thanks a lot for your help.
1°) I've done a mistake : the amplitude setting in spectrum analyzer was wrong (too low ---> big distortion!!)
2°) But I still have many problems :
a) mesurement of THD (in %) :
signal : 1,7kHz sinus 3V peak to peak
spectrum settings : 39,06kHz Hann Window average amplitude : +/-20V to +/-2V (see below)
+/- 2V ---> 0,07%
+/- 5V ---> 0,03%
+/- 10V ---> 0,05%
+/- 20V ---> 0,67%
With my HP8903E (setting with 30kHz filter to reduce bandwidth) : 0,01%
I put a 250Hz, 1V peak to peak in both channel (at the same time) with my Marconi generator :
results are completely different on channel A and on channel B...
I thought it could be my cables, so I invert the two cables : still the same!
Why do I don't have the same specrum view in both channels??? Why do I have so many harmonics?