In article ,
Look165 wrote:

selectivity is the opposite to sensivity.

By no means. They're almost orthogonal in principle, although in
practice they may tend to go in opposite directions.

Sensitivity is a measure of how well a tuner picks up the desired
(on-frequency) signal... that is, how weak a "wanted" signal it can
detect.

Selectivity is a measure of how well a tuner _rejects_ non-desired
signals (on adjacent channels, alternate channels, and further
away)... that is, how strong an "unwanted" signal it can filter out.

Cheap tuners may be both insensitive, and not very selective.

Really good FM tuners can be highly selective, _and_ highly sensitive.
This generally requires having at least one stage of selectivity
(i.e. one or more tuned stages) before the first RF amplifier - these
stages keep strong, off-frequency signals from overloading the amp.

Tuners that are beloved of the "FM DX" crowd will tend to have:

- Quite a few stages of selectivity in the front-end. You'll see
tuners described as having 3, 4, 5, or more "sections" in the front
end - each section is a tuned circuit. Commonly, one section tunes
the local oscillator, and the rest provide RF selectivity before
the mixer.

- Several different bandwidths available in the IF section. A narrow
IF (more stages of IF filtering, and/or more-narrowly-tuned filters
in each stage) provides better selectivity. This can be at the
cost of sensitivity (each IF filter stage adds some amount of loss,
even to the desired signal) so a good narrow-IF design will include
additional stages of IF amplification to make up the losses and
restore the sensitivity.

- Post-detection filters, after the FM detector and before the
stereo-multiplex decoder. These help filter out "birdies" and IBOC
digital sub-carrier noise, from FM stations on adjacent or
alternate channels.