Length transmission line in ltspice - Page 1
Forum section: electronics / beginners

author: moonz
author_rank: Contributor
timestamp: May 20, 2021, 03:31:09 pm
content: Hello every one!I tryed to simulate transmission line with parrametrs:Code:.model TL1 LTRA(LTRA(R=10m L=10n C=5p len= 1)But I can't find anywhere, the unit of measurement for length.According International System of Units, length should be measurment in metter. But if look at the waveform, you can see that it is not so. Becouse if signal propagation at thee speed of light, the differnt should be 1/3e8[m/s] = 1.3 nS. But it is not so!What is the unit of length in the model LTRA?

author: Zero999
author_rank: Super Contributor
timestamp: May 20, 2021, 04:13:01 pm
content: In LTSpice the lossy transmission line is modeled as a repeating pattern of, resistors, conductances, inductors and capacitors. The length is defined as the number of repeating units, not the physical length of the line.

author: jmw
author_rank: Frequent Contributor
timestamp: May 20, 2021, 04:40:15 pm
content: And because SPICE's R, L, and C elements have no physical extent, it can't model propagation delay. LTRA is for modeling transmission line loss; TLINE can model delay but is lossless.

author: penfold
author_rank: Frequent Contributor
timestamp: May 20, 2021, 05:03:18 pm
content: Quote from: jmw on May 20, 2021, 04:40:15 pmThe propagation delay will still be apparent with an RLC network, but perhaps using the R, L and C (90pF) in their per meter units, divide by 100 and use a network 100 elements long, itshoudlshow a propagation time so long as it is a true RLC equivilent circuit

author: moonz
author_rank: Contributor
timestamp: May 20, 2021, 07:32:56 pm
content: QuoteI agree with you about losses. But about the fact that the length is the number of repeating units, it is not so. I tried to simulation the same thing but from the usual RLC. What happened, see attached.QuoteIt looks like the truth, but the signal is still slightly distorted.

author: sigma_xi
author_rank: Contributor
timestamp: June 27, 2023, 10:38:42 am
content: QuoteThe problem is most likely your maximum timestep of the tran simulation, hence the signal looks distorted although it isn't. Try decreasing the maximum timestep to, say, a few picoseconds.

author: Ian.M
author_rank: Super Contributor
timestamp: June 27, 2023, 12:07:14 pm
content: Any time you suspect LTspice may be lying to you when zoomed in, temporarily turn on 'View' > 'MarkData Points' in the plot context menu and see how few sample points that trace is drawn from!Turning off waveform compression withCode:.opt plotwinsize=0can help, but the down-side is that can result in *VERY* large .raw files and slow plotting, so you may need to pause 'MarchingWaveforms' so it doesn't waste time redrawing the plot unnecessarily, and even reduce the number of traces you are plotting simultaneously.

author: radiolistener
author_rank: Super Contributor
timestamp: June 27, 2023, 12:07:32 pm
content: Quote from: moonz on May 20, 2021, 07:32:56 pmit seems that your signal is ok, just a poorly chosen simulation time step

author: WatchfulEye
author_rank: Regular Contributor
timestamp: June 27, 2023, 09:23:47 pm
content: Quote from: moonz on May 20, 2021, 03:31:09 pmThe same unit as your other parameters!If L is H/m and C is in F/m then Length is given in meters. If L is H/in and C is F/in then length is given in inches.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 28, 2023, 12:28:11 pm
content: Isn't it in nanoseconds rather than metres (3 feet and a bit USA)?

author: WatchfulEye
author_rank: Regular Contributor
timestamp: June 29, 2023, 04:40:39 pm
content: Quote from: Terry Bites on June 28, 2023, 12:28:11 pmThat's the lossless transmission line, which takes a delay time.The lossy model (LTRA) takes the LRCG parameters per unit length, as well as the length.