On Thu, 07 Jun 2018 20:32:03 -0400, micky
wrote:

I've known the basics of sensitivity and selectivity for a long time,
and I read more abou them on the web this week, but it hasn't helped.

I want a newer car radio for my 2005 Toyota Solara, and I live in
Baltimore and want to be able to receive WAMU, 88.5 and WCSP
(c-span)90.1, from DC.

Which value matters in predicting whether I'll be able to do so,
selectivity or sensitivity?

Both, and both have complications.
For selectivity, if your FM radio can hear HD Radio (IBOC), it needs
to have an IF bandwidth of at least 400 KHz. Here's what it looks
like on a spectrum analyzer:
http://www.learnbydestroying.com/jeffl/crud/KBRG-100_3.jpg
http://www.learnbydestroying.com/jeffl/crud/KCSM.jpg
It won't work if the IF bandwidth (-3dB down) were exactly 400 KHz as
it would be clipping the corners of the IF bandpass and probably have
horrible group delay. It has to be wider, typically about 500 to 600
KHz IF bandwidth. However, even if it was 400 KHz wide, you would
still have an adjacent channel problem. In the US, FM channels are on
200 KHz intervals. That means the digital part of the spectra
overlaps the conventional analog FM part of the spectrum in the
adjacent channel. If you were listening to a conventional FM station,
and there were an HD Radio digital station on the adjacent 200 KHz
slot, its digital signal would slop into your IF bandpass and all you
would hear is digital garbage.

The closest approximation to a solution are the digital FM receiver
and demodulator chips by SiLabs.
https://www.silabs.com/products/audio-and-radio/multi-band-radios
The IF bandwidth is programmable and very much a brick wall. The
designers can narrow up the IF bandwidth to exactly 200 KHz which will
remove most, but not all, of the HD Radio garbage in the adjacent
channel. You'll find these chips in radios by Tecsun and Meloson. Not
sure about car radios.

So the importance of selectivity depends on what you're listening to
(conventional FM, or HD Radio) and what's on the adjacent channel.

It's almost midnight and I'm beat. I'll dive into the sensitivity
part later, when I'm more awake. Incidentally, FM receiver
sensitivity is rather oddly measured in dBf (dB above 1 femtowatt).

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558