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

[Joe Subich, W4TV]

> 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.  

The information at 73.44(b) only applies to out of band emissions. 
You will note that the section applies additional limitations at 
20 KHz from carrier, 30 KHz from carrier, etc. 

> 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.

Any AM station that used a simply pi-network filter would have 
real problems since they would sound "muddy" an lack punch.  I 
think you will find that they are using filters much more complex 
than a simple pi-network - usually with a sharp cut-off ("brick 
wall") response to maintain response out to 10 KHz. For example, 
the audio response specification for the Harris AM transmitters 
is: +0.2/-0.8 dB, 20 Hz to 10 kHz. Ref. 1 kHz at 95% modulation.
See www.broadcast.harris.com/radio/transmission/analog.asp and 
look at the various specification sheets for the Analog AM 
transmitters.   

73, 

   ... Joe, W4TV 
   


> -----Original Message-----
> From: Alan Bloom [mailto:n1al at cds1.net] 
> Sent: Tuesday, January 15, 2008 1:50 AM
> To: Joe Subich, W4TV
> Cc: elecraft at mailman.qth.net
> Subject: AM bandwidth, the rest of the story :=)
> 
> 
> 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 
> >  
> 
> 
>