Unwanted modulation in crystal oscillator

[David Hess]

Such long-mem mode is less affected, and it's not quite clear to me
while it is not the scope default mode, I suppose there is some drawback
around.

It is a performance issue because processing long records takes longer so fewer records will be processes in a given amount of time.

As I understand based on your wise comments, with such cheap
instruments the choice is just as having the damn storage oscilloscope
to bite you on the left or right buttock.

Any DSO may suffer from this problem under the wrong circumstances and except for the sin(x)/x reconstruction issue on the Rigols, they are not any worse than most DSOs.

Back in the late 1980s, Tektronix came up for a way for a DSO to detect if aliasing was likely present; just compare the trigger and sample frequencies.  And it worked!  Unfortunately, it would have been used by marketing of the other oscilloscope companies to denigrate their products so the idea was dropped.  "Look at how bad Tektronix DSOs are!  They even tell you when aliasing is present!"  Marketing triumphs over engineering every time.

I also found out that setting Peak Detect the artifact vanishes
entirely, at all timebases (irrelevant of long-mem or not).
Actually, the manual says "To Avoid signal aliasing, select Peak Detect
Acquisition".  That seems to be ideal for analyzing a RF with long
timebases, which is exactly what I was trying to do when I bumped
into the artifact, I was looking for the rise and decay time of my
tx output.

Not all DSOs support peak detection but it is my first choice for some measurements and if I want a sanity check.  DSOs without it can still check for aliasing using persistence or envelope detection.

[mauri]

Not all DSOs support peak detection but it is my first choice for some measurements and if I want a sanity check.  DSOs without it can still check for aliasing using persistence or envelope detection.

so there is some workaround in the end.
Now that the picture is getting clearer to me, I can see the beauty of the trap I have fallen in
all that looks now like the most classic example of sync. sampling and aliasing as depicted in the literature,
everything matches perfectly.
In a sense, it has been a fortune to bump into this problem.  You know, the very few things I know I have learned
because something went wrong
It has also been a fortune to read all the comments posted in this thread, I have read and appreciate each of them.