THD calculations in multisim.

[giker]

I use Distortion Analyzer for calculating THD in multisim (version 14.0). I interested in two questions:

1. What defenition of FT does use Multisim to calculate THD?

2. I found the formula that should use the Analzer for calculating THD. Is f0 DC component or fundamental harmonic in the formula. If it is fundamental harmonic, how does the analyser use DC component for the THD calculations?

Thank you.

[DiegoH214]

Hello Giker. I believe you can find information related to your questions here.

Configuring a Fourier Analysis in Multisim

http://www.ni.com/tutorial/12784/en/

Diego H

National Instruments

[giker]

I'm trying to reproduce the result that Distortion Analyzer returns me in Excel. But I can't get the same result. The frequency is 100Hz. According data from oscilloscope (that connected to the same wire where Distortion Analyzer connected) multisim produces 1000 points per cycle. So, if multisym performs FFT, it should use amount of points that multiple power of two. I assume that multisim adds zeroes to perform FFT for given cycle.

according to manual the formula for THD in Distortion Analyzer is :

According to the message in Multisym the formula is:

Accordong to description on this page http://www.ni.com/tutorial/12784/en/ the formula is THD = [ (S_i=2V_i²)/V₁] x 100%

What is real formmula? Is there any additional factor that can affect on the THD calcultions for example Equivalent Noise Bandwidth?

Thank you.

[DiegoH214]

Giker

I see why this can be somehow confusing. All the formulas are correct. The first one is the definition of THD. Meaning that it sums up the harmonics frequencies by not taking into account the fundamental one.

The other two formulas are the actual formulas used in calculating THD. With the only exception that the one on the tutorial is a percentage. Since you can select the way to present results, percentage or dBs.

Diego H

National Instruments