Thank you for your questions.

1. It is caused by spectral leakage which is inherent in FFT. Spectral leakage occurs when a FFT segment does not contain exactly an integer number of the signal cycles. It causes the signal power / energy to leak from its original FFT bin into its adjacent FFT bins. As a result, the signal power / energy inside its original FFT bin drops. This is explained in details in Section 8.2 in

https://www.virtins.com/doc/Measurement ... rument.pdf. Multi-Instrument is able to work out the correct power / energy of spectral peaks even under spectral leakage conditions. Thus the signal power / energy values displayed on the top of the spectrum graph and in the DDP viewers (e.g. f1RMS_A(EU), f2RMS_A(EU)) will still be correct. To display the absolute signal power / energy at spectral peaks correctly in the spectrum graph under spectral leakage conditions, right click the spectrum graph and select [Spectrum Analyzer Chart Options]>”Mark Peaks”.

In your screenshot, although the "No Spectral Leakage" checkbox in the Signal Generator panel is ticked, the bottom left corner of the Spectrum Analyzer shows "Zero Padding", which implies that your FFT Size is greater than the Record Length of the Oscilloscope and thus the spectral leakage still occurs. Choose a Record Length equal to or greater than the FFT size will avoid this. Also, Rectangle Window function should be used for "No Spectral Leakage" spectrum analysis.

2. Your screenshot shows severe spectral leakage, mostly due to that the Record Length of the Oscilloscope is too short (10ms). As a result, the spectrum graph is dominated by the "wide" fundamental and harmonics, leaving very little space for frequency components to be considered as noise. Increasing the Record Length of the Oscilloscope (by righting click the Oscilloscope Window and select "T" Range at the bottom left corner of the screen) may solve the problem.

3. SFDR

As the second highest spectral peak is not necessarily a harmonic of the fundamental, the measurement of SFDR is parked under the "Peak" detection function of the Spectrum Analyzer (Right click the Spectrum Analyzer Window and select [Spectrum Analyzer Processing]>"Peaks").