[AMRadio] Trouble brewing ...


Donald Chester k4kyv at charter.net
Mon Dec 16 15:58:40 EST 2013


Bry Carling wrote:
> 24KW PEP for parity? I think not.
> 
> Bry Carling
> http://af4k.com


For parity in talk power, you would need the same total sideband power for
both modes. 1500 watts of peak sideband power for AM translates to 3000
watts of fully modulated carrier power, since only on voice PEAKS does the
modulation hit 100%. The modulating signal must be 50% of the carrier power
to modulate 100%, or 1500 watts of power in the two sidebands combined at
voice peaks to fully modulate the 3 kw carrier. 1500 watts of sideband power
is 1500 watts of sideband power, regardless of whether all the power is
concentrated into one sideband, or distributed into two sidebands. The
carrier is just "there" for a reference; it neither adds nor takes away
anything from the talk power

With SSB, the receiver passband need be only one half as wide as with DSB,
so you get a 3 dB s/n ratio advantage with SSB due to the narrower passband
at the receiver, which lets in only half the random noise. That would
represent a 3 dB improvement for SSB. But, with DSB, the voltages from two
sidebands add vectorally, so that the peak voltage at the receiver detector
from the  two  sidebands combined adds up to twice the peak voltage that
would result from each one of the sidebands received individually. P =
voltage squared X R. Doubling the voltage by vectorally combining the two
sidebands gives twice the voltage or 4 times the power of each sideband
taken individually, or 6 dB. But since the receiver passband allows in 3 dB
more noise with DSB, the actual improvement in s/n  ratio is only 3 dB. The
net improvement from combining the upper and lower sidebands is 3 dB over
each one of the sidebands transmitted alone, exactly what would be expected
since the total sideband power of both the sidebands is twice that of each
one.

Don k4kyv


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