Re: why the signal from a 50-ohm output impedance device should be checked by a scope channel with a 50-ohm input impedance?

If the source is designed to be operated into a 50-ohm load, that's the
way it should be tested. Most scopes these days have a switchable
50-ohm termination built-in to make this easy.

Nothing wrong with doing it with a scope with 1Mohm input impedance as
long as you've got a 50-ohm terminator Teed off at the input too
(that's the way it used to be done before scopes had built-in 50-ohm
terminations.)

Tim.

.