Why it is not valid? What is aliasing (it is not in my dictionary)?
With Owon (and many many other oscilloscopes including also some Rigol lowest end models)I have made lot lot of lab tests.)
(here borrowed for example picture from Owon thread page 57 where is also more).
For this measurement can use slow speeds for BW tests. It is quite normal method. This may avoid also some possible other fast sweep problems)
There is very slow samplerate and still no need think aliasing (in this case) but wait a moment...
(aliasing is not known?? and using digital oscilloscope... perhaps this is good to upgrade asap for avoid mistakes in measurements... but no problem, it is easy to learn. it is one very very basic fundamental what need know and know what all it is and what kind of affect it have... in different waveforms in normal oscilloscope trace and FFT. )
You see owon image I have used very slow speed (10s/div) and tested with sweep from 1MHz to 250MHz and "samplerate" is 50 samples per second (becouse in this case only 10k sampling buffer selected) This 50 samples/s mean that ALL analog frequency components what ADC input can see and what are over 25Hz produce exactly sure aliasing (if you see it or not but it is there). This is basic fundamental what can not break!
But example in Owon behind this 50sample/second ADC circuit works still (in this case 1GSa/s) full speed. For aliasing this do not matter and for alising it is 50 sample per second.
(ADC works full speed and systenm use only every 20000000's sample.)
How to do?
Turn sampling (acquire) mode to "peak" (it is used in this Owon test)
Now system select (inside this 1/50s window highest samples (different scopes use different methods perhaps (of course this have also bad effect becouse now this sample time is where ever inside this "window" but in this case it do not mean anything) and what happend... now you do not need put one penny for thinkin aliasing (in this case).
Of course if do some accurate tests also generator starting level need adjust with this frequency what want keep as reference level. Also it is good to use signal generator what have accurate flatness over used frequency band.