A quick question,

I have this pulse as an input to some device: Input pulse

I just need to simulate it in LTSpice. I have the values of voltage obviously, and I know how to simulate that fall time. I just need to know how to simulate that spike at the beginning of the pulse.

Edit: Here is a close up of the spike, measured:

enter image description here

Thank you!

  • 1
    \$\begingroup\$ PIecewise-linear source function. You're just going to have to make a guess about how long that spike lasts --- is it 2 us or 200 ns? \$\endgroup\$ – The Photon Nov 25 '18 at 6:12
  • \$\begingroup\$ I'm sorry but do you mean a voltage source in series? how can I do what you mean? About the spike, I assume I'll guess, I don't know if it matters that much. \$\endgroup\$ – Eran Nov 25 '18 at 6:20
  • \$\begingroup\$ Okay so I've thought about what you said about the spike so I added an image of it as well... \$\endgroup\$ – Eran Nov 25 '18 at 6:25

You could try something like this:


Where you can use the measured values (overshoot and ripple) to determine the frequency and damping. \$M\$ is the peak overshoot, \$\zeta\$ is the damping.

$$M\approx 1-\frac{\zeta}{0.6}$$ $$\omega_d=\omega_n\sqrt{1-\zeta^2}$$ $$T=\frac{2\pi}{\omega_d}$$

Note that your sampling frequency is not quite enough to precisely determine the parasitic oscillation, so you will have to rely on your "guts", as well. I used the dice to throw some values there, for exemplification. Also, I exaggerated a bit with the very small value for the rise time, compared to the period, but that's up to you to test. Optionally buffer the output if you're not comfortable with that parallel RLC driving whatever it is that comes afterwards.

You could also use a PWL source, as suggested in the comments, but that will onyl give you what you see on the oscilloscope, as oposed to what actually is. If you're fine with that, godspeed.

  • \$\begingroup\$ First, thank you for your answer. I don't really understand how I can find the values of these variables though. is M measured in volts? is damping measured in seconds? And what is w? 2*pi*f? \$\endgroup\$ – Eran Nov 27 '18 at 7:07
  • \$\begingroup\$ @Eran I would've edited my answer, but it may be more affective if you searched for "2nd order time response" and follow up some of the links, because it really is a bit of explaining. What I did is a mashup, a quick example, but another possibility is to simply use a voltage source with a lowpass RLC, set up properly. And all this implying that the overshoot you have is 2nd order, not more. Still, it's not a bad path to follow. \$\endgroup\$ – a concerned citizen Nov 27 '18 at 7:59

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