Two simulators in Qspice, Qspice64.exe (.option fastmath=1) or Qspice80.exe (.option fastmath=0), .option fastmath statement allows selection of which one to be used.
Qspice64 is 64 bits math (double) and Qspice80 with selective use of 80 bits math. Refer to this post for Mike Engelhardt answer (Qspice author).
Running qspice from command line? - QSPICE - Qorvo Tech Forum
.four is run with Qpost.exe (not in Qspice64.exe or Qspice80.exe). This is a post processing process from the computed data from simulator (i.e. post processing data in .qraw file). There is no reference what precision is used in Qpost.exe, but we have .option MEASDGT can specify number of significant figures in post processing output.
32 bits single precision floating point number with approximately 7 decimal digits of precision. In Qspice, you can specify to return number in measurement with up to 15 significant figures, therefore, I think we expect Qpost runs with 64 bits double precision.
Here is an example to generate a distorted sine wave with harmonics, with 15 significant figures specified.
Based on your screen capture, I believe you post video to compare excel and Qspice (https://www.youtube.com/watch?v=NcomvddNjb0), and I have a feeling that you quote results from other instead of running your own test. Therefore, I also upload this .qsch file and you can download Qspice and run a test on it.
four.qsch (3.3 KB)
