Hamateur wrote:
. . .
I tend to think of "stability" more in terms of random fluctuations.
Instability may or may not effect selectivity. As long as my desired
selection remains decipherable, I would say that selectivity has
been accomplished regardless of whether there's any kind of instabilty.

Filtering strong nearby signals seems more about "exclusivity"
than "selectivity". I would rather say that IF stages maintain
selectivity while they are excluding undesired mixing products
and other signals.
. . .

You're certainly free to make up interpretations of words any way you
choose. But if you want to communicate with others, that is, to have
them understand what you're saying and for you to understand what
they're saying, it's necessary to use common terms in the way they're
widely understood to mean.

In this context, "selectivity" is universally understood to mean the
ability to pass some signals and reject others, on the basis of their
frequencies, and is quite independent of stability. If this isn't what
you mean by "selectivity", you should use some other word or make up a
new one and define it, if your objective is to understand and be understood.

In a superhet receiver, most of the selectivity is achieved in the IF
stages, for a number of good reasons. One of the reasons is that it
prevents off-frequency signals from being amplified to a high level
where they can cause intermodulation and other problems. In a direct
conversion receiver, all the selectivity (other than relatively broad
selectivity from any bandpass filtering ahead of the mixer) is achieved
by audio filtering. Properly done, this filtering is near the input of
the high gain audio amplifier. Neither is inherently better than the
other at the basic job of providing selectivity.

Roy Lewallen, W7EL