Fix distortion analyses?

[gmack]

It appears that the Distortion Analyses in Multisims Versions 8 - 13 (and likely earlier) don't provide correct results (see my previous posts).  Can/will it ever be fixed?  If so, will it be limited to only certain devices like it was in Berkeley SPICE 3f*?: "Distortion analysis is supported for the following nonlinear devices: diodes (DIO), BJT, JFET, MOSFETs

(levels 1, 2, 3, 4/BSIM1, 5/BSIM2, and 6) and MESFETS. All linear devices are automatically supported by distortion analysis."

[Tien_P]

Hi,

Are you referring to the magnitude output of the distortion analysis vs. the Fourier? The distortion analysis is based on the AC frequency response and the magnitude from that analysis in a complex number, Fourier is based on the Transient Analysis and the output magnitude is peak voltage. 

[gmack]

Shouldn't the magnitudes of the 2nd and 3rd harmonic distortion components at any given frequency be the same as the "Norm. Mag" of the 2nd and 3rd Harmonics provided by a Fourier Analysis of a sine wave at that frequency with the same signal level?  I.e. for a 4Vp 1kHz Sinewave input and output at "Probe_output" of the "Distortion_HD" sample circuit, the Fourier Transform shows -37dB and -29dB 2nd and 3rd harmonic distortion components whereas the "Distorion Analysis" (and "Help" files and "User Manuals") show -51 dB and +27.2 dB for these components!

[gmack]

Tien_P

Understand my question/point?

[Tien_P]

Distrototion and Fourier preform different calculations. Distortion is based on AC analysis which prefrom small-signal analysis while the Fourier uses Transient and it performs large-signal analysis.

[gmack]

That does appear to be the case - so the distortion analysis sample circuits are inappropriate, right?  But, even the results for small signals aren't correct - try both approaches with a 1mVp 1kHz sinewave on a more appropriate/linear circuit!