Compare Qspice and LTspice


I had to look for a second but I finally see it, the plot axis is not the same for LT vs Q spice.

bordodynov, run again in QSPICE but this time with this:
.ac oct 100 1k 100k

Not in this case. See


What is the point here? Is there a question?

The question was that Qspice, instead of getting -6 dB, gave a different, incorrect result. Unlike LTspice. And then after the update, switching to increased accuracy gave the correct result. The question disappeared.
Other programs give incorrect results.

C1 and C2 show as 100nF with one teraohm series resistors, is it what you’d in mind when doing the exercise?
I mean, are trying to check if a combo of a 1T plus one tenth of a microfarad in series is treated equally as the component with a parasitic modeled in?
What’s the purpose of such test?

No. C1=C2=0.1=100mF=100000uF.
The scheme was invented by the author of LTspice (and Qspice). It demonstrated the superiority of LTspice over other programs.

OK, then the values are still more far from normal usage and the exercise is a synthetic circuit which only has the purpose to test the numerical treatment of the simulator vis-as-vis to the “competition”.

It remains to be seen if this “superiority” of LTspice is of pratical use, against, say the better diode model M. Engelhardt shows in his presentation in Youtube.

Given the author is the same for both simulators, maybe Mike will find where to tweak the code and get back the missing performance of the previous simulator.

I thought this discussion was closed when @bordodynov posted a screen where “Fast (less accurate) Math” is deselect as a solution.
Just to ensure everyone know exact reason why this option gives two different results, here is an explanation

In Qspice HELP > Simulator, it mentioned two simulator executables which are QSPICE64.exe and QSPICE80.exe

In Preference, you can decide simulation to run with 64 or 80.

  • Select “Fast (less accurate) Math” : use QSPICE64.exe
  • Deselect “Fast (less accurate) Math” : use QSPICE80.exe

Mike Engelhardt explained the different between QSPICE64 and QSPICE80 in this post : QSPICE80.exe uses 80 bit math in critical sections
Running qspice from command line? - QSPICE - Qorvo Tech Forum

It seems that this circuit with large number and require 80 bit math to handle it.
So, nothing goes wrong in here, as higher bit math normally rarely needed and in default Qspice use Qspice64.exe.

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