THD is different in RTA vs Stepped sine

[vasiliy]

I get significantly different THD numbers and harmonic levels when I measure in RTA vs Stepped Sine.
Stepped Sine vs Sine Sweep give me roughly the same results.
Settings seems to be the same - you can see on screenshots.

 

[John Mulcahy]

Is the distortion increasing over time? The stepped test doesn't spend long at each point and you have 3 seconds cool-down between points.

 

[vasiliy]

not increasing. very stable. the volume is pretty low.
3 sec cooldown left from previous measurements of small midranges.

 

[John Mulcahy]

Strange. Since you are using a UMIK-1 you shouldn't really use a rectangular window since it is unlikely the replay and receive clocks are identical, though it shouldn't make much difference at those frequencies and FFT lengths. Worth trying with BH-7 though.