Now I am confused. Are you referring to the analog antialiasing filter before the digitizer or the reconstruction filter after the digitizer? If aliasing occurs then the later cannot do anything about it.
Both are low-pass filters. Leakage is a phenomenon that is not associated with aliasing (since it happens below Nyquist) and is not preventable by antialiasing or reconstruction filters.
Yes, and we established that in the screenshots I made there were
no frequency components above Nyquist, so aliasing was not occurring.
But when reconstructing after the digitizer the scope is unable to reliably reconstruct the real 120 MHz signal, as this is too near the Nyquist frequency.
note: what results is not really an AM signal, but a DSB signal (or AM with suppressed carrier, the sine does not ride the wave) -equivalent to the summation of the original "real" 120 MHz and the leaked "mirror" at 130 MHz. In the FFT's a few pages ago and attached here you can see that the amplitude of the mirror grows as the sampled frequency approaches Fnyquist. The result is shown as a double-sideband signal, not really an AM signal as there is no power in the central frequency of 125 MHz.
All this is
after the digitizer and no aliasing occurring.
So as Marmad noticed this leakage is no issue till say Fsample/2.5 (my 100 MHz). There the leakage is so small is does not show any more.
As I understand it (now) up until Fnyquist you can reconstruct the frequency, but the amplitude information gets lost above Fsample/2.5. Just too few samples, and these samples "shift" along the wave resulting in the AM like waveform
Looking at the samples near Fnyquist themselves I am not even sure that a reliable reconstruction of the amplitude is even possible, and that it has nothing to do with errors in the reconstruction algorithms used by Rigol? At least that is what the documentation Marmad supplied is suggesting.
Interesting learning experience. This goes to show that one should be really really careful when interpreting displayed waveforms even long before FNyquist. So the DS1000Z's are 100 MHz scopes? Yes, but beware..