I got it work but I am not sure how you get the second Cursor reading, I can get Cursor 1 reading but not sure how to get the second cursor reading like in your example?
I have attached a picture, you will see only one cursor reading.
To do that, place the mouse cursor on the name of the trace of interest at the top of the plot, such as V(out), and when the pointer appears,
right click and a box will appear. In the upper right corner, click on the blue arrow of "Attached Cursor", select "1st & 2nd" and then click on OK to close the box. Those two cursors are now link to the values of the plot trace selected. If one wants to compare the value of another to that of the initial plot trace, say V(in), place the mouse cursor on that trace name and
left click once and move the traces to the points of interest. By left clicking a second time, both cursors will attach to the second trace. This will work on any plot, transient, FFT, etc.
When looking at the FFT on screen & the frequency spectrum of the output, what are we actually looking at in regards to say 10% -20% etc 2nd Harmonic distortion.
I presented this option so you might see the relationships of the harmonic distribution/content in the full spectrum given your initial inquiry. If one compares the voltage ratio of the fundamental to the total value of that in the harmonics, the TDH can be determined. For further background info. check out this link:
Total harmonic distortion - Wikipedia, the free encyclopedia
To get the actual THD from LT Spice modeling, you can use this basic spice directive I'll describe:
With the schematic selected, click on the ".op" icon in the very upper right of the toolbar. When the box opens and with the "spice directive" button selected (above the dialog area) type in
.four 4k V(out) . This instructs LT Spice to compute the fourier component of the fundamental frequency of 4kHz (the frequency in this particular case) of V(out) with the default value of nine points, the fundamental and eight harmonics. Look up "harmonics" in the help section to customize the spice directive for a number of options.
To see the result, run a transient response, and when the plot is displayed, follow the procedure to view the FFT plot, BUT select Spice Error log instead to view the values of the fundamental, the individual harmonics and the THD.
Example:
The Fundamental frequency on the attachment is around 7.7dB, how many dB does the second harmonic have to be to be audible with a definite difference in sound.
Just trying to understand what does & what doesn't make an audible difference.
I would think that a composite 30db or greater below the fundamental would be more than acceptable. That's a ratio of 1000:1.
Sorry, but I can't be of too much help beyond there. I haven't heard anything very well after a couple deployments to Southeast Asia forty odd years ago. Perhaps AG or another can help you with that comparison.
Hope the above give you some additional information.
Regards,
Merv