Reading THD, please help:)

[the19thbear]

Hi.
how do i read a thd graph?
It says total amount of thd is 0.6
But.. that doesnt makes sense? the 2nd harmonic is 0.6 something alone, and there are lots of other harmonics... shouldnt it add up to alot more than 0.6 - if 0.6 +0.2 + etc etc etc



thanks

[jksuperstar]

The graph might be a little misleading. THD is supposed to be a calculation as the sum of the Power of each harmonic, which is then divided by the Power of the fundamental. Your graph is labelled as Distortion (%).... not sure if it is actually Power, or if each harmonic is already calculated as a % relative to the fundamental. In that case, you'd have to multiply each harmonic by the fundamental (looks like 0.8 in the graph), add them up, then divide again by 0.8 to get the THD value.

[the19thbear]

So your saying that the total thd is correct but the individual harmonics aren't correct. The relationship to each other are right (2nd harmin is this much louder than 3rd harmonic as in the graph) but each individual graphs reported thd is incorrect?

Thanks alot!:)

[jksuperstar]

Yes, I'm saying that the graph reports "Harmonic Distortion" for each individual harmonic, so it should not be used as raw Power data to calculate the Total Harmonic Distortion. Not that the data is incorrect, it's just in a different form than is used for THD calculation.

I looked at the ymec.com site about RA, and it seems you can:

1.) switch from % to db reporting, to get the power of each harmonic so you can do the math as usual.
2.) in % mode: if possible, increase the measurement level (currently at -10db) until the fundamental is at a level of 1. This should make it easier to calculate THD from individual harmonic distortion %, as the math between THD and % Harmonic Distortion would be the same (dividing by 1 makes things nice & clean).

Look at the equations for THD on wikipedia. P1 is the fundamental, and is what you want to be "1" in % mode. Then it's just like adding up all the individual numbers like you want to do.

I hope that helps and doesn't make things more distorted