Most frequently asked questions for beginners

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Q: What are the different LTspice file types?


   Schematic:     name.asc   the drawing with your circuit
   Symbol:        name.asy   symbols for the schematic
   Logfile:       name.log   infos and results from .four, .measure, .op 
   Netlist:   you don't care about 
   Result:        name.raw   usually a binary file
   Result:        name.fft   result of an FFT, usually a binary file
   Plot settings: name.plt 
   Model file:   contains the models, any file name  
   Symbol:        name.cir   if you have a netlist input file

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

A: Please take a look to the many examples in the section "Files > Lib".

Q: How can I add subcircuits to LTspice?

A: You WILL find many answers when you search the 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: How do you copy and paste between schematics?

A: Click on "Copy" in the tool bar. Select what you want to copy. Make the other schematic active and then click on "paste" in the tool bar."

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 Yahoo Group. [1]

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 have to specify Trise and Tfall if you want a certain value.

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

Don't use steeper transitions as 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 after a new invocation of LTspice.