The FCC"s frequency assignment policy protected these Class 1A stations
from both co-channel and adjacent channel interference, if my
recollection of 40 year old rules is correct. Obviously greater
co-channel protection was provided than adjacent channel (+/- 10 KHz)
protection.
As far as frequency response, I worked my way through law school as a
transmitter engineer at a 50 KW AM station in Detroit 35 years or so
ago. We had an RCA 50KW ampliphase transmitter model BTA-50H, for
daytime operation. This particular transmitter design did not have a
modulation transformer, but rather had two independent signal paths that
were combined at the output port. When the signals were in phase, you
got modulation peaks; when out of phase 180 degrees, you got the
modulation minimum. There's a copy of the RCA review article on these
transmitters at http://www.fmamradios.com/Ampliphase.html.
In any event, when we ran an audio proof on the BTA-50H, we took it up
to 7.5 KHz, which I recall as being the upper limit required by the
then-current FCC rules. The BTA-50H would do quite well in both
distortion and in amplitude response over the full audio range, if the
exciter had been recently tuned, itself a challenging task, particularly
with the original vacuum tube exciter. RCA came out with a solid state
replacement exciter around 1975, which helped stability quite a bit.
Neither of the conventional transmitters (a 10KW RCA and a 1 kW Gates)
would do as well in the audio department as the ampliphase.
This was before the days of asymmetrical modulation, i.e., the limit was
100% for both positive and negative modulation. (Now 125% positive /
100% negative, as I recall.)
A number of AM stations now run AM digital mode, which uses two
multi-channel digital signals in the upper and lower 10 KHz adjacent
channel spectrum. I have some spectrum analyzer images of these signals
that I'll put up on my web site one of these days.
Jack K8ZOA
Don Wilhelm wrote:
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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