Measuring THD

[meconlen]

Every definition of the measurement of THD implies a sum from 1 to infinity. Unless you count the sound floor as 0 this is immediately meaningless since such a sum won't converge.

Google turns up pages of people who say things like the first three or first five harmonics only. One page even said that the subharmonics were harmonics and counted from 1 being one of the subharmonics (1k with a 7k fundamental, how he chose 1k as being the first I have no idea...).

When I take the output from a HP 331A and correct for the meter rage setting and the -20 dB level for full meter indication I get something that approximates the meter's reading when measuring with a fundamental of 1k at +4 dB from a HP 625A. On the other hand when I look at the corrected numbers, they are all below the sound floor, yet I can clearly see harmonics above the sound floor in the output of the 652A. Should I be converting the sound floor and distortion measurements to voltage and summing them at each harmonic?

1) When measuring THD on a spectrum analyzer how many measurements do you take?
2) How do you handle the sound floor? Do you just not count any measurement that is the sound floor?

Yes, I know that THD isn't the be all and end all of sound quality, but for my purposes right now I'm playing around with the right quiescent current levels for various topologies of differential amplifier and keeping the response flat with respect to voltage is a worthy goal.

Thanks

--
Mike