[Elecraft] AM bandwidth, the rest of the story :=)

[Jack Smith]

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).
>>  
> _______________________________________________
> Elecraft mailing list
> Post to: Elecraft at mailman.qth.net
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft   
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>