Back in the 'old days' of AM broadcasting in the US, the local stations
were spread out with greater than 10 KHz spacing so they would not
interfere even when received on wide bandwidth AM receivers. There were
a few 'clear channel' superstations that had no competition nationwide
and IIRC, they had a 20 kHz swath of spectrum. Those were mostly
Westinghouse stations which ran 50,000 watts and could be heard over
great distances. On normal stations, one would receive maximum fidelity
with a 10 kHz IF filter (if a straight sided filter would have been
available back then) because the modulation was supposed to be limited
to less 5 kHz (or so rumor had it in those days).
73,
Don W3FPR
Alan Bloom wrote:
OK< I looked it up. According to Title 47, part 73.44 of the FCC
regulations, <http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM> the
modulation of an AM broadcast station must be down 25 dB at 10.2 kHz
from the carrier. Assuming a 3-pole low-pass filter (e.g. a
pi-network), the filter attenuation is 18 dB per octave, which implies a
cutoff frequency of no more than 3.9 kHz. The -3 dB bandwidth would be
a little higher than that.
That's about what I remember from my broadcasting days many, many years
ago. If you think about it, a double-sideband AM signal can't have a
bandwidth greater than 1/2 the channel spacing without interfering with
adjacent channels. And it has to be somewhat less than that given
real-world filters. So there is not much point in having a receiver
with much more than 4 kHz or so audio response (8 kHz or so RF
bandwidth).
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