Cant get zero rise time in LT spice?

Thread Starter

MikeJacobs

Joined Dec 7, 2019
226
So I know such a thing doesn't exist, but for the purpose of testing something else, I need to get a pulse out of a voltage source that has essentially zero rise time.

I am finding that leaving the rise and fall time on a pulse voltage source give me 10ms rise and fall no matter what I do?

How do I get a pure 0 rise time pulse in lt spice?

Thanks all
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
226
ok well please disregard!
This is one of those posts that I found the answer like 5 seconds after I posted it. Sorry

Apparently leaving it blank defaults to 10ms.
putting something like .0000000001 in sure did the trick.
Thank you!
 

eetech00

Joined Jun 8, 2013
3,859
So I know such a thing doesn't exist, but for the purpose of testing something else, I need to get a pulse out of a voltage source that has essentially zero rise time.

I am finding that leaving the rise and fall time on a pulse voltage source give me 10ms rise and fall no matter what I do?

How do I get a pure 0 rise time pulse in lt spice?

Thanks all
Hi

LTspice is doing what it should.
Any "pulse" has rise time. LTspice faithfully honors that in the time domain.

eT
 

Papabravo

Joined Feb 24, 2006
21,159
The reason why LTspice cannot deal with a zero rise time is because the unit step is discontinuous at the origin and the derivative does not have a defined value at the discontinuity.
 

ebeowulf17

Joined Aug 12, 2014
3,307
ok well please disregard!
This is one of those posts that I found the answer like 5 seconds after I posted it. Sorry

Apparently leaving it blank defaults to 10ms.
putting something like .0000000001 in sure did the trick.
Thank you!
Not that it really matters either way, but you can use the abbreviations for prefixes like milli and micro. So, instead of all those zeroes, you can put 1u, 100n, etc.
 

crutschow

Joined Mar 14, 2008
34,285
I believe the default pulse rise and fall time is something like 10% of the pulse period (and zero rise time is not possible, either in simulation or a real circuit)
But you can specify 1ps (picosecond) or less if you like.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
226
The reason why LTspice cannot deal with a zero rise time is because the unit step is discontinuous at the origin and the derivative does not have a defined value at the discontinuity.
Ah yes, forgot about that!
Been a long time since I worked on that kinda stuff.

Thanks all. seriously good people here
 

Papabravo

Joined Feb 24, 2006
21,159
Interestingly I just discovered with respect to diode breakdown models there is a provision for a quadratic fit in the breakdown region. These guys who came up with this stuff were...quite forward thinking.
 
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