Latest LTspice giving crazy FFT results

ray_moth · 2nd May 2010, 01:39 PM

I update my version of LTspice every so often and I found that the FFT plots from the latest version are infested with harmonics. This wasn't true before, when the usual 3rd, 5th, 7th harmonics were present in diminishing quantities. Now there are literally dozens of the things and they don't decrease so much with increasing harmonic number. (This is for a simulation of a triode-strapped EL34 PP amp). Has anyone else noticed this?

jcx · 2nd May 2010, 07:53 PM

did you remember to turn off data compression?
I always add the spice directive to the schematic:

.options plotwinsize=0

ray_moth · 4th May 2010, 06:12 AM

Thanks for that. I find it difficult to understand mamny of the requirements of running LTspice. I haven't found the 'Help' to be particularly helpful - perhaps I'm just dumb

TheGimp · 4th May 2010, 12:25 PM

I understand what you mean about the help. I need to change the range of the FFT to make it more readable, but the help is less than helpfull on how to scale it.

ray_moth · 5th May 2010, 09:06 PM

I do that by amending the frequency range. I just left-click on the X-axis of the FFT plot and enter your own start and finish values (e.g. 1k and 100k).

revintage · 6th May 2010, 07:07 AM

I guess guys already know that "Blackman" is to be preferred.

2nd May 2010, 01:39 PM	#1
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Latest LTspice giving crazy FFT results I update my version of LTspice every so often and I found that the FFT plots from the latest version are infested with harmonics. This wasn't true before, when the usual 3rd, 5th, 7th harmonics were present in diminishing quantities. Now there are literally dozens of the things and they don't decrease so much with increasing harmonic number. (This is for a simulation of a triode-strapped EL34 PP amp). Has anyone else noticed this?

6th May 2010, 07:07 AM	#6
revintage diyAudio Member Join Date: May 2007 Location: Eskilstuna, Sweden	I guess guys already know that "Blackman" is to be preferred. __________________ Brgds Lars

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2nd May 2010, 07:53 PM	#2
jcx diyAudio Member Join Date: Feb 2003 Location: ..	did you remember to turn off data compression? I always add the spice directive to the schematic: .options plotwinsize=0

4th May 2010, 06:12 AM	#3
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Thanks for that. I find it difficult to understand mamny of the requirements of running LTspice. I haven't found the 'Help' to be particularly helpful - perhaps I'm just dumb

4th May 2010, 12:25 PM	#4
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	I understand what you mean about the help. I need to change the range of the FFT to make it more readable, but the help is less than helpfull on how to scale it.

5th May 2010, 09:06 PM	#5
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	I do that by amending the frequency range. I just left-click on the X-axis of the FFT plot and enter your own start and finish values (e.g. 1k and 100k).

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