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

[Alan Bloom]

Joe,

Somehow they have to limit their occupied bandwidth to less than 10
kHz.  If they don't there will be severe adjacent-channel interference. 
That means at most 5 kHz audio bandwidth, assuming a brick-wall filter.

Using a real-world filter, it has to be substantially less than 5 kHz. 
You can argue whether the right number is 4 kHz or 4.5 kHz, but for sure
it isn't 10 kHz.

73,

Al N1AL


On Tue, 2008-01-15 at 07:22, Joe Subich, W4TV 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.  
> 
> 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 
> > >  
> > 
> > 
> >