[AMRadio] Re: Screen Modulated 813


Bob Bruhns bbruhns at erols.com
Tue Dec 12 20:21:51 EST 2006


The maximum voice asymmetry possible without distortion is about
2:1.  Wityh 100% negative modulation, that would correspond to 200%
positive modulation.  So you could have a good sounding station
with 100% negative and 200% modulation.  Unless people are going to
listen with product detectors, that's about the limit for good
quality - and it requires receivers that have average AGC, and also
have the peak headroom in the IF amps to pass the peaks.  I find
that most receivers have the headroom, but some have peak-AGC, and
that screws up AM anyway.

With 200% mod, the PEP is nine times the carrier power.  So for a
carrier of 1 watt, 100% positive modulation would produce 4W PEP,
and 200% positive modulation would produce 9W pep.

With efficiency modulation (linear, screen mod), peak efficiency is
about 60%.  With 100% positive mod, carrier efficiency is half of
peak, or 30%.  With 200% mod, carrier efficiency is 1/3 of peak, or
20%.

So with 1 watt carrier 200% mod using a linear or screen
modulation, you have 2 watts dissipation at carrier level, and 3.6
watts dissipation at the peak level.  Dissipation is maximum
somewhere between carrier and peak level.

Efficiency modulation (linear, screen mod, etc) with an 813 at 200%
mod would permit a carrier level of about 30 watts, and peaks to
about 270 watts.  You can probably push the 813 a bit further if it
isn't getting too hot, but I don't think you can get 45 watts
carrier with 200% modulation in that service from an 813.

Efficiency mod with a 6146 at 200% mod would permit a carrier level
of about 8 watts, and peaks to about 72 watts.  You can probably
push the 6146 a bit further if it isn't getting too hot, but I
don't think you can get 12 watts carrier with 200% modulation in
that service from a 6146.

It is real impressive to have the positive peaks go way up, but you
might do better to use CPAM - Constant Peak AM.  You can get crude
CPAM by running AM through a linear amp with ALC.  When you don't
speak, the ALC will raise gain and your carrier will be at the peak
level.  When you speak, the ALC will drop the amplifier gain, and
the carrier level will drop, making room for positive peaks.  You
can modulate to any percentage you desire.  Good AM receivers will
quiet beteen words and syllables because of the high carrier level.
Dissipation will be a little higher, so an 813 might be able to do
250 watts PEP output max in that service, and a 6146 might be able
to do 60 watts pep output.  See
http://www.qsl.net/wa5bxo/asyam/aam3.html

  Bacon, WA3WDR



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