Hi Raf, I understoud and was aware of the 2 blades. But I still don't agree with you: - you agree with the 2,69 Hz OK - but this is the frequency scale and completely independant of the signal you are measuring, this is purely physics and maths - you can set the scale either in Hz or in RPM still independent of any singal Until now we have a signal analyser with above caracteristics. Now when you record a signal you must ask yourself: what frequencies should, could or would I see in the spectrum and only here you must take into account the 2 blades. So what are these frequencies with a rotor rotation speed for example 1200 rpm: - the rotation of the rotor 1200 rpm > 20Hz - the blade frequeny 2 times rotor 40 Hz - as the blade frequency is not a pure sine you will have a certain number of harmonics 80, 120, 160 Hz etc. - probably the blade frequency is modulated by the rotor frequency, in this case you will see 'side bands'. In an amplitude modulated signal you have the carrier frequeny, in your case the blades, and the modulating frequency in your case the rotor. In the frequency spectrum you will see 3 spectral lines: the carrier frequency and two side band lines with the carrier frequency plus and minus the modulating frequency. Your case is a special one because the carrier frequency (blades) is the double of the modulating frequency (rotor) this means that the frequency of the lower side band is the same as the modulating frequency. At the end you could find following frequencies: - 20 Hz rotor and lower blade side band - 40 Hz blade, 2nd rotor harmonic - 60 Hz upper side band, 3rd rotor harmonic - 80 Hz 2nd blade harmonic 4th rotor harmonic side band and so on. Now the amplitudes of the different peaks will depend on the 'purity' of the noise produced by the different elements. - Rotor probably relatively pure - Blades, anything but pure So probably the main peaks should be: 40 Hz baldes 80 hz 2nd blades harmonic. I hope all this stuff is clear enough. The amplitude modulated frequency example is already included in the (not yet posted) test program. Best regards.