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

[Alan Bloom]

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).

> I remember the AM guys doing proof to 10 KHz. 

Right, in order to confirm that the modulation is down 25 dB at 10.2
kHz.

Al N1AL


On Sun, 2008-01-13 at 20:40, Joe Subich, W4TV wrote:
> > Even with the 10 kHz channel spacing used in the USA, AM broadcast
> > stations do not have 5 kHz audio bandwidth.  The FCC requires a guard
> > band between stations.  As I recall, rgulations require that the audio
> > start to drop off at about 4 kHz so that it can be down 20 dB 
> > or so by 6 kHz (the passband edge of the adjacent station).
> 
> I don't think that's right ... or wasn't the last time I was around 
> an AM station (I spent most of my career in TV).  I remember the AM 
> guys doing proof to 10 KHz. 
> 
> Admittedly, many of the directional stations could not maintain 10 KHz 
> through the phasors and the high end got trashed at night but the old 
> allocation systems generally kept first adjacent situations far enough 
> apart that 10 KHz could be obtained on groundwave during the daytime.  
> 
> "In the day" most receivers would start to roll off somewhere around 
> 6 KHz and the better ones had a 10 KHz notch for nighttime conditions. 
> 
> Given the DSP demodulation in the K3, it's a shame that there isn't 
> an "offset" option to do "vestigial sideband" demodulation (offset the 
> AM filter to the upper sideband or lower sideband) and demodulate 
> carrier and one sideband for better fidelity.  This would work quite 
> well if the carrier were placed at the -6dB point on the composite 
> filter passband since it would keep the proper ratio between carrier 
> and sideband.  Alternatively, the carrier could be moved to 1 KHz 
> from the -6 dB point and the DSP could equalize out the 6 dB boost 
> in audio below 1 KHz from the "opposite" sideband. 
> 
> 73, 
> 
>    ... Joe, W4TV 
>