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Above is how I set up a pulse in LTspice.

When I set the rising and falling edges as zero or if I do not register any value, in that case LTspice automatically sets the rising and falling edge duration. In above case it is 25ms for a 2Hz pulse. So if I want a very sharp pulse I have to each time type something like 0.0000001 instead of 0.

Can there be a reason for this default settings or is this a bug?

  • \$\begingroup\$ Don't know Linear Tech's rationale for the default values, but you can use unit labels instead of typing all the zeros. For example, instead of writing 0.0000001 you can just write 100ns (or 0.1us) \$\endgroup\$ – Dan Laks May 29 '16 at 23:46

is this a bug?


Can there be a reason for this default settings

Yes. This is explained in this FAQ on the LTwiki website:

I have a pulse source in my schematic with zero transition times. LTspice only shows slow transition times of 2ns. What's going on here?

It explains:

LTspice automatically will use a default value for <trise> and <tfall> if these parameters are set to zero. Default value: 10% of Ton or 10% of Tperiod-Ton whatever is smaller. You must specify Trise and Tfall if you want a certain value.

  • \$\begingroup\$ could that be something to do with real life function generators? this %10 approximation I mean. \$\endgroup\$ – user16307 May 30 '16 at 0:52
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    \$\begingroup\$ I have an opinion, but I see no point in guessing about someone else's program :-) They have made a sensible default, when someone chooses a non-sensible input like a zero rise time :-) \$\endgroup\$ – SamGibson May 30 '16 at 1:00

Setting rise and/ or fall times to 'zero' is going to call up 'default' values every time. Zero is an irrational value for rise and fall times or even slew-rate. The software will not allow irrational values to be entered into the program.

Zero rise and fall would cause no vertical lines on your screen, so would zero slew rate. Be glad the software is not programmed to scold you for doing such things. ALWAYS insert rational values (>0) for rise time, fall time, and/or slew rate.

  • \$\begingroup\$ Yes I understand that, if it were zero dV/dt would be infinity hence rise time shouldnt be zero but what I dont get is the default setting is not short enough. It is 20ms for 2Hz. \$\endgroup\$ – user16307 May 29 '16 at 23:47
  • \$\begingroup\$ Your program should have a screen for default settings. If so, make them rational settings. If it does not offer that option, review the users manual or contact tech support. You will eventually find out what you can and cannot do with your program. \$\endgroup\$ – Sparky256 May 29 '16 at 23:59

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