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How to make a crisp square wave in LTSpice?

Hi - this seems like a simple thing... but I can't figure it out. I made a voltage source in LTSpice, set it to: PULSE(0V 5V 0 0ms 0ms

500ms 1000ms 2). However, rise and fall times seem to be about 50ms when driving a 1K load. I don't know how to make sense of this. Messing around with it, it seems to adjust the rise and fall times to have some sort of relationship with the period of the square wave. I just want a perfect square wave with infinite slew rate. Anybody know what I'm doing wrong?

Thanks!

-Michael

Reply to
Michael
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It seems that if you set the rise and fall times too slow, they default to something silly. So set them to 1n and all is well.

Reply to
Paul Burke

That's not an LTSpice problem specifically...you're giving it regularly spaced time samples of a square wave, and it's drawing straight lines between points. A 'crisp square wave' requires infinite time resolution. For plots, one justs adds points for the tops and bottoms of the cliffs, but SPICE won't like it if you try to make a voltage have two different values at the same time.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

I think you meant too fast, not too slow. Anyways - that was correct. Apparently if you set rise and fall times to 0 then it defaults to something like 5% of the period. I set it to 1us and now it works perfectly. It seems like a warning should be generated for this... I mean it is not exactly obvious. At least to me.

-Michael

Reply to
Michael

Hi Phil - that was one of my first thoughts as well, but it didn't work. Anyways - Paul was dead on. Setting rise and fall times to something very small but greater than zero fixed the problem.

-Michael

Reply to
Michael

"Michael" schrieb im Newsbeitrag news: snipped-for-privacy@28g2000hsw.googlegroups.com...

Hello Michael,

If you specify 0 for Trise or Tfall, LTspice will use a value of 10% of Ton or Toff whichever is smaller.

You have to specify a small number for Trise and Tfall.

PULSE(0 5 0 1n 1n 500m 1)

Best regards, Helmut

Reply to
Helmut Sennewald

It's going to be a long, long, LONG list enumerating all the things that are not obvious to you.

Reply to
Fred Bloggs

Hi Phil - that was one of my first thoughts as well, but it didn't work. Anyways - Paul was dead on. Setting rise and fall times to something very small but greater than zero fixed the problem.

-Michael

It's a common sense, where in the world can any one find a rising time of ZERO?

Ah... I know, in the bastard mind who went to war in Iraq.

Reply to
MooseFET

I played about with it a bit and found that .1p works but not .01p. What is the actual time resolution?

Reply to
Paul Burke

Hello Paul,

There is a limit in the ratio of about 1e12 between rise/fall time and the time span specified in .TRAN. I don't know the progam code, but that's my experience.

I never had a problem with that. A simulation over 1 sec with picosecond rise times would be nonsense at all in my opinion.

Best regards, Helmut

Reply to
Helmut Sennewald

It would give you time for a movie followed by dinner and drinks ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
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Reply to
Jim Thompson

There are both pulse sources and piecewise linear sources. Use whichever works best for you.

Reply to
JosephKK

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