Hello I am trying to understand the behaviour of a simulation of a simple transmission line.

Here is the Transmission line in question : enter image description here

The behaviour is quite confusing to me so I would like if someone explained it to me.

Here is the response. enter image description here

The thing that confuses me is why at the node Line_start the voltage is 3.6V. I was under the impression that this is obtained through a voltage divider of R1 and Zo, which be 0.5, hence the voltage should be 5V.


1 Answer 1


why at the node Line_start the voltage is 3.6V

The voltage source is only high for 10ns; Line_Start doesn't have enough time to reach the ultimate value. This is because of the distributed capacitance and inductance of the TL.

Note with a Td of 4µs, this means the line is simulated to be quite long! At 75% of the speed of light (guesstimating speed in simulated transmission line), it would travel 900m in that much time.

Increase the "on" period and this will become 5 Volts. Zoom in on that initial pulse and it should be evident what is happening.

Careful though, with an "open end" it will reflect a lot of that energy (you are starting to see a reflected pulse already.) Make R2 50Ω to adsorb all the reflections.

  • \$\begingroup\$ Thank you for your answer. How did you calculate 3km? I am very curious. My point is indeed to simulate an open circuit. \$\endgroup\$
    – DRF
    Commented Jun 7, 2023 at 21:18
  • 1
    \$\begingroup\$ Found a math error, fixing. \$c\$ = 300km/s, so 75% of C is 225km/s. \$4\text{µs}\times 225\text{km/s} = 900\text{m}\$. \$\endgroup\$
    – rdtsc
    Commented Jun 7, 2023 at 21:29
  • \$\begingroup\$ Thank you for the detail \$\endgroup\$
    – DRF
    Commented Jun 7, 2023 at 21:38
  • 2
    \$\begingroup\$ I don't think this is right. Line_start should ramp to half the value of the ideal voltage, which would be 5 V, in 2 ns (the rise time). Line_start would not reach it's final value until the reflection comes back from the open, 8 us later, but the voltage source needs needs to remain high for at least that long. If OP changes the high period to 10 us or longer, he should see the effect (the plateau) I think OP is looking for. \$\endgroup\$
    – SteveSh
    Commented Jun 7, 2023 at 22:11

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