M
Michael Black
Guest
On Sat, 9 Jun 2018, Phil Allison wrote:
FM tuners that could be switched between "wide" and "narrow", in relative
terms. So for strong signals, wider bandwidth is fine. But for weaker
signals, narrower bandwidth avoids interference from adjacent signals that
are stronger. It wasn't uncommon for FM DXers to swap the ceramic filters
in their FM receivers from the often 280KHz bandwidth to down about
180KHz, at one time one could go to a catalog and order Murata ceramic
filters in a range of bandwidths. If you don't need FM, you can get by
with narrower, though of course nt in the tens of KHz wide.
The scanner wants "narrow" for two way communication which is narrow
deviation, 10KHz or smaller in recent years. The wide is for broadcast FM
and maybe some other things, since yes, the "narrow" in this case is way
too narrow for FM broadcast. Of course, the wider bandwidth can be useful
for things like receiving weather satellites, which may have a wider
deviation of something like 40KHz, but also because of doppler shift, an
even wider bandwidth makes things easier. I know I've seen modifications
for scanners to use with weather satellites, and they bypass the narrow
filter at 455KHz, which leaves an FM broadcast band type ceramic filter at
the first IF of 10.7MHz.
There was a time twenty years ago when I was bringing home lots of Delco
car radios from garage sales. I'm not sure what the FM filter is in
there, but they certainly seemed to have better skirt selectivity than
other FM radios I'm familiar with. The AM filters seeemd sharper too.
Michael
Continuously variable for FM doesn't make sense. But there have been some
FM tuners that could be switched between "wide" and "narrow", in relative
terms. So for strong signals, wider bandwidth is fine. But for weaker
signals, narrower bandwidth avoids interference from adjacent signals that
are stronger. It wasn't uncommon for FM DXers to swap the ceramic filters
in their FM receivers from the often 280KHz bandwidth to down about
180KHz, at one time one could go to a catalog and order Murata ceramic
filters in a range of bandwidths. If you don't need FM, you can get by
with narrower, though of course nt in the tens of KHz wide.
The scanner wants "narrow" for two way communication which is narrow
deviation, 10KHz or smaller in recent years. The wide is for broadcast FM
and maybe some other things, since yes, the "narrow" in this case is way
too narrow for FM broadcast. Of course, the wider bandwidth can be useful
for things like receiving weather satellites, which may have a wider
deviation of something like 40KHz, but also because of doppler shift, an
even wider bandwidth makes things easier. I know I've seen modifications
for scanners to use with weather satellites, and they bypass the narrow
filter at 455KHz, which leaves an FM broadcast band type ceramic filter at
the first IF of 10.7MHz.
There was a time twenty years ago when I was bringing home lots of Delco
car radios from garage sales. I'm not sure what the FM filter is in
there, but they certainly seemed to have better skirt selectivity than
other FM radios I'm familiar with. The AM filters seeemd sharper too.
Michael