[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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