What is an harmonic?

[Bricolo]

ok

I wish I have an oscilloscope (one with FFT )
how does a such thing cost? (certainly well too much for me, poor little student )

 

[traderbam]

Brain in gear

Till wrote: "what is about mixing the signals?"
Till, you are very smart to question this. You are on to something very important here. Keep thinking about this.

THD is a very narrow way of looking at distortion. Of course, we really want to measure total distortion. THD is NOT total distortion. Not all distortion will be harmonically related to input signals. Not all of it will be integer multiples. Not all of it will show up on a spectrum analyzer...

BAM

 

[Bricolo]

Yes, I saw other kind of distortion at school, last week.
On an amplifier, you also can see "amplifier related" distortion, when a sinusoidal (in example) input signal isn't sinusoidal at the output. Or a s sinusoidal with "flatened" peaks (this is called saturation, at least in french)

 

[dhaen]

Bricolo,
The "saturation" distortion IS extreme harmonic distortion.
Maybe you should look for some articles on "Fourier"- in French. He invented the mathematics to analyse these phenomena.
This is a very interesing subject, and is perhaps too important and difficult to try and understand in a foreign language.

 

[Nelson Pass]

how can you conclude it's 2nd, 3rd or 4rd harmonic, looking the curve?

You look at the distortion output and count the bumps.
It's pretty easy to see.

A note on IMD vs THD: At reasonable levels, the ones
we are interested in, there is a distinctly proportional
relationship between THD and IMD, so both are equally
valid approaches. IMD takes noise out of the equation,
which is sometimes convenient, and IMD analyzers such
as the classic 60/7000 Hz / 4:1 type are extremely easy to
build and require no active tweaking.