the past but I could change the parameters. I'll embed into my answer a picture that might help. Commented Dec 26, 2020 at 14:47