Most frequently asked questions for beginners

From LTwiki-Wiki for LTspice
Revision as of 15:40, 17 August 2009 by Analogspiceman (talk | contribs)

Q: What are the different LTspice file types?

Schematic:     name.asc   the drawing with your circuit (a text file)
Symbol:        name.asy   symbols for the schematic (a text file)
Netlist:   unprocessed netlist (text file viewable from within LTspice & usable by other SPICE engines) 
Logfile:       name.log   info and results from .four, .measure, .op (text file viewable from within LTspice)
                          may also contain the fully processed and expanded netlist (a Control Panel setting)
Result:        name.raw   binary output data file (text format may be optionally specified)
Result:        name.fft   binary result of an FFT
Plot settings: name.plt   text file used to save and restore Waveform Viewer plot settings
Model file:   text file containing model(s) - may be any valid file name
Circuit file:  name.cir   frequently used extension for an external netlist input file (text)

Q: How do you copy and paste between schematics?

A: Click the Copy Icon in the tool bar or select Copy from the Edit drop down menu (or type ctrl-C).  Select (with the mouse) what you want to copy.  Make the target schematic active (click on it or its tab or type ctrl-Tab) and then click the Paste Icon in the tool bar or select Paste from the Edit drop down menu (or type ctrl-V).

Q: How do I copy and paste between symbols in the symbol editor?

A: It is not possible to use copy and paste in the symbol editor.  Symbol files are ascii text.  Merge the text as described in message 7201 in the LTspice Yahoo Group.

Q: How can I add intrinsic device models (BJTs, FETs, etc.) to LTspice?

A: Please take a look to the many examples in the files section of the LTspice Yahoo Group (Files => Lib).

Q: How can I add subcircuits to LTspice?

A: You will find many answers when you search the LTspice Yahoo Group messages for words like "library", "symbol" or "FAQ".

Please read first the programs help:
Help -> Schematic Capture -> Editing Components -> Creating New Symbols
Help -> Help Topics ->FAQs -> Third party models
Help -> Help Topics ->FAQs -> Mosfet

You will find also help in the linked documents from our LTspice Yahoo Group.

Links -> Spice Courseware And Tutorials

There is another document about symbols and models in our Files section.

Files -> Tut -> Symbol Types For Subcircuits

Q: 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?

PULSE(0 5 0 0 0 20n 100n)

A: 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.

PULSE(0 5 0 100p 100p 20n 100n)

Don't use steeper transitions than required by your application.

Q: I have used a pure sine source in my schematic, but the output signal of my circuit looks slightly different from cycle to cycle.

A: LTspice has waveform compression enabled as the default setting.  This compression reduces the amount of saved data during the simulation.  It's a lossy compression and thus it can distort the saved signals.  You can switch it off with the following command line in your schematic.

.options plotwinsize=0

It could be switched off in the "Control Panel -> Compression" pane as well, but this setting will be lost upon closing the current session of LTspice.