How to add distortion numbers

[DF96]

Couldn't you just convert into percentages and add those for a total?

Using percentages is the same as using a ratio, except that the decimal point is in a different place, so it saves no work. However you do it, the dB needs to be converted to a ratio as dB are meant for multiplying not adding.

Presumably, the harmonics of a signal are correlated, so add as voltage, not power.

To add as voltage you need to know relative phase. If the distortion arises from simple nonlinearity (e.g. with no phase shifting due to filters anywhere) then you can probably assume they will be in phase or antiphase.

Would that make sense?

Only if you make an assumption about phase.

If the sig gen produced 5% 2nd and after the amp you measure 15% second then it could mean:
a. in phase, the amp adds 10% 2nd
b. antiphase, the amp adds 20% 2nd
c. quadrature, the amp adds 14% 2nd
You get the picture? Option c is less likely, but a and b are equally likely.

 

[jan.didden]

Agree - I was for simplicity assuming no phase difference.

Jan