[AMRadio] Re: AM vs SSB???


D. Chester k4kyv at charter.net
Fri Apr 6 01:53:23 EDT 2007


>180W PEP SSB is four times as powerful, or 6dB more power than the
>comparable 45W AM transmitter Add another 3dB for the receiver where he
>bandwidth is also narrower. Total SSB advantage over the comparable AM
>signal is approximately 9 dB.

But if you copy both sidebands of the AM signal,  the two sidebands add to 
vectorally to double the audio output voltage at the dectector, therefore 
quadrupling the power of the recovered signal at the detector.  So the 
vector addition of both sidebands on AM gives 3 extra dB just as the reduced 
bandwidth of the SSB receiver gives 3 extra dB.  Therefore, there is no 
power advantage with either mode, if each has the same total sideband power. 
It doesn't matter whether the power is concentrated into one sideband or 
divided between two sidebands.

180 watts PEP of AM is equavalent to 45 watts of carrier modulated 100%. 
The sideband power is 22.5 watts PEP, or 50% of the carrier power.  So there 
is 8 times as much sideband power in the SSB signal as in the AM signal, or 
9 dB more.

But comparing 180 watts PEP of SSB to 180 watts of AM is like comparing 
apples to oranges.  It is average, or mean power that determines the 
effective power of a signal, not PEP.  With voice modulation, 180 watts PEP 
is approximately 90 watts average power output.  The equivalent power on AM 
would be a 90 watt carrier.  When that carrier is modulated 100%, the 
sideband power would be 45 watts PEP.  In this case, the SSB signal has 4 
times, or 6 dB more sideband power than an AM signal at the same average 
(mean) power output.

So if you are talking about equivalent effective power outputs, that would 
light up a lamp to the same brilliance, or deliver the same amount of heat 
from a dummy load, SSB has a 6 dB advantage.  If you are talking about using 
final amplifiers with the same peak power output capability, SSB has a 9 dB 
advantage, because you can run twice as much average power output on SSB as 
with AM, with the same headroom limitation at the amplifier.

PEP becomes significant in the case of a linear amplifier, since the limit 
to the output is the maximum peak power capability of the amplifier before 
the signal begins to flat top.

Controlled carrier AM reduces the duty cycle demand on the amplifier power 
supply and the plate dissipation of the final tubes by allowing the 
amplifier to rest between syllables, much as in the case of SSB.

Don, k4kyv 




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