"Richard" ) writes:
Went into a shop today and tried a small (5" x 3"x 3/4") PLL AM/FM portable.
Tuned to a local AM station. Audio excellent for the size of speaker. Tuned
up 9Khz and station still there, very much audible, with a bit of splatter.
Tuned up18Khz from station, station still audible, but much slatter. Gosh,
they make the AM filters wide these days don't they in your pocket
portable!

I suppose that in practical use as a non DX radio, the wide bandwith has
little significance. Not likely that another favourite station is going to
be within 18Khz. Audio quality over selectivity obviously. You feel that
the radio you are listening to is a peice of junk because of the wide
bandwidth, but I guess not, if it gives such good audio quality for such a
small size.



I had a digitally tuned pocket radio from Radio Shack, and after I dropped
it in water, I had the chance to take it apart. It only used IF transformers
on AM, so it wasn't anything much in the selectivity department.

Keep in mind, though, that if the PLL isn't well designed, you will get
spurs and a signal can turn up in more than one place on the dial. In
this case, IF selectivity is not the problem, since the same station is
being converted to the IF due to spurs from the PLL. If you've got a local
station that's stronger than the more distant stations, even the reduced
level of the spurs means that the local station can wipe out the weak station.

I've had digitally tuned AM broadcast radios where a local station will
appear up and down the dial from where it belongs, with perfect fidelity
but with reduced amplitude. But at night when some of the strong signals
come booming in, they wipe out those spurs.

Michael VE2BVW